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Diffstat (limited to 'llvm/lib/Transforms/Utils/LoopUnroll.cpp')
| -rw-r--r-- | llvm/lib/Transforms/Utils/LoopUnroll.cpp | 980 |
1 files changed, 980 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/Utils/LoopUnroll.cpp b/llvm/lib/Transforms/Utils/LoopUnroll.cpp new file mode 100644 index 0000000000000..a7590fc325457 --- /dev/null +++ b/llvm/lib/Transforms/Utils/LoopUnroll.cpp @@ -0,0 +1,980 @@ +//===-- 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/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/OptimizationRemarkEmitter.h" +#include "llvm/Analysis/ScalarEvolution.h" +#include "llvm/Transforms/Utils/Local.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/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)"); +STATISTIC(NumUnrolledWithHeader, "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 + ); + +/// Convert the instruction operands from referencing the current values into +/// those specified by VMap. +void llvm::remapInstruction(Instruction *I, ValueToValueMapTy &VMap) { + for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) { + Value *Op = I->getOperand(op); + + // Unwrap arguments of dbg.value intrinsics. + bool Wrapped = false; + if (auto *V = dyn_cast<MetadataAsValue>(Op)) + if (auto *Unwrapped = dyn_cast<ValueAsMetadata>(V->getMetadata())) { + Op = Unwrapped->getValue(); + Wrapped = true; + } + + auto wrap = [&](Value *V) { + auto &C = I->getContext(); + return Wrapped ? MetadataAsValue::get(C, ValueAsMetadata::get(V)) : V; + }; + + ValueToValueMapTy::iterator It = VMap.find(Op); + if (It != VMap.end()) + I->setOperand(op, wrap(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)); + } + } +} + +/// 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 = 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) { + // Simplify any new induction variables in the partially unrolled loop. + if (SE && SimplifyIVs) { + 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 = L->getHeader()->getModule()->getDataLayout(); + for (BasicBlock *BB : L->getBlocks()) { + 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. +} + +/// 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. +/// +/// 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. +/// +/// 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, + OptimizationRemarkEmitter *ORE, + bool PreserveLCSSA, Loop **RemainderLoop) { + + BasicBlock *Preheader = L->getLoopPreheader(); + if (!Preheader) { + LLVM_DEBUG(dbgs() << " Can't unroll; loop preheader-insertion failed.\n"); + return LoopUnrollResult::Unmodified; + } + + BasicBlock *LatchBlock = L->getLoopLatch(); + if (!LatchBlock) { + 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; + } + + // The current loop unroll pass can unroll loops with a single latch or header + // 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 *HeaderBI = dyn_cast<BranchInst>(Header->getTerminator()); + BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator()); + + // FIXME: Support loops without conditional latch and multiple exiting blocks. + if (!BI || + (BI->isUnconditional() && (!HeaderBI || HeaderBI->isUnconditional() || + L->getExitingBlock() != Header))) { + LLVM_DEBUG(dbgs() << " Can't unroll; loop not terminated by a conditional " + "branch in the latch or header.\n"); + return LoopUnrollResult::Unmodified; + } + + auto CheckLatchSuccessors = [&](unsigned S1, unsigned S2) { + return BI->isConditional() && BI->getSuccessor(S1) == Header && + !L->contains(BI->getSuccessor(S2)); + }; + + // If we have a conditional latch, it must exit the loop. + if (BI && BI->isConditional() && !CheckLatchSuccessors(0, 1) && + !CheckLatchSuccessors(1, 0)) { + LLVM_DEBUG( + dbgs() << "Can't unroll; a conditional latch must exit the loop"); + return LoopUnrollResult::Unmodified; + } + + auto CheckHeaderSuccessors = [&](unsigned S1, unsigned S2) { + return HeaderBI && HeaderBI->isConditional() && + L->contains(HeaderBI->getSuccessor(S1)) && + !L->contains(HeaderBI->getSuccessor(S2)); + }; + + // If we do not have a conditional latch, the header must exit the loop. + if (BI && !BI->isConditional() && HeaderBI && HeaderBI->isConditional() && + !CheckHeaderSuccessors(0, 1) && !CheckHeaderSuccessors(1, 0)) { + LLVM_DEBUG(dbgs() << "Can't unroll; conditional header must exit the loop"); + return LoopUnrollResult::Unmodified; + } + + if (Header->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; + } + + if (ULO.TripCount != 0) + LLVM_DEBUG(dbgs() << " Trip Count = " << ULO.TripCount << "\n"); + if (ULO.TripMultiple != 1) + LLVM_DEBUG(dbgs() << " Trip Multiple = " << ULO.TripMultiple << "\n"); + + // Effectively "DCE" unrolled iterations that are beyond the tripcount + // and will never be executed. + if (ULO.TripCount != 0 && ULO.Count > ULO.TripCount) + ULO.Count = ULO.TripCount; + + // Don't enter the unroll code if there is nothing to do. + if (ULO.TripCount == 0 && ULO.Count < 2 && ULO.PeelCount == 0) { + LLVM_DEBUG(dbgs() << "Won't unroll; almost nothing to do\n"); + return LoopUnrollResult::Unmodified; + } + + assert(ULO.Count > 0); + assert(ULO.TripMultiple > 0); + assert(ULO.TripCount == 0 || ULO.TripCount % ULO.TripMultiple == 0); + + // Are we eliminating the loop control altogether? + bool CompletelyUnroll = ULO.Count == ULO.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 = + (ULO.TripCount == 0 && ULO.Count > 0 && ULO.AllowRuntime); + + assert((!RuntimeTripCount || !ULO.PeelCount) && + "Did not expect runtime trip-count unrolling " + "and peeling for the same loop"); + + bool Peeled = false; + if (ULO.PeelCount) { + Peeled = peelLoop(L, ULO.PeelCount, LI, SE, DT, AC, PreserveLCSSA); + + // Successful peeling may result in a change in the loop preheader/trip + // counts. If we later unroll the loop, we want these to be updated. + if (Peeled) { + // According to our guards and profitability checks the only + // meaningful exit should be latch block. Other exits go to deopt, + // so we do not worry about them. + BasicBlock *ExitingBlock = L->getLoopLatch(); + assert(ExitingBlock && "Loop without exiting block?"); + assert(L->isLoopExiting(ExitingBlock) && "Latch is not exiting?"); + Preheader = L->getLoopPreheader(); + ULO.TripCount = SE->getSmallConstantTripCount(L, ExitingBlock); + ULO.TripMultiple = SE->getSmallConstantTripMultiple(L, ExitingBlock); + } + } + + // Loops containing convergent instructions must have a count that divides + // their TripMultiple. + LLVM_DEBUG( + { + bool HasConvergent = false; + for (auto &BB : L->blocks()) + for (auto &I : *BB) + if (auto CS = CallSite(&I)) + HasConvergent |= CS.isConvergent(); + assert((!HasConvergent || ULO.TripMultiple % ULO.Count == 0) && + "Unroll count must divide trip multiple if loop contains a " + "convergent operation."); + }); + + bool EpilogProfitability = + UnrollRuntimeEpilog.getNumOccurrences() ? UnrollRuntimeEpilog + : isEpilogProfitable(L); + + if (RuntimeTripCount && ULO.TripMultiple % ULO.Count != 0 && + !UnrollRuntimeLoopRemainder(L, ULO.Count, ULO.AllowExpensiveTripCount, + EpilogProfitability, ULO.UnrollRemainder, + ULO.ForgetAllSCEV, LI, SE, DT, AC, + PreserveLCSSA, RemainderLoop)) { + if (ULO.Force) + RuntimeTripCount = false; + else { + LLVM_DEBUG(dbgs() << "Won't unroll; remainder loop could not be " + "generated when assuming runtime trip count\n"); + return LoopUnrollResult::Unmodified; + } + } + + // If we know the trip count, we know the multiple... + unsigned BreakoutTrip = 0; + if (ULO.TripCount != 0) { + BreakoutTrip = ULO.TripCount % ULO.Count; + ULO.TripMultiple = 0; + } else { + // Figure out what multiple to use. + BreakoutTrip = ULO.TripMultiple = + (unsigned)GreatestCommonDivisor64(ULO.Count, ULO.TripMultiple); + } + + using namespace ore; + // Report the unrolling decision. + if (CompletelyUnroll) { + LLVM_DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName() + << " with trip count " << ULO.TripCount << "!\n"); + if (ORE) + ORE->emit([&]() { + return OptimizationRemark(DEBUG_TYPE, "FullyUnrolled", L->getStartLoc(), + L->getHeader()) + << "completely unrolled loop with " + << NV("UnrollCount", ULO.TripCount) << " iterations"; + }); + } else if (ULO.PeelCount) { + LLVM_DEBUG(dbgs() << "PEELING loop %" << Header->getName() + << " with iteration count " << ULO.PeelCount << "!\n"); + if (ORE) + ORE->emit([&]() { + return OptimizationRemark(DEBUG_TYPE, "Peeled", L->getStartLoc(), + L->getHeader()) + << " peeled loop by " << NV("PeelCount", ULO.PeelCount) + << " iterations"; + }); + } else { + auto DiagBuilder = [&]() { + OptimizationRemark Diag(DEBUG_TYPE, "PartialUnrolled", L->getStartLoc(), + L->getHeader()); + return Diag << "unrolled loop by a factor of " + << NV("UnrollCount", ULO.Count); + }; + + LLVM_DEBUG(dbgs() << "UNROLLING loop %" << Header->getName() << " by " + << ULO.Count); + if (ULO.TripMultiple == 0 || BreakoutTrip != ULO.TripMultiple) { + LLVM_DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip); + if (ORE) + ORE->emit([&]() { + return DiagBuilder() << " with a breakout at trip " + << NV("BreakoutTrip", BreakoutTrip); + }); + } else if (ULO.TripMultiple != 1) { + LLVM_DEBUG(dbgs() << " with " << ULO.TripMultiple << " trips per branch"); + if (ORE) + ORE->emit([&]() { + return DiagBuilder() + << " with " << NV("TripMultiple", ULO.TripMultiple) + << " trips per branch"; + }); + } else if (RuntimeTripCount) { + LLVM_DEBUG(dbgs() << " with run-time trip count"); + if (ORE) + ORE->emit( + [&]() { return DiagBuilder() << " with run-time trip count"; }); + } + LLVM_DEBUG(dbgs() << "!\n"); + } + + // 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); + } + + bool ContinueOnTrue; + bool LatchIsExiting = BI->isConditional(); + BasicBlock *LoopExit = nullptr; + if (LatchIsExiting) { + ContinueOnTrue = L->contains(BI->getSuccessor(0)); + LoopExit = BI->getSuccessor(ContinueOnTrue); + } else { + NumUnrolledWithHeader++; + ContinueOnTrue = L->contains(HeaderBI->getSuccessor(0)); + LoopExit = HeaderBI->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 *> HeaderSucc; + std::vector<BasicBlock *> Latches; + Headers.push_back(Header); + Latches.push_back(LatchBlock); + + if (!LatchIsExiting) { + auto *Term = cast<BranchInst>(Header->getTerminator()); + if (Term->isUnconditional() || L->contains(Term->getSuccessor(0))) { + assert(L->contains(Term->getSuccessor(0))); + HeaderSucc.push_back(Term->getSuccessor(0)); + } else { + assert(L->contains(Term->getSuccessor(1))); + HeaderSucc.push_back(Term->getSuccessor(1)); + } + } + + // 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 (!isa<DbgInfoIntrinsic>(&I)) + if (const DILocation *DIL = I.getDebugLoc()) { + auto NewDIL = DIL->cloneByMultiplyingDuplicationFactor(ULO.Count); + if (NewDIL) + I.setDebugLoc(NewDIL.getValue()); + else + LLVM_DEBUG(dbgs() + << "Failed to create new discriminator: " + << DIL->getFilename() << " Line: " << DIL->getLine()); + } + + for (unsigned It = 1; It != ULO.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]); + + 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 (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); + } + } + // 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 successor of the new header in the current iteration. + for (auto *Pred : predecessors(*BB)) + if (Pred == Header) { + HeaderSucc.push_back(New); + break; + } + + 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 (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()); + } + } + + auto setDest = [LoopExit, ContinueOnTrue](BasicBlock *Src, BasicBlock *Dest, + ArrayRef<BasicBlock *> NextBlocks, + BasicBlock *BlockInLoop, + bool NeedConditional) { + auto *Term = cast<BranchInst>(Src->getTerminator()); + 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 = Src; + for (BasicBlock *Succ : successors(BB)) { + // Preserve the incoming value from BB if we are jumping to the block + // in the current loop. + if (Succ == BlockInLoop) + continue; + for (PHINode &Phi : Succ->phis()) + Phi.removeIncomingValue(BB, false); + } + } + // Replace the conditional branch with an unconditional one. + BranchInst::Create(Dest, Term); + Term->eraseFromParent(); + } + }; + + // Now that all the basic blocks for the unrolled iterations are in place, + // set up the branches to connect them. + if (LatchIsExiting) { + // Set up latches to branch to the new header in the unrolled iterations or + // the loop exit for the last latch in a fully unrolled loop. + for (unsigned i = 0, e = Latches.size(); i != e; ++i) { + // 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 = + (ULO.PreserveCondBr && j && !(ULO.PreserveOnlyFirst && i != 0)); + } else if (j != BreakoutTrip && + (ULO.TripMultiple == 0 || j % ULO.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; + } + + setDest(Latches[i], Dest, Headers, Headers[i], NeedConditional); + } + } else { + // Setup headers to branch to their new successors in the unrolled + // iterations. + for (unsigned i = 0, e = Headers.size(); i != e; ++i) { + // The branch destination. + unsigned j = (i + 1) % e; + BasicBlock *Dest = HeaderSucc[i]; + bool NeedConditional = true; + + if (RuntimeTripCount && j != 0) + NeedConditional = false; + + if (CompletelyUnroll) + // We cannot drop the conditional branch for the last condition, as we + // may have to execute the loop body depending on the condition. + NeedConditional = j == 0 || ULO.PreserveCondBr; + else if (j != BreakoutTrip && + (ULO.TripMultiple == 0 || j % ULO.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; + + setDest(Headers[i], Dest, Headers, HeaderSucc[i], NeedConditional); + } + + // Set up latches to branch to the new header in the unrolled iterations or + // the loop exit for the last latch in a fully unrolled loop. + + 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]; + + // When completely unrolling, the last latch becomes unreachable. + if (CompletelyUnroll && j == 0) + new UnreachableInst(Term->getContext(), Term); + else + // 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 && ULO.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; + BasicBlock *&TermBlock = LatchIsExiting ? LatchBlock : Header; + auto &TermBlocks = LatchIsExiting ? Latches : Headers; + if (BB == TermBlock) { + // 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. + // For loops exiting from the header, we limit the supported loops + // to have a single exiting block. + NewIDom = TermBlocks.back(); + for (BasicBlock *Iter : TermBlocks) { + Instruction *Term = Iter->getTerminator(); + if (isa<BranchInst>(Term) && cast<BranchInst>(Term)->isConditional()) { + NewIDom = Iter; + 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); + } + } + + assert(!DT || !UnrollVerifyDomtree || + DT->verify(DominatorTree::VerificationLevel::Fast)); + + DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy); + // 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, LI)) { + // 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()); + } + } + } + // Apply updates to the DomTree. + DT = &DTU.getDomTree(); + + // 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 || Peeled), LI, + SE, DT, AC); + + NumCompletelyUnrolled += CompletelyUnroll; + ++NumUnrolled; + + Loop *OuterL = L->getParentLoop(); + // Update LoopInfo if the loop is completely removed. + if (CompletelyUnroll) + LI->erase(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 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; +} |