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llvm-project/llvm/lib/Transforms/Utils/LCSSA.cpp

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//===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass transforms loops by placing phi nodes at the end of the loops for
// all values that are live across the loop boundary. For example, it turns
// the left into the right code:
//
// for (...) for (...)
// if (c) if (c)
// X1 = ... X1 = ...
// else else
// X2 = ... X2 = ...
// X3 = phi(X1, X2) X3 = phi(X1, X2)
// ... = X3 + 4 X4 = phi(X3)
// ... = X4 + 4
//
// This is still valid LLVM; the extra phi nodes are purely redundant, and will
// be trivially eliminated by InstCombine. The major benefit of this
// transformation is that it makes many other loop optimizations, such as
// LoopUnswitching, simpler.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/LCSSA.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/PredIteratorCache.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
using namespace llvm;
#define DEBUG_TYPE "lcssa"
STATISTIC(NumLCSSA, "Number of live out of a loop variables");
/// Return true if the specified block is in the list.
static bool isExitBlock(BasicBlock *BB,
const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
return is_contained(ExitBlocks, BB);
}
/// For every instruction from the worklist, check to see if it has any uses
/// that are outside the current loop. If so, insert LCSSA PHI nodes and
/// rewrite the uses.
bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
DominatorTree &DT, LoopInfo &LI) {
SmallVector<Use *, 16> UsesToRewrite;
SmallSetVector<PHINode *, 16> PHIsToRemove;
PredIteratorCache PredCache;
bool Changed = false;
// Cache the Loop ExitBlocks across this loop. We expect to get a lot of
// instructions within the same loops, computing the exit blocks is
// expensive, and we're not mutating the loop structure.
SmallDenseMap<Loop*, SmallVector<BasicBlock *,1>> LoopExitBlocks;
while (!Worklist.empty()) {
UsesToRewrite.clear();
Instruction *I = Worklist.pop_back_val();
BasicBlock *InstBB = I->getParent();
Loop *L = LI.getLoopFor(InstBB);
if (!LoopExitBlocks.count(L))
L->getExitBlocks(LoopExitBlocks[L]);
assert(LoopExitBlocks.count(L));
const SmallVectorImpl<BasicBlock *> &ExitBlocks = LoopExitBlocks[L];
if (ExitBlocks.empty())
continue;
// Tokens cannot be used in PHI nodes, so we skip over them.
// We can run into tokens which are live out of a loop with catchswitch
// instructions in Windows EH if the catchswitch has one catchpad which
// is inside the loop and another which is not.
if (I->getType()->isTokenTy())
continue;
for (Use &U : I->uses()) {
Instruction *User = cast<Instruction>(U.getUser());
BasicBlock *UserBB = User->getParent();
if (PHINode *PN = dyn_cast<PHINode>(User))
UserBB = PN->getIncomingBlock(U);
if (InstBB != UserBB && !L->contains(UserBB))
UsesToRewrite.push_back(&U);
}
// If there are no uses outside the loop, exit with no change.
if (UsesToRewrite.empty())
continue;
++NumLCSSA; // We are applying the transformation
// Invoke instructions are special in that their result value is not
// available along their unwind edge. The code below tests to see whether
// DomBB dominates the value, so adjust DomBB to the normal destination
// block, which is effectively where the value is first usable.
BasicBlock *DomBB = InstBB;
if (InvokeInst *Inv = dyn_cast<InvokeInst>(I))
DomBB = Inv->getNormalDest();
DomTreeNode *DomNode = DT.getNode(DomBB);
SmallVector<PHINode *, 16> AddedPHIs;
SmallVector<PHINode *, 8> PostProcessPHIs;
SmallVector<PHINode *, 4> InsertedPHIs;
SSAUpdater SSAUpdate(&InsertedPHIs);
SSAUpdate.Initialize(I->getType(), I->getName());
// Insert the LCSSA phi's into all of the exit blocks dominated by the
// value, and add them to the Phi's map.
for (BasicBlock *ExitBB : ExitBlocks) {
if (!DT.dominates(DomNode, DT.getNode(ExitBB)))
continue;
// If we already inserted something for this BB, don't reprocess it.
if (SSAUpdate.HasValueForBlock(ExitBB))
continue;
PHINode *PN = PHINode::Create(I->getType(), PredCache.size(ExitBB),
I->getName() + ".lcssa", &ExitBB->front());
// Add inputs from inside the loop for this PHI.
for (BasicBlock *Pred : PredCache.get(ExitBB)) {
PN->addIncoming(I, Pred);
// If the exit block has a predecessor not within the loop, arrange for
// the incoming value use corresponding to that predecessor to be
// rewritten in terms of a different LCSSA PHI.
if (!L->contains(Pred))
UsesToRewrite.push_back(
&PN->getOperandUse(PN->getOperandNumForIncomingValue(
PN->getNumIncomingValues() - 1)));
}
AddedPHIs.push_back(PN);
// Remember that this phi makes the value alive in this block.
SSAUpdate.AddAvailableValue(ExitBB, PN);
// LoopSimplify might fail to simplify some loops (e.g. when indirect
// branches are involved). In such situations, it might happen that an
// exit for Loop L1 is the header of a disjoint Loop L2. Thus, when we
// create PHIs in such an exit block, we are also inserting PHIs into L2's
// header. This could break LCSSA form for L2 because these inserted PHIs
// can also have uses outside of L2. Remember all PHIs in such situation
// as to revisit than later on. FIXME: Remove this if indirectbr support
// into LoopSimplify gets improved.
if (auto *OtherLoop = LI.getLoopFor(ExitBB))
if (!L->contains(OtherLoop))
PostProcessPHIs.push_back(PN);
}
// Rewrite all uses outside the loop in terms of the new PHIs we just
// inserted.
for (Use *UseToRewrite : UsesToRewrite) {
// If this use is in an exit block, rewrite to use the newly inserted PHI.
// This is required for correctness because SSAUpdate doesn't handle uses
// in the same block. It assumes the PHI we inserted is at the end of the
// block.
Instruction *User = cast<Instruction>(UseToRewrite->getUser());
BasicBlock *UserBB = User->getParent();
if (PHINode *PN = dyn_cast<PHINode>(User))
UserBB = PN->getIncomingBlock(*UseToRewrite);
if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
// Tell the VHs that the uses changed. This updates SCEV's caches.
if (UseToRewrite->get()->hasValueHandle())
ValueHandleBase::ValueIsRAUWd(*UseToRewrite, &UserBB->front());
UseToRewrite->set(&UserBB->front());
continue;
}
// Otherwise, do full PHI insertion.
SSAUpdate.RewriteUse(*UseToRewrite);
}
// SSAUpdater might have inserted phi-nodes inside other loops. We'll need
// to post-process them to keep LCSSA form.
for (PHINode *InsertedPN : InsertedPHIs) {
if (auto *OtherLoop = LI.getLoopFor(InsertedPN->getParent()))
if (!L->contains(OtherLoop))
PostProcessPHIs.push_back(InsertedPN);
}
// Post process PHI instructions that were inserted into another disjoint
// loop and update their exits properly.
for (auto *PostProcessPN : PostProcessPHIs) {
if (PostProcessPN->use_empty())
continue;
// Reprocess each PHI instruction.
Worklist.push_back(PostProcessPN);
}
// Keep track of PHI nodes that we want to remove because they did not have
// any uses rewritten.
for (PHINode *PN : AddedPHIs)
if (PN->use_empty())
PHIsToRemove.insert(PN);
Changed = true;
}
// Remove PHI nodes that did not have any uses rewritten.
for (PHINode *PN : PHIsToRemove) {
assert (PN->use_empty() && "Trying to remove a phi with uses.");
PN->eraseFromParent();
}
return Changed;
}
/// Return true if the specified block dominates at least
/// one of the blocks in the specified list.
static bool
blockDominatesAnExit(BasicBlock *BB,
DominatorTree &DT,
const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
DomTreeNode *DomNode = DT.getNode(BB);
return any_of(ExitBlocks, [&](BasicBlock *EB) {
return DT.dominates(DomNode, DT.getNode(EB));
});
}
bool llvm::formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI,
ScalarEvolution *SE) {
bool Changed = false;
// Get the set of exiting blocks.
SmallVector<BasicBlock *, 8> ExitBlocks;
L.getExitBlocks(ExitBlocks);
if (ExitBlocks.empty())
return false;
SmallVector<Instruction *, 8> Worklist;
// Look at all the instructions in the loop, checking to see if they have uses
// outside the loop. If so, put them into the worklist to rewrite those uses.
for (BasicBlock *BB : L.blocks()) {
// For large loops, avoid use-scanning by using dominance information: In
// particular, if a block does not dominate any of the loop exits, then none
// of the values defined in the block could be used outside the loop.
if (!blockDominatesAnExit(BB, DT, ExitBlocks))
continue;
for (Instruction &I : *BB) {
// Reject two common cases fast: instructions with no uses (like stores)
// and instructions with one use that is in the same block as this.
if (I.use_empty() ||
(I.hasOneUse() && I.user_back()->getParent() == BB &&
!isa<PHINode>(I.user_back())))
continue;
Worklist.push_back(&I);
}
}
Changed = formLCSSAForInstructions(Worklist, DT, *LI);
// If we modified the code, remove any caches about the loop from SCEV to
// avoid dangling entries.
// FIXME: This is a big hammer, can we clear the cache more selectively?
if (SE && Changed)
SE->forgetLoop(&L);
assert(L.isLCSSAForm(DT));
return Changed;
}
/// Process a loop nest depth first.
bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT, LoopInfo *LI,
ScalarEvolution *SE) {
bool Changed = false;
// Recurse depth-first through inner loops.
for (Loop *SubLoop : L.getSubLoops())
Changed |= formLCSSARecursively(*SubLoop, DT, LI, SE);
Changed |= formLCSSA(L, DT, LI, SE);
return Changed;
}
/// Process all loops in the function, inner-most out.
static bool formLCSSAOnAllLoops(LoopInfo *LI, DominatorTree &DT,
ScalarEvolution *SE) {
bool Changed = false;
for (auto &L : *LI)
Changed |= formLCSSARecursively(*L, DT, LI, SE);
return Changed;
}
namespace {
struct LCSSAWrapperPass : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
LCSSAWrapperPass() : FunctionPass(ID) {
initializeLCSSAWrapperPassPass(*PassRegistry::getPassRegistry());
}
// Cached analysis information for the current function.
DominatorTree *DT;
LoopInfo *LI;
ScalarEvolution *SE;
bool runOnFunction(Function &F) override;
void verifyAnalysis() const override {
assert(
all_of(*LI, [&](Loop *L) { return L->isRecursivelyLCSSAForm(*DT); }) &&
"LCSSA form is broken!");
};
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG. It maintains both of these,
/// as well as the CFG. It also requires dominator information.
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<LoopInfoWrapperPass>();
AU.addPreservedID(LoopSimplifyID);
AU.addPreserved<AAResultsWrapperPass>();
AU.addPreserved<BasicAAWrapperPass>();
AU.addPreserved<GlobalsAAWrapperPass>();
AU.addPreserved<ScalarEvolutionWrapperPass>();
AU.addPreserved<SCEVAAWrapperPass>();
}
};
}
char LCSSAWrapperPass::ID = 0;
INITIALIZE_PASS_BEGIN(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_END(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
false, false)
Pass *llvm::createLCSSAPass() { return new LCSSAWrapperPass(); }
char &llvm::LCSSAID = LCSSAWrapperPass::ID;
/// Transform \p F into loop-closed SSA form.
bool LCSSAWrapperPass::runOnFunction(Function &F) {
LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
SE = SEWP ? &SEWP->getSE() : nullptr;
return formLCSSAOnAllLoops(LI, *DT, SE);
}
PreservedAnalyses LCSSAPass::run(Function &F, FunctionAnalysisManager &AM) {
auto &LI = AM.getResult<LoopAnalysis>(F);
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
auto *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F);
if (!formLCSSAOnAllLoops(&LI, DT, SE))
return PreservedAnalyses::all();
// FIXME: This should also 'preserve the CFG'.
PreservedAnalyses PA;
PA.preserve<BasicAA>();
PA.preserve<GlobalsAA>();
PA.preserve<SCEVAA>();
PA.preserve<ScalarEvolutionAnalysis>();
return PA;
}
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