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Diffstat (limited to 'contrib/llvm-project/llvm/lib/Transforms/Utils/LoopUnroll.cpp')
| -rw-r--r-- | contrib/llvm-project/llvm/lib/Transforms/Utils/LoopUnroll.cpp | 908 |
1 files changed, 908 insertions, 0 deletions
diff --git a/contrib/llvm-project/llvm/lib/Transforms/Utils/LoopUnroll.cpp b/contrib/llvm-project/llvm/lib/Transforms/Utils/LoopUnroll.cpp new file mode 100644 index 000000000000..e8f585b4a94d --- /dev/null +++ b/contrib/llvm-project/llvm/lib/Transforms/Utils/LoopUnroll.cpp @@ -0,0 +1,908 @@ +//===-- UnrollLoop.cpp - Loop unrolling utilities -------------------------===// +// +// 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 +// +//===----------------------------------------------------------------------===// +// +// 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/ArrayRef.h" +#include "llvm/ADT/DenseMap.h" +#include "llvm/ADT/STLExtras.h" +#include "llvm/ADT/SetVector.h" +#include "llvm/ADT/SmallVector.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/ADT/StringRef.h" +#include "llvm/ADT/Twine.h" +#include "llvm/ADT/ilist_iterator.h" +#include "llvm/ADT/iterator_range.h" +#include "llvm/Analysis/AssumptionCache.h" +#include "llvm/Analysis/DomTreeUpdater.h" +#include "llvm/Analysis/InstructionSimplify.h" +#include "llvm/Analysis/LoopInfo.h" +#include "llvm/Analysis/LoopIterator.h" +#include "llvm/Analysis/OptimizationRemarkEmitter.h" +#include "llvm/Analysis/ScalarEvolution.h" +#include "llvm/IR/BasicBlock.h" +#include "llvm/IR/CFG.h" +#include "llvm/IR/Constants.h" +#include "llvm/IR/DebugInfoMetadata.h" +#include "llvm/IR/DebugLoc.h" +#include "llvm/IR/DiagnosticInfo.h" +#include "llvm/IR/Dominators.h" +#include "llvm/IR/Function.h" +#include "llvm/IR/Instruction.h" +#include "llvm/IR/Instructions.h" +#include "llvm/IR/IntrinsicInst.h" +#include "llvm/IR/Metadata.h" +#include "llvm/IR/Module.h" +#include "llvm/IR/Use.h" +#include "llvm/IR/User.h" +#include "llvm/IR/ValueHandle.h" +#include "llvm/IR/ValueMap.h" +#include "llvm/Support/Casting.h" +#include "llvm/Support/CommandLine.h" +#include "llvm/Support/Debug.h" +#include "llvm/Support/GenericDomTree.h" +#include "llvm/Support/MathExtras.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" +#include "llvm/Transforms/Utils/ValueMapper.h" +#include <algorithm> +#include <assert.h> +#include <numeric> +#include <type_traits> +#include <vector> + +namespace llvm { +class DataLayout; +class Value; +} // namespace llvm + +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)"); +STATISTIC(NumUnrolledNotLatch, "Number of loops unrolled without a conditional " + "latch (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 EXPENSIVE_CHECKS + cl::init(true) +#else + cl::init(false) +#endif + ); + +static cl::opt<bool> +UnrollVerifyLoopInfo("unroll-verify-loopinfo", cl::Hidden, + cl::desc("Verify loopinfo after unrolling"), +#ifdef EXPENSIVE_CHECKS + cl::init(true) +#else + cl::init(false) +#endif + ); + + +/// 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, + const 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 (const 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 = LI->AllocateLoop(); + 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 (const PHINode &PN : Header->phis()) { + if (isa<ConstantInt>(PN.getIncomingValueForBlock(PreHeader))) + return true; + } + return false; +} + +/// Perform some cleanup and simplifications on loops after unrolling. It is +/// useful to simplify the IV's in the new loop, as well as do a quick +/// simplify/dce pass of the instructions. +void llvm::simplifyLoopAfterUnroll(Loop *L, bool SimplifyIVs, LoopInfo *LI, + ScalarEvolution *SE, DominatorTree *DT, + AssumptionCache *AC, + const TargetTransformInfo *TTI) { + // Simplify any new induction variables in the partially unrolled loop. + if (SE && SimplifyIVs) { + SmallVector<WeakTrackingVH, 16> DeadInsts; + simplifyLoopIVs(L, SE, DT, LI, TTI, DeadInsts); + + // Aggressively clean up dead instructions that simplifyLoopIVs already + // identified. Any remaining should be cleaned up below. + while (!DeadInsts.empty()) { + Value *V = DeadInsts.pop_back_val(); + if (Instruction *Inst = dyn_cast_or_null<Instruction>(V)) + RecursivelyDeleteTriviallyDeadInstructions(Inst); + } + } + + // At this point, the code is well formed. Perform constprop, instsimplify, + // and dce. + const DataLayout &DL = L->getHeader()->getModule()->getDataLayout(); + SmallVector<WeakTrackingVH, 16> DeadInsts; + for (BasicBlock *BB : L->getBlocks()) { + for (Instruction &Inst : llvm::make_early_inc_range(*BB)) { + if (Value *V = simplifyInstruction(&Inst, {DL, nullptr, DT, AC})) + if (LI->replacementPreservesLCSSAForm(&Inst, V)) + Inst.replaceAllUsesWith(V); + if (isInstructionTriviallyDead(&Inst)) + DeadInsts.emplace_back(&Inst); + } + // We can't do recursive deletion until we're done iterating, as we might + // have a phi which (potentially indirectly) uses instructions later in + // the block we're iterating through. + RecursivelyDeleteTriviallyDeadInstructions(DeadInsts); + } +} + +/// Unroll the given loop by Count. The loop must be in LCSSA form. 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. +/// +/// If Runtime is true then UnrollLoop will try to insert a prologue or +/// epilogue that ensures the latch has a trip multiple of Count. UnrollLoop +/// will not runtime-unroll the loop if computing the run-time trip count will +/// be expensive and AllowExpensiveTripCount is false. +/// +/// 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. +/// +/// If RemainderLoop is non-null, it will receive the remainder loop (if +/// required and not fully unrolled). +LoopUnrollResult llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI, + ScalarEvolution *SE, DominatorTree *DT, + AssumptionCache *AC, + const TargetTransformInfo *TTI, + OptimizationRemarkEmitter *ORE, + bool PreserveLCSSA, Loop **RemainderLoop) { + assert(DT && "DomTree is required"); + + if (!L->getLoopPreheader()) { + LLVM_DEBUG(dbgs() << " Can't unroll; loop preheader-insertion failed.\n"); + return LoopUnrollResult::Unmodified; + } + + if (!L->getLoopLatch()) { + LLVM_DEBUG(dbgs() << " Can't unroll; loop exit-block-insertion failed.\n"); + return LoopUnrollResult::Unmodified; + } + + // Loops with indirectbr cannot be cloned. + if (!L->isSafeToClone()) { + LLVM_DEBUG(dbgs() << " Can't unroll; Loop body cannot be cloned.\n"); + return LoopUnrollResult::Unmodified; + } + + if (L->getHeader()->hasAddressTaken()) { + // The loop-rotate pass can be helpful to avoid this in many cases. + LLVM_DEBUG( + dbgs() << " Won't unroll loop: address of header block is taken.\n"); + return LoopUnrollResult::Unmodified; + } + + assert(ULO.Count > 0); + + // All these values should be taken only after peeling because they might have + // changed. + BasicBlock *Preheader = L->getLoopPreheader(); + BasicBlock *Header = L->getHeader(); + BasicBlock *LatchBlock = L->getLoopLatch(); + SmallVector<BasicBlock *, 4> ExitBlocks; + L->getExitBlocks(ExitBlocks); + std::vector<BasicBlock *> OriginalLoopBlocks = L->getBlocks(); + + const unsigned MaxTripCount = SE->getSmallConstantMaxTripCount(L); + const bool MaxOrZero = SE->isBackedgeTakenCountMaxOrZero(L); + + // Effectively "DCE" unrolled iterations that are beyond the max tripcount + // and will never be executed. + if (MaxTripCount && ULO.Count > MaxTripCount) + ULO.Count = MaxTripCount; + + struct ExitInfo { + unsigned TripCount; + unsigned TripMultiple; + unsigned BreakoutTrip; + bool ExitOnTrue; + BasicBlock *FirstExitingBlock = nullptr; + SmallVector<BasicBlock *> ExitingBlocks; + }; + DenseMap<BasicBlock *, ExitInfo> ExitInfos; + SmallVector<BasicBlock *, 4> ExitingBlocks; + L->getExitingBlocks(ExitingBlocks); + for (auto *ExitingBlock : ExitingBlocks) { + // The folding code is not prepared to deal with non-branch instructions + // right now. + auto *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator()); + if (!BI) + continue; + + ExitInfo &Info = ExitInfos.try_emplace(ExitingBlock).first->second; + Info.TripCount = SE->getSmallConstantTripCount(L, ExitingBlock); + Info.TripMultiple = SE->getSmallConstantTripMultiple(L, ExitingBlock); + if (Info.TripCount != 0) { + Info.BreakoutTrip = Info.TripCount % ULO.Count; + Info.TripMultiple = 0; + } else { + Info.BreakoutTrip = Info.TripMultiple = + (unsigned)std::gcd(ULO.Count, Info.TripMultiple); + } + Info.ExitOnTrue = !L->contains(BI->getSuccessor(0)); + Info.ExitingBlocks.push_back(ExitingBlock); + LLVM_DEBUG(dbgs() << " Exiting block %" << ExitingBlock->getName() + << ": TripCount=" << Info.TripCount + << ", TripMultiple=" << Info.TripMultiple + << ", BreakoutTrip=" << Info.BreakoutTrip << "\n"); + } + + // Are we eliminating the loop control altogether? Note that we can know + // we're eliminating the backedge without knowing exactly which iteration + // of the unrolled body exits. + const bool CompletelyUnroll = ULO.Count == MaxTripCount; + + const bool PreserveOnlyFirst = CompletelyUnroll && MaxOrZero; + + // There's no point in performing runtime unrolling if this unroll count + // results in a full unroll. + if (CompletelyUnroll) + ULO.Runtime = false; + + // 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()); }); + + // The current loop unroll pass can unroll loops that have + // (1) single latch; and + // (2a) latch is unconditional; or + // (2b) latch is conditional and is an exiting block + // FIXME: The implementation can be extended to work with more complicated + // cases, e.g. loops with multiple latches. + BranchInst *LatchBI = dyn_cast<BranchInst>(LatchBlock->getTerminator()); + + // A conditional branch which exits the loop, which can be optimized to an + // unconditional branch in the unrolled loop in some cases. + bool LatchIsExiting = L->isLoopExiting(LatchBlock); + if (!LatchBI || (LatchBI->isConditional() && !LatchIsExiting)) { + LLVM_DEBUG( + dbgs() << "Can't unroll; a conditional latch must exit the loop"); + return LoopUnrollResult::Unmodified; + } + + // Loops containing convergent instructions cannot use runtime unrolling, + // as the prologue/epilogue may add additional control-dependencies to + // convergent operations. + LLVM_DEBUG( + { + bool HasConvergent = false; + for (auto &BB : L->blocks()) + for (auto &I : *BB) + if (auto *CB = dyn_cast<CallBase>(&I)) + HasConvergent |= CB->isConvergent(); + assert((!HasConvergent || !ULO.Runtime) && + "Can't runtime unroll if loop contains a convergent operation."); + }); + + bool EpilogProfitability = + UnrollRuntimeEpilog.getNumOccurrences() ? UnrollRuntimeEpilog + : isEpilogProfitable(L); + + if (ULO.Runtime && + !UnrollRuntimeLoopRemainder(L, ULO.Count, ULO.AllowExpensiveTripCount, + EpilogProfitability, ULO.UnrollRemainder, + ULO.ForgetAllSCEV, LI, SE, DT, AC, TTI, + PreserveLCSSA, RemainderLoop)) { + if (ULO.Force) + ULO.Runtime = false; + else { + LLVM_DEBUG(dbgs() << "Won't unroll; remainder loop could not be " + "generated when assuming runtime trip count\n"); + return LoopUnrollResult::Unmodified; + } + } + + using namespace ore; + // Report the unrolling decision. + if (CompletelyUnroll) { + LLVM_DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName() + << " with trip count " << ULO.Count << "!\n"); + if (ORE) + ORE->emit([&]() { + return OptimizationRemark(DEBUG_TYPE, "FullyUnrolled", L->getStartLoc(), + L->getHeader()) + << "completely unrolled loop with " + << NV("UnrollCount", ULO.Count) << " iterations"; + }); + } else { + LLVM_DEBUG(dbgs() << "UNROLLING loop %" << Header->getName() << " by " + << ULO.Count); + if (ULO.Runtime) + LLVM_DEBUG(dbgs() << " with run-time trip count"); + LLVM_DEBUG(dbgs() << "!\n"); + + if (ORE) + ORE->emit([&]() { + OptimizationRemark Diag(DEBUG_TYPE, "PartialUnrolled", L->getStartLoc(), + L->getHeader()); + Diag << "unrolled loop by a factor of " << NV("UnrollCount", ULO.Count); + if (ULO.Runtime) + Diag << " with run-time trip count"; + return Diag; + }); + } + + // We are going to make changes to this loop. SCEV may be keeping cached info + // about it, in particular about backedge taken count. The changes we make + // are guaranteed to invalidate this information for our loop. It is tempting + // to only invalidate the loop being unrolled, but it is incorrect as long as + // all exiting branches from all inner loops have impact on the outer loops, + // and if something changes inside them then any of outer loops may also + // change. When we forget outermost loop, we also forget all contained loops + // and this is what we need here. + if (SE) { + if (ULO.ForgetAllSCEV) + SE->forgetAllLoops(); + else { + SE->forgetTopmostLoop(L); + SE->forgetBlockAndLoopDispositions(); + } + } + + if (!LatchIsExiting) + ++NumUnrolledNotLatch; + + // 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); + + // When a FSDiscriminator is enabled, we don't need to add the multiply + // factors to the discriminators. + if (Header->getParent()->shouldEmitDebugInfoForProfiling() && + !EnableFSDiscriminator) + for (BasicBlock *BB : L->getBlocks()) + for (Instruction &I : *BB) + if (!isa<DbgInfoIntrinsic>(&I)) + if (const DILocation *DIL = I.getDebugLoc()) { + auto NewDIL = DIL->cloneByMultiplyingDuplicationFactor(ULO.Count); + if (NewDIL) + I.setDebugLoc(*NewDIL); + else + LLVM_DEBUG(dbgs() + << "Failed to create new discriminator: " + << DIL->getFilename() << " Line: " << DIL->getLine()); + } + + // Identify what noalias metadata is inside the loop: if it is inside the + // loop, the associated metadata must be cloned for each iteration. + SmallVector<MDNode *, 6> LoopLocalNoAliasDeclScopes; + identifyNoAliasScopesToClone(L->getBlocks(), LoopLocalNoAliasDeclScopes); + + // We place the unrolled iterations immediately after the original loop + // latch. This is a reasonable default placement if we don't have block + // frequencies, and if we do, well the layout will be adjusted later. + auto BlockInsertPt = std::next(LatchBlock->getIterator()); + for (unsigned It = 1; It != ULO.Count; ++It) { + SmallVector<BasicBlock *, 8> 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()->insert(BlockInsertPt, 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]); + + 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; + NewPHI->eraseFromParent(); + } + + // 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 (PHINode &PHI : Succ->phis()) { + Value *Incoming = PHI.getIncomingValueForBlock(*BB); + ValueToValueMapTy::iterator It = LastValueMap.find(Incoming); + if (It != LastValueMap.end()) + Incoming = It->second; + PHI.addIncoming(Incoming, New); + SE->forgetValue(&PHI); + } + } + // 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); + + // Keep track of the exiting block and its successor block contained in + // the loop for the current iteration. + auto ExitInfoIt = ExitInfos.find(*BB); + if (ExitInfoIt != ExitInfos.end()) + ExitInfoIt->second.ExitingBlocks.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 (*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 + remapInstructionsInBlocks(NewBlocks, LastValueMap); + for (BasicBlock *NewBlock : NewBlocks) + for (Instruction &I : *NewBlock) + if (auto *II = dyn_cast<AssumeInst>(&I)) + AC->registerAssumption(II); + + { + // Identify what other metadata depends on the cloned version. After + // cloning, replace the metadata with the corrected version for both + // memory instructions and noalias intrinsics. + std::string ext = (Twine("It") + Twine(It)).str(); + cloneAndAdaptNoAliasScopes(LoopLocalNoAliasDeclScopes, NewBlocks, + Header->getContext(), ext); + } + } + + // Loop over the PHI nodes in the original block, setting incoming values. + for (PHINode *PN : OrigPHINode) { + if (CompletelyUnroll) { + PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader)); + PN->eraseFromParent(); + } else if (ULO.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()); + } + } + + // Connect latches of the unrolled iterations to the headers of the next + // iteration. Currently they point to the header of the same iteration. + for (unsigned i = 0, e = Latches.size(); i != e; ++i) { + unsigned j = (i + 1) % e; + Latches[i]->getTerminator()->replaceSuccessorWith(Headers[i], Headers[j]); + } + + // 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 (ULO.Count > 1) { + for (auto *BB : OriginalLoopBlocks) { + auto *BBDomNode = DT->getNode(BB); + SmallVector<BasicBlock *, 16> ChildrenToUpdate; + for (auto *ChildDomNode : BBDomNode->children()) { + auto *ChildBB = ChildDomNode->getBlock(); + if (!L->contains(ChildBB)) + ChildrenToUpdate.push_back(ChildBB); + } + // 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. + BasicBlock *NewIDom = DT->findNearestCommonDominator(BB, LatchBlock); + for (auto *ChildBB : ChildrenToUpdate) + DT->changeImmediateDominator(ChildBB, NewIDom); + } + } + + assert(!UnrollVerifyDomtree || + DT->verify(DominatorTree::VerificationLevel::Fast)); + + SmallVector<DominatorTree::UpdateType> DTUpdates; + auto SetDest = [&](BasicBlock *Src, bool WillExit, bool ExitOnTrue) { + auto *Term = cast<BranchInst>(Src->getTerminator()); + const unsigned Idx = ExitOnTrue ^ WillExit; + BasicBlock *Dest = Term->getSuccessor(Idx); + BasicBlock *DeadSucc = Term->getSuccessor(1-Idx); + + // Remove predecessors from all non-Dest successors. + DeadSucc->removePredecessor(Src, /* KeepOneInputPHIs */ true); + + // Replace the conditional branch with an unconditional one. + BranchInst::Create(Dest, Term); + Term->eraseFromParent(); + + DTUpdates.emplace_back(DominatorTree::Delete, Src, DeadSucc); + }; + + auto WillExit = [&](const ExitInfo &Info, unsigned i, unsigned j, + bool IsLatch) -> std::optional<bool> { + if (CompletelyUnroll) { + if (PreserveOnlyFirst) { + if (i == 0) + return std::nullopt; + return j == 0; + } + // Complete (but possibly inexact) unrolling + if (j == 0) + return true; + if (Info.TripCount && j != Info.TripCount) + return false; + return std::nullopt; + } + + if (ULO.Runtime) { + // If runtime unrolling inserts a prologue, information about non-latch + // exits may be stale. + if (IsLatch && j != 0) + return false; + return std::nullopt; + } + + if (j != Info.BreakoutTrip && + (Info.TripMultiple == 0 || j % Info.TripMultiple != 0)) { + // If we know the trip count or a multiple of it, we can safely use an + // unconditional branch for some iterations. + return false; + } + return std::nullopt; + }; + + // Fold branches for iterations where we know that they will exit or not + // exit. + for (auto &Pair : ExitInfos) { + ExitInfo &Info = Pair.second; + for (unsigned i = 0, e = Info.ExitingBlocks.size(); i != e; ++i) { + // The branch destination. + unsigned j = (i + 1) % e; + bool IsLatch = Pair.first == LatchBlock; + std::optional<bool> KnownWillExit = WillExit(Info, i, j, IsLatch); + if (!KnownWillExit) { + if (!Info.FirstExitingBlock) + Info.FirstExitingBlock = Info.ExitingBlocks[i]; + continue; + } + + // We don't fold known-exiting branches for non-latch exits here, + // because this ensures that both all loop blocks and all exit blocks + // remain reachable in the CFG. + // TODO: We could fold these branches, but it would require much more + // sophisticated updates to LoopInfo. + if (*KnownWillExit && !IsLatch) { + if (!Info.FirstExitingBlock) + Info.FirstExitingBlock = Info.ExitingBlocks[i]; + continue; + } + + SetDest(Info.ExitingBlocks[i], *KnownWillExit, Info.ExitOnTrue); + } + } + + DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy); + DomTreeUpdater *DTUToUse = &DTU; + if (ExitingBlocks.size() == 1 && ExitInfos.size() == 1) { + // Manually update the DT if there's a single exiting node. In that case + // there's a single exit node and it is sufficient to update the nodes + // immediately dominated by the original exiting block. They will become + // dominated by the first exiting block that leaves the loop after + // unrolling. Note that the CFG inside the loop does not change, so there's + // no need to update the DT inside the unrolled loop. + DTUToUse = nullptr; + auto &[OriginalExit, Info] = *ExitInfos.begin(); + if (!Info.FirstExitingBlock) + Info.FirstExitingBlock = Info.ExitingBlocks.back(); + for (auto *C : to_vector(DT->getNode(OriginalExit)->children())) { + if (L->contains(C->getBlock())) + continue; + C->setIDom(DT->getNode(Info.FirstExitingBlock)); + } + } else { + DTU.applyUpdates(DTUpdates); + } + + // When completely unrolling, the last latch becomes unreachable. + if (!LatchIsExiting && CompletelyUnroll) { + // There is no need to update the DT here, because there must be a unique + // latch. Hence if the latch is not exiting it must directly branch back to + // the original loop header and does not dominate any nodes. + assert(LatchBlock->getSingleSuccessor() && "Loop with multiple latches?"); + changeToUnreachable(Latches.back()->getTerminator(), PreserveLCSSA); + } + + // Merge adjacent basic blocks, if possible. + for (BasicBlock *Latch : Latches) { + BranchInst *Term = dyn_cast<BranchInst>(Latch->getTerminator()); + assert((Term || + (CompletelyUnroll && !LatchIsExiting && Latch == Latches.back())) && + "Need a branch as terminator, except when fully unrolling with " + "unconditional latch"); + if (Term && Term->isUnconditional()) { + BasicBlock *Dest = Term->getSuccessor(0); + BasicBlock *Fold = Dest->getUniquePredecessor(); + if (MergeBlockIntoPredecessor(Dest, /*DTU=*/DTUToUse, LI, + /*MSSAU=*/nullptr, /*MemDep=*/nullptr, + /*PredecessorWithTwoSuccessors=*/false, + DTUToUse ? nullptr : DT)) { + // Dest has been folded into Fold. Update our worklists accordingly. + std::replace(Latches.begin(), Latches.end(), Dest, Fold); + llvm::erase_value(UnrolledLoopBlocks, Dest); + } + } + } + + if (DTUToUse) { + // Apply updates to the DomTree. + DT = &DTU.getDomTree(); + } + assert(!UnrollVerifyDomtree || + DT->verify(DominatorTree::VerificationLevel::Fast)); + + // At this point, the code is well formed. We now simplify the unrolled loop, + // doing constant propagation and dead code elimination as we go. + simplifyLoopAfterUnroll(L, !CompletelyUnroll && ULO.Count > 1, LI, SE, DT, AC, + TTI); + + NumCompletelyUnrolled += CompletelyUnroll; + ++NumUnrolled; + + Loop *OuterL = L->getParentLoop(); + // Update LoopInfo if the loop is completely removed. + if (CompletelyUnroll) + LI->erase(L); + + // LoopInfo should not be valid, confirm that. + if (UnrollVerifyLoopInfo) + LI->verify(*DT); + + // 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); + + // Make sure that loop-simplify form is preserved. 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 (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 LoopInfo::erase. + 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, nullptr, PreserveLCSSA); + } else { + // Simplify loops for which we might've broken loop-simplify form. + for (Loop *SubLoop : LoopsToSimplify) + simplifyLoop(SubLoop, DT, LI, SE, AC, nullptr, PreserveLCSSA); + } + + return CompletelyUnroll ? LoopUnrollResult::FullyUnrolled + : LoopUnrollResult::PartiallyUnrolled; +} + +/// 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; +} |