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Diffstat (limited to 'contrib/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp')
| -rw-r--r-- | contrib/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp | 1342 |
1 files changed, 1342 insertions, 0 deletions
diff --git a/contrib/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp b/contrib/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp new file mode 100644 index 0000000..95d7f8a --- /dev/null +++ b/contrib/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp @@ -0,0 +1,1342 @@ +//===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This pass transforms loops that contain branches on loop-invariant conditions +// to have 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/Transforms/Scalar.h" +#include "llvm/ADT/STLExtras.h" +#include "llvm/ADT/SmallPtrSet.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/Analysis/GlobalsModRef.h" +#include "llvm/Analysis/AssumptionCache.h" +#include "llvm/Analysis/CodeMetrics.h" +#include "llvm/Analysis/InstructionSimplify.h" +#include "llvm/Analysis/LoopInfo.h" +#include "llvm/Analysis/LoopPass.h" +#include "llvm/Analysis/ScalarEvolution.h" +#include "llvm/Analysis/TargetTransformInfo.h" +#include "llvm/Analysis/BlockFrequencyInfoImpl.h" +#include "llvm/Analysis/BlockFrequencyInfo.h" +#include "llvm/Analysis/BranchProbabilityInfo.h" +#include "llvm/Support/BranchProbability.h" +#include "llvm/IR/Constants.h" +#include "llvm/IR/DerivedTypes.h" +#include "llvm/IR/Dominators.h" +#include "llvm/IR/Function.h" +#include "llvm/IR/Instructions.h" +#include "llvm/IR/Module.h" +#include "llvm/IR/MDBuilder.h" +#include "llvm/Support/CommandLine.h" +#include "llvm/Support/Debug.h" +#include "llvm/Support/raw_ostream.h" +#include "llvm/Transforms/Utils/BasicBlockUtils.h" +#include "llvm/Transforms/Utils/Cloning.h" +#include "llvm/Transforms/Utils/Local.h" +#include <algorithm> +#include <map> +#include <set> +using namespace llvm; + +#define DEBUG_TYPE "loop-unswitch" + +STATISTIC(NumBranches, "Number of branches unswitched"); +STATISTIC(NumSwitches, "Number of switches 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); + +static cl::opt<bool> +LoopUnswitchWithBlockFrequency("loop-unswitch-with-block-frequency", + cl::init(false), cl::Hidden, + cl::desc("Enable the use of the block frequency analysis to access PGO " + "heuristics to minimize code growth in cold regions.")); + +static cl::opt<unsigned> +ColdnessThreshold("loop-unswitch-coldness-threshold", cl::init(1), cl::Hidden, + cl::desc("Coldness threshold in percentage. The loop header frequency " + "(relative to the entry frequency) is compared with this " + "threshold to determine if non-trivial unswitching should be " + "enabled.")); + +namespace { + + class LUAnalysisCache { + + typedef DenseMap<const SwitchInst*, SmallPtrSet<const Value *, 8> > + UnswitchedValsMap; + + typedef UnswitchedValsMap::iterator UnswitchedValsIt; + + 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. + typedef std::map<const Loop*, LoopProperties> LoopPropsMap; + typedef LoopPropsMap::iterator LoopPropsMapIt; + + LoopPropsMap LoopsProperties; + UnswitchedValsMap *CurLoopInstructions; + LoopProperties *CurrentLoopProperties; + + // 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() + : CurLoopInstructions(nullptr), CurrentLoopProperties(nullptr), + 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 EnabledPGO; + + // BFI and ColdEntryFreq are only used when PGO and + // LoopUnswitchWithBlockFrequency are enabled. + BlockFrequencyInfo BFI; + BlockFrequency ColdEntryFreq; + + bool OptimizeForSize; + bool redoLoop; + + Loop *currentLoop; + DominatorTree *DT; + BasicBlock *loopHeader; + BasicBlock *loopPreheader; + + // 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; + + public: + static char ID; // Pass ID, replacement for typeid + explicit LoopUnswitch(bool Os = false) : + LoopPass(ID), OptimizeForSize(Os), redoLoop(false), + currentLoop(nullptr), DT(nullptr), loopHeader(nullptr), + loopPreheader(nullptr) { + initializeLoopUnswitchPass(*PassRegistry::getPassRegistry()); + } + + bool runOnLoop(Loop *L, LPPassManager &LPM) override; + bool processCurrentLoop(); + + /// 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.addRequiredID(LoopSimplifyID); + AU.addPreservedID(LoopSimplifyID); + AU.addRequired<LoopInfoWrapperPass>(); + AU.addPreserved<LoopInfoWrapperPass>(); + AU.addRequiredID(LCSSAID); + AU.addPreservedID(LCSSAID); + AU.addRequired<DominatorTreeWrapperPass>(); + AU.addPreserved<DominatorTreeWrapperPass>(); + AU.addPreserved<ScalarEvolutionWrapperPass>(); + AU.addRequired<TargetTransformInfoWrapperPass>(); + AU.addPreserved<GlobalsAAWrapperPass>(); + } + + 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, + TerminatorInst *TI = nullptr); + void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, + BasicBlock *ExitBlock, TerminatorInst *TI); + void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L, + TerminatorInst *TI); + + void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, + Constant *Val, bool isEqual); + + void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, + BasicBlock *TrueDest, + BasicBlock *FalseDest, + Instruction *InsertPt, + TerminatorInst *TI); + + void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L); + }; +} + +// 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) { + 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(TargetTransformInfoWrapperPass) +INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) +INITIALIZE_PASS_DEPENDENCY(LoopSimplify) +INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) +INITIALIZE_PASS_DEPENDENCY(LCSSA) +INITIALIZE_PASS_END(LoopUnswitch, "loop-unswitch", "Unswitch loops", + false, false) + +Pass *llvm::createLoopUnswitchPass(bool Os) { + return new LoopUnswitch(Os); +} + +/// 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. +static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) { + + // 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)) + return Cond; + + if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond)) + if (BO->getOpcode() == Instruction::And || + BO->getOpcode() == Instruction::Or) { + // 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)) + return LHS; + if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed)) + return RHS; + } + + return nullptr; +} + +bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) { + if (skipOptnoneFunction(L)) + return false; + + AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache( + *L->getHeader()->getParent()); + LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); + LPM = &LPM_Ref; + DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); + currentLoop = L; + Function *F = currentLoop->getHeader()->getParent(); + + EnabledPGO = F->getEntryCount().hasValue(); + + if (LoopUnswitchWithBlockFrequency && EnabledPGO) { + BranchProbabilityInfo BPI(*F, *LI); + BFI.calculate(*L->getHeader()->getParent(), BPI, *LI); + + // Use BranchProbability to compute a minimum frequency based on + // function entry baseline frequency. Loops with headers below this + // frequency are considered as cold. + const BranchProbability ColdProb(ColdnessThreshold, 100); + ColdEntryFreq = BlockFrequency(BFI.getEntryFreq()) * ColdProb; + } + + bool Changed = false; + do { + assert(currentLoop->isLCSSAForm(*DT)); + redoLoop = false; + Changed |= processCurrentLoop(); + } while(redoLoop); + + // FIXME: Reconstruct dom info, because it is not preserved properly. + if (Changed) + DT->recalculate(*F); + return Changed; +} + +/// 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 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. + for (const auto BB : currentLoop->blocks()) { + for (auto &I : *BB) { + auto CS = CallSite(&I); + if (!CS) continue; + if (CS.hasFnAttr(Attribute::Convergent)) + return false; + } + } + + // Do not do non-trivial unswitch while optimizing for size. + // FIXME: Use Function::optForSize(). + if (OptimizeForSize || + loopHeader->getParent()->hasFnAttribute(Attribute::OptimizeForSize)) + return false; + + if (LoopUnswitchWithBlockFrequency && EnabledPGO) { + // Compute the weighted frequency of the hottest block in the + // loop (loopHeader in this case since inner loops should be + // processed before outer loop). If it is less than ColdFrequency, + // we should not unswitch. + BlockFrequency LoopEntryFreq = BFI.getBlockFreq(loopHeader); + if (LoopEntryFreq < ColdEntryFreq) + return false; + } + + // 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) { + TerminatorInst *TI = (*I)->getTerminator(); + if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { + // 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); + if (LoopCond && + UnswitchIfProfitable(LoopCond, ConstantInt::getTrue(Context), TI)) { + ++NumBranches; + return true; + } + } + } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { + Value *LoopCond = FindLIVLoopCondition(SI->getCondition(), + currentLoop, Changed); + 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; + + // 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 (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end(); + i != e; ++i) { + Constant *UnswitchValCandidate = i.getCaseValue(); + if (!BranchesInfo.isUnswitched(SI, UnswitchValCandidate)) { + UnswitchVal = UnswitchValCandidate; + break; + } + } + + if (!UnswitchVal) + continue; + + if (UnswitchIfProfitable(LoopCond, UnswitchVal)) { + ++NumSwitches; + 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); + 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 (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) + 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, + TerminatorInst *TI) { + // Check to see if it would be profitable to unswitch current loop. + if (!BranchesInfo.CostAllowsUnswitching()) { + DEBUG(dbgs() << "NOT unswitching loop %" + << currentLoop->getHeader()->getName() + << " at non-trivial condition '" << *Val + << "' == " << *LoopCond << "\n" + << ". Cost too high.\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 = LPM->addLoop(PL); + + // 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::iterator I = L->begin(), E = L->end(); I != E; ++I) + CloneLoop(*I, &New, VM, LI, LPM); + + return &New; +} + +static void copyMetadata(Instruction *DstInst, const Instruction *SrcInst, + bool Swapped) { + if (!SrcInst || !SrcInst->hasMetadata()) + return; + + SmallVector<std::pair<unsigned, MDNode *>, 4> MDs; + SrcInst->getAllMetadata(MDs); + for (auto &MD : MDs) { + switch (MD.first) { + default: + break; + case LLVMContext::MD_prof: + if (Swapped && MD.second->getNumOperands() == 3 && + isa<MDString>(MD.second->getOperand(0))) { + MDString *MDName = cast<MDString>(MD.second->getOperand(0)); + if (MDName->getString() == "branch_weights") { + auto *ValT = cast_or_null<ConstantAsMetadata>( + MD.second->getOperand(1))->getValue(); + auto *ValF = cast_or_null<ConstantAsMetadata>( + MD.second->getOperand(2))->getValue(); + assert(ValT && ValF && "Invalid Operands of branch_weights"); + auto NewMD = + MDBuilder(DstInst->getParent()->getContext()) + .createBranchWeights(cast<ConstantInt>(ValF)->getZExtValue(), + cast<ConstantInt>(ValT)->getZExtValue()); + MD.second = NewMD; + } + } + // fallthrough. + case LLVMContext::MD_make_implicit: + case LLVMContext::MD_dbg: + DstInst->setMetadata(MD.first, MD.second); + } + } +} + +/// Emit a conditional branch on two values if LIC == Val, branch to TrueDst, +/// otherwise branch to FalseDest. Insert the code immediately before InsertPt. +void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, + BasicBlock *TrueDest, + BasicBlock *FalseDest, + Instruction *InsertPt, + TerminatorInst *TI) { + // 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(InsertPt, 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; + } + + // Insert the new branch. + BranchInst *BI = BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt); + copyMetadata(BI, TI, Swapped); + + // If either edge is critical, split it. This helps preserve LoopSimplify + // form for enclosing loops. + auto Options = CriticalEdgeSplittingOptions(DT, LI).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, + TerminatorInst *TI) { + DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %" + << loopHeader->getName() << " [" << L->getBlocks().size() + << " blocks] in Function " + << L->getHeader()->getParent()->getName() << " on cond: " << *Val + << " == " << *Cond << "\n"); + + // 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); + + // 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); + + // Okay, now we have a position to branch from and a position to branch to, + // insert the new conditional branch. + EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH, + loopPreheader->getTerminator(), TI); + LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L); + loopPreheader->getTerminator()->eraseFromParent(); + + // 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(); + TerminatorInst *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. + SmallSet<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; + + // FIXME: add check for constant foldable switch instructions. + 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 { + 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); + + // 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. + + 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); + + // 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 (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end(); + i != e; ++i) { + BasicBlock *LoopExitCandidate; + if ((LoopExitCandidate = isTrivialLoopExitBlock(currentLoop, + i.getCaseSuccessor()))) { + // Okay, we found a trivial case, remember the value that is trivial. + ConstantInt *CaseVal = i.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); + ++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, + /*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, TerminatorInst *TI) { + Function *F = loopHeader->getParent(); + DEBUG(dbgs() << "loop-unswitch: Unswitching loop %" + << loopHeader->getName() << " [" << L->getBlocks().size() + << " blocks] in Function " << F->getName() + << " when '" << *Val << "' == " << *LIC << "\n"); + + if (auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>()) + SEWP->getSE().forgetLoop(L); + + LoopBlocks.clear(); + NewBlocks.clear(); + + // First step, split the preheader and exit blocks, and add these blocks to + // the LoopBlocks list. + BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, DT, LI); + 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()); + + // FIXME: We could register any cloned assumptions instead of clearing the + // whole function's cache. + AC->clear(); + + // 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 (BasicBlock::iterator I = ExitSucc->begin(); + PHINode *PN = dyn_cast<PHINode>(I); ++I) { + 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 (BasicBlock::iterator I = NewBlocks[i]->begin(), + E = NewBlocks[i]->end(); I != E; ++I) + RemapInstruction(&*I, VMap, + RF_NoModuleLevelChanges | RF_IgnoreMissingEntries); + + // 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!"); + + // 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); + OldBR->eraseFromParent(); + + LoopProcessWorklist.push_back(NewLoop); + redoLoop = true; + + // Keep a WeakVH 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. + WeakVH 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); +} + +/// 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) { + DEBUG(dbgs() << "Replace with '" << *V << "': " << *I); + + // 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); + 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 (std::vector<Instruction*>::iterator UI = Worklist.begin(), + UE = Worklist.end(); UI != UE; ++UI) + (*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; + + Worklist.push_back(UI); + + // TODO: We could do other simplifications, for example, turning + // 'icmp eq LIC, Val' -> false. + + // 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; + + SwitchInst::CaseIt 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(); + + BranchesInfo.setUnswitched(SI, Val); + + 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); + // 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 (BasicBlock::iterator II = NewSISucc->begin(); + PHINode *PN = dyn_cast<PHINode>(II); ++II) + PN->setIncomingValue(PN->getBasicBlockIndex(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)) { + DEBUG(dbgs() << "Remove dead instruction '" << *I); + + // 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); + 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); + 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(); + BasicBlock *Succ = BI->getSuccessor(0); + BasicBlock *SinglePred = Succ->getSinglePredecessor(); + if (!SinglePred) continue; // Nothing to do. + assert(SinglePred == Pred && "CFG broken"); + + DEBUG(dbgs() << "Merging blocks: " << Pred->getName() << " <- " + << Succ->getName() << "\n"); + + // Resolve any single entry PHI nodes in Succ. + while (PHINode *PN = dyn_cast<PHINode>(Succ->begin())) + ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM); + + // If Succ has any successors with PHI nodes, update them to have + // entries coming from Pred instead of Succ. + Succ->replaceAllUsesWith(Pred); + + // Move all of the successor contents from Succ to Pred. + Pred->getInstList().splice(BI->getIterator(), Succ->getInstList(), + Succ->begin(), Succ->end()); + LPM->deleteSimpleAnalysisValue(BI, L); + BI->eraseFromParent(); + RemoveFromWorklist(BI, Worklist); + + // Remove Succ from the loop tree. + LI->removeBlock(Succ); + LPM->deleteSimpleAnalysisValue(Succ, L); + Succ->eraseFromParent(); + ++NumSimplify; + continue; + } + + continue; + } + } +} |
