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

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//===-- UnrollLoop.cpp - Loop unrolling utilities -------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements some loop unrolling utilities. It does not define any
// actual pass or policy, but provides a single function to perform loop
// unrolling.
//
// The process of unrolling can produce extraneous basic blocks linked with
// unconditional branches. This will be corrected in the future.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/OptimizationDiagnosticInfo.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopSimplify.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/SimplifyIndVar.h"
#include "llvm/Transforms/Utils/UnrollLoop.h"
using namespace llvm;
#define DEBUG_TYPE "loop-unroll"
// TODO: Should these be here or in LoopUnroll?
STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled");
STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)");
static cl::opt<bool>
UnrollRuntimeEpilog("unroll-runtime-epilog", cl::init(false), cl::Hidden,
cl::desc("Allow runtime unrolled loops to be unrolled "
"with epilog instead of prolog."));
static cl::opt<bool>
UnrollVerifyDomtree("unroll-verify-domtree", cl::Hidden,
cl::desc("Verify domtree after unrolling"),
#ifdef NDEBUG
cl::init(false)
#else
cl::init(true)
#endif
);
/// Convert the instruction operands from referencing the current values into
/// those specified by VMap.
static inline void remapInstruction(Instruction *I,
ValueToValueMapTy &VMap) {
for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
Value *Op = I->getOperand(op);
ValueToValueMapTy::iterator It = VMap.find(Op);
if (It != VMap.end())
I->setOperand(op, It->second);
}
if (PHINode *PN = dyn_cast<PHINode>(I)) {
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
ValueToValueMapTy::iterator It = VMap.find(PN->getIncomingBlock(i));
if (It != VMap.end())
PN->setIncomingBlock(i, cast<BasicBlock>(It->second));
}
}
}
/// Folds a basic block into its predecessor if it only has one predecessor, and
/// that predecessor only has one successor.
/// The LoopInfo Analysis that is passed will be kept consistent. If folding is
/// successful references to the containing loop must be removed from
/// ScalarEvolution by calling ScalarEvolution::forgetLoop because SE may have
/// references to the eliminated BB. The argument ForgottenLoops contains a set
/// of loops that have already been forgotten to prevent redundant, expensive
/// calls to ScalarEvolution::forgetLoop. Returns the new combined block.
static BasicBlock *
foldBlockIntoPredecessor(BasicBlock *BB, LoopInfo *LI, ScalarEvolution *SE,
SmallPtrSetImpl<Loop *> &ForgottenLoops,
DominatorTree *DT) {
// Merge basic blocks into their predecessor if there is only one distinct
// pred, and if there is only one distinct successor of the predecessor, and
// if there are no PHI nodes.
BasicBlock *OnlyPred = BB->getSinglePredecessor();
if (!OnlyPred) return nullptr;
if (OnlyPred->getTerminator()->getNumSuccessors() != 1)
return nullptr;
DEBUG(dbgs() << "Merging: " << *BB << "into: " << *OnlyPred);
// Resolve any PHI nodes at the start of the block. They are all
// guaranteed to have exactly one entry if they exist, unless there are
// multiple duplicate (but guaranteed to be equal) entries for the
// incoming edges. This occurs when there are multiple edges from
// OnlyPred to OnlySucc.
FoldSingleEntryPHINodes(BB);
// Delete the unconditional branch from the predecessor...
OnlyPred->getInstList().pop_back();
// Make all PHI nodes that referred to BB now refer to Pred as their
// source...
BB->replaceAllUsesWith(OnlyPred);
// Move all definitions in the successor to the predecessor...
OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
// OldName will be valid until erased.
StringRef OldName = BB->getName();
// Erase the old block and update dominator info.
if (DT)
if (DomTreeNode *DTN = DT->getNode(BB)) {
DomTreeNode *PredDTN = DT->getNode(OnlyPred);
SmallVector<DomTreeNode *, 8> Children(DTN->begin(), DTN->end());
for (auto *DI : Children)
DT->changeImmediateDominator(DI, PredDTN);
DT->eraseNode(BB);
}
// ScalarEvolution holds references to loop exit blocks.
if (SE) {
if (Loop *L = LI->getLoopFor(BB)) {
if (ForgottenLoops.insert(L).second)
SE->forgetLoop(L);
}
}
LI->removeBlock(BB);
// Inherit predecessor's name if it exists...
if (!OldName.empty() && !OnlyPred->hasName())
OnlyPred->setName(OldName);
BB->eraseFromParent();
return OnlyPred;
}
/// Check if unrolling created a situation where we need to insert phi nodes to
/// preserve LCSSA form.
/// \param Blocks is a vector of basic blocks representing unrolled loop.
/// \param L is the outer loop.
/// It's possible that some of the blocks are in L, and some are not. In this
/// case, if there is a use is outside L, and definition is inside L, we need to
/// insert a phi-node, otherwise LCSSA will be broken.
/// The function is just a helper function for llvm::UnrollLoop that returns
/// true if this situation occurs, indicating that LCSSA needs to be fixed.
static bool needToInsertPhisForLCSSA(Loop *L, std::vector<BasicBlock *> Blocks,
LoopInfo *LI) {
for (BasicBlock *BB : Blocks) {
if (LI->getLoopFor(BB) == L)
continue;
for (Instruction &I : *BB) {
for (Use &U : I.operands()) {
if (auto Def = dyn_cast<Instruction>(U)) {
Loop *DefLoop = LI->getLoopFor(Def->getParent());
if (!DefLoop)
continue;
if (DefLoop->contains(L))
return true;
}
}
}
}
return false;
}
/// Adds ClonedBB to LoopInfo, creates a new loop for ClonedBB if necessary
/// and adds a mapping from the original loop to the new loop to NewLoops.
/// Returns nullptr if no new loop was created and a pointer to the
/// original loop OriginalBB was part of otherwise.
const Loop* llvm::addClonedBlockToLoopInfo(BasicBlock *OriginalBB,
BasicBlock *ClonedBB, LoopInfo *LI,
NewLoopsMap &NewLoops) {
// Figure out which loop New is in.
const Loop *OldLoop = LI->getLoopFor(OriginalBB);
assert(OldLoop && "Should (at least) be in the loop being unrolled!");
Loop *&NewLoop = NewLoops[OldLoop];
if (!NewLoop) {
// Found a new sub-loop.
assert(OriginalBB == OldLoop->getHeader() &&
"Header should be first in RPO");
NewLoop = new Loop();
Loop *NewLoopParent = NewLoops.lookup(OldLoop->getParentLoop());
if (NewLoopParent)
NewLoopParent->addChildLoop(NewLoop);
else
LI->addTopLevelLoop(NewLoop);
NewLoop->addBasicBlockToLoop(ClonedBB, *LI);
return OldLoop;
} else {
NewLoop->addBasicBlockToLoop(ClonedBB, *LI);
return nullptr;
}
}
/// The function chooses which type of unroll (epilog or prolog) is more
/// profitabale.
/// Epilog unroll is more profitable when there is PHI that starts from
/// constant. In this case epilog will leave PHI start from constant,
/// but prolog will convert it to non-constant.
///
/// loop:
/// PN = PHI [I, Latch], [CI, PreHeader]
/// I = foo(PN)
/// ...
///
/// Epilog unroll case.
/// loop:
/// PN = PHI [I2, Latch], [CI, PreHeader]
/// I1 = foo(PN)
/// I2 = foo(I1)
/// ...
/// Prolog unroll case.
/// NewPN = PHI [PrologI, Prolog], [CI, PreHeader]
/// loop:
/// PN = PHI [I2, Latch], [NewPN, PreHeader]
/// I1 = foo(PN)
/// I2 = foo(I1)
/// ...
///
static bool isEpilogProfitable(Loop *L) {
BasicBlock *PreHeader = L->getLoopPreheader();
BasicBlock *Header = L->getHeader();
assert(PreHeader && Header);
for (Instruction &BBI : *Header) {
PHINode *PN = dyn_cast<PHINode>(&BBI);
if (!PN)
break;
if (isa<ConstantInt>(PN->getIncomingValueForBlock(PreHeader)))
return true;
}
return false;
}
/// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true
/// if unrolling was successful, or false if the loop was unmodified. Unrolling
/// can only fail when the loop's latch block is not terminated by a conditional
/// branch instruction. However, if the trip count (and multiple) are not known,
/// loop unrolling will mostly produce more code that is no faster.
///
/// TripCount is the upper bound of the iteration on which control exits
/// LatchBlock. Control may exit the loop prior to TripCount iterations either
/// via an early branch in other loop block or via LatchBlock terminator. This
/// is relaxed from the general definition of trip count which is the number of
/// times the loop header executes. Note that UnrollLoop assumes that the loop
/// counter test is in LatchBlock in order to remove unnecesssary instances of
/// the test. If control can exit the loop from the LatchBlock's terminator
/// prior to TripCount iterations, flag PreserveCondBr needs to be set.
///
/// PreserveCondBr indicates whether the conditional branch of the LatchBlock
/// needs to be preserved. It is needed when we use trip count upper bound to
/// fully unroll the loop. If PreserveOnlyFirst is also set then only the first
/// conditional branch needs to be preserved.
///
/// Similarly, TripMultiple divides the number of times that the LatchBlock may
/// execute without exiting the loop.
///
/// If AllowRuntime is true then UnrollLoop will consider unrolling loops that
/// have a runtime (i.e. not compile time constant) trip count. Unrolling these
/// loops require a unroll "prologue" that runs "RuntimeTripCount % Count"
/// iterations before branching into the unrolled loop. UnrollLoop will not
/// runtime-unroll the loop if computing RuntimeTripCount will be expensive and
/// AllowExpensiveTripCount is false.
///
/// If we want to perform PGO-based loop peeling, PeelCount is set to the
/// number of iterations we want to peel off.
///
/// The LoopInfo Analysis that is passed will be kept consistent.
///
/// This utility preserves LoopInfo. It will also preserve ScalarEvolution and
/// DominatorTree if they are non-null.
bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount, bool Force,
bool AllowRuntime, bool AllowExpensiveTripCount,
bool PreserveCondBr, bool PreserveOnlyFirst,
unsigned TripMultiple, unsigned PeelCount, LoopInfo *LI,
ScalarEvolution *SE, DominatorTree *DT,
AssumptionCache *AC, OptimizationRemarkEmitter *ORE,
bool PreserveLCSSA) {
BasicBlock *Preheader = L->getLoopPreheader();
if (!Preheader) {
DEBUG(dbgs() << " Can't unroll; loop preheader-insertion failed.\n");
return false;
}
BasicBlock *LatchBlock = L->getLoopLatch();
if (!LatchBlock) {
DEBUG(dbgs() << " Can't unroll; loop exit-block-insertion failed.\n");
return false;
}
// Loops with indirectbr cannot be cloned.
if (!L->isSafeToClone()) {
DEBUG(dbgs() << " Can't unroll; Loop body cannot be cloned.\n");
return false;
}
// The current loop unroll pass can only unroll loops with a single latch
// that's a conditional branch exiting the loop.
// FIXME: The implementation can be extended to work with more complicated
// cases, e.g. loops with multiple latches.
BasicBlock *Header = L->getHeader();
BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
if (!BI || BI->isUnconditional()) {
// The loop-rotate pass can be helpful to avoid this in many cases.
DEBUG(dbgs() <<
" Can't unroll; loop not terminated by a conditional branch.\n");
return false;
}
auto CheckSuccessors = [&](unsigned S1, unsigned S2) {
return BI->getSuccessor(S1) == Header && !L->contains(BI->getSuccessor(S2));
};
if (!CheckSuccessors(0, 1) && !CheckSuccessors(1, 0)) {
DEBUG(dbgs() << "Can't unroll; only loops with one conditional latch"
" exiting the loop can be unrolled\n");
return false;
}
if (Header->hasAddressTaken()) {
// The loop-rotate pass can be helpful to avoid this in many cases.
DEBUG(dbgs() <<
" Won't unroll loop: address of header block is taken.\n");
return false;
}
if (TripCount != 0)
DEBUG(dbgs() << " Trip Count = " << TripCount << "\n");
if (TripMultiple != 1)
DEBUG(dbgs() << " Trip Multiple = " << TripMultiple << "\n");
// Effectively "DCE" unrolled iterations that are beyond the tripcount
// and will never be executed.
if (TripCount != 0 && Count > TripCount)
Count = TripCount;
// Don't enter the unroll code if there is nothing to do.
if (TripCount == 0 && Count < 2 && PeelCount == 0) {
DEBUG(dbgs() << "Won't unroll; almost nothing to do\n");
return false;
}
assert(Count > 0);
assert(TripMultiple > 0);
assert(TripCount == 0 || TripCount % TripMultiple == 0);
// Are we eliminating the loop control altogether?
bool CompletelyUnroll = Count == TripCount;
SmallVector<BasicBlock *, 4> ExitBlocks;
L->getExitBlocks(ExitBlocks);
std::vector<BasicBlock*> OriginalLoopBlocks = L->getBlocks();
// Go through all exits of L and see if there are any phi-nodes there. We just
// conservatively assume that they're inserted to preserve LCSSA form, which
// means that complete unrolling might break this form. We need to either fix
// it in-place after the transformation, or entirely rebuild LCSSA. TODO: For
// now we just recompute LCSSA for the outer loop, but it should be possible
// to fix it in-place.
bool NeedToFixLCSSA = PreserveLCSSA && CompletelyUnroll &&
any_of(ExitBlocks, [](const BasicBlock *BB) {
return isa<PHINode>(BB->begin());
});
// We assume a run-time trip count if the compiler cannot
// figure out the loop trip count and the unroll-runtime
// flag is specified.
bool RuntimeTripCount = (TripCount == 0 && Count > 0 && AllowRuntime);
assert((!RuntimeTripCount || !PeelCount) &&
"Did not expect runtime trip-count unrolling "
"and peeling for the same loop");
if (PeelCount)
peelLoop(L, PeelCount, LI, SE, DT, AC, PreserveLCSSA);
// Loops containing convergent instructions must have a count that divides
// their TripMultiple.
DEBUG(
{
bool HasConvergent = false;
for (auto &BB : L->blocks())
for (auto &I : *BB)
if (auto CS = CallSite(&I))
HasConvergent |= CS.isConvergent();
assert((!HasConvergent || TripMultiple % Count == 0) &&
"Unroll count must divide trip multiple if loop contains a "
"convergent operation.");
});
bool EpilogProfitability =
UnrollRuntimeEpilog.getNumOccurrences() ? UnrollRuntimeEpilog
: isEpilogProfitable(L);
if (RuntimeTripCount && TripMultiple % Count != 0 &&
!UnrollRuntimeLoopRemainder(L, Count, AllowExpensiveTripCount,
EpilogProfitability, LI, SE, DT,
PreserveLCSSA)) {
if (Force)
RuntimeTripCount = false;
else {
DEBUG(
dbgs() << "Wont unroll; remainder loop could not be generated"
"when assuming runtime trip count\n");
return false;
}
}
// Notify ScalarEvolution that the loop will be substantially changed,
// if not outright eliminated.
if (SE)
SE->forgetLoop(L);
// If we know the trip count, we know the multiple...
unsigned BreakoutTrip = 0;
if (TripCount != 0) {
BreakoutTrip = TripCount % Count;
TripMultiple = 0;
} else {
// Figure out what multiple to use.
BreakoutTrip = TripMultiple =
(unsigned)GreatestCommonDivisor64(Count, TripMultiple);
}
using namespace ore;
// Report the unrolling decision.
if (CompletelyUnroll) {
DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName()
<< " with trip count " << TripCount << "!\n");
ORE->emit(OptimizationRemark(DEBUG_TYPE, "FullyUnrolled", L->getStartLoc(),
L->getHeader())
<< "completely unrolled loop with "
<< NV("UnrollCount", TripCount) << " iterations");
} else if (PeelCount) {
DEBUG(dbgs() << "PEELING loop %" << Header->getName()
<< " with iteration count " << PeelCount << "!\n");
ORE->emit(OptimizationRemark(DEBUG_TYPE, "Peeled", L->getStartLoc(),
L->getHeader())
<< " peeled loop by " << NV("PeelCount", PeelCount)
<< " iterations");
} else {
OptimizationRemark Diag(DEBUG_TYPE, "PartialUnrolled", L->getStartLoc(),
L->getHeader());
Diag << "unrolled loop by a factor of " << NV("UnrollCount", Count);
DEBUG(dbgs() << "UNROLLING loop %" << Header->getName()
<< " by " << Count);
if (TripMultiple == 0 || BreakoutTrip != TripMultiple) {
DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip);
ORE->emit(Diag << " with a breakout at trip "
<< NV("BreakoutTrip", BreakoutTrip));
} else if (TripMultiple != 1) {
DEBUG(dbgs() << " with " << TripMultiple << " trips per branch");
ORE->emit(Diag << " with " << NV("TripMultiple", TripMultiple)
<< " trips per branch");
} else if (RuntimeTripCount) {
DEBUG(dbgs() << " with run-time trip count");
ORE->emit(Diag << " with run-time trip count");
}
DEBUG(dbgs() << "!\n");
}
bool ContinueOnTrue = L->contains(BI->getSuccessor(0));
BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue);
// For the first iteration of the loop, we should use the precloned values for
// PHI nodes. Insert associations now.
ValueToValueMapTy LastValueMap;
std::vector<PHINode*> OrigPHINode;
for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
OrigPHINode.push_back(cast<PHINode>(I));
}
std::vector<BasicBlock*> Headers;
std::vector<BasicBlock*> Latches;
Headers.push_back(Header);
Latches.push_back(LatchBlock);
// The current on-the-fly SSA update requires blocks to be processed in
// reverse postorder so that LastValueMap contains the correct value at each
// exit.
LoopBlocksDFS DFS(L);
DFS.perform(LI);
// Stash the DFS iterators before adding blocks to the loop.
LoopBlocksDFS::RPOIterator BlockBegin = DFS.beginRPO();
LoopBlocksDFS::RPOIterator BlockEnd = DFS.endRPO();
std::vector<BasicBlock*> UnrolledLoopBlocks = L->getBlocks();
// Loop Unrolling might create new loops. While we do preserve LoopInfo, we
// might break loop-simplified form for these loops (as they, e.g., would
// share the same exit blocks). We'll keep track of loops for which we can
// break this so that later we can re-simplify them.
SmallSetVector<Loop *, 4> LoopsToSimplify;
for (Loop *SubLoop : *L)
LoopsToSimplify.insert(SubLoop);
if (Header->getParent()->isDebugInfoForProfiling())
for (BasicBlock *BB : L->getBlocks())
for (Instruction &I : *BB)
if (const DILocation *DIL = I.getDebugLoc())
I.setDebugLoc(DIL->cloneWithDuplicationFactor(Count));
for (unsigned It = 1; It != Count; ++It) {
std::vector<BasicBlock*> NewBlocks;
SmallDenseMap<const Loop *, Loop *, 4> NewLoops;
NewLoops[L] = L;
for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
ValueToValueMapTy VMap;
BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It));
Header->getParent()->getBasicBlockList().push_back(New);
assert((*BB != Header || LI->getLoopFor(*BB) == L) &&
"Header should not be in a sub-loop");
// Tell LI about New.
const Loop *OldLoop = addClonedBlockToLoopInfo(*BB, New, LI, NewLoops);
if (OldLoop) {
LoopsToSimplify.insert(NewLoops[OldLoop]);
// Forget the old loop, since its inputs may have changed.
if (SE)
SE->forgetLoop(OldLoop);
}
if (*BB == Header)
// Loop over all of the PHI nodes in the block, changing them to use
// the incoming values from the previous block.
for (PHINode *OrigPHI : OrigPHINode) {
PHINode *NewPHI = cast<PHINode>(VMap[OrigPHI]);
Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock);
if (Instruction *InValI = dyn_cast<Instruction>(InVal))
if (It > 1 && L->contains(InValI))
InVal = LastValueMap[InValI];
VMap[OrigPHI] = InVal;
New->getInstList().erase(NewPHI);
}
// Update our running map of newest clones
LastValueMap[*BB] = New;
for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
VI != VE; ++VI)
LastValueMap[VI->first] = VI->second;
// Add phi entries for newly created values to all exit blocks.
for (BasicBlock *Succ : successors(*BB)) {
if (L->contains(Succ))
continue;
for (BasicBlock::iterator BBI = Succ->begin();
PHINode *phi = dyn_cast<PHINode>(BBI); ++BBI) {
Value *Incoming = phi->getIncomingValueForBlock(*BB);
ValueToValueMapTy::iterator It = LastValueMap.find(Incoming);
if (It != LastValueMap.end())
Incoming = It->second;
phi->addIncoming(Incoming, New);
}
}
// Keep track of new headers and latches as we create them, so that
// we can insert the proper branches later.
if (*BB == Header)
Headers.push_back(New);
if (*BB == LatchBlock)
Latches.push_back(New);
NewBlocks.push_back(New);
UnrolledLoopBlocks.push_back(New);
// Update DomTree: since we just copy the loop body, and each copy has a
// dedicated entry block (copy of the header block), this header's copy
// dominates all copied blocks. That means, dominance relations in the
// copied body are the same as in the original body.
if (DT) {
if (*BB == Header)
DT->addNewBlock(New, Latches[It - 1]);
else {
auto BBDomNode = DT->getNode(*BB);
auto BBIDom = BBDomNode->getIDom();
BasicBlock *OriginalBBIDom = BBIDom->getBlock();
DT->addNewBlock(
New, cast<BasicBlock>(LastValueMap[cast<Value>(OriginalBBIDom)]));
}
}
}
// Remap all instructions in the most recent iteration
for (BasicBlock *NewBlock : NewBlocks) {
for (Instruction &I : *NewBlock) {
::remapInstruction(&I, LastValueMap);
if (auto *II = dyn_cast<IntrinsicInst>(&I))
if (II->getIntrinsicID() == Intrinsic::assume)
AC->registerAssumption(II);
}
}
}
// Loop over the PHI nodes in the original block, setting incoming values.
for (PHINode *PN : OrigPHINode) {
if (CompletelyUnroll) {
PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
Header->getInstList().erase(PN);
}
else if (Count > 1) {
Value *InVal = PN->removeIncomingValue(LatchBlock, false);
// If this value was defined in the loop, take the value defined by the
// last iteration of the loop.
if (Instruction *InValI = dyn_cast<Instruction>(InVal)) {
if (L->contains(InValI))
InVal = LastValueMap[InVal];
}
assert(Latches.back() == LastValueMap[LatchBlock] && "bad last latch");
PN->addIncoming(InVal, Latches.back());
}
}
// Now that all the basic blocks for the unrolled iterations are in place,
// set up the branches to connect them.
for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
// The original branch was replicated in each unrolled iteration.
BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());
// The branch destination.
unsigned j = (i + 1) % e;
BasicBlock *Dest = Headers[j];
bool NeedConditional = true;
if (RuntimeTripCount && j != 0) {
NeedConditional = false;
}
// For a complete unroll, make the last iteration end with a branch
// to the exit block.
if (CompletelyUnroll) {
if (j == 0)
Dest = LoopExit;
// If using trip count upper bound to completely unroll, we need to keep
// the conditional branch except the last one because the loop may exit
// after any iteration.
assert(NeedConditional &&
"NeedCondition cannot be modified by both complete "
"unrolling and runtime unrolling");
NeedConditional = (PreserveCondBr && j && !(PreserveOnlyFirst && i != 0));
} else if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) {
// If we know the trip count or a multiple of it, we can safely use an
// unconditional branch for some iterations.
NeedConditional = false;
}
if (NeedConditional) {
// Update the conditional branch's successor for the following
// iteration.
Term->setSuccessor(!ContinueOnTrue, Dest);
} else {
// Remove phi operands at this loop exit
if (Dest != LoopExit) {
BasicBlock *BB = Latches[i];
for (BasicBlock *Succ: successors(BB)) {
if (Succ == Headers[i])
continue;
for (BasicBlock::iterator BBI = Succ->begin();
PHINode *Phi = dyn_cast<PHINode>(BBI); ++BBI) {
Phi->removeIncomingValue(BB, false);
}
}
}
// Replace the conditional branch with an unconditional one.
BranchInst::Create(Dest, Term);
Term->eraseFromParent();
}
}
// Update dominators of blocks we might reach through exits.
// Immediate dominator of such block might change, because we add more
// routes which can lead to the exit: we can now reach it from the copied
// iterations too.
if (DT && Count > 1) {
for (auto *BB : OriginalLoopBlocks) {
auto *BBDomNode = DT->getNode(BB);
SmallVector<BasicBlock *, 16> ChildrenToUpdate;
for (auto *ChildDomNode : BBDomNode->getChildren()) {
auto *ChildBB = ChildDomNode->getBlock();
if (!L->contains(ChildBB))
ChildrenToUpdate.push_back(ChildBB);
}
BasicBlock *NewIDom;
if (BB == LatchBlock) {
// The latch is special because we emit unconditional branches in
// some cases where the original loop contained a conditional branch.
// Since the latch is always at the bottom of the loop, if the latch
// dominated an exit before unrolling, the new dominator of that exit
// must also be a latch. Specifically, the dominator is the first
// latch which ends in a conditional branch, or the last latch if
// there is no such latch.
NewIDom = Latches.back();
for (BasicBlock *IterLatch : Latches) {
TerminatorInst *Term = IterLatch->getTerminator();
if (isa<BranchInst>(Term) && cast<BranchInst>(Term)->isConditional()) {
NewIDom = IterLatch;
break;
}
}
} else {
// The new idom of the block will be the nearest common dominator
// of all copies of the previous idom. This is equivalent to the
// nearest common dominator of the previous idom and the first latch,
// which dominates all copies of the previous idom.
NewIDom = DT->findNearestCommonDominator(BB, LatchBlock);
}
for (auto *ChildBB : ChildrenToUpdate)
DT->changeImmediateDominator(ChildBB, NewIDom);
}
}
if (DT && UnrollVerifyDomtree)
DT->verifyDomTree();
// Merge adjacent basic blocks, if possible.
SmallPtrSet<Loop *, 4> ForgottenLoops;
for (BasicBlock *Latch : Latches) {
BranchInst *Term = cast<BranchInst>(Latch->getTerminator());
if (Term->isUnconditional()) {
BasicBlock *Dest = Term->getSuccessor(0);
if (BasicBlock *Fold =
foldBlockIntoPredecessor(Dest, LI, SE, ForgottenLoops, DT)) {
// Dest has been folded into Fold. Update our worklists accordingly.
std::replace(Latches.begin(), Latches.end(), Dest, Fold);
UnrolledLoopBlocks.erase(std::remove(UnrolledLoopBlocks.begin(),
UnrolledLoopBlocks.end(), Dest),
UnrolledLoopBlocks.end());
}
}
}
// Simplify any new induction variables in the partially unrolled loop.
if (SE && !CompletelyUnroll && Count > 1) {
SmallVector<WeakTrackingVH, 16> DeadInsts;
simplifyLoopIVs(L, SE, DT, LI, DeadInsts);
// Aggressively clean up dead instructions that simplifyLoopIVs already
// identified. Any remaining should be cleaned up below.
while (!DeadInsts.empty())
if (Instruction *Inst =
dyn_cast_or_null<Instruction>(&*DeadInsts.pop_back_val()))
RecursivelyDeleteTriviallyDeadInstructions(Inst);
}
// At this point, the code is well formed. We now do a quick sweep over the
// inserted code, doing constant propagation and dead code elimination as we
// go.
const DataLayout &DL = Header->getModule()->getDataLayout();
const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks();
for (BasicBlock *BB : NewLoopBlocks) {
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
Instruction *Inst = &*I++;
if (Value *V = SimplifyInstruction(Inst, {DL, nullptr, DT, AC}))
if (LI->replacementPreservesLCSSAForm(Inst, V))
Inst->replaceAllUsesWith(V);
if (isInstructionTriviallyDead(Inst))
BB->getInstList().erase(Inst);
}
}
// TODO: after peeling or unrolling, previously loop variant conditions are
// likely to fold to constants, eagerly propagating those here will require
// fewer cleanup passes to be run. Alternatively, a LoopEarlyCSE might be
// appropriate.
NumCompletelyUnrolled += CompletelyUnroll;
++NumUnrolled;
Loop *OuterL = L->getParentLoop();
// Update LoopInfo if the loop is completely removed.
if (CompletelyUnroll)
LI->markAsRemoved(L);
// After complete unrolling most of the blocks should be contained in OuterL.
// However, some of them might happen to be out of OuterL (e.g. if they
// precede a loop exit). In this case we might need to insert PHI nodes in
// order to preserve LCSSA form.
// We don't need to check this if we already know that we need to fix LCSSA
// form.
// TODO: For now we just recompute LCSSA for the outer loop in this case, but
// it should be possible to fix it in-place.
if (PreserveLCSSA && OuterL && CompletelyUnroll && !NeedToFixLCSSA)
NeedToFixLCSSA |= ::needToInsertPhisForLCSSA(OuterL, UnrolledLoopBlocks, LI);
// If we have a pass and a DominatorTree we should re-simplify impacted loops
// to ensure subsequent analyses can rely on this form. We want to simplify
// at least one layer outside of the loop that was unrolled so that any
// changes to the parent loop exposed by the unrolling are considered.
if (DT) {
if (OuterL) {
// OuterL includes all loops for which we can break loop-simplify, so
// it's sufficient to simplify only it (it'll recursively simplify inner
// loops too).
if (NeedToFixLCSSA) {
// LCSSA must be performed on the outermost affected loop. The unrolled
// loop's last loop latch is guaranteed to be in the outermost loop
// after LoopInfo's been updated by markAsRemoved.
Loop *LatchLoop = LI->getLoopFor(Latches.back());
Loop *FixLCSSALoop = OuterL;
if (!FixLCSSALoop->contains(LatchLoop))
while (FixLCSSALoop->getParentLoop() != LatchLoop)
FixLCSSALoop = FixLCSSALoop->getParentLoop();
formLCSSARecursively(*FixLCSSALoop, *DT, LI, SE);
} else if (PreserveLCSSA) {
assert(OuterL->isLCSSAForm(*DT) &&
"Loops should be in LCSSA form after loop-unroll.");
}
// TODO: That potentially might be compile-time expensive. We should try
// to fix the loop-simplified form incrementally.
simplifyLoop(OuterL, DT, LI, SE, AC, PreserveLCSSA);
} else {
// Simplify loops for which we might've broken loop-simplify form.
for (Loop *SubLoop : LoopsToSimplify)
simplifyLoop(SubLoop, DT, LI, SE, AC, PreserveLCSSA);
}
}
return true;
}
/// Given an llvm.loop loop id metadata node, returns the loop hint metadata
/// node with the given name (for example, "llvm.loop.unroll.count"). If no
/// such metadata node exists, then nullptr is returned.
MDNode *llvm::GetUnrollMetadata(MDNode *LoopID, StringRef Name) {
// First operand should refer to the loop id itself.
assert(LoopID->getNumOperands() > 0 && "requires at least one operand");
assert(LoopID->getOperand(0) == LoopID && "invalid loop id");
for (unsigned i = 1, e = LoopID->getNumOperands(); i < e; ++i) {
MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
if (!MD)
continue;
MDString *S = dyn_cast<MDString>(MD->getOperand(0));
if (!S)
continue;
if (Name.equals(S->getString()))
return MD;
}
return nullptr;
}
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