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llvm-project/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp

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///===- SimpleLoopUnswitch.cpp - Hoist loop-invariant control flow ---------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/SimpleLoopUnswitch.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Sequence.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/GuardUtils.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopAnalysisManager.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/Utils/Local.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/GenericDomTree.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar/SimpleLoopUnswitch.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <numeric>
#include <utility>
#define DEBUG_TYPE "simple-loop-unswitch"
using namespace llvm;
STATISTIC(NumBranches, "Number of branches unswitched");
STATISTIC(NumSwitches, "Number of switches unswitched");
STATISTIC(NumGuards, "Number of guards turned into branches for unswitching");
STATISTIC(NumTrivial, "Number of unswitches that are trivial");
STATISTIC(
NumCostMultiplierSkipped,
"Number of unswitch candidates that had their cost multiplier skipped");
static cl::opt<bool> EnableNonTrivialUnswitch(
"enable-nontrivial-unswitch", cl::init(false), cl::Hidden,
cl::desc("Forcibly enables non-trivial loop unswitching rather than "
"following the configuration passed into the pass."));
static cl::opt<int>
UnswitchThreshold("unswitch-threshold", cl::init(50), cl::Hidden,
cl::desc("The cost threshold for unswitching a loop."));
static cl::opt<bool> EnableUnswitchCostMultiplier(
"enable-unswitch-cost-multiplier", cl::init(true), cl::Hidden,
cl::desc("Enable unswitch cost multiplier that prohibits exponential "
"explosion in nontrivial unswitch."));
static cl::opt<int> UnswitchSiblingsToplevelDiv(
"unswitch-siblings-toplevel-div", cl::init(2), cl::Hidden,
cl::desc("Toplevel siblings divisor for cost multiplier."));
static cl::opt<int> UnswitchNumInitialUnscaledCandidates(
"unswitch-num-initial-unscaled-candidates", cl::init(8), cl::Hidden,
cl::desc("Number of unswitch candidates that are ignored when calculating "
"cost multiplier."));
static cl::opt<bool> UnswitchGuards(
"simple-loop-unswitch-guards", cl::init(true), cl::Hidden,
cl::desc("If enabled, simple loop unswitching will also consider "
"llvm.experimental.guard intrinsics as unswitch candidates."));
/// Collect all of the loop invariant input values transitively used by the
/// homogeneous instruction graph from a given root.
///
/// This essentially walks from a root recursively through loop variant operands
/// which have the exact same opcode and finds all inputs which are loop
/// invariant. For some operations these can be re-associated and unswitched out
/// of the loop entirely.
static TinyPtrVector<Value *>
collectHomogenousInstGraphLoopInvariants(Loop &L, Instruction &Root,
LoopInfo &LI) {
assert(!L.isLoopInvariant(&Root) &&
"Only need to walk the graph if root itself is not invariant.");
TinyPtrVector<Value *> Invariants;
// Build a worklist and recurse through operators collecting invariants.
SmallVector<Instruction *, 4> Worklist;
SmallPtrSet<Instruction *, 8> Visited;
Worklist.push_back(&Root);
Visited.insert(&Root);
do {
Instruction &I = *Worklist.pop_back_val();
for (Value *OpV : I.operand_values()) {
// Skip constants as unswitching isn't interesting for them.
if (isa<Constant>(OpV))
continue;
// Add it to our result if loop invariant.
if (L.isLoopInvariant(OpV)) {
Invariants.push_back(OpV);
continue;
}
// If not an instruction with the same opcode, nothing we can do.
Instruction *OpI = dyn_cast<Instruction>(OpV);
if (!OpI || OpI->getOpcode() != Root.getOpcode())
continue;
// Visit this operand.
if (Visited.insert(OpI).second)
Worklist.push_back(OpI);
}
} while (!Worklist.empty());
return Invariants;
}
static void replaceLoopInvariantUses(Loop &L, Value *Invariant,
Constant &Replacement) {
assert(!isa<Constant>(Invariant) && "Why are we unswitching on a constant?");
// Replace uses of LIC in the loop with the given constant.
for (auto UI = Invariant->use_begin(), UE = Invariant->use_end(); UI != UE;) {
// Grab the use and walk past it so we can clobber it in the use list.
Use *U = &*UI++;
Instruction *UserI = dyn_cast<Instruction>(U->getUser());
// Replace this use within the loop body.
if (UserI && L.contains(UserI))
U->set(&Replacement);
}
}
/// Check that all the LCSSA PHI nodes in the loop exit block have trivial
/// incoming values along this edge.
static bool areLoopExitPHIsLoopInvariant(Loop &L, BasicBlock &ExitingBB,
BasicBlock &ExitBB) {
for (Instruction &I : ExitBB) {
auto *PN = dyn_cast<PHINode>(&I);
if (!PN)
// No more PHIs to check.
return true;
// If the incoming value for this edge isn't loop invariant the unswitch
// won't be trivial.
if (!L.isLoopInvariant(PN->getIncomingValueForBlock(&ExitingBB)))
return false;
}
llvm_unreachable("Basic blocks should never be empty!");
}
/// Insert code to test a set of loop invariant values, and conditionally branch
/// on them.
static void buildPartialUnswitchConditionalBranch(BasicBlock &BB,
ArrayRef<Value *> Invariants,
bool Direction,
BasicBlock &UnswitchedSucc,
BasicBlock &NormalSucc) {
IRBuilder<> IRB(&BB);
Value *Cond = Direction ? IRB.CreateOr(Invariants) :
IRB.CreateAnd(Invariants);
IRB.CreateCondBr(Cond, Direction ? &UnswitchedSucc : &NormalSucc,
Direction ? &NormalSucc : &UnswitchedSucc);
}
/// Rewrite the PHI nodes in an unswitched loop exit basic block.
///
/// Requires that the loop exit and unswitched basic block are the same, and
/// that the exiting block was a unique predecessor of that block. Rewrites the
/// PHI nodes in that block such that what were LCSSA PHI nodes become trivial
/// PHI nodes from the old preheader that now contains the unswitched
/// terminator.
static void rewritePHINodesForUnswitchedExitBlock(BasicBlock &UnswitchedBB,
BasicBlock &OldExitingBB,
BasicBlock &OldPH) {
for (PHINode &PN : UnswitchedBB.phis()) {
// When the loop exit is directly unswitched we just need to update the
// incoming basic block. We loop to handle weird cases with repeated
// incoming blocks, but expect to typically only have one operand here.
for (auto i : seq<int>(0, PN.getNumOperands())) {
assert(PN.getIncomingBlock(i) == &OldExitingBB &&
"Found incoming block different from unique predecessor!");
PN.setIncomingBlock(i, &OldPH);
}
}
}
/// Rewrite the PHI nodes in the loop exit basic block and the split off
/// unswitched block.
///
/// Because the exit block remains an exit from the loop, this rewrites the
/// LCSSA PHI nodes in it to remove the unswitched edge and introduces PHI
/// nodes into the unswitched basic block to select between the value in the
/// old preheader and the loop exit.
static void rewritePHINodesForExitAndUnswitchedBlocks(BasicBlock &ExitBB,
BasicBlock &UnswitchedBB,
BasicBlock &OldExitingBB,
BasicBlock &OldPH,
bool FullUnswitch) {
assert(&ExitBB != &UnswitchedBB &&
"Must have different loop exit and unswitched blocks!");
Instruction *InsertPt = &*UnswitchedBB.begin();
for (PHINode &PN : ExitBB.phis()) {
auto *NewPN = PHINode::Create(PN.getType(), /*NumReservedValues*/ 2,
PN.getName() + ".split", InsertPt);
// Walk backwards over the old PHI node's inputs to minimize the cost of
// removing each one. We have to do this weird loop manually so that we
// create the same number of new incoming edges in the new PHI as we expect
// each case-based edge to be included in the unswitched switch in some
// cases.
// FIXME: This is really, really gross. It would be much cleaner if LLVM
// allowed us to create a single entry for a predecessor block without
// having separate entries for each "edge" even though these edges are
// required to produce identical results.
for (int i = PN.getNumIncomingValues() - 1; i >= 0; --i) {
if (PN.getIncomingBlock(i) != &OldExitingBB)
continue;
Value *Incoming = PN.getIncomingValue(i);
if (FullUnswitch)
// No more edge from the old exiting block to the exit block.
PN.removeIncomingValue(i);
NewPN->addIncoming(Incoming, &OldPH);
}
// Now replace the old PHI with the new one and wire the old one in as an
// input to the new one.
PN.replaceAllUsesWith(NewPN);
NewPN->addIncoming(&PN, &ExitBB);
}
}
/// Hoist the current loop up to the innermost loop containing a remaining exit.
///
/// Because we've removed an exit from the loop, we may have changed the set of
/// loops reachable and need to move the current loop up the loop nest or even
/// to an entirely separate nest.
static void hoistLoopToNewParent(Loop &L, BasicBlock &Preheader,
DominatorTree &DT, LoopInfo &LI,
MemorySSAUpdater *MSSAU) {
// If the loop is already at the top level, we can't hoist it anywhere.
Loop *OldParentL = L.getParentLoop();
if (!OldParentL)
return;
SmallVector<BasicBlock *, 4> Exits;
L.getExitBlocks(Exits);
Loop *NewParentL = nullptr;
for (auto *ExitBB : Exits)
if (Loop *ExitL = LI.getLoopFor(ExitBB))
if (!NewParentL || NewParentL->contains(ExitL))
NewParentL = ExitL;
if (NewParentL == OldParentL)
return;
// The new parent loop (if different) should always contain the old one.
if (NewParentL)
assert(NewParentL->contains(OldParentL) &&
"Can only hoist this loop up the nest!");
// The preheader will need to move with the body of this loop. However,
// because it isn't in this loop we also need to update the primary loop map.
assert(OldParentL == LI.getLoopFor(&Preheader) &&
"Parent loop of this loop should contain this loop's preheader!");
LI.changeLoopFor(&Preheader, NewParentL);
// Remove this loop from its old parent.
OldParentL->removeChildLoop(&L);
// Add the loop either to the new parent or as a top-level loop.
if (NewParentL)
NewParentL->addChildLoop(&L);
else
LI.addTopLevelLoop(&L);
// Remove this loops blocks from the old parent and every other loop up the
// nest until reaching the new parent. Also update all of these
// no-longer-containing loops to reflect the nesting change.
for (Loop *OldContainingL = OldParentL; OldContainingL != NewParentL;
OldContainingL = OldContainingL->getParentLoop()) {
llvm::erase_if(OldContainingL->getBlocksVector(),
[&](const BasicBlock *BB) {
return BB == &Preheader || L.contains(BB);
});
OldContainingL->getBlocksSet().erase(&Preheader);
for (BasicBlock *BB : L.blocks())
OldContainingL->getBlocksSet().erase(BB);
// Because we just hoisted a loop out of this one, we have essentially
// created new exit paths from it. That means we need to form LCSSA PHI
// nodes for values used in the no-longer-nested loop.
formLCSSA(*OldContainingL, DT, &LI, nullptr);
// We shouldn't need to form dedicated exits because the exit introduced
// here is the (just split by unswitching) preheader. However, after trivial
// unswitching it is possible to get new non-dedicated exits out of parent
// loop so let's conservatively form dedicated exit blocks and figure out
// if we can optimize later.
formDedicatedExitBlocks(OldContainingL, &DT, &LI, MSSAU,
/*PreserveLCSSA*/ true);
}
}
/// Unswitch a trivial branch if the condition is loop invariant.
///
/// This routine should only be called when loop code leading to the branch has
/// been validated as trivial (no side effects). This routine checks if the
/// condition is invariant and one of the successors is a loop exit. This
/// allows us to unswitch without duplicating the loop, making it trivial.
///
/// If this routine fails to unswitch the branch it returns false.
///
/// If the branch can be unswitched, this routine splits the preheader and
/// hoists the branch above that split. Preserves loop simplified form
/// (splitting the exit block as necessary). It simplifies the branch within
/// the loop to an unconditional branch but doesn't remove it entirely. Further
/// cleanup can be done with some simplify-cfg like pass.
///
/// If `SE` is not null, it will be updated based on the potential loop SCEVs
/// invalidated by this.
static bool unswitchTrivialBranch(Loop &L, BranchInst &BI, DominatorTree &DT,
LoopInfo &LI, ScalarEvolution *SE,
MemorySSAUpdater *MSSAU) {
assert(BI.isConditional() && "Can only unswitch a conditional branch!");
LLVM_DEBUG(dbgs() << " Trying to unswitch branch: " << BI << "\n");
// The loop invariant values that we want to unswitch.
TinyPtrVector<Value *> Invariants;
// When true, we're fully unswitching the branch rather than just unswitching
// some input conditions to the branch.
bool FullUnswitch = false;
if (L.isLoopInvariant(BI.getCondition())) {
Invariants.push_back(BI.getCondition());
FullUnswitch = true;
} else {
if (auto *CondInst = dyn_cast<Instruction>(BI.getCondition()))
Invariants = collectHomogenousInstGraphLoopInvariants(L, *CondInst, LI);
if (Invariants.empty())
// Couldn't find invariant inputs!
return false;
}
// Check that one of the branch's successors exits, and which one.
bool ExitDirection = true;
int LoopExitSuccIdx = 0;
auto *LoopExitBB = BI.getSuccessor(0);
if (L.contains(LoopExitBB)) {
ExitDirection = false;
LoopExitSuccIdx = 1;
LoopExitBB = BI.getSuccessor(1);
if (L.contains(LoopExitBB))
return false;
}
auto *ContinueBB = BI.getSuccessor(1 - LoopExitSuccIdx);
auto *ParentBB = BI.getParent();
if (!areLoopExitPHIsLoopInvariant(L, *ParentBB, *LoopExitBB))
return false;
// When unswitching only part of the branch's condition, we need the exit
// block to be reached directly from the partially unswitched input. This can
// be done when the exit block is along the true edge and the branch condition
// is a graph of `or` operations, or the exit block is along the false edge
// and the condition is a graph of `and` operations.
if (!FullUnswitch) {
if (ExitDirection) {
if (cast<Instruction>(BI.getCondition())->getOpcode() != Instruction::Or)
return false;
} else {
if (cast<Instruction>(BI.getCondition())->getOpcode() != Instruction::And)
return false;
}
}
LLVM_DEBUG({
dbgs() << " unswitching trivial invariant conditions for: " << BI
<< "\n";
for (Value *Invariant : Invariants) {
dbgs() << " " << *Invariant << " == true";
if (Invariant != Invariants.back())
dbgs() << " ||";
dbgs() << "\n";
}
});
// If we have scalar evolutions, we need to invalidate them including this
// loop and the loop containing the exit block.
if (SE) {
if (Loop *ExitL = LI.getLoopFor(LoopExitBB))
SE->forgetLoop(ExitL);
else
// Forget the entire nest as this exits the entire nest.
SE->forgetTopmostLoop(&L);
}
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// Split the preheader, so that we know that there is a safe place to insert
// the conditional branch. We will change the preheader to have a conditional
// branch on LoopCond.
BasicBlock *OldPH = L.getLoopPreheader();
BasicBlock *NewPH = SplitEdge(OldPH, L.getHeader(), &DT, &LI, MSSAU);
// Now that we have a place to insert the conditional branch, create a place
// to branch to: this is the exit block out of the loop that we are
// unswitching. We need to split this if there are other loop predecessors.
// Because the loop is in simplified form, *any* other predecessor is enough.
BasicBlock *UnswitchedBB;
if (FullUnswitch && LoopExitBB->getUniquePredecessor()) {
assert(LoopExitBB->getUniquePredecessor() == BI.getParent() &&
"A branch's parent isn't a predecessor!");
UnswitchedBB = LoopExitBB;
} else {
UnswitchedBB =
SplitBlock(LoopExitBB, &LoopExitBB->front(), &DT, &LI, MSSAU);
}
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// Actually move the invariant uses into the unswitched position. If possible,
// we do this by moving the instructions, but when doing partial unswitching
// we do it by building a new merge of the values in the unswitched position.
OldPH->getTerminator()->eraseFromParent();
if (FullUnswitch) {
// If fully unswitching, we can use the existing branch instruction.
// Splice it into the old PH to gate reaching the new preheader and re-point
// its successors.
OldPH->getInstList().splice(OldPH->end(), BI.getParent()->getInstList(),
BI);
if (MSSAU) {
// Temporarily clone the terminator, to make MSSA update cheaper by
// separating "insert edge" updates from "remove edge" ones.
ParentBB->getInstList().push_back(BI.clone());
} else {
// Create a new unconditional branch that will continue the loop as a new
// terminator.
BranchInst::Create(ContinueBB, ParentBB);
}
BI.setSuccessor(LoopExitSuccIdx, UnswitchedBB);
BI.setSuccessor(1 - LoopExitSuccIdx, NewPH);
} else {
// Only unswitching a subset of inputs to the condition, so we will need to
// build a new branch that merges the invariant inputs.
if (ExitDirection)
assert(cast<Instruction>(BI.getCondition())->getOpcode() ==
Instruction::Or &&
"Must have an `or` of `i1`s for the condition!");
else
assert(cast<Instruction>(BI.getCondition())->getOpcode() ==
Instruction::And &&
"Must have an `and` of `i1`s for the condition!");
buildPartialUnswitchConditionalBranch(*OldPH, Invariants, ExitDirection,
*UnswitchedBB, *NewPH);
}
// Update the dominator tree with the added edge.
DT.insertEdge(OldPH, UnswitchedBB);
// After the dominator tree was updated with the added edge, update MemorySSA
// if available.
if (MSSAU) {
SmallVector<CFGUpdate, 1> Updates;
Updates.push_back({cfg::UpdateKind::Insert, OldPH, UnswitchedBB});
MSSAU->applyInsertUpdates(Updates, DT);
}
// Finish updating dominator tree and memory ssa for full unswitch.
if (FullUnswitch) {
if (MSSAU) {
// Remove the cloned branch instruction.
ParentBB->getTerminator()->eraseFromParent();
// Create unconditional branch now.
BranchInst::Create(ContinueBB, ParentBB);
MSSAU->removeEdge(ParentBB, LoopExitBB);
}
DT.deleteEdge(ParentBB, LoopExitBB);
}
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// Rewrite the relevant PHI nodes.
if (UnswitchedBB == LoopExitBB)
rewritePHINodesForUnswitchedExitBlock(*UnswitchedBB, *ParentBB, *OldPH);
else
rewritePHINodesForExitAndUnswitchedBlocks(*LoopExitBB, *UnswitchedBB,
*ParentBB, *OldPH, FullUnswitch);
// The constant we can replace all of our invariants with inside the loop
// body. If any of the invariants have a value other than this the loop won't
// be entered.
ConstantInt *Replacement = ExitDirection
? ConstantInt::getFalse(BI.getContext())
: ConstantInt::getTrue(BI.getContext());
// Since this is an i1 condition we can also trivially replace uses of it
// within the loop with a constant.
for (Value *Invariant : Invariants)
replaceLoopInvariantUses(L, Invariant, *Replacement);
// If this was full unswitching, we may have changed the nesting relationship
// for this loop so hoist it to its correct parent if needed.
if (FullUnswitch)
hoistLoopToNewParent(L, *NewPH, DT, LI, MSSAU);
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
LLVM_DEBUG(dbgs() << " done: unswitching trivial branch...\n");
++NumTrivial;
++NumBranches;
return true;
}
/// Unswitch a trivial switch if the condition is loop invariant.
///
/// This routine should only be called when loop code leading to the switch has
/// been validated as trivial (no side effects). This routine checks if the
/// condition is invariant and that at least one of the successors is a loop
/// exit. This allows us to unswitch without duplicating the loop, making it
/// trivial.
///
/// If this routine fails to unswitch the switch it returns false.
///
/// If the switch can be unswitched, this routine splits the preheader and
/// copies the switch above that split. If the default case is one of the
/// exiting cases, it copies the non-exiting cases and points them at the new
/// preheader. If the default case is not exiting, it copies the exiting cases
/// and points the default at the preheader. It preserves loop simplified form
/// (splitting the exit blocks as necessary). It simplifies the switch within
/// the loop by removing now-dead cases. If the default case is one of those
/// unswitched, it replaces its destination with a new basic block containing
/// only unreachable. Such basic blocks, while technically loop exits, are not
/// considered for unswitching so this is a stable transform and the same
/// switch will not be revisited. If after unswitching there is only a single
/// in-loop successor, the switch is further simplified to an unconditional
/// branch. Still more cleanup can be done with some simplify-cfg like pass.
///
/// If `SE` is not null, it will be updated based on the potential loop SCEVs
/// invalidated by this.
static bool unswitchTrivialSwitch(Loop &L, SwitchInst &SI, DominatorTree &DT,
LoopInfo &LI, ScalarEvolution *SE,
MemorySSAUpdater *MSSAU) {
LLVM_DEBUG(dbgs() << " Trying to unswitch switch: " << SI << "\n");
Value *LoopCond = SI.getCondition();
// If this isn't switching on an invariant condition, we can't unswitch it.
if (!L.isLoopInvariant(LoopCond))
return false;
auto *ParentBB = SI.getParent();
SmallVector<int, 4> ExitCaseIndices;
for (auto Case : SI.cases()) {
auto *SuccBB = Case.getCaseSuccessor();
if (!L.contains(SuccBB) &&
areLoopExitPHIsLoopInvariant(L, *ParentBB, *SuccBB))
ExitCaseIndices.push_back(Case.getCaseIndex());
}
BasicBlock *DefaultExitBB = nullptr;
SwitchInstProfUpdateWrapper::CaseWeightOpt DefaultCaseWeight =
SwitchInstProfUpdateWrapper::getSuccessorWeight(SI, 0);
if (!L.contains(SI.getDefaultDest()) &&
areLoopExitPHIsLoopInvariant(L, *ParentBB, *SI.getDefaultDest()) &&
!isa<UnreachableInst>(SI.getDefaultDest()->getTerminator())) {
DefaultExitBB = SI.getDefaultDest();
} else if (ExitCaseIndices.empty())
return false;
LLVM_DEBUG(dbgs() << " unswitching trivial switch...\n");
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// We may need to invalidate SCEVs for the outermost loop reached by any of
// the exits.
Loop *OuterL = &L;
if (DefaultExitBB) {
// Clear out the default destination temporarily to allow accurate
// predecessor lists to be examined below.
SI.setDefaultDest(nullptr);
// Check the loop containing this exit.
Loop *ExitL = LI.getLoopFor(DefaultExitBB);
if (!ExitL || ExitL->contains(OuterL))
OuterL = ExitL;
}
// Store the exit cases into a separate data structure and remove them from
// the switch.
SmallVector<std::tuple<ConstantInt *, BasicBlock *,
SwitchInstProfUpdateWrapper::CaseWeightOpt>,
4> ExitCases;
ExitCases.reserve(ExitCaseIndices.size());
SwitchInstProfUpdateWrapper SIW(SI);
// We walk the case indices backwards so that we remove the last case first
// and don't disrupt the earlier indices.
for (unsigned Index : reverse(ExitCaseIndices)) {
auto CaseI = SI.case_begin() + Index;
// Compute the outer loop from this exit.
Loop *ExitL = LI.getLoopFor(CaseI->getCaseSuccessor());
if (!ExitL || ExitL->contains(OuterL))
OuterL = ExitL;
// Save the value of this case.
auto W = SIW.getSuccessorWeight(CaseI->getSuccessorIndex());
ExitCases.emplace_back(CaseI->getCaseValue(), CaseI->getCaseSuccessor(), W);
// Delete the unswitched cases.
SIW.removeCase(CaseI);
}
if (SE) {
if (OuterL)
SE->forgetLoop(OuterL);
else
SE->forgetTopmostLoop(&L);
}
// Check if after this all of the remaining cases point at the same
// successor.
BasicBlock *CommonSuccBB = nullptr;
if (SI.getNumCases() > 0 &&
std::all_of(std::next(SI.case_begin()), SI.case_end(),
[&SI](const SwitchInst::CaseHandle &Case) {
return Case.getCaseSuccessor() ==
SI.case_begin()->getCaseSuccessor();
}))
CommonSuccBB = SI.case_begin()->getCaseSuccessor();
if (!DefaultExitBB) {
// If we're not unswitching the default, we need it to match any cases to
// have a common successor or if we have no cases it is the common
// successor.
if (SI.getNumCases() == 0)
CommonSuccBB = SI.getDefaultDest();
else if (SI.getDefaultDest() != CommonSuccBB)
CommonSuccBB = nullptr;
}
// Split the preheader, so that we know that there is a safe place to insert
// the switch.
BasicBlock *OldPH = L.getLoopPreheader();
BasicBlock *NewPH = SplitEdge(OldPH, L.getHeader(), &DT, &LI, MSSAU);
OldPH->getTerminator()->eraseFromParent();
// Now add the unswitched switch.
auto *NewSI = SwitchInst::Create(LoopCond, NewPH, ExitCases.size(), OldPH);
SwitchInstProfUpdateWrapper NewSIW(*NewSI);
// Rewrite the IR for the unswitched basic blocks. This requires two steps.
// First, we split any exit blocks with remaining in-loop predecessors. Then
// we update the PHIs in one of two ways depending on if there was a split.
// We walk in reverse so that we split in the same order as the cases
// appeared. This is purely for convenience of reading the resulting IR, but
// it doesn't cost anything really.
SmallPtrSet<BasicBlock *, 2> UnswitchedExitBBs;
SmallDenseMap<BasicBlock *, BasicBlock *, 2> SplitExitBBMap;
// Handle the default exit if necessary.
// FIXME: It'd be great if we could merge this with the loop below but LLVM's
// ranges aren't quite powerful enough yet.
if (DefaultExitBB) {
if (pred_empty(DefaultExitBB)) {
UnswitchedExitBBs.insert(DefaultExitBB);
rewritePHINodesForUnswitchedExitBlock(*DefaultExitBB, *ParentBB, *OldPH);
} else {
auto *SplitBB =
SplitBlock(DefaultExitBB, &DefaultExitBB->front(), &DT, &LI, MSSAU);
rewritePHINodesForExitAndUnswitchedBlocks(*DefaultExitBB, *SplitBB,
*ParentBB, *OldPH,
/*FullUnswitch*/ true);
DefaultExitBB = SplitExitBBMap[DefaultExitBB] = SplitBB;
}
}
// Note that we must use a reference in the for loop so that we update the
// container.
for (auto &ExitCase : reverse(ExitCases)) {
// Grab a reference to the exit block in the pair so that we can update it.
BasicBlock *ExitBB = std::get<1>(ExitCase);
// If this case is the last edge into the exit block, we can simply reuse it
// as it will no longer be a loop exit. No mapping necessary.
if (pred_empty(ExitBB)) {
// Only rewrite once.
if (UnswitchedExitBBs.insert(ExitBB).second)
rewritePHINodesForUnswitchedExitBlock(*ExitBB, *ParentBB, *OldPH);
continue;
}
// Otherwise we need to split the exit block so that we retain an exit
// block from the loop and a target for the unswitched condition.
BasicBlock *&SplitExitBB = SplitExitBBMap[ExitBB];
if (!SplitExitBB) {
// If this is the first time we see this, do the split and remember it.
SplitExitBB = SplitBlock(ExitBB, &ExitBB->front(), &DT, &LI, MSSAU);
rewritePHINodesForExitAndUnswitchedBlocks(*ExitBB, *SplitExitBB,
*ParentBB, *OldPH,
/*FullUnswitch*/ true);
}
// Update the case pair to point to the split block.
std::get<1>(ExitCase) = SplitExitBB;
}
// Now add the unswitched cases. We do this in reverse order as we built them
// in reverse order.
for (auto &ExitCase : reverse(ExitCases)) {
ConstantInt *CaseVal = std::get<0>(ExitCase);
BasicBlock *UnswitchedBB = std::get<1>(ExitCase);
NewSIW.addCase(CaseVal, UnswitchedBB, std::get<2>(ExitCase));
}
// If the default was unswitched, re-point it and add explicit cases for
// entering the loop.
if (DefaultExitBB) {
NewSIW->setDefaultDest(DefaultExitBB);
NewSIW.setSuccessorWeight(0, DefaultCaseWeight);
// We removed all the exit cases, so we just copy the cases to the
// unswitched switch.
for (const auto &Case : SI.cases())
NewSIW.addCase(Case.getCaseValue(), NewPH,
SIW.getSuccessorWeight(Case.getSuccessorIndex()));
} else if (DefaultCaseWeight) {
// We have to set branch weight of the default case.
uint64_t SW = *DefaultCaseWeight;
for (const auto &Case : SI.cases()) {
auto W = SIW.getSuccessorWeight(Case.getSuccessorIndex());
assert(W &&
"case weight must be defined as default case weight is defined");
SW += *W;
}
NewSIW.setSuccessorWeight(0, SW);
}
// If we ended up with a common successor for every path through the switch
// after unswitching, rewrite it to an unconditional branch to make it easy
// to recognize. Otherwise we potentially have to recognize the default case
// pointing at unreachable and other complexity.
if (CommonSuccBB) {
BasicBlock *BB = SI.getParent();
// We may have had multiple edges to this common successor block, so remove
// them as predecessors. We skip the first one, either the default or the
// actual first case.
bool SkippedFirst = DefaultExitBB == nullptr;
for (auto Case : SI.cases()) {
assert(Case.getCaseSuccessor() == CommonSuccBB &&
"Non-common successor!");
(void)Case;
if (!SkippedFirst) {
SkippedFirst = true;
continue;
}
CommonSuccBB->removePredecessor(BB,
/*KeepOneInputPHIs*/ true);
}
// Now nuke the switch and replace it with a direct branch.
SIW.eraseFromParent();
BranchInst::Create(CommonSuccBB, BB);
} else if (DefaultExitBB) {
assert(SI.getNumCases() > 0 &&
"If we had no cases we'd have a common successor!");
// Move the last case to the default successor. This is valid as if the
// default got unswitched it cannot be reached. This has the advantage of
// being simple and keeping the number of edges from this switch to
// successors the same, and avoiding any PHI update complexity.
auto LastCaseI = std::prev(SI.case_end());
SI.setDefaultDest(LastCaseI->getCaseSuccessor());
SIW.setSuccessorWeight(
0, SIW.getSuccessorWeight(LastCaseI->getSuccessorIndex()));
SIW.removeCase(LastCaseI);
}
// Walk the unswitched exit blocks and the unswitched split blocks and update
// the dominator tree based on the CFG edits. While we are walking unordered
// containers here, the API for applyUpdates takes an unordered list of
// updates and requires them to not contain duplicates.
SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
for (auto *UnswitchedExitBB : UnswitchedExitBBs) {
DTUpdates.push_back({DT.Delete, ParentBB, UnswitchedExitBB});
DTUpdates.push_back({DT.Insert, OldPH, UnswitchedExitBB});
}
for (auto SplitUnswitchedPair : SplitExitBBMap) {
DTUpdates.push_back({DT.Delete, ParentBB, SplitUnswitchedPair.first});
DTUpdates.push_back({DT.Insert, OldPH, SplitUnswitchedPair.second});
}
DT.applyUpdates(DTUpdates);
if (MSSAU) {
MSSAU->applyUpdates(DTUpdates, DT);
if (VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
}
assert(DT.verify(DominatorTree::VerificationLevel::Fast));
// We may have changed the nesting relationship for this loop so hoist it to
// its correct parent if needed.
hoistLoopToNewParent(L, *NewPH, DT, LI, MSSAU);
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
++NumTrivial;
++NumSwitches;
LLVM_DEBUG(dbgs() << " done: unswitching trivial switch...\n");
return true;
}
/// This routine scans the loop to find a branch or switch which occurs before
/// any side effects occur. These can potentially be unswitched without
/// duplicating the loop. If a branch or switch is successfully unswitched the
/// scanning continues to see if subsequent branches or switches have become
/// trivial. Once all trivial candidates have been unswitched, this routine
/// returns.
///
/// The return value indicates whether anything was unswitched (and therefore
/// changed).
///
/// If `SE` is not null, it will be updated based on the potential loop SCEVs
/// invalidated by this.
static bool unswitchAllTrivialConditions(Loop &L, DominatorTree &DT,
LoopInfo &LI, ScalarEvolution *SE,
MemorySSAUpdater *MSSAU) {
bool Changed = false;
// If loop header has only one reachable successor we should keep looking for
// trivial condition candidates in the successor as well. An alternative is
// to constant fold conditions and merge successors into loop header (then we
// only need to check header's terminator). The reason for not doing this in
// LoopUnswitch pass is that it could potentially break LoopPassManager's
// invariants. Folding dead branches could either eliminate the current loop
// or make other loops unreachable. LCSSA form might also not be preserved
// after deleting branches. The following code keeps traversing loop header's
// successors until it finds the trivial condition candidate (condition that
// is not a constant). Since unswitching generates branches with constant
// conditions, this scenario could be very common in practice.
BasicBlock *CurrentBB = L.getHeader();
SmallPtrSet<BasicBlock *, 8> Visited;
Visited.insert(CurrentBB);
do {
// Check if there are any side-effecting instructions (e.g. stores, calls,
// volatile loads) in the part of the loop that the code *would* execute
// without unswitching.
if (MSSAU) // Possible early exit with MSSA
if (auto *Defs = MSSAU->getMemorySSA()->getBlockDefs(CurrentBB))
if (!isa<MemoryPhi>(*Defs->begin()) || (++Defs->begin() != Defs->end()))
return Changed;
if (llvm::any_of(*CurrentBB,
[](Instruction &I) { return I.mayHaveSideEffects(); }))
return Changed;
Instruction *CurrentTerm = CurrentBB->getTerminator();
if (auto *SI = dyn_cast<SwitchInst>(CurrentTerm)) {
// Don't bother trying to unswitch past a switch with a constant
// condition. This should be removed prior to running this pass by
// simplify-cfg.
if (isa<Constant>(SI->getCondition()))
return Changed;
if (!unswitchTrivialSwitch(L, *SI, DT, LI, SE, MSSAU))
// Couldn't unswitch this one so we're done.
return Changed;
// Mark that we managed to unswitch something.
Changed = true;
// If unswitching turned the terminator into an unconditional branch then
// we can continue. The unswitching logic specifically works to fold any
// cases it can into an unconditional branch to make it easier to
// recognize here.
auto *BI = dyn_cast<BranchInst>(CurrentBB->getTerminator());
if (!BI || BI->isConditional())
return Changed;
CurrentBB = BI->getSuccessor(0);
continue;
}
auto *BI = dyn_cast<BranchInst>(CurrentTerm);
if (!BI)
// We do not understand other terminator instructions.
return Changed;
// Don't bother trying to unswitch past an unconditional branch or a branch
// with a constant value. These should be removed by simplify-cfg prior to
// running this pass.
if (!BI->isConditional() || isa<Constant>(BI->getCondition()))
return Changed;
// Found a trivial condition candidate: non-foldable conditional branch. If
// we fail to unswitch this, we can't do anything else that is trivial.
if (!unswitchTrivialBranch(L, *BI, DT, LI, SE, MSSAU))
return Changed;
// Mark that we managed to unswitch something.
Changed = true;
// If we only unswitched some of the conditions feeding the branch, we won't
// have collapsed it to a single successor.
BI = cast<BranchInst>(CurrentBB->getTerminator());
if (BI->isConditional())
return Changed;
// Follow the newly unconditional branch into its successor.
CurrentBB = BI->getSuccessor(0);
// When continuing, if we exit the loop or reach a previous visited block,
// then we can not reach any trivial condition candidates (unfoldable
// branch instructions or switch instructions) and no unswitch can happen.
} while (L.contains(CurrentBB) && Visited.insert(CurrentBB).second);
return Changed;
}
/// Build the cloned blocks for an unswitched copy of the given loop.
///
/// The cloned blocks are inserted before the loop preheader (`LoopPH`) and
/// after the split block (`SplitBB`) that will be used to select between the
/// cloned and original loop.
///
/// This routine handles cloning all of the necessary loop blocks and exit
/// blocks including rewriting their instructions and the relevant PHI nodes.
/// Any loop blocks or exit blocks which are dominated by a different successor
/// than the one for this clone of the loop blocks can be trivially skipped. We
/// use the `DominatingSucc` map to determine whether a block satisfies that
/// property with a simple map lookup.
///
/// It also correctly creates the unconditional branch in the cloned
/// unswitched parent block to only point at the unswitched successor.
///
/// This does not handle most of the necessary updates to `LoopInfo`. Only exit
/// block splitting is correctly reflected in `LoopInfo`, essentially all of
/// the cloned blocks (and their loops) are left without full `LoopInfo`
/// updates. This also doesn't fully update `DominatorTree`. It adds the cloned
/// blocks to them but doesn't create the cloned `DominatorTree` structure and
/// instead the caller must recompute an accurate DT. It *does* correctly
/// update the `AssumptionCache` provided in `AC`.
static BasicBlock *buildClonedLoopBlocks(
Loop &L, BasicBlock *LoopPH, BasicBlock *SplitBB,
ArrayRef<BasicBlock *> ExitBlocks, BasicBlock *ParentBB,
BasicBlock *UnswitchedSuccBB, BasicBlock *ContinueSuccBB,
const SmallDenseMap<BasicBlock *, BasicBlock *, 16> &DominatingSucc,
ValueToValueMapTy &VMap,
SmallVectorImpl<DominatorTree::UpdateType> &DTUpdates, AssumptionCache &AC,
DominatorTree &DT, LoopInfo &LI, MemorySSAUpdater *MSSAU) {
SmallVector<BasicBlock *, 4> NewBlocks;
NewBlocks.reserve(L.getNumBlocks() + ExitBlocks.size());
// We will need to clone a bunch of blocks, wrap up the clone operation in
// a helper.
auto CloneBlock = [&](BasicBlock *OldBB) {
// Clone the basic block and insert it before the new preheader.
BasicBlock *NewBB = CloneBasicBlock(OldBB, VMap, ".us", OldBB->getParent());
NewBB->moveBefore(LoopPH);
// Record this block and the mapping.
NewBlocks.push_back(NewBB);
VMap[OldBB] = NewBB;
return NewBB;
};
// We skip cloning blocks when they have a dominating succ that is not the
// succ we are cloning for.
auto SkipBlock = [&](BasicBlock *BB) {
auto It = DominatingSucc.find(BB);
return It != DominatingSucc.end() && It->second != UnswitchedSuccBB;
};
// First, clone the preheader.
auto *ClonedPH = CloneBlock(LoopPH);
// Then clone all the loop blocks, skipping the ones that aren't necessary.
for (auto *LoopBB : L.blocks())
if (!SkipBlock(LoopBB))
CloneBlock(LoopBB);
// Split all the loop exit edges so that when we clone the exit blocks, if
// any of the exit blocks are *also* a preheader for some other loop, we
// don't create multiple predecessors entering the loop header.
for (auto *ExitBB : ExitBlocks) {
if (SkipBlock(ExitBB))
continue;
// When we are going to clone an exit, we don't need to clone all the
// instructions in the exit block and we want to ensure we have an easy
// place to merge the CFG, so split the exit first. This is always safe to
// do because there cannot be any non-loop predecessors of a loop exit in
// loop simplified form.
auto *MergeBB = SplitBlock(ExitBB, &ExitBB->front(), &DT, &LI, MSSAU);
// Rearrange the names to make it easier to write test cases by having the
// exit block carry the suffix rather than the merge block carrying the
// suffix.
MergeBB->takeName(ExitBB);
ExitBB->setName(Twine(MergeBB->getName()) + ".split");
// Now clone the original exit block.
auto *ClonedExitBB = CloneBlock(ExitBB);
assert(ClonedExitBB->getTerminator()->getNumSuccessors() == 1 &&
"Exit block should have been split to have one successor!");
assert(ClonedExitBB->getTerminator()->getSuccessor(0) == MergeBB &&
"Cloned exit block has the wrong successor!");
// Remap any cloned instructions and create a merge phi node for them.
for (auto ZippedInsts : llvm::zip_first(
llvm::make_range(ExitBB->begin(), std::prev(ExitBB->end())),
llvm::make_range(ClonedExitBB->begin(),
std::prev(ClonedExitBB->end())))) {
Instruction &I = std::get<0>(ZippedInsts);
Instruction &ClonedI = std::get<1>(ZippedInsts);
// The only instructions in the exit block should be PHI nodes and
// potentially a landing pad.
assert(
(isa<PHINode>(I) || isa<LandingPadInst>(I) || isa<CatchPadInst>(I)) &&
"Bad instruction in exit block!");
// We should have a value map between the instruction and its clone.
assert(VMap.lookup(&I) == &ClonedI && "Mismatch in the value map!");
auto *MergePN =
PHINode::Create(I.getType(), /*NumReservedValues*/ 2, ".us-phi",
&*MergeBB->getFirstInsertionPt());
I.replaceAllUsesWith(MergePN);
MergePN->addIncoming(&I, ExitBB);
MergePN->addIncoming(&ClonedI, ClonedExitBB);
}
}
// Rewrite the instructions in the cloned blocks to refer to the instructions
// in the cloned blocks. We have to do this as a second pass so that we have
// everything available. Also, we have inserted new instructions which may
// include assume intrinsics, so we update the assumption cache while
// processing this.
for (auto *ClonedBB : NewBlocks)
for (Instruction &I : *ClonedBB) {
RemapInstruction(&I, VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
if (auto *II = dyn_cast<IntrinsicInst>(&I))
if (II->getIntrinsicID() == Intrinsic::assume)
AC.registerAssumption(II);
}
// Update any PHI nodes in the cloned successors of the skipped blocks to not
// have spurious incoming values.
for (auto *LoopBB : L.blocks())
if (SkipBlock(LoopBB))
for (auto *SuccBB : successors(LoopBB))
if (auto *ClonedSuccBB = cast_or_null<BasicBlock>(VMap.lookup(SuccBB)))
for (PHINode &PN : ClonedSuccBB->phis())
PN.removeIncomingValue(LoopBB, /*DeletePHIIfEmpty*/ false);
// Remove the cloned parent as a predecessor of any successor we ended up
// cloning other than the unswitched one.
auto *ClonedParentBB = cast<BasicBlock>(VMap.lookup(ParentBB));
for (auto *SuccBB : successors(ParentBB)) {
if (SuccBB == UnswitchedSuccBB)
continue;
auto *ClonedSuccBB = cast_or_null<BasicBlock>(VMap.lookup(SuccBB));
if (!ClonedSuccBB)
continue;
ClonedSuccBB->removePredecessor(ClonedParentBB,
/*KeepOneInputPHIs*/ true);
}
// Replace the cloned branch with an unconditional branch to the cloned
// unswitched successor.
auto *ClonedSuccBB = cast<BasicBlock>(VMap.lookup(UnswitchedSuccBB));
ClonedParentBB->getTerminator()->eraseFromParent();
BranchInst::Create(ClonedSuccBB, ClonedParentBB);
// If there are duplicate entries in the PHI nodes because of multiple edges
// to the unswitched successor, we need to nuke all but one as we replaced it
// with a direct branch.
for (PHINode &PN : ClonedSuccBB->phis()) {
bool Found = false;
// Loop over the incoming operands backwards so we can easily delete as we
// go without invalidating the index.
for (int i = PN.getNumOperands() - 1; i >= 0; --i) {
if (PN.getIncomingBlock(i) != ClonedParentBB)
continue;
if (!Found) {
Found = true;
continue;
}
PN.removeIncomingValue(i, /*DeletePHIIfEmpty*/ false);
}
}
// Record the domtree updates for the new blocks.
SmallPtrSet<BasicBlock *, 4> SuccSet;
for (auto *ClonedBB : NewBlocks) {
for (auto *SuccBB : successors(ClonedBB))
if (SuccSet.insert(SuccBB).second)
DTUpdates.push_back({DominatorTree::Insert, ClonedBB, SuccBB});
SuccSet.clear();
}
return ClonedPH;
}
/// Recursively clone the specified loop and all of its children.
///
/// The target parent loop for the clone should be provided, or can be null if
/// the clone is a top-level loop. While cloning, all the blocks are mapped
/// with the provided value map. The entire original loop must be present in
/// the value map. The cloned loop is returned.
static Loop *cloneLoopNest(Loop &OrigRootL, Loop *RootParentL,
const ValueToValueMapTy &VMap, LoopInfo &LI) {
auto AddClonedBlocksToLoop = [&](Loop &OrigL, Loop &ClonedL) {
assert(ClonedL.getBlocks().empty() && "Must start with an empty loop!");
ClonedL.reserveBlocks(OrigL.getNumBlocks());
for (auto *BB : OrigL.blocks()) {
auto *ClonedBB = cast<BasicBlock>(VMap.lookup(BB));
ClonedL.addBlockEntry(ClonedBB);
if (LI.getLoopFor(BB) == &OrigL)
LI.changeLoopFor(ClonedBB, &ClonedL);
}
};
// We specially handle the first loop because it may get cloned into
// a different parent and because we most commonly are cloning leaf loops.
Loop *ClonedRootL = LI.AllocateLoop();
if (RootParentL)
RootParentL->addChildLoop(ClonedRootL);
else
LI.addTopLevelLoop(ClonedRootL);
AddClonedBlocksToLoop(OrigRootL, *ClonedRootL);
if (OrigRootL.empty())
return ClonedRootL;
// If we have a nest, we can quickly clone the entire loop nest using an
// iterative approach because it is a tree. We keep the cloned parent in the
// data structure to avoid repeatedly querying through a map to find it.
SmallVector<std::pair<Loop *, Loop *>, 16> LoopsToClone;
// Build up the loops to clone in reverse order as we'll clone them from the
// back.
for (Loop *ChildL : llvm::reverse(OrigRootL))
LoopsToClone.push_back({ClonedRootL, ChildL});
do {
Loop *ClonedParentL, *L;
std::tie(ClonedParentL, L) = LoopsToClone.pop_back_val();
Loop *ClonedL = LI.AllocateLoop();
ClonedParentL->addChildLoop(ClonedL);
AddClonedBlocksToLoop(*L, *ClonedL);
for (Loop *ChildL : llvm::reverse(*L))
LoopsToClone.push_back({ClonedL, ChildL});
} while (!LoopsToClone.empty());
return ClonedRootL;
}
/// Build the cloned loops of an original loop from unswitching.
///
/// Because unswitching simplifies the CFG of the loop, this isn't a trivial
/// operation. We need to re-verify that there even is a loop (as the backedge
/// may not have been cloned), and even if there are remaining backedges the
/// backedge set may be different. However, we know that each child loop is
/// undisturbed, we only need to find where to place each child loop within
/// either any parent loop or within a cloned version of the original loop.
///
/// Because child loops may end up cloned outside of any cloned version of the
/// original loop, multiple cloned sibling loops may be created. All of them
/// are returned so that the newly introduced loop nest roots can be
/// identified.
static void buildClonedLoops(Loop &OrigL, ArrayRef<BasicBlock *> ExitBlocks,
const ValueToValueMapTy &VMap, LoopInfo &LI,
SmallVectorImpl<Loop *> &NonChildClonedLoops) {
Loop *ClonedL = nullptr;
auto *OrigPH = OrigL.getLoopPreheader();
auto *OrigHeader = OrigL.getHeader();
auto *ClonedPH = cast<BasicBlock>(VMap.lookup(OrigPH));
auto *ClonedHeader = cast<BasicBlock>(VMap.lookup(OrigHeader));
// We need to know the loops of the cloned exit blocks to even compute the
// accurate parent loop. If we only clone exits to some parent of the
// original parent, we want to clone into that outer loop. We also keep track
// of the loops that our cloned exit blocks participate in.
Loop *ParentL = nullptr;
SmallVector<BasicBlock *, 4> ClonedExitsInLoops;
SmallDenseMap<BasicBlock *, Loop *, 16> ExitLoopMap;
ClonedExitsInLoops.reserve(ExitBlocks.size());
for (auto *ExitBB : ExitBlocks)
if (auto *ClonedExitBB = cast_or_null<BasicBlock>(VMap.lookup(ExitBB)))
if (Loop *ExitL = LI.getLoopFor(ExitBB)) {
ExitLoopMap[ClonedExitBB] = ExitL;
ClonedExitsInLoops.push_back(ClonedExitBB);
if (!ParentL || (ParentL != ExitL && ParentL->contains(ExitL)))
ParentL = ExitL;
}
assert((!ParentL || ParentL == OrigL.getParentLoop() ||
ParentL->contains(OrigL.getParentLoop())) &&
"The computed parent loop should always contain (or be) the parent of "
"the original loop.");
// We build the set of blocks dominated by the cloned header from the set of
// cloned blocks out of the original loop. While not all of these will
// necessarily be in the cloned loop, it is enough to establish that they
// aren't in unreachable cycles, etc.
SmallSetVector<BasicBlock *, 16> ClonedLoopBlocks;
for (auto *BB : OrigL.blocks())
if (auto *ClonedBB = cast_or_null<BasicBlock>(VMap.lookup(BB)))
ClonedLoopBlocks.insert(ClonedBB);
// Rebuild the set of blocks that will end up in the cloned loop. We may have
// skipped cloning some region of this loop which can in turn skip some of
// the backedges so we have to rebuild the blocks in the loop based on the
// backedges that remain after cloning.
SmallVector<BasicBlock *, 16> Worklist;
SmallPtrSet<BasicBlock *, 16> BlocksInClonedLoop;
for (auto *Pred : predecessors(ClonedHeader)) {
// The only possible non-loop header predecessor is the preheader because
// we know we cloned the loop in simplified form.
if (Pred == ClonedPH)
continue;
// Because the loop was in simplified form, the only non-loop predecessor
// should be the preheader.
assert(ClonedLoopBlocks.count(Pred) && "Found a predecessor of the loop "
"header other than the preheader "
"that is not part of the loop!");
// Insert this block into the loop set and on the first visit (and if it
// isn't the header we're currently walking) put it into the worklist to
// recurse through.
if (BlocksInClonedLoop.insert(Pred).second && Pred != ClonedHeader)
Worklist.push_back(Pred);
}
// If we had any backedges then there *is* a cloned loop. Put the header into
// the loop set and then walk the worklist backwards to find all the blocks
// that remain within the loop after cloning.
if (!BlocksInClonedLoop.empty()) {
BlocksInClonedLoop.insert(ClonedHeader);
while (!Worklist.empty()) {
BasicBlock *BB = Worklist.pop_back_val();
assert(BlocksInClonedLoop.count(BB) &&
"Didn't put block into the loop set!");
// Insert any predecessors that are in the possible set into the cloned
// set, and if the insert is successful, add them to the worklist. Note
// that we filter on the blocks that are definitely reachable via the
// backedge to the loop header so we may prune out dead code within the
// cloned loop.
for (auto *Pred : predecessors(BB))
if (ClonedLoopBlocks.count(Pred) &&
BlocksInClonedLoop.insert(Pred).second)
Worklist.push_back(Pred);
}
ClonedL = LI.AllocateLoop();
if (ParentL) {
ParentL->addBasicBlockToLoop(ClonedPH, LI);
ParentL->addChildLoop(ClonedL);
} else {
LI.addTopLevelLoop(ClonedL);
}
NonChildClonedLoops.push_back(ClonedL);
ClonedL->reserveBlocks(BlocksInClonedLoop.size());
// We don't want to just add the cloned loop blocks based on how we
// discovered them. The original order of blocks was carefully built in
// a way that doesn't rely on predecessor ordering. Rather than re-invent
// that logic, we just re-walk the original blocks (and those of the child
// loops) and filter them as we add them into the cloned loop.
for (auto *BB : OrigL.blocks()) {
auto *ClonedBB = cast_or_null<BasicBlock>(VMap.lookup(BB));
if (!ClonedBB || !BlocksInClonedLoop.count(ClonedBB))
continue;
// Directly add the blocks that are only in this loop.
if (LI.getLoopFor(BB) == &OrigL) {
ClonedL->addBasicBlockToLoop(ClonedBB, LI);
continue;
}
// We want to manually add it to this loop and parents.
// Registering it with LoopInfo will happen when we clone the top
// loop for this block.
for (Loop *PL = ClonedL; PL; PL = PL->getParentLoop())
PL->addBlockEntry(ClonedBB);
}
// Now add each child loop whose header remains within the cloned loop. All
// of the blocks within the loop must satisfy the same constraints as the
// header so once we pass the header checks we can just clone the entire
// child loop nest.
for (Loop *ChildL : OrigL) {
auto *ClonedChildHeader =
cast_or_null<BasicBlock>(VMap.lookup(ChildL->getHeader()));
if (!ClonedChildHeader || !BlocksInClonedLoop.count(ClonedChildHeader))
continue;
#ifndef NDEBUG
// We should never have a cloned child loop header but fail to have
// all of the blocks for that child loop.
for (auto *ChildLoopBB : ChildL->blocks())
assert(BlocksInClonedLoop.count(
cast<BasicBlock>(VMap.lookup(ChildLoopBB))) &&
"Child cloned loop has a header within the cloned outer "
"loop but not all of its blocks!");
#endif
cloneLoopNest(*ChildL, ClonedL, VMap, LI);
}
}
// Now that we've handled all the components of the original loop that were
// cloned into a new loop, we still need to handle anything from the original
// loop that wasn't in a cloned loop.
// Figure out what blocks are left to place within any loop nest containing
// the unswitched loop. If we never formed a loop, the cloned PH is one of
// them.
SmallPtrSet<BasicBlock *, 16> UnloopedBlockSet;
if (BlocksInClonedLoop.empty())
UnloopedBlockSet.insert(ClonedPH);
for (auto *ClonedBB : ClonedLoopBlocks)
if (!BlocksInClonedLoop.count(ClonedBB))
UnloopedBlockSet.insert(ClonedBB);
// Copy the cloned exits and sort them in ascending loop depth, we'll work
// backwards across these to process them inside out. The order shouldn't
// matter as we're just trying to build up the map from inside-out; we use
// the map in a more stably ordered way below.
auto OrderedClonedExitsInLoops = ClonedExitsInLoops;
llvm::sort(OrderedClonedExitsInLoops, [&](BasicBlock *LHS, BasicBlock *RHS) {
return ExitLoopMap.lookup(LHS)->getLoopDepth() <
ExitLoopMap.lookup(RHS)->getLoopDepth();
});
// Populate the existing ExitLoopMap with everything reachable from each
// exit, starting from the inner most exit.
while (!UnloopedBlockSet.empty() && !OrderedClonedExitsInLoops.empty()) {
assert(Worklist.empty() && "Didn't clear worklist!");
BasicBlock *ExitBB = OrderedClonedExitsInLoops.pop_back_val();
Loop *ExitL = ExitLoopMap.lookup(ExitBB);
// Walk the CFG back until we hit the cloned PH adding everything reachable
// and in the unlooped set to this exit block's loop.
Worklist.push_back(ExitBB);
do {
BasicBlock *BB = Worklist.pop_back_val();
// We can stop recursing at the cloned preheader (if we get there).
if (BB == ClonedPH)
continue;
for (BasicBlock *PredBB : predecessors(BB)) {
// If this pred has already been moved to our set or is part of some
// (inner) loop, no update needed.
if (!UnloopedBlockSet.erase(PredBB)) {
assert(
(BlocksInClonedLoop.count(PredBB) || ExitLoopMap.count(PredBB)) &&
"Predecessor not mapped to a loop!");
continue;
}
// We just insert into the loop set here. We'll add these blocks to the
// exit loop after we build up the set in an order that doesn't rely on
// predecessor order (which in turn relies on use list order).
bool Inserted = ExitLoopMap.insert({PredBB, ExitL}).second;
(void)Inserted;
assert(Inserted && "Should only visit an unlooped block once!");
// And recurse through to its predecessors.
Worklist.push_back(PredBB);
}
} while (!Worklist.empty());
}
// Now that the ExitLoopMap gives as mapping for all the non-looping cloned
// blocks to their outer loops, walk the cloned blocks and the cloned exits
// in their original order adding them to the correct loop.
// We need a stable insertion order. We use the order of the original loop
// order and map into the correct parent loop.
for (auto *BB : llvm::concat<BasicBlock *const>(
makeArrayRef(ClonedPH), ClonedLoopBlocks, ClonedExitsInLoops))
if (Loop *OuterL = ExitLoopMap.lookup(BB))
OuterL->addBasicBlockToLoop(BB, LI);
#ifndef NDEBUG
for (auto &BBAndL : ExitLoopMap) {
auto *BB = BBAndL.first;
auto *OuterL = BBAndL.second;
assert(LI.getLoopFor(BB) == OuterL &&
"Failed to put all blocks into outer loops!");
}
#endif
// Now that all the blocks are placed into the correct containing loop in the
// absence of child loops, find all the potentially cloned child loops and
// clone them into whatever outer loop we placed their header into.
for (Loop *ChildL : OrigL) {
auto *ClonedChildHeader =
cast_or_null<BasicBlock>(VMap.lookup(ChildL->getHeader()));
if (!ClonedChildHeader || BlocksInClonedLoop.count(ClonedChildHeader))
continue;
#ifndef NDEBUG
for (auto *ChildLoopBB : ChildL->blocks())
assert(VMap.count(ChildLoopBB) &&
"Cloned a child loop header but not all of that loops blocks!");
#endif
NonChildClonedLoops.push_back(cloneLoopNest(
*ChildL, ExitLoopMap.lookup(ClonedChildHeader), VMap, LI));
}
}
static void
deleteDeadClonedBlocks(Loop &L, ArrayRef<BasicBlock *> ExitBlocks,
ArrayRef<std::unique_ptr<ValueToValueMapTy>> VMaps,
DominatorTree &DT, MemorySSAUpdater *MSSAU) {
// Find all the dead clones, and remove them from their successors.
SmallVector<BasicBlock *, 16> DeadBlocks;
for (BasicBlock *BB : llvm::concat<BasicBlock *const>(L.blocks(), ExitBlocks))
for (auto &VMap : VMaps)
if (BasicBlock *ClonedBB = cast_or_null<BasicBlock>(VMap->lookup(BB)))
if (!DT.isReachableFromEntry(ClonedBB)) {
for (BasicBlock *SuccBB : successors(ClonedBB))
SuccBB->removePredecessor(ClonedBB);
DeadBlocks.push_back(ClonedBB);
}
// Remove all MemorySSA in the dead blocks
if (MSSAU) {
SmallSetVector<BasicBlock *, 8> DeadBlockSet(DeadBlocks.begin(),
DeadBlocks.end());
MSSAU->removeBlocks(DeadBlockSet);
}
// Drop any remaining references to break cycles.
for (BasicBlock *BB : DeadBlocks)
BB->dropAllReferences();
// Erase them from the IR.
for (BasicBlock *BB : DeadBlocks)
BB->eraseFromParent();
}
static void deleteDeadBlocksFromLoop(Loop &L,
SmallVectorImpl<BasicBlock *> &ExitBlocks,
DominatorTree &DT, LoopInfo &LI,
MemorySSAUpdater *MSSAU) {
// Find all the dead blocks tied to this loop, and remove them from their
// successors.
SmallSetVector<BasicBlock *, 8> DeadBlockSet;
// Start with loop/exit blocks and get a transitive closure of reachable dead
// blocks.
SmallVector<BasicBlock *, 16> DeathCandidates(ExitBlocks.begin(),
ExitBlocks.end());
DeathCandidates.append(L.blocks().begin(), L.blocks().end());
while (!DeathCandidates.empty()) {
auto *BB = DeathCandidates.pop_back_val();
if (!DeadBlockSet.count(BB) && !DT.isReachableFromEntry(BB)) {
for (BasicBlock *SuccBB : successors(BB)) {
SuccBB->removePredecessor(BB);
DeathCandidates.push_back(SuccBB);
}
DeadBlockSet.insert(BB);
}
}
// Remove all MemorySSA in the dead blocks
if (MSSAU)
MSSAU->removeBlocks(DeadBlockSet);
// Filter out the dead blocks from the exit blocks list so that it can be
// used in the caller.
llvm::erase_if(ExitBlocks,
[&](BasicBlock *BB) { return DeadBlockSet.count(BB); });
// Walk from this loop up through its parents removing all of the dead blocks.
for (Loop *ParentL = &L; ParentL; ParentL = ParentL->getParentLoop()) {
for (auto *BB : DeadBlockSet)
ParentL->getBlocksSet().erase(BB);
llvm::erase_if(ParentL->getBlocksVector(),
[&](BasicBlock *BB) { return DeadBlockSet.count(BB); });
}
// Now delete the dead child loops. This raw delete will clear them
// recursively.
llvm::erase_if(L.getSubLoopsVector(), [&](Loop *ChildL) {
if (!DeadBlockSet.count(ChildL->getHeader()))
return false;
assert(llvm::all_of(ChildL->blocks(),
[&](BasicBlock *ChildBB) {
return DeadBlockSet.count(ChildBB);
}) &&
"If the child loop header is dead all blocks in the child loop must "
"be dead as well!");
LI.destroy(ChildL);
return true;
});
// Remove the loop mappings for the dead blocks and drop all the references
// from these blocks to others to handle cyclic references as we start
// deleting the blocks themselves.
for (auto *BB : DeadBlockSet) {
// Check that the dominator tree has already been updated.
assert(!DT.getNode(BB) && "Should already have cleared domtree!");
LI.changeLoopFor(BB, nullptr);
BB->dropAllReferences();
}
// Actually delete the blocks now that they've been fully unhooked from the
// IR.
for (auto *BB : DeadBlockSet)
BB->eraseFromParent();
}
/// Recompute the set of blocks in a loop after unswitching.
///
/// This walks from the original headers predecessors to rebuild the loop. We
/// take advantage of the fact that new blocks can't have been added, and so we
/// filter by the original loop's blocks. This also handles potentially
/// unreachable code that we don't want to explore but might be found examining
/// the predecessors of the header.
///
/// If the original loop is no longer a loop, this will return an empty set. If
/// it remains a loop, all the blocks within it will be added to the set
/// (including those blocks in inner loops).
static SmallPtrSet<const BasicBlock *, 16> recomputeLoopBlockSet(Loop &L,
LoopInfo &LI) {
SmallPtrSet<const BasicBlock *, 16> LoopBlockSet;
auto *PH = L.getLoopPreheader();
auto *Header = L.getHeader();
// A worklist to use while walking backwards from the header.
SmallVector<BasicBlock *, 16> Worklist;
// First walk the predecessors of the header to find the backedges. This will
// form the basis of our walk.
for (auto *Pred : predecessors(Header)) {
// Skip the preheader.
if (Pred == PH)
continue;
// Because the loop was in simplified form, the only non-loop predecessor
// is the preheader.
assert(L.contains(Pred) && "Found a predecessor of the loop header other "
"than the preheader that is not part of the "
"loop!");
// Insert this block into the loop set and on the first visit and, if it
// isn't the header we're currently walking, put it into the worklist to
// recurse through.
if (LoopBlockSet.insert(Pred).second && Pred != Header)
Worklist.push_back(Pred);
}
// If no backedges were found, we're done.
if (LoopBlockSet.empty())
return LoopBlockSet;
// We found backedges, recurse through them to identify the loop blocks.
while (!Worklist.empty()) {
BasicBlock *BB = Worklist.pop_back_val();
assert(LoopBlockSet.count(BB) && "Didn't put block into the loop set!");
// No need to walk past the header.
if (BB == Header)
continue;
// Because we know the inner loop structure remains valid we can use the
// loop structure to jump immediately across the entire nested loop.
// Further, because it is in loop simplified form, we can directly jump
// to its preheader afterward.
if (Loop *InnerL = LI.getLoopFor(BB))
if (InnerL != &L) {
assert(L.contains(InnerL) &&
"Should not reach a loop *outside* this loop!");
// The preheader is the only possible predecessor of the loop so
// insert it into the set and check whether it was already handled.
auto *InnerPH = InnerL->getLoopPreheader();
assert(L.contains(InnerPH) && "Cannot contain an inner loop block "
"but not contain the inner loop "
"preheader!");
if (!LoopBlockSet.insert(InnerPH).second)
// The only way to reach the preheader is through the loop body
// itself so if it has been visited the loop is already handled.
continue;
// Insert all of the blocks (other than those already present) into
// the loop set. We expect at least the block that led us to find the
// inner loop to be in the block set, but we may also have other loop
// blocks if they were already enqueued as predecessors of some other
// outer loop block.
for (auto *InnerBB : InnerL->blocks()) {
if (InnerBB == BB) {
assert(LoopBlockSet.count(InnerBB) &&
"Block should already be in the set!");
continue;
}
LoopBlockSet.insert(InnerBB);
}
// Add the preheader to the worklist so we will continue past the
// loop body.
Worklist.push_back(InnerPH);
continue;
}
// Insert any predecessors that were in the original loop into the new
// set, and if the insert is successful, add them to the worklist.
for (auto *Pred : predecessors(BB))
if (L.contains(Pred) && LoopBlockSet.insert(Pred).second)
Worklist.push_back(Pred);
}
assert(LoopBlockSet.count(Header) && "Cannot fail to add the header!");
// We've found all the blocks participating in the loop, return our completed
// set.
return LoopBlockSet;
}
/// Rebuild a loop after unswitching removes some subset of blocks and edges.
///
/// The removal may have removed some child loops entirely but cannot have
/// disturbed any remaining child loops. However, they may need to be hoisted
/// to the parent loop (or to be top-level loops). The original loop may be
/// completely removed.
///
/// The sibling loops resulting from this update are returned. If the original
/// loop remains a valid loop, it will be the first entry in this list with all
/// of the newly sibling loops following it.
///
/// Returns true if the loop remains a loop after unswitching, and false if it
/// is no longer a loop after unswitching (and should not continue to be
/// referenced).
static bool rebuildLoopAfterUnswitch(Loop &L, ArrayRef<BasicBlock *> ExitBlocks,
LoopInfo &LI,
SmallVectorImpl<Loop *> &HoistedLoops) {
auto *PH = L.getLoopPreheader();
// Compute the actual parent loop from the exit blocks. Because we may have
// pruned some exits the loop may be different from the original parent.
Loop *ParentL = nullptr;
SmallVector<Loop *, 4> ExitLoops;
SmallVector<BasicBlock *, 4> ExitsInLoops;
ExitsInLoops.reserve(ExitBlocks.size());
for (auto *ExitBB : ExitBlocks)
if (Loop *ExitL = LI.getLoopFor(ExitBB)) {
ExitLoops.push_back(ExitL);
ExitsInLoops.push_back(ExitBB);
if (!ParentL || (ParentL != ExitL && ParentL->contains(ExitL)))
ParentL = ExitL;
}
// Recompute the blocks participating in this loop. This may be empty if it
// is no longer a loop.
auto LoopBlockSet = recomputeLoopBlockSet(L, LI);
// If we still have a loop, we need to re-set the loop's parent as the exit
// block set changing may have moved it within the loop nest. Note that this
// can only happen when this loop has a parent as it can only hoist the loop
// *up* the nest.
if (!LoopBlockSet.empty() && L.getParentLoop() != ParentL) {
// Remove this loop's (original) blocks from all of the intervening loops.
for (Loop *IL = L.getParentLoop(); IL != ParentL;
IL = IL->getParentLoop()) {
IL->getBlocksSet().erase(PH);
for (auto *BB : L.blocks())
IL->getBlocksSet().erase(BB);
llvm::erase_if(IL->getBlocksVector(), [&](BasicBlock *BB) {
return BB == PH || L.contains(BB);
});
}
LI.changeLoopFor(PH, ParentL);
L.getParentLoop()->removeChildLoop(&L);
if (ParentL)
ParentL->addChildLoop(&L);
else
LI.addTopLevelLoop(&L);
}
// Now we update all the blocks which are no longer within the loop.
auto &Blocks = L.getBlocksVector();
auto BlocksSplitI =
LoopBlockSet.empty()
? Blocks.begin()
: std::stable_partition(
Blocks.begin(), Blocks.end(),
[&](BasicBlock *BB) { return LoopBlockSet.count(BB); });
// Before we erase the list of unlooped blocks, build a set of them.
SmallPtrSet<BasicBlock *, 16> UnloopedBlocks(BlocksSplitI, Blocks.end());
if (LoopBlockSet.empty())
UnloopedBlocks.insert(PH);
// Now erase these blocks from the loop.
for (auto *BB : make_range(BlocksSplitI, Blocks.end()))
L.getBlocksSet().erase(BB);
Blocks.erase(BlocksSplitI, Blocks.end());
// Sort the exits in ascending loop depth, we'll work backwards across these
// to process them inside out.
llvm::stable_sort(ExitsInLoops, [&](BasicBlock *LHS, BasicBlock *RHS) {
return LI.getLoopDepth(LHS) < LI.getLoopDepth(RHS);
});
// We'll build up a set for each exit loop.
SmallPtrSet<BasicBlock *, 16> NewExitLoopBlocks;
Loop *PrevExitL = L.getParentLoop(); // The deepest possible exit loop.
auto RemoveUnloopedBlocksFromLoop =
[](Loop &L, SmallPtrSetImpl<BasicBlock *> &UnloopedBlocks) {
for (auto *BB : UnloopedBlocks)
L.getBlocksSet().erase(BB);
llvm::erase_if(L.getBlocksVector(), [&](BasicBlock *BB) {
return UnloopedBlocks.count(BB);
});
};
SmallVector<BasicBlock *, 16> Worklist;
while (!UnloopedBlocks.empty() && !ExitsInLoops.empty()) {
assert(Worklist.empty() && "Didn't clear worklist!");
assert(NewExitLoopBlocks.empty() && "Didn't clear loop set!");
// Grab the next exit block, in decreasing loop depth order.
BasicBlock *ExitBB = ExitsInLoops.pop_back_val();
Loop &ExitL = *LI.getLoopFor(ExitBB);
assert(ExitL.contains(&L) && "Exit loop must contain the inner loop!");
// Erase all of the unlooped blocks from the loops between the previous
// exit loop and this exit loop. This works because the ExitInLoops list is
// sorted in increasing order of loop depth and thus we visit loops in
// decreasing order of loop depth.
for (; PrevExitL != &ExitL; PrevExitL = PrevExitL->getParentLoop())
RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks);
// Walk the CFG back until we hit the cloned PH adding everything reachable
// and in the unlooped set to this exit block's loop.
Worklist.push_back(ExitBB);
do {
BasicBlock *BB = Worklist.pop_back_val();
// We can stop recursing at the cloned preheader (if we get there).
if (BB == PH)
continue;
for (BasicBlock *PredBB : predecessors(BB)) {
// If this pred has already been moved to our set or is part of some
// (inner) loop, no update needed.
if (!UnloopedBlocks.erase(PredBB)) {
assert((NewExitLoopBlocks.count(PredBB) ||
ExitL.contains(LI.getLoopFor(PredBB))) &&
"Predecessor not in a nested loop (or already visited)!");
continue;
}
// We just insert into the loop set here. We'll add these blocks to the
// exit loop after we build up the set in a deterministic order rather
// than the predecessor-influenced visit order.
bool Inserted = NewExitLoopBlocks.insert(PredBB).second;
(void)Inserted;
assert(Inserted && "Should only visit an unlooped block once!");
// And recurse through to its predecessors.
Worklist.push_back(PredBB);
}
} while (!Worklist.empty());
// If blocks in this exit loop were directly part of the original loop (as
// opposed to a child loop) update the map to point to this exit loop. This
// just updates a map and so the fact that the order is unstable is fine.
for (auto *BB : NewExitLoopBlocks)
if (Loop *BBL = LI.getLoopFor(BB))
if (BBL == &L || !L.contains(BBL))
LI.changeLoopFor(BB, &ExitL);
// We will remove the remaining unlooped blocks from this loop in the next
// iteration or below.
NewExitLoopBlocks.clear();
}
// Any remaining unlooped blocks are no longer part of any loop unless they
// are part of some child loop.
for (; PrevExitL; PrevExitL = PrevExitL->getParentLoop())
RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks);
for (auto *BB : UnloopedBlocks)
if (Loop *BBL = LI.getLoopFor(BB))
if (BBL == &L || !L.contains(BBL))
LI.changeLoopFor(BB, nullptr);
// Sink all the child loops whose headers are no longer in the loop set to
// the parent (or to be top level loops). We reach into the loop and directly
// update its subloop vector to make this batch update efficient.
auto &SubLoops = L.getSubLoopsVector();
auto SubLoopsSplitI =
LoopBlockSet.empty()
? SubLoops.begin()
: std::stable_partition(
SubLoops.begin(), SubLoops.end(), [&](Loop *SubL) {
return LoopBlockSet.count(SubL->getHeader());
});
for (auto *HoistedL : make_range(SubLoopsSplitI, SubLoops.end())) {
HoistedLoops.push_back(HoistedL);
HoistedL->setParentLoop(nullptr);
// To compute the new parent of this hoisted loop we look at where we
// placed the preheader above. We can't lookup the header itself because we
// retained the mapping from the header to the hoisted loop. But the
// preheader and header should have the exact same new parent computed
// based on the set of exit blocks from the original loop as the preheader
// is a predecessor of the header and so reached in the reverse walk. And
// because the loops were all in simplified form the preheader of the
// hoisted loop can't be part of some *other* loop.
if (auto *NewParentL = LI.getLoopFor(HoistedL->getLoopPreheader()))
NewParentL->addChildLoop(HoistedL);
else
LI.addTopLevelLoop(HoistedL);
}
SubLoops.erase(SubLoopsSplitI, SubLoops.end());
// Actually delete the loop if nothing remained within it.
if (Blocks.empty()) {
assert(SubLoops.empty() &&
"Failed to remove all subloops from the original loop!");
if (Loop *ParentL = L.getParentLoop())
ParentL->removeChildLoop(llvm::find(*ParentL, &L));
else
LI.removeLoop(llvm::find(LI, &L));
LI.destroy(&L);
return false;
}
return true;
}
/// Helper to visit a dominator subtree, invoking a callable on each node.
///
/// Returning false at any point will stop walking past that node of the tree.
template <typename CallableT>
void visitDomSubTree(DominatorTree &DT, BasicBlock *BB, CallableT Callable) {
SmallVector<DomTreeNode *, 4> DomWorklist;
DomWorklist.push_back(DT[BB]);
#ifndef NDEBUG
SmallPtrSet<DomTreeNode *, 4> Visited;
Visited.insert(DT[BB]);
#endif
do {
DomTreeNode *N = DomWorklist.pop_back_val();
// Visit this node.
if (!Callable(N->getBlock()))
continue;
// Accumulate the child nodes.
for (DomTreeNode *ChildN : *N) {
assert(Visited.insert(ChildN).second &&
"Cannot visit a node twice when walking a tree!");
DomWorklist.push_back(ChildN);
}
} while (!DomWorklist.empty());
}
static void unswitchNontrivialInvariants(
Loop &L, Instruction &TI, ArrayRef<Value *> Invariants,
SmallVectorImpl<BasicBlock *> &ExitBlocks, DominatorTree &DT, LoopInfo &LI,
AssumptionCache &AC, function_ref<void(bool, ArrayRef<Loop *>)> UnswitchCB,
ScalarEvolution *SE, MemorySSAUpdater *MSSAU) {
auto *ParentBB = TI.getParent();
BranchInst *BI = dyn_cast<BranchInst>(&TI);
SwitchInst *SI = BI ? nullptr : cast<SwitchInst>(&TI);
// We can only unswitch switches, conditional branches with an invariant
// condition, or combining invariant conditions with an instruction.
assert((SI || (BI && BI->isConditional())) &&
"Can only unswitch switches and conditional branch!");
bool FullUnswitch = SI || BI->getCondition() == Invariants[0];
if (FullUnswitch)
assert(Invariants.size() == 1 &&
"Cannot have other invariants with full unswitching!");
else
assert(isa<Instruction>(BI->getCondition()) &&
"Partial unswitching requires an instruction as the condition!");
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// Constant and BBs tracking the cloned and continuing successor. When we are
// unswitching the entire condition, this can just be trivially chosen to
// unswitch towards `true`. However, when we are unswitching a set of
// invariants combined with `and` or `or`, the combining operation determines
// the best direction to unswitch: we want to unswitch the direction that will
// collapse the branch.
bool Direction = true;
int ClonedSucc = 0;
if (!FullUnswitch) {
if (cast<Instruction>(BI->getCondition())->getOpcode() != Instruction::Or) {
assert(cast<Instruction>(BI->getCondition())->getOpcode() ==
Instruction::And &&
"Only `or` and `and` instructions can combine invariants being "
"unswitched.");
Direction = false;
ClonedSucc = 1;
}
}
BasicBlock *RetainedSuccBB =
BI ? BI->getSuccessor(1 - ClonedSucc) : SI->getDefaultDest();
SmallSetVector<BasicBlock *, 4> UnswitchedSuccBBs;
if (BI)
UnswitchedSuccBBs.insert(BI->getSuccessor(ClonedSucc));
else
for (auto Case : SI->cases())
if (Case.getCaseSuccessor() != RetainedSuccBB)
UnswitchedSuccBBs.insert(Case.getCaseSuccessor());
assert(!UnswitchedSuccBBs.count(RetainedSuccBB) &&
"Should not unswitch the same successor we are retaining!");
// The branch should be in this exact loop. Any inner loop's invariant branch
// should be handled by unswitching that inner loop. The caller of this
// routine should filter out any candidates that remain (but were skipped for
// whatever reason).
assert(LI.getLoopFor(ParentBB) == &L && "Branch in an inner loop!");
// Compute the parent loop now before we start hacking on things.
Loop *ParentL = L.getParentLoop();
// Get blocks in RPO order for MSSA update, before changing the CFG.
LoopBlocksRPO LBRPO(&L);
if (MSSAU)
LBRPO.perform(&LI);
// Compute the outer-most loop containing one of our exit blocks. This is the
// furthest up our loopnest which can be mutated, which we will use below to
// update things.
Loop *OuterExitL = &L;
for (auto *ExitBB : ExitBlocks) {
Loop *NewOuterExitL = LI.getLoopFor(ExitBB);
if (!NewOuterExitL) {
// We exited the entire nest with this block, so we're done.
OuterExitL = nullptr;
break;
}
if (NewOuterExitL != OuterExitL && NewOuterExitL->contains(OuterExitL))
OuterExitL = NewOuterExitL;
}
// At this point, we're definitely going to unswitch something so invalidate
// any cached information in ScalarEvolution for the outer most loop
// containing an exit block and all nested loops.
if (SE) {
if (OuterExitL)
SE->forgetLoop(OuterExitL);
else
SE->forgetTopmostLoop(&L);
}
// If the edge from this terminator to a successor dominates that successor,
// store a map from each block in its dominator subtree to it. This lets us
// tell when cloning for a particular successor if a block is dominated by
// some *other* successor with a single data structure. We use this to
// significantly reduce cloning.
SmallDenseMap<BasicBlock *, BasicBlock *, 16> DominatingSucc;
for (auto *SuccBB : llvm::concat<BasicBlock *const>(
makeArrayRef(RetainedSuccBB), UnswitchedSuccBBs))
if (SuccBB->getUniquePredecessor() ||
llvm::all_of(predecessors(SuccBB), [&](BasicBlock *PredBB) {
return PredBB == ParentBB || DT.dominates(SuccBB, PredBB);
}))
visitDomSubTree(DT, SuccBB, [&](BasicBlock *BB) {
DominatingSucc[BB] = SuccBB;
return true;
});
// Split the preheader, so that we know that there is a safe place to insert
// the conditional branch. We will change the preheader to have a conditional
// branch on LoopCond. The original preheader will become the split point
// between the unswitched versions, and we will have a new preheader for the
// original loop.
BasicBlock *SplitBB = L.getLoopPreheader();
BasicBlock *LoopPH = SplitEdge(SplitBB, L.getHeader(), &DT, &LI, MSSAU);
// Keep track of the dominator tree updates needed.
SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
// Clone the loop for each unswitched successor.
SmallVector<std::unique_ptr<ValueToValueMapTy>, 4> VMaps;
VMaps.reserve(UnswitchedSuccBBs.size());
SmallDenseMap<BasicBlock *, BasicBlock *, 4> ClonedPHs;
for (auto *SuccBB : UnswitchedSuccBBs) {
VMaps.emplace_back(new ValueToValueMapTy());
ClonedPHs[SuccBB] = buildClonedLoopBlocks(
L, LoopPH, SplitBB, ExitBlocks, ParentBB, SuccBB, RetainedSuccBB,
DominatingSucc, *VMaps.back(), DTUpdates, AC, DT, LI, MSSAU);
}
// The stitching of the branched code back together depends on whether we're
// doing full unswitching or not with the exception that we always want to
// nuke the initial terminator placed in the split block.
SplitBB->getTerminator()->eraseFromParent();
if (FullUnswitch) {
// Splice the terminator from the original loop and rewrite its
// successors.
SplitBB->getInstList().splice(SplitBB->end(), ParentBB->getInstList(), TI);
// Keep a clone of the terminator for MSSA updates.
Instruction *NewTI = TI.clone();
ParentBB->getInstList().push_back(NewTI);
// First wire up the moved terminator to the preheaders.
if (BI) {
BasicBlock *ClonedPH = ClonedPHs.begin()->second;
BI->setSuccessor(ClonedSucc, ClonedPH);
BI->setSuccessor(1 - ClonedSucc, LoopPH);
DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH});
} else {
assert(SI && "Must either be a branch or switch!");
// Walk the cases and directly update their successors.
assert(SI->getDefaultDest() == RetainedSuccBB &&
"Not retaining default successor!");
SI->setDefaultDest(LoopPH);
for (auto &Case : SI->cases())
if (Case.getCaseSuccessor() == RetainedSuccBB)
Case.setSuccessor(LoopPH);
else
Case.setSuccessor(ClonedPHs.find(Case.getCaseSuccessor())->second);
// We need to use the set to populate domtree updates as even when there
// are multiple cases pointing at the same successor we only want to
// remove and insert one edge in the domtree.
for (BasicBlock *SuccBB : UnswitchedSuccBBs)
DTUpdates.push_back(
{DominatorTree::Insert, SplitBB, ClonedPHs.find(SuccBB)->second});
}
if (MSSAU) {
DT.applyUpdates(DTUpdates);
DTUpdates.clear();
// Remove all but one edge to the retained block and all unswitched
// blocks. This is to avoid having duplicate entries in the cloned Phis,
// when we know we only keep a single edge for each case.
MSSAU->removeDuplicatePhiEdgesBetween(ParentBB, RetainedSuccBB);
for (BasicBlock *SuccBB : UnswitchedSuccBBs)
MSSAU->removeDuplicatePhiEdgesBetween(ParentBB, SuccBB);
for (auto &VMap : VMaps)
MSSAU->updateForClonedLoop(LBRPO, ExitBlocks, *VMap,
/*IgnoreIncomingWithNoClones=*/true);
MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMaps, DT);
// Remove all edges to unswitched blocks.
for (BasicBlock *SuccBB : UnswitchedSuccBBs)
MSSAU->removeEdge(ParentBB, SuccBB);
}
// Now unhook the successor relationship as we'll be replacing
// the terminator with a direct branch. This is much simpler for branches
// than switches so we handle those first.
if (BI) {
// Remove the parent as a predecessor of the unswitched successor.
assert(UnswitchedSuccBBs.size() == 1 &&
"Only one possible unswitched block for a branch!");
BasicBlock *UnswitchedSuccBB = *UnswitchedSuccBBs.begin();
UnswitchedSuccBB->removePredecessor(ParentBB,
/*KeepOneInputPHIs*/ true);
DTUpdates.push_back({DominatorTree::Delete, ParentBB, UnswitchedSuccBB});
} else {
// Note that we actually want to remove the parent block as a predecessor
// of *every* case successor. The case successor is either unswitched,
// completely eliminating an edge from the parent to that successor, or it
// is a duplicate edge to the retained successor as the retained successor
// is always the default successor and as we'll replace this with a direct
// branch we no longer need the duplicate entries in the PHI nodes.
SwitchInst *NewSI = cast<SwitchInst>(NewTI);
assert(NewSI->getDefaultDest() == RetainedSuccBB &&
"Not retaining default successor!");
for (auto &Case : NewSI->cases())
Case.getCaseSuccessor()->removePredecessor(
ParentBB,
/*KeepOneInputPHIs*/ true);
// We need to use the set to populate domtree updates as even when there
// are multiple cases pointing at the same successor we only want to
// remove and insert one edge in the domtree.
for (BasicBlock *SuccBB : UnswitchedSuccBBs)
DTUpdates.push_back({DominatorTree::Delete, ParentBB, SuccBB});
}
// After MSSAU update, remove the cloned terminator instruction NewTI.
ParentBB->getTerminator()->eraseFromParent();
// Create a new unconditional branch to the continuing block (as opposed to
// the one cloned).
BranchInst::Create(RetainedSuccBB, ParentBB);
} else {
assert(BI && "Only branches have partial unswitching.");
assert(UnswitchedSuccBBs.size() == 1 &&
"Only one possible unswitched block for a branch!");
BasicBlock *ClonedPH = ClonedPHs.begin()->second;
// When doing a partial unswitch, we have to do a bit more work to build up
// the branch in the split block.
buildPartialUnswitchConditionalBranch(*SplitBB, Invariants, Direction,
*ClonedPH, *LoopPH);
DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH});
if (MSSAU) {
DT.applyUpdates(DTUpdates);
DTUpdates.clear();
// Perform MSSA cloning updates.
for (auto &VMap : VMaps)
MSSAU->updateForClonedLoop(LBRPO, ExitBlocks, *VMap,
/*IgnoreIncomingWithNoClones=*/true);
MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMaps, DT);
}
}
// Apply the updates accumulated above to get an up-to-date dominator tree.
DT.applyUpdates(DTUpdates);
// Now that we have an accurate dominator tree, first delete the dead cloned
// blocks so that we can accurately build any cloned loops. It is important to
// not delete the blocks from the original loop yet because we still want to
// reference the original loop to understand the cloned loop's structure.
deleteDeadClonedBlocks(L, ExitBlocks, VMaps, DT, MSSAU);
// Build the cloned loop structure itself. This may be substantially
// different from the original structure due to the simplified CFG. This also
// handles inserting all the cloned blocks into the correct loops.
SmallVector<Loop *, 4> NonChildClonedLoops;
for (std::unique_ptr<ValueToValueMapTy> &VMap : VMaps)
buildClonedLoops(L, ExitBlocks, *VMap, LI, NonChildClonedLoops);
// Now that our cloned loops have been built, we can update the original loop.
// First we delete the dead blocks from it and then we rebuild the loop
// structure taking these deletions into account.
deleteDeadBlocksFromLoop(L, ExitBlocks, DT, LI, MSSAU);
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
SmallVector<Loop *, 4> HoistedLoops;
bool IsStillLoop = rebuildLoopAfterUnswitch(L, ExitBlocks, LI, HoistedLoops);
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// This transformation has a high risk of corrupting the dominator tree, and
// the below steps to rebuild loop structures will result in hard to debug
// errors in that case so verify that the dominator tree is sane first.
// FIXME: Remove this when the bugs stop showing up and rely on existing
// verification steps.
assert(DT.verify(DominatorTree::VerificationLevel::Fast));
if (BI) {
// If we unswitched a branch which collapses the condition to a known
// constant we want to replace all the uses of the invariants within both
// the original and cloned blocks. We do this here so that we can use the
// now updated dominator tree to identify which side the users are on.
assert(UnswitchedSuccBBs.size() == 1 &&
"Only one possible unswitched block for a branch!");
BasicBlock *ClonedPH = ClonedPHs.begin()->second;
// When considering multiple partially-unswitched invariants
// we cant just go replace them with constants in both branches.
//
// For 'AND' we infer that true branch ("continue") means true
// for each invariant operand.
// For 'OR' we can infer that false branch ("continue") means false
// for each invariant operand.
// So it happens that for multiple-partial case we dont replace
// in the unswitched branch.
bool ReplaceUnswitched = FullUnswitch || (Invariants.size() == 1);
ConstantInt *UnswitchedReplacement =
Direction ? ConstantInt::getTrue(BI->getContext())
: ConstantInt::getFalse(BI->getContext());
ConstantInt *ContinueReplacement =
Direction ? ConstantInt::getFalse(BI->getContext())
: ConstantInt::getTrue(BI->getContext());
for (Value *Invariant : Invariants)
for (auto UI = Invariant->use_begin(), UE = Invariant->use_end();
UI != UE;) {
// Grab the use and walk past it so we can clobber it in the use list.
Use *U = &*UI++;
Instruction *UserI = dyn_cast<Instruction>(U->getUser());
if (!UserI)
continue;
// Replace it with the 'continue' side if in the main loop body, and the
// unswitched if in the cloned blocks.
if (DT.dominates(LoopPH, UserI->getParent()))
U->set(ContinueReplacement);
else if (ReplaceUnswitched &&
DT.dominates(ClonedPH, UserI->getParent()))
U->set(UnswitchedReplacement);
}
}
// We can change which blocks are exit blocks of all the cloned sibling
// loops, the current loop, and any parent loops which shared exit blocks
// with the current loop. As a consequence, we need to re-form LCSSA for
// them. But we shouldn't need to re-form LCSSA for any child loops.
// FIXME: This could be made more efficient by tracking which exit blocks are
// new, and focusing on them, but that isn't likely to be necessary.
//
// In order to reasonably rebuild LCSSA we need to walk inside-out across the
// loop nest and update every loop that could have had its exits changed. We
// also need to cover any intervening loops. We add all of these loops to
// a list and sort them by loop depth to achieve this without updating
// unnecessary loops.
auto UpdateLoop = [&](Loop &UpdateL) {
#ifndef NDEBUG
UpdateL.verifyLoop();
for (Loop *ChildL : UpdateL) {
ChildL->verifyLoop();
assert(ChildL->isRecursivelyLCSSAForm(DT, LI) &&
"Perturbed a child loop's LCSSA form!");
}
#endif
// First build LCSSA for this loop so that we can preserve it when
// forming dedicated exits. We don't want to perturb some other loop's
// LCSSA while doing that CFG edit.
formLCSSA(UpdateL, DT, &LI, nullptr);
// For loops reached by this loop's original exit blocks we may
// introduced new, non-dedicated exits. At least try to re-form dedicated
// exits for these loops. This may fail if they couldn't have dedicated
// exits to start with.
formDedicatedExitBlocks(&UpdateL, &DT, &LI, MSSAU, /*PreserveLCSSA*/ true);
};
// For non-child cloned loops and hoisted loops, we just need to update LCSSA
// and we can do it in any order as they don't nest relative to each other.
//
// Also check if any of the loops we have updated have become top-level loops
// as that will necessitate widening the outer loop scope.
for (Loop *UpdatedL :
llvm::concat<Loop *>(NonChildClonedLoops, HoistedLoops)) {
UpdateLoop(*UpdatedL);
if (!UpdatedL->getParentLoop())
OuterExitL = nullptr;
}
if (IsStillLoop) {
UpdateLoop(L);
if (!L.getParentLoop())
OuterExitL = nullptr;
}
// If the original loop had exit blocks, walk up through the outer most loop
// of those exit blocks to update LCSSA and form updated dedicated exits.
if (OuterExitL != &L)
for (Loop *OuterL = ParentL; OuterL != OuterExitL;
OuterL = OuterL->getParentLoop())
UpdateLoop(*OuterL);
#ifndef NDEBUG
// Verify the entire loop structure to catch any incorrect updates before we
// progress in the pass pipeline.
LI.verify(DT);
#endif
// Now that we've unswitched something, make callbacks to report the changes.
// For that we need to merge together the updated loops and the cloned loops
// and check whether the original loop survived.
SmallVector<Loop *, 4> SibLoops;
for (Loop *UpdatedL : llvm::concat<Loop *>(NonChildClonedLoops, HoistedLoops))
if (UpdatedL->getParentLoop() == ParentL)
SibLoops.push_back(UpdatedL);
UnswitchCB(IsStillLoop, SibLoops);
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
if (BI)
++NumBranches;
else
++NumSwitches;
}
/// Recursively compute the cost of a dominator subtree based on the per-block
/// cost map provided.
///
/// The recursive computation is memozied into the provided DT-indexed cost map
/// to allow querying it for most nodes in the domtree without it becoming
/// quadratic.
static int
computeDomSubtreeCost(DomTreeNode &N,
const SmallDenseMap<BasicBlock *, int, 4> &BBCostMap,
SmallDenseMap<DomTreeNode *, int, 4> &DTCostMap) {
// Don't accumulate cost (or recurse through) blocks not in our block cost
// map and thus not part of the duplication cost being considered.
auto BBCostIt = BBCostMap.find(N.getBlock());
if (BBCostIt == BBCostMap.end())
return 0;
// Lookup this node to see if we already computed its cost.
auto DTCostIt = DTCostMap.find(&N);
if (DTCostIt != DTCostMap.end())
return DTCostIt->second;
// If not, we have to compute it. We can't use insert above and update
// because computing the cost may insert more things into the map.
int Cost = std::accumulate(
N.begin(), N.end(), BBCostIt->second, [&](int Sum, DomTreeNode *ChildN) {
return Sum + computeDomSubtreeCost(*ChildN, BBCostMap, DTCostMap);
});
bool Inserted = DTCostMap.insert({&N, Cost}).second;
(void)Inserted;
assert(Inserted && "Should not insert a node while visiting children!");
return Cost;
}
/// Turns a llvm.experimental.guard intrinsic into implicit control flow branch,
/// making the following replacement:
///
/// --code before guard--
/// call void (i1, ...) @llvm.experimental.guard(i1 %cond) [ "deopt"() ]
/// --code after guard--
///
/// into
///
/// --code before guard--
/// br i1 %cond, label %guarded, label %deopt
///
/// guarded:
/// --code after guard--
///
/// deopt:
/// call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
/// unreachable
///
/// It also makes all relevant DT and LI updates, so that all structures are in
/// valid state after this transform.
static BranchInst *
turnGuardIntoBranch(IntrinsicInst *GI, Loop &L,
SmallVectorImpl<BasicBlock *> &ExitBlocks,
DominatorTree &DT, LoopInfo &LI, MemorySSAUpdater *MSSAU) {
SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
LLVM_DEBUG(dbgs() << "Turning " << *GI << " into a branch.\n");
BasicBlock *CheckBB = GI->getParent();
if (MSSAU && VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
// Remove all CheckBB's successors from DomTree. A block can be seen among
// successors more than once, but for DomTree it should be added only once.
SmallPtrSet<BasicBlock *, 4> Successors;
for (auto *Succ : successors(CheckBB))
if (Successors.insert(Succ).second)
DTUpdates.push_back({DominatorTree::Delete, CheckBB, Succ});
Instruction *DeoptBlockTerm =
SplitBlockAndInsertIfThen(GI->getArgOperand(0), GI, true);
BranchInst *CheckBI = cast<BranchInst>(CheckBB->getTerminator());
// SplitBlockAndInsertIfThen inserts control flow that branches to
// DeoptBlockTerm if the condition is true. We want the opposite.
CheckBI->swapSuccessors();
BasicBlock *GuardedBlock = CheckBI->getSuccessor(0);
GuardedBlock->setName("guarded");
CheckBI->getSuccessor(1)->setName("deopt");
BasicBlock *DeoptBlock = CheckBI->getSuccessor(1);
// We now have a new exit block.
ExitBlocks.push_back(CheckBI->getSuccessor(1));
if (MSSAU)
MSSAU->moveAllAfterSpliceBlocks(CheckBB, GuardedBlock, GI);
GI->moveBefore(DeoptBlockTerm);
GI->setArgOperand(0, ConstantInt::getFalse(GI->getContext()));
// Add new successors of CheckBB into DomTree.
for (auto *Succ : successors(CheckBB))
DTUpdates.push_back({DominatorTree::Insert, CheckBB, Succ});
// Now the blocks that used to be CheckBB's successors are GuardedBlock's
// successors.
for (auto *Succ : Successors)
DTUpdates.push_back({DominatorTree::Insert, GuardedBlock, Succ});
// Make proper changes to DT.
DT.applyUpdates(DTUpdates);
// Inform LI of a new loop block.
L.addBasicBlockToLoop(GuardedBlock, LI);
if (MSSAU) {
MemoryDef *MD = cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(GI));
MSSAU->moveToPlace(MD, DeoptBlock, MemorySSA::End);
if (VerifyMemorySSA)
MSSAU->getMemorySSA()->verifyMemorySSA();
}
++NumGuards;
return CheckBI;
}
/// Cost multiplier is a way to limit potentially exponential behavior
/// of loop-unswitch. Cost is multipied in proportion of 2^number of unswitch
/// candidates available. Also accounting for the number of "sibling" loops with
/// the idea to account for previous unswitches that already happened on this
/// cluster of loops. There was an attempt to keep this formula simple,
/// just enough to limit the worst case behavior. Even if it is not that simple
/// now it is still not an attempt to provide a detailed heuristic size
/// prediction.
///
/// TODO: Make a proper accounting of "explosion" effect for all kinds of
/// unswitch candidates, making adequate predictions instead of wild guesses.
/// That requires knowing not just the number of "remaining" candidates but
/// also costs of unswitching for each of these candidates.
static int calculateUnswitchCostMultiplier(
Instruction &TI, Loop &L, LoopInfo &LI, DominatorTree &DT,
ArrayRef<std::pair<Instruction *, TinyPtrVector<Value *>>>
UnswitchCandidates) {
// Guards and other exiting conditions do not contribute to exponential
// explosion as soon as they dominate the latch (otherwise there might be
// another path to the latch remaining that does not allow to eliminate the
// loop copy on unswitch).
BasicBlock *Latch = L.getLoopLatch();
BasicBlock *CondBlock = TI.getParent();
if (DT.dominates(CondBlock, Latch) &&
(isGuard(&TI) ||
llvm::count_if(successors(&TI), [&L](BasicBlock *SuccBB) {
return L.contains(SuccBB);
}) <= 1)) {
NumCostMultiplierSkipped++;
return 1;
}
auto *ParentL = L.getParentLoop();
int SiblingsCount = (ParentL ? ParentL->getSubLoopsVector().size()
: std::distance(LI.begin(), LI.end()));
// Count amount of clones that all the candidates might cause during
// unswitching. Branch/guard counts as 1, switch counts as log2 of its cases.
int UnswitchedClones = 0;
for (auto Candidate : UnswitchCandidates) {
Instruction *CI = Candidate.first;
BasicBlock *CondBlock = CI->getParent();
bool SkipExitingSuccessors = DT.dominates(CondBlock, Latch);
if (isGuard(CI)) {
if (!SkipExitingSuccessors)
UnswitchedClones++;
continue;
}
int NonExitingSuccessors = llvm::count_if(
successors(CondBlock), [SkipExitingSuccessors, &L](BasicBlock *SuccBB) {
return !SkipExitingSuccessors || L.contains(SuccBB);
});
UnswitchedClones += Log2_32(NonExitingSuccessors);
}
// Ignore up to the "unscaled candidates" number of unswitch candidates
// when calculating the power-of-two scaling of the cost. The main idea
// with this control is to allow a small number of unswitches to happen
// and rely more on siblings multiplier (see below) when the number
// of candidates is small.
unsigned ClonesPower =
std::max(UnswitchedClones - (int)UnswitchNumInitialUnscaledCandidates, 0);
// Allowing top-level loops to spread a bit more than nested ones.
int SiblingsMultiplier =
std::max((ParentL ? SiblingsCount
: SiblingsCount / (int)UnswitchSiblingsToplevelDiv),
1);
// Compute the cost multiplier in a way that won't overflow by saturating
// at an upper bound.
int CostMultiplier;
if (ClonesPower > Log2_32(UnswitchThreshold) ||
SiblingsMultiplier > UnswitchThreshold)
CostMultiplier = UnswitchThreshold;
else
CostMultiplier = std::min(SiblingsMultiplier * (1 << ClonesPower),
(int)UnswitchThreshold);
LLVM_DEBUG(dbgs() << " Computed multiplier " << CostMultiplier
<< " (siblings " << SiblingsMultiplier << " * clones "
<< (1 << ClonesPower) << ")"
<< " for unswitch candidate: " << TI << "\n");
return CostMultiplier;
}
static bool
unswitchBestCondition(Loop &L, DominatorTree &DT, LoopInfo &LI,
AssumptionCache &AC, TargetTransformInfo &TTI,
function_ref<void(bool, ArrayRef<Loop *>)> UnswitchCB,
ScalarEvolution *SE, MemorySSAUpdater *MSSAU) {
// Collect all invariant conditions within this loop (as opposed to an inner
// loop which would be handled when visiting that inner loop).
SmallVector<std::pair<Instruction *, TinyPtrVector<Value *>>, 4>
UnswitchCandidates;
// Whether or not we should also collect guards in the loop.
bool CollectGuards = false;
if (UnswitchGuards) {
auto *GuardDecl = L.getHeader()->getParent()->getParent()->getFunction(
Intrinsic::getName(Intrinsic::experimental_guard));
if (GuardDecl && !GuardDecl->use_empty())
CollectGuards = true;
}
for (auto *BB : L.blocks()) {
if (LI.getLoopFor(BB) != &L)
continue;
if (CollectGuards)
for (auto &I : *BB)
if (isGuard(&I)) {
auto *Cond = cast<IntrinsicInst>(&I)->getArgOperand(0);
// TODO: Support AND, OR conditions and partial unswitching.
if (!isa<Constant>(Cond) && L.isLoopInvariant(Cond))
UnswitchCandidates.push_back({&I, {Cond}});
}
if (auto *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
// We can only consider fully loop-invariant switch conditions as we need
// to completely eliminate the switch after unswitching.
if (!isa<Constant>(SI->getCondition()) &&
L.isLoopInvariant(SI->getCondition()) && !BB->getUniqueSuccessor())
UnswitchCandidates.push_back({SI, {SI->getCondition()}});
continue;
}
auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
if (!BI || !BI->isConditional() || isa<Constant>(BI->getCondition()) ||
BI->getSuccessor(0) == BI->getSuccessor(1))
continue;
if (L.isLoopInvariant(BI->getCondition())) {
UnswitchCandidates.push_back({BI, {BI->getCondition()}});
continue;
}
Instruction &CondI = *cast<Instruction>(BI->getCondition());
if (CondI.getOpcode() != Instruction::And &&
CondI.getOpcode() != Instruction::Or)
continue;
TinyPtrVector<Value *> Invariants =
collectHomogenousInstGraphLoopInvariants(L, CondI, LI);
if (Invariants.empty())
continue;
UnswitchCandidates.push_back({BI, std::move(Invariants)});
}
// If we didn't find any candidates, we're done.
if (UnswitchCandidates.empty())
return false;
// Check if there are irreducible CFG cycles in this loop. If so, we cannot
// easily unswitch non-trivial edges out of the loop. Doing so might turn the
// irreducible control flow into reducible control flow and introduce new
// loops "out of thin air". If we ever discover important use cases for doing
// this, we can add support to loop unswitch, but it is a lot of complexity
// for what seems little or no real world benefit.
LoopBlocksRPO RPOT(&L);
RPOT.perform(&LI);
if (containsIrreducibleCFG<const BasicBlock *>(RPOT, LI))
return false;
SmallVector<BasicBlock *, 4> ExitBlocks;
L.getUniqueExitBlocks(ExitBlocks);
// We cannot unswitch if exit blocks contain a cleanuppad instruction as we
// don't know how to split those exit blocks.
// FIXME: We should teach SplitBlock to handle this and remove this
// restriction.
for (auto *ExitBB : ExitBlocks)
if (isa<CleanupPadInst>(ExitBB->getFirstNonPHI())) {
dbgs() << "Cannot unswitch because of cleanuppad in exit block\n";
return false;
}
LLVM_DEBUG(
dbgs() << "Considering " << UnswitchCandidates.size()
<< " non-trivial loop invariant conditions for unswitching.\n");
// Given that unswitching these terminators will require duplicating parts of
// the loop, so we need to be able to model that cost. Compute the ephemeral
// values and set up a data structure to hold per-BB costs. We cache each
// block's cost so that we don't recompute this when considering different
// subsets of the loop for duplication during unswitching.
SmallPtrSet<const Value *, 4> EphValues;
CodeMetrics::collectEphemeralValues(&L, &AC, EphValues);
SmallDenseMap<BasicBlock *, int, 4> BBCostMap;
// Compute the cost of each block, as well as the total loop cost. Also, bail
// out if we see instructions which are incompatible with loop unswitching
// (convergent, noduplicate, or cross-basic-block tokens).
// FIXME: We might be able to safely handle some of these in non-duplicated
// regions.
int LoopCost = 0;
for (auto *BB : L.blocks()) {
int Cost = 0;
for (auto &I : *BB) {
if (EphValues.count(&I))
continue;
if (I.getType()->isTokenTy() && I.isUsedOutsideOfBlock(BB))
return false;
if (auto CS = CallSite(&I))
if (CS.isConvergent() || CS.cannotDuplicate())
return false;
Cost += TTI.getUserCost(&I);
}
assert(Cost >= 0 && "Must not have negative costs!");
LoopCost += Cost;
assert(LoopCost >= 0 && "Must not have negative loop costs!");
BBCostMap[BB] = Cost;
}
LLVM_DEBUG(dbgs() << " Total loop cost: " << LoopCost << "\n");
// Now we find the best candidate by searching for the one with the following
// properties in order:
//
// 1) An unswitching cost below the threshold
// 2) The smallest number of duplicated unswitch candidates (to avoid
// creating redundant subsequent unswitching)
// 3) The smallest cost after unswitching.
//
// We prioritize reducing fanout of unswitch candidates provided the cost
// remains below the threshold because this has a multiplicative effect.
//
// This requires memoizing each dominator subtree to avoid redundant work.
//
// FIXME: Need to actually do the number of candidates part above.
SmallDenseMap<DomTreeNode *, int, 4> DTCostMap;
// Given a terminator which might be unswitched, computes the non-duplicated
// cost for that terminator.
auto ComputeUnswitchedCost = [&](Instruction &TI, bool FullUnswitch) {
BasicBlock &BB = *TI.getParent();
SmallPtrSet<BasicBlock *, 4> Visited;
int Cost = LoopCost;
for (BasicBlock *SuccBB : successors(&BB)) {
// Don't count successors more than once.
if (!Visited.insert(SuccBB).second)
continue;
// If this is a partial unswitch candidate, then it must be a conditional
// branch with a condition of either `or` or `and`. In that case, one of
// the successors is necessarily duplicated, so don't even try to remove
// its cost.
if (!FullUnswitch) {
auto &BI = cast<BranchInst>(TI);
if (cast<Instruction>(BI.getCondition())->getOpcode() ==
Instruction::And) {
if (SuccBB == BI.getSuccessor(1))
continue;
} else {
assert(cast<Instruction>(BI.getCondition())->getOpcode() ==
Instruction::Or &&
"Only `and` and `or` conditions can result in a partial "
"unswitch!");
if (SuccBB == BI.getSuccessor(0))
continue;
}
}
// This successor's domtree will not need to be duplicated after
// unswitching if the edge to the successor dominates it (and thus the
// entire tree). This essentially means there is no other path into this
// subtree and so it will end up live in only one clone of the loop.
if (SuccBB->getUniquePredecessor() ||
llvm::all_of(predecessors(SuccBB), [&](BasicBlock *PredBB) {
return PredBB == &BB || DT.dominates(SuccBB, PredBB);
})) {
Cost -= computeDomSubtreeCost(*DT[SuccBB], BBCostMap, DTCostMap);
assert(Cost >= 0 &&
"Non-duplicated cost should never exceed total loop cost!");
}
}
// Now scale the cost by the number of unique successors minus one. We
// subtract one because there is already at least one copy of the entire
// loop. This is computing the new cost of unswitching a condition.
// Note that guards always have 2 unique successors that are implicit and
// will be materialized if we decide to unswitch it.
int SuccessorsCount = isGuard(&TI) ? 2 : Visited.size();
assert(SuccessorsCount > 1 &&
"Cannot unswitch a condition without multiple distinct successors!");
return Cost * (SuccessorsCount - 1);
};
Instruction *BestUnswitchTI = nullptr;
int BestUnswitchCost = 0;
ArrayRef<Value *> BestUnswitchInvariants;
for (auto &TerminatorAndInvariants : UnswitchCandidates) {
Instruction &TI = *TerminatorAndInvariants.first;
ArrayRef<Value *> Invariants = TerminatorAndInvariants.second;
BranchInst *BI = dyn_cast<BranchInst>(&TI);
int CandidateCost = ComputeUnswitchedCost(
TI, /*FullUnswitch*/ !BI || (Invariants.size() == 1 &&
Invariants[0] == BI->getCondition()));
// Calculate cost multiplier which is a tool to limit potentially
// exponential behavior of loop-unswitch.
if (EnableUnswitchCostMultiplier) {
int CostMultiplier =
calculateUnswitchCostMultiplier(TI, L, LI, DT, UnswitchCandidates);
assert(
(CostMultiplier > 0 && CostMultiplier <= UnswitchThreshold) &&
"cost multiplier needs to be in the range of 1..UnswitchThreshold");
CandidateCost *= CostMultiplier;
LLVM_DEBUG(dbgs() << " Computed cost of " << CandidateCost
<< " (multiplier: " << CostMultiplier << ")"
<< " for unswitch candidate: " << TI << "\n");
} else {
LLVM_DEBUG(dbgs() << " Computed cost of " << CandidateCost
<< " for unswitch candidate: " << TI << "\n");
}
if (!BestUnswitchTI || CandidateCost < BestUnswitchCost) {
BestUnswitchTI = &TI;
BestUnswitchCost = CandidateCost;
BestUnswitchInvariants = Invariants;
}
}
assert(BestUnswitchTI && "Failed to find loop unswitch candidate");
if (BestUnswitchCost >= UnswitchThreshold) {
LLVM_DEBUG(dbgs() << "Cannot unswitch, lowest cost found: "
<< BestUnswitchCost << "\n");
return false;
}
// If the best candidate is a guard, turn it into a branch.
if (isGuard(BestUnswitchTI))
BestUnswitchTI = turnGuardIntoBranch(cast<IntrinsicInst>(BestUnswitchTI), L,
ExitBlocks, DT, LI, MSSAU);
LLVM_DEBUG(dbgs() << " Unswitching non-trivial (cost = "
<< BestUnswitchCost << ") terminator: " << *BestUnswitchTI
<< "\n");
unswitchNontrivialInvariants(L, *BestUnswitchTI, BestUnswitchInvariants,
ExitBlocks, DT, LI, AC, UnswitchCB, SE, MSSAU);
return true;
}
/// Unswitch control flow predicated on loop invariant conditions.
///
/// This first hoists all branches or switches which are trivial (IE, do not
/// require duplicating any part of the loop) out of the loop body. It then
/// looks at other loop invariant control flows and tries to unswitch those as
/// well by cloning the loop if the result is small enough.
///
/// The `DT`, `LI`, `AC`, `TTI` parameters are required analyses that are also
/// updated based on the unswitch.
/// The `MSSA` analysis is also updated if valid (i.e. its use is enabled).
///
/// If either `NonTrivial` is true or the flag `EnableNonTrivialUnswitch` is
/// true, we will attempt to do non-trivial unswitching as well as trivial
/// unswitching.
///
/// The `UnswitchCB` callback provided will be run after unswitching is
/// complete, with the first parameter set to `true` if the provided loop
/// remains a loop, and a list of new sibling loops created.
///
/// If `SE` is non-null, we will update that analysis based on the unswitching
/// done.
static bool unswitchLoop(Loop &L, DominatorTree &DT, LoopInfo &LI,
AssumptionCache &AC, TargetTransformInfo &TTI,
bool NonTrivial,
function_ref<void(bool, ArrayRef<Loop *>)> UnswitchCB,
ScalarEvolution *SE, MemorySSAUpdater *MSSAU) {
assert(L.isRecursivelyLCSSAForm(DT, LI) &&
"Loops must be in LCSSA form before unswitching.");
bool Changed = false;
// Must be in loop simplified form: we need a preheader and dedicated exits.
if (!L.isLoopSimplifyForm())
return false;
// Try trivial unswitch first before loop over other basic blocks in the loop.
if (unswitchAllTrivialConditions(L, DT, LI, SE, MSSAU)) {
// If we unswitched successfully we will want to clean up the loop before
// processing it further so just mark it as unswitched and return.
UnswitchCB(/*CurrentLoopValid*/ true, {});
return true;
}
// If we're not doing non-trivial unswitching, we're done. We both accept
// a parameter but also check a local flag that can be used for testing
// a debugging.
if (!NonTrivial && !EnableNonTrivialUnswitch)
return false;
// For non-trivial unswitching, because it often creates new loops, we rely on
// the pass manager to iterate on the loops rather than trying to immediately
// reach a fixed point. There is no substantial advantage to iterating
// internally, and if any of the new loops are simplified enough to contain
// trivial unswitching we want to prefer those.
// Try to unswitch the best invariant condition. We prefer this full unswitch to
// a partial unswitch when possible below the threshold.
if (unswitchBestCondition(L, DT, LI, AC, TTI, UnswitchCB, SE, MSSAU))
return true;
// No other opportunities to unswitch.
return Changed;
}
PreservedAnalyses SimpleLoopUnswitchPass::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR,
LPMUpdater &U) {
Function &F = *L.getHeader()->getParent();
(void)F;
LLVM_DEBUG(dbgs() << "Unswitching loop in " << F.getName() << ": " << L
<< "\n");
// Save the current loop name in a variable so that we can report it even
// after it has been deleted.
std::string LoopName = L.getName();
auto UnswitchCB = [&L, &U, &LoopName](bool CurrentLoopValid,
ArrayRef<Loop *> NewLoops) {
// If we did a non-trivial unswitch, we have added new (cloned) loops.
if (!NewLoops.empty())
U.addSiblingLoops(NewLoops);
// If the current loop remains valid, we should revisit it to catch any
// other unswitch opportunities. Otherwise, we need to mark it as deleted.
if (CurrentLoopValid)
U.revisitCurrentLoop();
else
U.markLoopAsDeleted(L, LoopName);
};
Optional<MemorySSAUpdater> MSSAU;
if (AR.MSSA) {
MSSAU = MemorySSAUpdater(AR.MSSA);
if (VerifyMemorySSA)
AR.MSSA->verifyMemorySSA();
}
if (!unswitchLoop(L, AR.DT, AR.LI, AR.AC, AR.TTI, NonTrivial, UnswitchCB,
&AR.SE, MSSAU.hasValue() ? MSSAU.getPointer() : nullptr))
return PreservedAnalyses::all();
if (AR.MSSA && VerifyMemorySSA)
AR.MSSA->verifyMemorySSA();
// Historically this pass has had issues with the dominator tree so verify it
// in asserts builds.
assert(AR.DT.verify(DominatorTree::VerificationLevel::Fast));
auto PA = getLoopPassPreservedAnalyses();
if (AR.MSSA)
PA.preserve<MemorySSAAnalysis>();
return PA;
}
namespace {
class SimpleLoopUnswitchLegacyPass : public LoopPass {
bool NonTrivial;
public:
static char ID; // Pass ID, replacement for typeid
explicit SimpleLoopUnswitchLegacyPass(bool NonTrivial = false)
: LoopPass(ID), NonTrivial(NonTrivial) {
initializeSimpleLoopUnswitchLegacyPassPass(
*PassRegistry::getPassRegistry());
}
bool runOnLoop(Loop *L, LPPassManager &LPM) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<AssumptionCacheTracker>();
AU.addRequired<TargetTransformInfoWrapperPass>();
if (EnableMSSALoopDependency) {
AU.addRequired<MemorySSAWrapperPass>();
AU.addPreserved<MemorySSAWrapperPass>();
}
getLoopAnalysisUsage(AU);
}
};
} // end anonymous namespace
bool SimpleLoopUnswitchLegacyPass::runOnLoop(Loop *L, LPPassManager &LPM) {
if (skipLoop(L))
return false;
Function &F = *L->getHeader()->getParent();
LLVM_DEBUG(dbgs() << "Unswitching loop in " << F.getName() << ": " << *L
<< "\n");
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
MemorySSA *MSSA = nullptr;
Optional<MemorySSAUpdater> MSSAU;
if (EnableMSSALoopDependency) {
MSSA = &getAnalysis<MemorySSAWrapperPass>().getMSSA();
MSSAU = MemorySSAUpdater(MSSA);
}
auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
auto *SE = SEWP ? &SEWP->getSE() : nullptr;
auto UnswitchCB = [&L, &LPM](bool CurrentLoopValid,
ArrayRef<Loop *> NewLoops) {
// If we did a non-trivial unswitch, we have added new (cloned) loops.
for (auto *NewL : NewLoops)
LPM.addLoop(*NewL);
// If the current loop remains valid, re-add it to the queue. This is
// a little wasteful as we'll finish processing the current loop as well,
// but it is the best we can do in the old PM.
if (CurrentLoopValid)
LPM.addLoop(*L);
else
LPM.markLoopAsDeleted(*L);
};
if (MSSA && VerifyMemorySSA)
MSSA->verifyMemorySSA();
bool Changed = unswitchLoop(*L, DT, LI, AC, TTI, NonTrivial, UnswitchCB, SE,
MSSAU.hasValue() ? MSSAU.getPointer() : nullptr);
if (MSSA && VerifyMemorySSA)
MSSA->verifyMemorySSA();
// If anything was unswitched, also clear any cached information about this
// loop.
LPM.deleteSimpleAnalysisLoop(L);
// Historically this pass has had issues with the dominator tree so verify it
// in asserts builds.
assert(DT.verify(DominatorTree::VerificationLevel::Fast));
return Changed;
}
char SimpleLoopUnswitchLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(SimpleLoopUnswitchLegacyPass, "simple-loop-unswitch",
"Simple unswitch loops", false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopPass)
INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_END(SimpleLoopUnswitchLegacyPass, "simple-loop-unswitch",
"Simple unswitch loops", false, false)
Pass *llvm::createSimpleLoopUnswitchLegacyPass(bool NonTrivial) {
return new SimpleLoopUnswitchLegacyPass(NonTrivial);
}
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