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Diffstat (limited to 'llvm/lib/Transforms/Scalar/LoopUnswitch.cpp')
| -rw-r--r-- | llvm/lib/Transforms/Scalar/LoopUnswitch.cpp | 1674 |
1 files changed, 1674 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp new file mode 100644 index 0000000000000..b410df0c5f68c --- /dev/null +++ b/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp @@ -0,0 +1,1674 @@ +//===- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop -------===// +// +// 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 pass transforms loops that contain branches on loop-invariant conditions +// to multiple loops. For example, it turns the left into the right code: +// +// for (...) if (lic) +// A for (...) +// if (lic) A; B; C +// B else +// C for (...) +// A; C +// +// This can increase the size of the code exponentially (doubling it every time +// a loop is unswitched) so we only unswitch if the resultant code will be +// smaller than a threshold. +// +// This pass expects LICM to be run before it to hoist invariant conditions out +// of the loop, to make the unswitching opportunity obvious. +// +//===----------------------------------------------------------------------===// + +#include "llvm/ADT/DenseMap.h" +#include "llvm/ADT/SmallPtrSet.h" +#include "llvm/ADT/SmallVector.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/Analysis/AssumptionCache.h" +#include "llvm/Analysis/CodeMetrics.h" +#include "llvm/Analysis/InstructionSimplify.h" +#include "llvm/Analysis/LegacyDivergenceAnalysis.h" +#include "llvm/Analysis/LoopInfo.h" +#include "llvm/Analysis/LoopIterator.h" +#include "llvm/Analysis/LoopPass.h" +#include "llvm/Analysis/MemorySSA.h" +#include "llvm/Analysis/MemorySSAUpdater.h" +#include "llvm/Analysis/ScalarEvolution.h" +#include "llvm/Analysis/TargetTransformInfo.h" +#include "llvm/IR/Attributes.h" +#include "llvm/IR/BasicBlock.h" +#include "llvm/IR/CallSite.h" +#include "llvm/IR/Constant.h" +#include "llvm/IR/Constants.h" +#include "llvm/IR/DerivedTypes.h" +#include "llvm/IR/Dominators.h" +#include "llvm/IR/Function.h" +#include "llvm/IR/IRBuilder.h" +#include "llvm/IR/InstrTypes.h" +#include "llvm/IR/Instruction.h" +#include "llvm/IR/Instructions.h" +#include "llvm/IR/IntrinsicInst.h" +#include "llvm/IR/Intrinsics.h" +#include "llvm/IR/Module.h" +#include "llvm/IR/Type.h" +#include "llvm/IR/User.h" +#include "llvm/IR/Value.h" +#include "llvm/IR/ValueHandle.h" +#include "llvm/Pass.h" +#include "llvm/Support/Casting.h" +#include "llvm/Support/CommandLine.h" +#include "llvm/Support/Debug.h" +#include "llvm/Support/raw_ostream.h" +#include "llvm/Transforms/Scalar.h" +#include "llvm/Transforms/Scalar/LoopPassManager.h" +#include "llvm/Transforms/Utils/BasicBlockUtils.h" +#include "llvm/Transforms/Utils/Cloning.h" +#include "llvm/Transforms/Utils/Local.h" +#include "llvm/Transforms/Utils/LoopUtils.h" +#include "llvm/Transforms/Utils/ValueMapper.h" +#include <algorithm> +#include <cassert> +#include <map> +#include <set> +#include <tuple> +#include <utility> +#include <vector> + +using namespace llvm; + +#define DEBUG_TYPE "loop-unswitch" + +STATISTIC(NumBranches, "Number of branches unswitched"); +STATISTIC(NumSwitches, "Number of switches unswitched"); +STATISTIC(NumGuards, "Number of guards unswitched"); +STATISTIC(NumSelects , "Number of selects unswitched"); +STATISTIC(NumTrivial , "Number of unswitches that are trivial"); +STATISTIC(NumSimplify, "Number of simplifications of unswitched code"); +STATISTIC(TotalInsts, "Total number of instructions analyzed"); + +// The specific value of 100 here was chosen based only on intuition and a +// few specific examples. +static cl::opt<unsigned> +Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"), + cl::init(100), cl::Hidden); + +namespace { + + class LUAnalysisCache { + using UnswitchedValsMap = + DenseMap<const SwitchInst *, SmallPtrSet<const Value *, 8>>; + using UnswitchedValsIt = UnswitchedValsMap::iterator; + + struct LoopProperties { + unsigned CanBeUnswitchedCount; + unsigned WasUnswitchedCount; + unsigned SizeEstimation; + UnswitchedValsMap UnswitchedVals; + }; + + // Here we use std::map instead of DenseMap, since we need to keep valid + // LoopProperties pointer for current loop for better performance. + using LoopPropsMap = std::map<const Loop *, LoopProperties>; + using LoopPropsMapIt = LoopPropsMap::iterator; + + LoopPropsMap LoopsProperties; + UnswitchedValsMap *CurLoopInstructions = nullptr; + LoopProperties *CurrentLoopProperties = nullptr; + + // A loop unswitching with an estimated cost above this threshold + // is not performed. MaxSize is turned into unswitching quota for + // the current loop, and reduced correspondingly, though note that + // the quota is returned by releaseMemory() when the loop has been + // processed, so that MaxSize will return to its previous + // value. So in most cases MaxSize will equal the Threshold flag + // when a new loop is processed. An exception to that is that + // MaxSize will have a smaller value while processing nested loops + // that were introduced due to loop unswitching of an outer loop. + // + // FIXME: The way that MaxSize works is subtle and depends on the + // pass manager processing loops and calling releaseMemory() in a + // specific order. It would be good to find a more straightforward + // way of doing what MaxSize does. + unsigned MaxSize; + + public: + LUAnalysisCache() : MaxSize(Threshold) {} + + // Analyze loop. Check its size, calculate is it possible to unswitch + // it. Returns true if we can unswitch this loop. + bool countLoop(const Loop *L, const TargetTransformInfo &TTI, + AssumptionCache *AC); + + // Clean all data related to given loop. + void forgetLoop(const Loop *L); + + // Mark case value as unswitched. + // Since SI instruction can be partly unswitched, in order to avoid + // extra unswitching in cloned loops keep track all unswitched values. + void setUnswitched(const SwitchInst *SI, const Value *V); + + // Check was this case value unswitched before or not. + bool isUnswitched(const SwitchInst *SI, const Value *V); + + // Returns true if another unswitching could be done within the cost + // threshold. + bool CostAllowsUnswitching(); + + // Clone all loop-unswitch related loop properties. + // Redistribute unswitching quotas. + // Note, that new loop data is stored inside the VMap. + void cloneData(const Loop *NewLoop, const Loop *OldLoop, + const ValueToValueMapTy &VMap); + }; + + class LoopUnswitch : public LoopPass { + LoopInfo *LI; // Loop information + LPPassManager *LPM; + AssumptionCache *AC; + + // Used to check if second loop needs processing after + // RewriteLoopBodyWithConditionConstant rewrites first loop. + std::vector<Loop*> LoopProcessWorklist; + + LUAnalysisCache BranchesInfo; + + bool OptimizeForSize; + bool redoLoop = false; + + Loop *currentLoop = nullptr; + DominatorTree *DT = nullptr; + MemorySSA *MSSA = nullptr; + std::unique_ptr<MemorySSAUpdater> MSSAU; + BasicBlock *loopHeader = nullptr; + BasicBlock *loopPreheader = nullptr; + + bool SanitizeMemory; + SimpleLoopSafetyInfo SafetyInfo; + + // LoopBlocks contains all of the basic blocks of the loop, including the + // preheader of the loop, the body of the loop, and the exit blocks of the + // loop, in that order. + std::vector<BasicBlock*> LoopBlocks; + // NewBlocks contained cloned copy of basic blocks from LoopBlocks. + std::vector<BasicBlock*> NewBlocks; + + bool hasBranchDivergence; + + public: + static char ID; // Pass ID, replacement for typeid + + explicit LoopUnswitch(bool Os = false, bool hasBranchDivergence = false) + : LoopPass(ID), OptimizeForSize(Os), + hasBranchDivergence(hasBranchDivergence) { + initializeLoopUnswitchPass(*PassRegistry::getPassRegistry()); + } + + bool runOnLoop(Loop *L, LPPassManager &LPM) override; + bool processCurrentLoop(); + bool isUnreachableDueToPreviousUnswitching(BasicBlock *); + + /// This transformation requires natural loop information & requires that + /// loop preheaders be inserted into the CFG. + /// + void getAnalysisUsage(AnalysisUsage &AU) const override { + AU.addRequired<AssumptionCacheTracker>(); + AU.addRequired<TargetTransformInfoWrapperPass>(); + if (EnableMSSALoopDependency) { + AU.addRequired<MemorySSAWrapperPass>(); + AU.addPreserved<MemorySSAWrapperPass>(); + } + if (hasBranchDivergence) + AU.addRequired<LegacyDivergenceAnalysis>(); + getLoopAnalysisUsage(AU); + } + + private: + void releaseMemory() override { + BranchesInfo.forgetLoop(currentLoop); + } + + void initLoopData() { + loopHeader = currentLoop->getHeader(); + loopPreheader = currentLoop->getLoopPreheader(); + } + + /// Split all of the edges from inside the loop to their exit blocks. + /// Update the appropriate Phi nodes as we do so. + void SplitExitEdges(Loop *L, + const SmallVectorImpl<BasicBlock *> &ExitBlocks); + + bool TryTrivialLoopUnswitch(bool &Changed); + + bool UnswitchIfProfitable(Value *LoopCond, Constant *Val, + Instruction *TI = nullptr); + void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, + BasicBlock *ExitBlock, Instruction *TI); + void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L, + Instruction *TI); + + void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, + Constant *Val, bool isEqual); + + void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, + BasicBlock *TrueDest, + BasicBlock *FalseDest, + BranchInst *OldBranch, Instruction *TI); + + void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L); + + /// Given that the Invariant is not equal to Val. Simplify instructions + /// in the loop. + Value *SimplifyInstructionWithNotEqual(Instruction *Inst, Value *Invariant, + Constant *Val); + }; + +} // end anonymous namespace + +// Analyze loop. Check its size, calculate is it possible to unswitch +// it. Returns true if we can unswitch this loop. +bool LUAnalysisCache::countLoop(const Loop *L, const TargetTransformInfo &TTI, + AssumptionCache *AC) { + LoopPropsMapIt PropsIt; + bool Inserted; + std::tie(PropsIt, Inserted) = + LoopsProperties.insert(std::make_pair(L, LoopProperties())); + + LoopProperties &Props = PropsIt->second; + + if (Inserted) { + // New loop. + + // Limit the number of instructions to avoid causing significant code + // expansion, and the number of basic blocks, to avoid loops with + // large numbers of branches which cause loop unswitching to go crazy. + // This is a very ad-hoc heuristic. + + SmallPtrSet<const Value *, 32> EphValues; + CodeMetrics::collectEphemeralValues(L, AC, EphValues); + + // FIXME: This is overly conservative because it does not take into + // consideration code simplification opportunities and code that can + // be shared by the resultant unswitched loops. + CodeMetrics Metrics; + for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); I != E; + ++I) + Metrics.analyzeBasicBlock(*I, TTI, EphValues); + + Props.SizeEstimation = Metrics.NumInsts; + Props.CanBeUnswitchedCount = MaxSize / (Props.SizeEstimation); + Props.WasUnswitchedCount = 0; + MaxSize -= Props.SizeEstimation * Props.CanBeUnswitchedCount; + + if (Metrics.notDuplicatable) { + LLVM_DEBUG(dbgs() << "NOT unswitching loop %" << L->getHeader()->getName() + << ", contents cannot be " + << "duplicated!\n"); + return false; + } + } + + // Be careful. This links are good only before new loop addition. + CurrentLoopProperties = &Props; + CurLoopInstructions = &Props.UnswitchedVals; + + return true; +} + +// Clean all data related to given loop. +void LUAnalysisCache::forgetLoop(const Loop *L) { + LoopPropsMapIt LIt = LoopsProperties.find(L); + + if (LIt != LoopsProperties.end()) { + LoopProperties &Props = LIt->second; + MaxSize += (Props.CanBeUnswitchedCount + Props.WasUnswitchedCount) * + Props.SizeEstimation; + LoopsProperties.erase(LIt); + } + + CurrentLoopProperties = nullptr; + CurLoopInstructions = nullptr; +} + +// Mark case value as unswitched. +// Since SI instruction can be partly unswitched, in order to avoid +// extra unswitching in cloned loops keep track all unswitched values. +void LUAnalysisCache::setUnswitched(const SwitchInst *SI, const Value *V) { + (*CurLoopInstructions)[SI].insert(V); +} + +// Check was this case value unswitched before or not. +bool LUAnalysisCache::isUnswitched(const SwitchInst *SI, const Value *V) { + return (*CurLoopInstructions)[SI].count(V); +} + +bool LUAnalysisCache::CostAllowsUnswitching() { + return CurrentLoopProperties->CanBeUnswitchedCount > 0; +} + +// Clone all loop-unswitch related loop properties. +// Redistribute unswitching quotas. +// Note, that new loop data is stored inside the VMap. +void LUAnalysisCache::cloneData(const Loop *NewLoop, const Loop *OldLoop, + const ValueToValueMapTy &VMap) { + LoopProperties &NewLoopProps = LoopsProperties[NewLoop]; + LoopProperties &OldLoopProps = *CurrentLoopProperties; + UnswitchedValsMap &Insts = OldLoopProps.UnswitchedVals; + + // Reallocate "can-be-unswitched quota" + + --OldLoopProps.CanBeUnswitchedCount; + ++OldLoopProps.WasUnswitchedCount; + NewLoopProps.WasUnswitchedCount = 0; + unsigned Quota = OldLoopProps.CanBeUnswitchedCount; + NewLoopProps.CanBeUnswitchedCount = Quota / 2; + OldLoopProps.CanBeUnswitchedCount = Quota - Quota / 2; + + NewLoopProps.SizeEstimation = OldLoopProps.SizeEstimation; + + // Clone unswitched values info: + // for new loop switches we clone info about values that was + // already unswitched and has redundant successors. + for (UnswitchedValsIt I = Insts.begin(); I != Insts.end(); ++I) { + const SwitchInst *OldInst = I->first; + Value *NewI = VMap.lookup(OldInst); + const SwitchInst *NewInst = cast_or_null<SwitchInst>(NewI); + assert(NewInst && "All instructions that are in SrcBB must be in VMap."); + + NewLoopProps.UnswitchedVals[NewInst] = OldLoopProps.UnswitchedVals[OldInst]; + } +} + +char LoopUnswitch::ID = 0; + +INITIALIZE_PASS_BEGIN(LoopUnswitch, "loop-unswitch", "Unswitch loops", + false, false) +INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) +INITIALIZE_PASS_DEPENDENCY(LoopPass) +INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) +INITIALIZE_PASS_DEPENDENCY(LegacyDivergenceAnalysis) +INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass) +INITIALIZE_PASS_END(LoopUnswitch, "loop-unswitch", "Unswitch loops", + false, false) + +Pass *llvm::createLoopUnswitchPass(bool Os, bool hasBranchDivergence) { + return new LoopUnswitch(Os, hasBranchDivergence); +} + +/// Operator chain lattice. +enum OperatorChain { + OC_OpChainNone, ///< There is no operator. + OC_OpChainOr, ///< There are only ORs. + OC_OpChainAnd, ///< There are only ANDs. + OC_OpChainMixed ///< There are ANDs and ORs. +}; + +/// Cond is a condition that occurs in L. If it is invariant in the loop, or has +/// an invariant piece, return the invariant. Otherwise, return null. +// +/// NOTE: FindLIVLoopCondition will not return a partial LIV by walking up a +/// mixed operator chain, as we can not reliably find a value which will simplify +/// the operator chain. If the chain is AND-only or OR-only, we can use 0 or ~0 +/// to simplify the chain. +/// +/// NOTE: In case a partial LIV and a mixed operator chain, we may be able to +/// simplify the condition itself to a loop variant condition, but at the +/// cost of creating an entirely new loop. +static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed, + OperatorChain &ParentChain, + DenseMap<Value *, Value *> &Cache, + MemorySSAUpdater *MSSAU) { + auto CacheIt = Cache.find(Cond); + if (CacheIt != Cache.end()) + return CacheIt->second; + + // We started analyze new instruction, increment scanned instructions counter. + ++TotalInsts; + + // We can never unswitch on vector conditions. + if (Cond->getType()->isVectorTy()) + return nullptr; + + // Constants should be folded, not unswitched on! + if (isa<Constant>(Cond)) return nullptr; + + // TODO: Handle: br (VARIANT|INVARIANT). + + // Hoist simple values out. + if (L->makeLoopInvariant(Cond, Changed, nullptr, MSSAU)) { + Cache[Cond] = Cond; + return Cond; + } + + // Walk up the operator chain to find partial invariant conditions. + if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond)) + if (BO->getOpcode() == Instruction::And || + BO->getOpcode() == Instruction::Or) { + // Given the previous operator, compute the current operator chain status. + OperatorChain NewChain; + switch (ParentChain) { + case OC_OpChainNone: + NewChain = BO->getOpcode() == Instruction::And ? OC_OpChainAnd : + OC_OpChainOr; + break; + case OC_OpChainOr: + NewChain = BO->getOpcode() == Instruction::Or ? OC_OpChainOr : + OC_OpChainMixed; + break; + case OC_OpChainAnd: + NewChain = BO->getOpcode() == Instruction::And ? OC_OpChainAnd : + OC_OpChainMixed; + break; + case OC_OpChainMixed: + NewChain = OC_OpChainMixed; + break; + } + + // If we reach a Mixed state, we do not want to keep walking up as we can not + // reliably find a value that will simplify the chain. With this check, we + // will return null on the first sight of mixed chain and the caller will + // either backtrack to find partial LIV in other operand or return null. + if (NewChain != OC_OpChainMixed) { + // Update the current operator chain type before we search up the chain. + ParentChain = NewChain; + // If either the left or right side is invariant, we can unswitch on this, + // which will cause the branch to go away in one loop and the condition to + // simplify in the other one. + if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed, + ParentChain, Cache, MSSAU)) { + Cache[Cond] = LHS; + return LHS; + } + // We did not manage to find a partial LIV in operand(0). Backtrack and try + // operand(1). + ParentChain = NewChain; + if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed, + ParentChain, Cache, MSSAU)) { + Cache[Cond] = RHS; + return RHS; + } + } + } + + Cache[Cond] = nullptr; + return nullptr; +} + +/// Cond is a condition that occurs in L. If it is invariant in the loop, or has +/// an invariant piece, return the invariant along with the operator chain type. +/// Otherwise, return null. +static std::pair<Value *, OperatorChain> +FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed, + MemorySSAUpdater *MSSAU) { + DenseMap<Value *, Value *> Cache; + OperatorChain OpChain = OC_OpChainNone; + Value *FCond = FindLIVLoopCondition(Cond, L, Changed, OpChain, Cache, MSSAU); + + // In case we do find a LIV, it can not be obtained by walking up a mixed + // operator chain. + assert((!FCond || OpChain != OC_OpChainMixed) && + "Do not expect a partial LIV with mixed operator chain"); + return {FCond, OpChain}; +} + +bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) { + if (skipLoop(L)) + return false; + + AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache( + *L->getHeader()->getParent()); + LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); + LPM = &LPM_Ref; + DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); + if (EnableMSSALoopDependency) { + MSSA = &getAnalysis<MemorySSAWrapperPass>().getMSSA(); + MSSAU = std::make_unique<MemorySSAUpdater>(MSSA); + assert(DT && "Cannot update MemorySSA without a valid DomTree."); + } + currentLoop = L; + Function *F = currentLoop->getHeader()->getParent(); + + SanitizeMemory = F->hasFnAttribute(Attribute::SanitizeMemory); + if (SanitizeMemory) + SafetyInfo.computeLoopSafetyInfo(L); + + if (MSSA && VerifyMemorySSA) + MSSA->verifyMemorySSA(); + + bool Changed = false; + do { + assert(currentLoop->isLCSSAForm(*DT)); + if (MSSA && VerifyMemorySSA) + MSSA->verifyMemorySSA(); + redoLoop = false; + Changed |= processCurrentLoop(); + } while(redoLoop); + + if (MSSA && VerifyMemorySSA) + MSSA->verifyMemorySSA(); + + return Changed; +} + +// Return true if the BasicBlock BB is unreachable from the loop header. +// Return false, otherwise. +bool LoopUnswitch::isUnreachableDueToPreviousUnswitching(BasicBlock *BB) { + auto *Node = DT->getNode(BB)->getIDom(); + BasicBlock *DomBB = Node->getBlock(); + while (currentLoop->contains(DomBB)) { + BranchInst *BInst = dyn_cast<BranchInst>(DomBB->getTerminator()); + + Node = DT->getNode(DomBB)->getIDom(); + DomBB = Node->getBlock(); + + if (!BInst || !BInst->isConditional()) + continue; + + Value *Cond = BInst->getCondition(); + if (!isa<ConstantInt>(Cond)) + continue; + + BasicBlock *UnreachableSucc = + Cond == ConstantInt::getTrue(Cond->getContext()) + ? BInst->getSuccessor(1) + : BInst->getSuccessor(0); + + if (DT->dominates(UnreachableSucc, BB)) + return true; + } + return false; +} + +/// FIXME: Remove this workaround when freeze related patches are done. +/// LoopUnswitch and Equality propagation in GVN have discrepancy about +/// whether branch on undef/poison has undefine behavior. Here it is to +/// rule out some common cases that we found such discrepancy already +/// causing problems. Detail could be found in PR31652. Note if the +/// func returns true, it is unsafe. But if it is false, it doesn't mean +/// it is necessarily safe. +static bool EqualityPropUnSafe(Value &LoopCond) { + ICmpInst *CI = dyn_cast<ICmpInst>(&LoopCond); + if (!CI || !CI->isEquality()) + return false; + + Value *LHS = CI->getOperand(0); + Value *RHS = CI->getOperand(1); + if (isa<UndefValue>(LHS) || isa<UndefValue>(RHS)) + return true; + + auto hasUndefInPHI = [](PHINode &PN) { + for (Value *Opd : PN.incoming_values()) { + if (isa<UndefValue>(Opd)) + return true; + } + return false; + }; + PHINode *LPHI = dyn_cast<PHINode>(LHS); + PHINode *RPHI = dyn_cast<PHINode>(RHS); + if ((LPHI && hasUndefInPHI(*LPHI)) || (RPHI && hasUndefInPHI(*RPHI))) + return true; + + auto hasUndefInSelect = [](SelectInst &SI) { + if (isa<UndefValue>(SI.getTrueValue()) || + isa<UndefValue>(SI.getFalseValue())) + return true; + return false; + }; + SelectInst *LSI = dyn_cast<SelectInst>(LHS); + SelectInst *RSI = dyn_cast<SelectInst>(RHS); + if ((LSI && hasUndefInSelect(*LSI)) || (RSI && hasUndefInSelect(*RSI))) + return true; + return false; +} + +/// Do actual work and unswitch loop if possible and profitable. +bool LoopUnswitch::processCurrentLoop() { + bool Changed = false; + + initLoopData(); + + // If LoopSimplify was unable to form a preheader, don't do any unswitching. + if (!loopPreheader) + return false; + + // Loops with indirectbr cannot be cloned. + if (!currentLoop->isSafeToClone()) + return false; + + // Without dedicated exits, splitting the exit edge may fail. + if (!currentLoop->hasDedicatedExits()) + return false; + + LLVMContext &Context = loopHeader->getContext(); + + // Analyze loop cost, and stop unswitching if loop content can not be duplicated. + if (!BranchesInfo.countLoop( + currentLoop, getAnalysis<TargetTransformInfoWrapperPass>().getTTI( + *currentLoop->getHeader()->getParent()), + AC)) + return false; + + // Try trivial unswitch first before loop over other basic blocks in the loop. + if (TryTrivialLoopUnswitch(Changed)) { + return true; + } + + // Do not do non-trivial unswitch while optimizing for size. + // FIXME: Use Function::hasOptSize(). + if (OptimizeForSize || + loopHeader->getParent()->hasFnAttribute(Attribute::OptimizeForSize)) + return false; + + // Run through the instructions in the loop, keeping track of three things: + // + // - That we do not unswitch loops containing convergent operations, as we + // might be making them control dependent on the unswitch value when they + // were not before. + // FIXME: This could be refined to only bail if the convergent operation is + // not already control-dependent on the unswitch value. + // + // - That basic blocks in the loop contain invokes whose predecessor edges we + // cannot split. + // + // - The set of guard intrinsics encountered (these are non terminator + // instructions that are also profitable to be unswitched). + + SmallVector<IntrinsicInst *, 4> Guards; + + for (const auto BB : currentLoop->blocks()) { + for (auto &I : *BB) { + auto CS = CallSite(&I); + if (!CS) continue; + if (CS.hasFnAttr(Attribute::Convergent)) + return false; + if (auto *II = dyn_cast<InvokeInst>(&I)) + if (!II->getUnwindDest()->canSplitPredecessors()) + return false; + if (auto *II = dyn_cast<IntrinsicInst>(&I)) + if (II->getIntrinsicID() == Intrinsic::experimental_guard) + Guards.push_back(II); + } + } + + for (IntrinsicInst *Guard : Guards) { + Value *LoopCond = FindLIVLoopCondition(Guard->getOperand(0), currentLoop, + Changed, MSSAU.get()) + .first; + if (LoopCond && + UnswitchIfProfitable(LoopCond, ConstantInt::getTrue(Context))) { + // NB! Unswitching (if successful) could have erased some of the + // instructions in Guards leaving dangling pointers there. This is fine + // because we're returning now, and won't look at Guards again. + ++NumGuards; + return true; + } + } + + // Loop over all of the basic blocks in the loop. If we find an interior + // block that is branching on a loop-invariant condition, we can unswitch this + // loop. + for (Loop::block_iterator I = currentLoop->block_begin(), + E = currentLoop->block_end(); I != E; ++I) { + Instruction *TI = (*I)->getTerminator(); + + // Unswitching on a potentially uninitialized predicate is not + // MSan-friendly. Limit this to the cases when the original predicate is + // guaranteed to execute, to avoid creating a use-of-uninitialized-value + // in the code that did not have one. + // This is a workaround for the discrepancy between LLVM IR and MSan + // semantics. See PR28054 for more details. + if (SanitizeMemory && + !SafetyInfo.isGuaranteedToExecute(*TI, DT, currentLoop)) + continue; + + if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { + // Some branches may be rendered unreachable because of previous + // unswitching. + // Unswitch only those branches that are reachable. + if (isUnreachableDueToPreviousUnswitching(*I)) + continue; + + // If this isn't branching on an invariant condition, we can't unswitch + // it. + if (BI->isConditional()) { + // See if this, or some part of it, is loop invariant. If so, we can + // unswitch on it if we desire. + Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), currentLoop, + Changed, MSSAU.get()) + .first; + if (LoopCond && !EqualityPropUnSafe(*LoopCond) && + UnswitchIfProfitable(LoopCond, ConstantInt::getTrue(Context), TI)) { + ++NumBranches; + return true; + } + } + } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { + Value *SC = SI->getCondition(); + Value *LoopCond; + OperatorChain OpChain; + std::tie(LoopCond, OpChain) = + FindLIVLoopCondition(SC, currentLoop, Changed, MSSAU.get()); + + unsigned NumCases = SI->getNumCases(); + if (LoopCond && NumCases) { + // Find a value to unswitch on: + // FIXME: this should chose the most expensive case! + // FIXME: scan for a case with a non-critical edge? + Constant *UnswitchVal = nullptr; + // Find a case value such that at least one case value is unswitched + // out. + if (OpChain == OC_OpChainAnd) { + // If the chain only has ANDs and the switch has a case value of 0. + // Dropping in a 0 to the chain will unswitch out the 0-casevalue. + auto *AllZero = cast<ConstantInt>(Constant::getNullValue(SC->getType())); + if (BranchesInfo.isUnswitched(SI, AllZero)) + continue; + // We are unswitching 0 out. + UnswitchVal = AllZero; + } else if (OpChain == OC_OpChainOr) { + // If the chain only has ORs and the switch has a case value of ~0. + // Dropping in a ~0 to the chain will unswitch out the ~0-casevalue. + auto *AllOne = cast<ConstantInt>(Constant::getAllOnesValue(SC->getType())); + if (BranchesInfo.isUnswitched(SI, AllOne)) + continue; + // We are unswitching ~0 out. + UnswitchVal = AllOne; + } else { + assert(OpChain == OC_OpChainNone && + "Expect to unswitch on trivial chain"); + // Do not process same value again and again. + // At this point we have some cases already unswitched and + // some not yet unswitched. Let's find the first not yet unswitched one. + for (auto Case : SI->cases()) { + Constant *UnswitchValCandidate = Case.getCaseValue(); + if (!BranchesInfo.isUnswitched(SI, UnswitchValCandidate)) { + UnswitchVal = UnswitchValCandidate; + break; + } + } + } + + if (!UnswitchVal) + continue; + + if (UnswitchIfProfitable(LoopCond, UnswitchVal)) { + ++NumSwitches; + // In case of a full LIV, UnswitchVal is the value we unswitched out. + // In case of a partial LIV, we only unswitch when its an AND-chain + // or OR-chain. In both cases switch input value simplifies to + // UnswitchVal. + BranchesInfo.setUnswitched(SI, UnswitchVal); + return true; + } + } + } + + // Scan the instructions to check for unswitchable values. + for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end(); + BBI != E; ++BBI) + if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) { + Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), currentLoop, + Changed, MSSAU.get()) + .first; + if (LoopCond && UnswitchIfProfitable(LoopCond, + ConstantInt::getTrue(Context))) { + ++NumSelects; + return true; + } + } + } + return Changed; +} + +/// Check to see if all paths from BB exit the loop with no side effects +/// (including infinite loops). +/// +/// If true, we return true and set ExitBB to the block we +/// exit through. +/// +static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB, + BasicBlock *&ExitBB, + std::set<BasicBlock*> &Visited) { + if (!Visited.insert(BB).second) { + // Already visited. Without more analysis, this could indicate an infinite + // loop. + return false; + } + if (!L->contains(BB)) { + // Otherwise, this is a loop exit, this is fine so long as this is the + // first exit. + if (ExitBB) return false; + ExitBB = BB; + return true; + } + + // Otherwise, this is an unvisited intra-loop node. Check all successors. + for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) { + // Check to see if the successor is a trivial loop exit. + if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited)) + return false; + } + + // Okay, everything after this looks good, check to make sure that this block + // doesn't include any side effects. + for (Instruction &I : *BB) + if (I.mayHaveSideEffects()) + return false; + + return true; +} + +/// Return true if the specified block unconditionally leads to an exit from +/// the specified loop, and has no side-effects in the process. If so, return +/// the block that is exited to, otherwise return null. +static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) { + std::set<BasicBlock*> Visited; + Visited.insert(L->getHeader()); // Branches to header make infinite loops. + BasicBlock *ExitBB = nullptr; + if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited)) + return ExitBB; + return nullptr; +} + +/// We have found that we can unswitch currentLoop when LoopCond == Val to +/// simplify the loop. If we decide that this is profitable, +/// unswitch the loop, reprocess the pieces, then return true. +bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val, + Instruction *TI) { + // Check to see if it would be profitable to unswitch current loop. + if (!BranchesInfo.CostAllowsUnswitching()) { + LLVM_DEBUG(dbgs() << "NOT unswitching loop %" + << currentLoop->getHeader()->getName() + << " at non-trivial condition '" << *Val + << "' == " << *LoopCond << "\n" + << ". Cost too high.\n"); + return false; + } + if (hasBranchDivergence && + getAnalysis<LegacyDivergenceAnalysis>().isDivergent(LoopCond)) { + LLVM_DEBUG(dbgs() << "NOT unswitching loop %" + << currentLoop->getHeader()->getName() + << " at non-trivial condition '" << *Val + << "' == " << *LoopCond << "\n" + << ". Condition is divergent.\n"); + return false; + } + + UnswitchNontrivialCondition(LoopCond, Val, currentLoop, TI); + return true; +} + +/// Recursively clone the specified loop and all of its children, +/// mapping the blocks with the specified map. +static Loop *CloneLoop(Loop *L, Loop *PL, ValueToValueMapTy &VM, + LoopInfo *LI, LPPassManager *LPM) { + Loop &New = *LI->AllocateLoop(); + if (PL) + PL->addChildLoop(&New); + else + LI->addTopLevelLoop(&New); + LPM->addLoop(New); + + // Add all of the blocks in L to the new loop. + for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); + I != E; ++I) + if (LI->getLoopFor(*I) == L) + New.addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI); + + // Add all of the subloops to the new loop. + for (Loop *I : *L) + CloneLoop(I, &New, VM, LI, LPM); + + return &New; +} + +/// Emit a conditional branch on two values if LIC == Val, branch to TrueDst, +/// otherwise branch to FalseDest. Insert the code immediately before OldBranch +/// and remove (but not erase!) it from the function. +void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, + BasicBlock *TrueDest, + BasicBlock *FalseDest, + BranchInst *OldBranch, + Instruction *TI) { + assert(OldBranch->isUnconditional() && "Preheader is not split correctly"); + assert(TrueDest != FalseDest && "Branch targets should be different"); + // Insert a conditional branch on LIC to the two preheaders. The original + // code is the true version and the new code is the false version. + Value *BranchVal = LIC; + bool Swapped = false; + if (!isa<ConstantInt>(Val) || + Val->getType() != Type::getInt1Ty(LIC->getContext())) + BranchVal = new ICmpInst(OldBranch, ICmpInst::ICMP_EQ, LIC, Val); + else if (Val != ConstantInt::getTrue(Val->getContext())) { + // We want to enter the new loop when the condition is true. + std::swap(TrueDest, FalseDest); + Swapped = true; + } + + // Old branch will be removed, so save its parent and successor to update the + // DomTree. + auto *OldBranchSucc = OldBranch->getSuccessor(0); + auto *OldBranchParent = OldBranch->getParent(); + + // Insert the new branch. + BranchInst *BI = + IRBuilder<>(OldBranch).CreateCondBr(BranchVal, TrueDest, FalseDest, TI); + if (Swapped) + BI->swapProfMetadata(); + + // Remove the old branch so there is only one branch at the end. This is + // needed to perform DomTree's internal DFS walk on the function's CFG. + OldBranch->removeFromParent(); + + // Inform the DT about the new branch. + if (DT) { + // First, add both successors. + SmallVector<DominatorTree::UpdateType, 3> Updates; + if (TrueDest != OldBranchSucc) + Updates.push_back({DominatorTree::Insert, OldBranchParent, TrueDest}); + if (FalseDest != OldBranchSucc) + Updates.push_back({DominatorTree::Insert, OldBranchParent, FalseDest}); + // If both of the new successors are different from the old one, inform the + // DT that the edge was deleted. + if (OldBranchSucc != TrueDest && OldBranchSucc != FalseDest) { + Updates.push_back({DominatorTree::Delete, OldBranchParent, OldBranchSucc}); + } + DT->applyUpdates(Updates); + + if (MSSAU) + MSSAU->applyUpdates(Updates, *DT); + } + + // If either edge is critical, split it. This helps preserve LoopSimplify + // form for enclosing loops. + auto Options = + CriticalEdgeSplittingOptions(DT, LI, MSSAU.get()).setPreserveLCSSA(); + SplitCriticalEdge(BI, 0, Options); + SplitCriticalEdge(BI, 1, Options); +} + +/// Given a loop that has a trivial unswitchable condition in it (a cond branch +/// from its header block to its latch block, where the path through the loop +/// that doesn't execute its body has no side-effects), unswitch it. This +/// doesn't involve any code duplication, just moving the conditional branch +/// outside of the loop and updating loop info. +void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, + BasicBlock *ExitBlock, + Instruction *TI) { + LLVM_DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %" + << loopHeader->getName() << " [" << L->getBlocks().size() + << " blocks] in Function " + << L->getHeader()->getParent()->getName() + << " on cond: " << *Val << " == " << *Cond << "\n"); + // We are going to make essential changes to CFG. This may invalidate cached + // information for L or one of its parent loops in SCEV. + if (auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>()) + SEWP->getSE().forgetTopmostLoop(L); + + // First step, split the preheader, so that we know that there is a safe place + // to insert the conditional branch. We will change loopPreheader to have a + // conditional branch on Cond. + BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, DT, LI, MSSAU.get()); + + // Now that we have a place to insert the conditional branch, create a place + // to branch to: this is the exit block out of the loop that we should + // short-circuit to. + + // Split this block now, so that the loop maintains its exit block, and so + // that the jump from the preheader can execute the contents of the exit block + // without actually branching to it (the exit block should be dominated by the + // loop header, not the preheader). + assert(!L->contains(ExitBlock) && "Exit block is in the loop?"); + BasicBlock *NewExit = + SplitBlock(ExitBlock, &ExitBlock->front(), DT, LI, MSSAU.get()); + + // Okay, now we have a position to branch from and a position to branch to, + // insert the new conditional branch. + auto *OldBranch = dyn_cast<BranchInst>(loopPreheader->getTerminator()); + assert(OldBranch && "Failed to split the preheader"); + EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH, OldBranch, TI); + LPM->deleteSimpleAnalysisValue(OldBranch, L); + + // EmitPreheaderBranchOnCondition removed the OldBranch from the function. + // Delete it, as it is no longer needed. + delete OldBranch; + + // We need to reprocess this loop, it could be unswitched again. + redoLoop = true; + + // Now that we know that the loop is never entered when this condition is a + // particular value, rewrite the loop with this info. We know that this will + // at least eliminate the old branch. + RewriteLoopBodyWithConditionConstant(L, Cond, Val, false); + + ++NumTrivial; +} + +/// Check if the first non-constant condition starting from the loop header is +/// a trivial unswitch condition: that is, a condition controls whether or not +/// the loop does anything at all. If it is a trivial condition, unswitching +/// produces no code duplications (equivalently, it produces a simpler loop and +/// a new empty loop, which gets deleted). Therefore always unswitch trivial +/// condition. +bool LoopUnswitch::TryTrivialLoopUnswitch(bool &Changed) { + BasicBlock *CurrentBB = currentLoop->getHeader(); + Instruction *CurrentTerm = CurrentBB->getTerminator(); + LLVMContext &Context = CurrentBB->getContext(); + + // If loop header has only one reachable successor (currently via an + // unconditional branch or constant foldable conditional branch, but + // should also consider adding constant foldable switch instruction in + // future), we should keep looking for trivial condition candidates in + // the successor as well. An alternative is to constant fold conditions + // and merge successors into loop header (then we only need to check header's + // terminator). The reason for not doing this in LoopUnswitch pass is that + // it could potentially break LoopPassManager's invariants. Folding dead + // branches could either eliminate the current loop or make other loops + // unreachable. LCSSA form might also not be preserved after deleting + // branches. The following code keeps traversing loop header's successors + // until it finds the trivial condition candidate (condition that is not a + // constant). Since unswitching generates branches with constant conditions, + // this scenario could be very common in practice. + SmallPtrSet<BasicBlock*, 8> Visited; + + while (true) { + // If we exit loop or reach a previous visited block, then + // we can not reach any trivial condition candidates (unfoldable + // branch instructions or switch instructions) and no unswitch + // can happen. Exit and return false. + if (!currentLoop->contains(CurrentBB) || !Visited.insert(CurrentBB).second) + return false; + + // Check if this loop will execute any side-effecting instructions (e.g. + // stores, calls, volatile loads) in the part of the loop that the code + // *would* execute. Check the header first. + for (Instruction &I : *CurrentBB) + if (I.mayHaveSideEffects()) + return false; + + if (BranchInst *BI = dyn_cast<BranchInst>(CurrentTerm)) { + if (BI->isUnconditional()) { + CurrentBB = BI->getSuccessor(0); + } else if (BI->getCondition() == ConstantInt::getTrue(Context)) { + CurrentBB = BI->getSuccessor(0); + } else if (BI->getCondition() == ConstantInt::getFalse(Context)) { + CurrentBB = BI->getSuccessor(1); + } else { + // Found a trivial condition candidate: non-foldable conditional branch. + break; + } + } else if (SwitchInst *SI = dyn_cast<SwitchInst>(CurrentTerm)) { + // At this point, any constant-foldable instructions should have probably + // been folded. + ConstantInt *Cond = dyn_cast<ConstantInt>(SI->getCondition()); + if (!Cond) + break; + // Find the target block we are definitely going to. + CurrentBB = SI->findCaseValue(Cond)->getCaseSuccessor(); + } else { + // We do not understand these terminator instructions. + break; + } + + CurrentTerm = CurrentBB->getTerminator(); + } + + // CondVal is the condition that controls the trivial condition. + // LoopExitBB is the BasicBlock that loop exits when meets trivial condition. + Constant *CondVal = nullptr; + BasicBlock *LoopExitBB = nullptr; + + if (BranchInst *BI = dyn_cast<BranchInst>(CurrentTerm)) { + // If this isn't branching on an invariant condition, we can't unswitch it. + if (!BI->isConditional()) + return false; + + Value *LoopCond = FindLIVLoopCondition(BI->getCondition(), currentLoop, + Changed, MSSAU.get()) + .first; + + // Unswitch only if the trivial condition itself is an LIV (not + // partial LIV which could occur in and/or) + if (!LoopCond || LoopCond != BI->getCondition()) + return false; + + // Check to see if a successor of the branch is guaranteed to + // exit through a unique exit block without having any + // side-effects. If so, determine the value of Cond that causes + // it to do this. + if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, + BI->getSuccessor(0)))) { + CondVal = ConstantInt::getTrue(Context); + } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, + BI->getSuccessor(1)))) { + CondVal = ConstantInt::getFalse(Context); + } + + // If we didn't find a single unique LoopExit block, or if the loop exit + // block contains phi nodes, this isn't trivial. + if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin())) + return false; // Can't handle this. + + if (EqualityPropUnSafe(*LoopCond)) + return false; + + UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, LoopExitBB, + CurrentTerm); + ++NumBranches; + return true; + } else if (SwitchInst *SI = dyn_cast<SwitchInst>(CurrentTerm)) { + // If this isn't switching on an invariant condition, we can't unswitch it. + Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), currentLoop, + Changed, MSSAU.get()) + .first; + + // Unswitch only if the trivial condition itself is an LIV (not + // partial LIV which could occur in and/or) + if (!LoopCond || LoopCond != SI->getCondition()) + return false; + + // Check to see if a successor of the switch is guaranteed to go to the + // latch block or exit through a one exit block without having any + // side-effects. If so, determine the value of Cond that causes it to do + // this. + // Note that we can't trivially unswitch on the default case or + // on already unswitched cases. + for (auto Case : SI->cases()) { + BasicBlock *LoopExitCandidate; + if ((LoopExitCandidate = + isTrivialLoopExitBlock(currentLoop, Case.getCaseSuccessor()))) { + // Okay, we found a trivial case, remember the value that is trivial. + ConstantInt *CaseVal = Case.getCaseValue(); + + // Check that it was not unswitched before, since already unswitched + // trivial vals are looks trivial too. + if (BranchesInfo.isUnswitched(SI, CaseVal)) + continue; + LoopExitBB = LoopExitCandidate; + CondVal = CaseVal; + break; + } + } + + // If we didn't find a single unique LoopExit block, or if the loop exit + // block contains phi nodes, this isn't trivial. + if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin())) + return false; // Can't handle this. + + UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, LoopExitBB, + nullptr); + + // We are only unswitching full LIV. + BranchesInfo.setUnswitched(SI, CondVal); + ++NumSwitches; + return true; + } + return false; +} + +/// Split all of the edges from inside the loop to their exit blocks. +/// Update the appropriate Phi nodes as we do so. +void LoopUnswitch::SplitExitEdges(Loop *L, + const SmallVectorImpl<BasicBlock *> &ExitBlocks){ + + for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { + BasicBlock *ExitBlock = ExitBlocks[i]; + SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock), + pred_end(ExitBlock)); + + // Although SplitBlockPredecessors doesn't preserve loop-simplify in + // general, if we call it on all predecessors of all exits then it does. + SplitBlockPredecessors(ExitBlock, Preds, ".us-lcssa", DT, LI, MSSAU.get(), + /*PreserveLCSSA*/ true); + } +} + +/// We determined that the loop is profitable to unswitch when LIC equal Val. +/// Split it into loop versions and test the condition outside of either loop. +/// Return the loops created as Out1/Out2. +void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val, + Loop *L, Instruction *TI) { + Function *F = loopHeader->getParent(); + LLVM_DEBUG(dbgs() << "loop-unswitch: Unswitching loop %" + << loopHeader->getName() << " [" << L->getBlocks().size() + << " blocks] in Function " << F->getName() << " when '" + << *Val << "' == " << *LIC << "\n"); + + // We are going to make essential changes to CFG. This may invalidate cached + // information for L or one of its parent loops in SCEV. + if (auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>()) + SEWP->getSE().forgetTopmostLoop(L); + + LoopBlocks.clear(); + NewBlocks.clear(); + + if (MSSAU && VerifyMemorySSA) + MSSA->verifyMemorySSA(); + + // First step, split the preheader and exit blocks, and add these blocks to + // the LoopBlocks list. + BasicBlock *NewPreheader = + SplitEdge(loopPreheader, loopHeader, DT, LI, MSSAU.get()); + LoopBlocks.push_back(NewPreheader); + + // We want the loop to come after the preheader, but before the exit blocks. + LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end()); + + SmallVector<BasicBlock*, 8> ExitBlocks; + L->getUniqueExitBlocks(ExitBlocks); + + // Split all of the edges from inside the loop to their exit blocks. Update + // the appropriate Phi nodes as we do so. + SplitExitEdges(L, ExitBlocks); + + // The exit blocks may have been changed due to edge splitting, recompute. + ExitBlocks.clear(); + L->getUniqueExitBlocks(ExitBlocks); + + // Add exit blocks to the loop blocks. + LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end()); + + // Next step, clone all of the basic blocks that make up the loop (including + // the loop preheader and exit blocks), keeping track of the mapping between + // the instructions and blocks. + NewBlocks.reserve(LoopBlocks.size()); + ValueToValueMapTy VMap; + for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) { + BasicBlock *NewBB = CloneBasicBlock(LoopBlocks[i], VMap, ".us", F); + + NewBlocks.push_back(NewBB); + VMap[LoopBlocks[i]] = NewBB; // Keep the BB mapping. + LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], NewBB, L); + } + + // Splice the newly inserted blocks into the function right before the + // original preheader. + F->getBasicBlockList().splice(NewPreheader->getIterator(), + F->getBasicBlockList(), + NewBlocks[0]->getIterator(), F->end()); + + // Now we create the new Loop object for the versioned loop. + Loop *NewLoop = CloneLoop(L, L->getParentLoop(), VMap, LI, LPM); + + // Recalculate unswitching quota, inherit simplified switches info for NewBB, + // Probably clone more loop-unswitch related loop properties. + BranchesInfo.cloneData(NewLoop, L, VMap); + + Loop *ParentLoop = L->getParentLoop(); + if (ParentLoop) { + // Make sure to add the cloned preheader and exit blocks to the parent loop + // as well. + ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI); + } + + for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { + BasicBlock *NewExit = cast<BasicBlock>(VMap[ExitBlocks[i]]); + // The new exit block should be in the same loop as the old one. + if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i])) + ExitBBLoop->addBasicBlockToLoop(NewExit, *LI); + + assert(NewExit->getTerminator()->getNumSuccessors() == 1 && + "Exit block should have been split to have one successor!"); + BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0); + + // If the successor of the exit block had PHI nodes, add an entry for + // NewExit. + for (PHINode &PN : ExitSucc->phis()) { + Value *V = PN.getIncomingValueForBlock(ExitBlocks[i]); + ValueToValueMapTy::iterator It = VMap.find(V); + if (It != VMap.end()) V = It->second; + PN.addIncoming(V, NewExit); + } + + if (LandingPadInst *LPad = NewExit->getLandingPadInst()) { + PHINode *PN = PHINode::Create(LPad->getType(), 0, "", + &*ExitSucc->getFirstInsertionPt()); + + for (pred_iterator I = pred_begin(ExitSucc), E = pred_end(ExitSucc); + I != E; ++I) { + BasicBlock *BB = *I; + LandingPadInst *LPI = BB->getLandingPadInst(); + LPI->replaceAllUsesWith(PN); + PN->addIncoming(LPI, BB); + } + } + } + + // Rewrite the code to refer to itself. + for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i) { + for (Instruction &I : *NewBlocks[i]) { + RemapInstruction(&I, VMap, + RF_NoModuleLevelChanges | RF_IgnoreMissingLocals); + if (auto *II = dyn_cast<IntrinsicInst>(&I)) + if (II->getIntrinsicID() == Intrinsic::assume) + AC->registerAssumption(II); + } + } + + // Rewrite the original preheader to select between versions of the loop. + BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator()); + assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] && + "Preheader splitting did not work correctly!"); + + if (MSSAU) { + // Update MemorySSA after cloning, and before splitting to unreachables, + // since that invalidates the 1:1 mapping of clones in VMap. + LoopBlocksRPO LBRPO(L); + LBRPO.perform(LI); + MSSAU->updateForClonedLoop(LBRPO, ExitBlocks, VMap); + } + + // Emit the new branch that selects between the two versions of this loop. + EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR, + TI); + LPM->deleteSimpleAnalysisValue(OldBR, L); + if (MSSAU) { + // Update MemoryPhis in Exit blocks. + MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMap, *DT); + if (VerifyMemorySSA) + MSSA->verifyMemorySSA(); + } + + // The OldBr was replaced by a new one and removed (but not erased) by + // EmitPreheaderBranchOnCondition. It is no longer needed, so delete it. + delete OldBR; + + LoopProcessWorklist.push_back(NewLoop); + redoLoop = true; + + // Keep a WeakTrackingVH holding onto LIC. If the first call to + // RewriteLoopBody + // deletes the instruction (for example by simplifying a PHI that feeds into + // the condition that we're unswitching on), we don't rewrite the second + // iteration. + WeakTrackingVH LICHandle(LIC); + + // Now we rewrite the original code to know that the condition is true and the + // new code to know that the condition is false. + RewriteLoopBodyWithConditionConstant(L, LIC, Val, false); + + // It's possible that simplifying one loop could cause the other to be + // changed to another value or a constant. If its a constant, don't simplify + // it. + if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop && + LICHandle && !isa<Constant>(LICHandle)) + RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val, true); + + if (MSSA && VerifyMemorySSA) + MSSA->verifyMemorySSA(); +} + +/// Remove all instances of I from the worklist vector specified. +static void RemoveFromWorklist(Instruction *I, + std::vector<Instruction*> &Worklist) { + + Worklist.erase(std::remove(Worklist.begin(), Worklist.end(), I), + Worklist.end()); +} + +/// When we find that I really equals V, remove I from the +/// program, replacing all uses with V and update the worklist. +static void ReplaceUsesOfWith(Instruction *I, Value *V, + std::vector<Instruction *> &Worklist, Loop *L, + LPPassManager *LPM, MemorySSAUpdater *MSSAU) { + LLVM_DEBUG(dbgs() << "Replace with '" << *V << "': " << *I << "\n"); + + // Add uses to the worklist, which may be dead now. + for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) + if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) + Worklist.push_back(Use); + + // Add users to the worklist which may be simplified now. + for (User *U : I->users()) + Worklist.push_back(cast<Instruction>(U)); + LPM->deleteSimpleAnalysisValue(I, L); + RemoveFromWorklist(I, Worklist); + I->replaceAllUsesWith(V); + if (!I->mayHaveSideEffects()) { + if (MSSAU) + MSSAU->removeMemoryAccess(I); + I->eraseFromParent(); + } + ++NumSimplify; +} + +/// We know either that the value LIC has the value specified by Val in the +/// specified loop, or we know it does NOT have that value. +/// Rewrite any uses of LIC or of properties correlated to it. +void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, + Constant *Val, + bool IsEqual) { + assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?"); + + // FIXME: Support correlated properties, like: + // for (...) + // if (li1 < li2) + // ... + // if (li1 > li2) + // ... + + // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches, + // selects, switches. + std::vector<Instruction*> Worklist; + LLVMContext &Context = Val->getContext(); + + // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC + // in the loop with the appropriate one directly. + if (IsEqual || (isa<ConstantInt>(Val) && + Val->getType()->isIntegerTy(1))) { + Value *Replacement; + if (IsEqual) + Replacement = Val; + else + Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()), + !cast<ConstantInt>(Val)->getZExtValue()); + + for (User *U : LIC->users()) { + Instruction *UI = dyn_cast<Instruction>(U); + if (!UI || !L->contains(UI)) + continue; + Worklist.push_back(UI); + } + + for (Instruction *UI : Worklist) + UI->replaceUsesOfWith(LIC, Replacement); + + SimplifyCode(Worklist, L); + return; + } + + // Otherwise, we don't know the precise value of LIC, but we do know that it + // is certainly NOT "Val". As such, simplify any uses in the loop that we + // can. This case occurs when we unswitch switch statements. + for (User *U : LIC->users()) { + Instruction *UI = dyn_cast<Instruction>(U); + if (!UI || !L->contains(UI)) + continue; + + // At this point, we know LIC is definitely not Val. Try to use some simple + // logic to simplify the user w.r.t. to the context. + if (Value *Replacement = SimplifyInstructionWithNotEqual(UI, LIC, Val)) { + if (LI->replacementPreservesLCSSAForm(UI, Replacement)) { + // This in-loop instruction has been simplified w.r.t. its context, + // i.e. LIC != Val, make sure we propagate its replacement value to + // all its users. + // + // We can not yet delete UI, the LIC user, yet, because that would invalidate + // the LIC->users() iterator !. However, we can make this instruction + // dead by replacing all its users and push it onto the worklist so that + // it can be properly deleted and its operands simplified. + UI->replaceAllUsesWith(Replacement); + } + } + + // This is a LIC user, push it into the worklist so that SimplifyCode can + // attempt to simplify it. + Worklist.push_back(UI); + + // If we know that LIC is not Val, use this info to simplify code. + SwitchInst *SI = dyn_cast<SwitchInst>(UI); + if (!SI || !isa<ConstantInt>(Val)) continue; + + // NOTE: if a case value for the switch is unswitched out, we record it + // after the unswitch finishes. We can not record it here as the switch + // is not a direct user of the partial LIV. + SwitchInst::CaseHandle DeadCase = + *SI->findCaseValue(cast<ConstantInt>(Val)); + // Default case is live for multiple values. + if (DeadCase == *SI->case_default()) + continue; + + // Found a dead case value. Don't remove PHI nodes in the + // successor if they become single-entry, those PHI nodes may + // be in the Users list. + + BasicBlock *Switch = SI->getParent(); + BasicBlock *SISucc = DeadCase.getCaseSuccessor(); + BasicBlock *Latch = L->getLoopLatch(); + + if (!SI->findCaseDest(SISucc)) continue; // Edge is critical. + // If the DeadCase successor dominates the loop latch, then the + // transformation isn't safe since it will delete the sole predecessor edge + // to the latch. + if (Latch && DT->dominates(SISucc, Latch)) + continue; + + // FIXME: This is a hack. We need to keep the successor around + // and hooked up so as to preserve the loop structure, because + // trying to update it is complicated. So instead we preserve the + // loop structure and put the block on a dead code path. + SplitEdge(Switch, SISucc, DT, LI, MSSAU.get()); + // Compute the successors instead of relying on the return value + // of SplitEdge, since it may have split the switch successor + // after PHI nodes. + BasicBlock *NewSISucc = DeadCase.getCaseSuccessor(); + BasicBlock *OldSISucc = *succ_begin(NewSISucc); + // Create an "unreachable" destination. + BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable", + Switch->getParent(), + OldSISucc); + new UnreachableInst(Context, Abort); + // Force the new case destination to branch to the "unreachable" + // block while maintaining a (dead) CFG edge to the old block. + NewSISucc->getTerminator()->eraseFromParent(); + BranchInst::Create(Abort, OldSISucc, + ConstantInt::getTrue(Context), NewSISucc); + // Release the PHI operands for this edge. + for (PHINode &PN : NewSISucc->phis()) + PN.setIncomingValueForBlock(Switch, UndefValue::get(PN.getType())); + // Tell the domtree about the new block. We don't fully update the + // domtree here -- instead we force it to do a full recomputation + // after the pass is complete -- but we do need to inform it of + // new blocks. + DT->addNewBlock(Abort, NewSISucc); + } + + SimplifyCode(Worklist, L); +} + +/// Now that we have simplified some instructions in the loop, walk over it and +/// constant prop, dce, and fold control flow where possible. Note that this is +/// effectively a very simple loop-structure-aware optimizer. During processing +/// of this loop, L could very well be deleted, so it must not be used. +/// +/// FIXME: When the loop optimizer is more mature, separate this out to a new +/// pass. +/// +void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) { + const DataLayout &DL = L->getHeader()->getModule()->getDataLayout(); + while (!Worklist.empty()) { + Instruction *I = Worklist.back(); + Worklist.pop_back(); + + // Simple DCE. + if (isInstructionTriviallyDead(I)) { + LLVM_DEBUG(dbgs() << "Remove dead instruction '" << *I << "\n"); + + // Add uses to the worklist, which may be dead now. + for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) + if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i))) + Worklist.push_back(Use); + LPM->deleteSimpleAnalysisValue(I, L); + RemoveFromWorklist(I, Worklist); + if (MSSAU) + MSSAU->removeMemoryAccess(I); + I->eraseFromParent(); + ++NumSimplify; + continue; + } + + // See if instruction simplification can hack this up. This is common for + // things like "select false, X, Y" after unswitching made the condition be + // 'false'. TODO: update the domtree properly so we can pass it here. + if (Value *V = SimplifyInstruction(I, DL)) + if (LI->replacementPreservesLCSSAForm(I, V)) { + ReplaceUsesOfWith(I, V, Worklist, L, LPM, MSSAU.get()); + continue; + } + + // Special case hacks that appear commonly in unswitched code. + if (BranchInst *BI = dyn_cast<BranchInst>(I)) { + if (BI->isUnconditional()) { + // If BI's parent is the only pred of the successor, fold the two blocks + // together. + BasicBlock *Pred = BI->getParent(); + (void)Pred; + BasicBlock *Succ = BI->getSuccessor(0); + BasicBlock *SinglePred = Succ->getSinglePredecessor(); + if (!SinglePred) continue; // Nothing to do. + assert(SinglePred == Pred && "CFG broken"); + + // Make the LPM and Worklist updates specific to LoopUnswitch. + LPM->deleteSimpleAnalysisValue(BI, L); + RemoveFromWorklist(BI, Worklist); + LPM->deleteSimpleAnalysisValue(Succ, L); + auto SuccIt = Succ->begin(); + while (PHINode *PN = dyn_cast<PHINode>(SuccIt++)) { + for (unsigned It = 0, E = PN->getNumOperands(); It != E; ++It) + if (Instruction *Use = dyn_cast<Instruction>(PN->getOperand(It))) + Worklist.push_back(Use); + for (User *U : PN->users()) + Worklist.push_back(cast<Instruction>(U)); + LPM->deleteSimpleAnalysisValue(PN, L); + RemoveFromWorklist(PN, Worklist); + ++NumSimplify; + } + // Merge the block and make the remaining analyses updates. + DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager); + MergeBlockIntoPredecessor(Succ, &DTU, LI, MSSAU.get()); + ++NumSimplify; + continue; + } + + continue; + } + } +} + +/// Simple simplifications we can do given the information that Cond is +/// definitely not equal to Val. +Value *LoopUnswitch::SimplifyInstructionWithNotEqual(Instruction *Inst, + Value *Invariant, + Constant *Val) { + // icmp eq cond, val -> false + ICmpInst *CI = dyn_cast<ICmpInst>(Inst); + if (CI && CI->isEquality()) { + Value *Op0 = CI->getOperand(0); + Value *Op1 = CI->getOperand(1); + if ((Op0 == Invariant && Op1 == Val) || (Op0 == Val && Op1 == Invariant)) { + LLVMContext &Ctx = Inst->getContext(); + if (CI->getPredicate() == CmpInst::ICMP_EQ) + return ConstantInt::getFalse(Ctx); + else + return ConstantInt::getTrue(Ctx); + } + } + + // FIXME: there may be other opportunities, e.g. comparison with floating + // point, or Invariant - Val != 0, etc. + return nullptr; +} |