Mercurial > hg > Members > tobaru > cbc > CbC_llvm
view lib/Transforms/Scalar/LoopUnswitch.cpp @ 83:60c9769439b8
LLVM 3.7
author | Tatsuki IHA <e125716@ie.u-ryukyu.ac.jp> |
---|---|
date | Wed, 18 Feb 2015 14:55:36 +0900 |
parents | 54457678186b |
children | afa8332a0e37 |
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line source
//===-- 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/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/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/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); namespace { class LUAnalysisCache { typedef DenseMap<const SwitchInst*, SmallPtrSet<const Value *, 8> > UnswitchedValsMap; typedef UnswitchedValsMap::iterator UnswitchedValsIt; struct LoopProperties { unsigned CanBeUnswitchedCount; 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; // Max size of code we can produce on remained iterations. 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); // 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; // LoopProcessWorklist - Used to check if second loop needs processing // after RewriteLoopBodyWithConditionConstant rewrites first loop. std::vector<Loop*> LoopProcessWorklist; LUAnalysisCache BranchesInfo; 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.addPreserved<DominatorTreeWrapperPass>(); AU.addPreserved<ScalarEvolution>(); AU.addRequired<TargetTransformInfoWrapperPass>(); } 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 UnswitchIfProfitable(Value *LoopCond, Constant *Val); void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val, BasicBlock *ExitBlock); void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L); void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, Constant *Val, bool isEqual); void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val, BasicBlock *TrueDest, BasicBlock *FalseDest, Instruction *InsertPt); void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L); bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = nullptr, BasicBlock **LoopExit = nullptr); }; } // 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 = std::min(Metrics.NumInsts, Metrics.NumBlocks * 5); Props.CanBeUnswitchedCount = MaxSize / (Props.SizeEstimation); MaxSize -= Props.SizeEstimation * Props.CanBeUnswitchedCount; if (Metrics.notDuplicatable) { DEBUG(dbgs() << "NOT unswitching loop %" << L->getHeader()->getName() << ", contents cannot be " << "duplicated!\n"); return false; } } if (!Props.CanBeUnswitchedCount) { DEBUG(dbgs() << "NOT unswitching loop %" << L->getHeader()->getName() << ", cost too high: " << L->getBlocks().size() << "\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.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); } // 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; 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); } /// FindLIVLoopCondition - 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; DominatorTreeWrapperPass *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>(); DT = DTWP ? &DTWP->getDomTree() : nullptr; currentLoop = L; Function *F = currentLoop->getHeader()->getParent(); bool Changed = false; do { assert(currentLoop->isLCSSAForm(*DT)); redoLoop = false; Changed |= processCurrentLoop(); } while(redoLoop); if (Changed) { // FIXME: Reconstruct dom info, because it is not preserved properly. if (DT) DT->recalculate(*F); } return Changed; } /// processCurrentLoop - 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(); // Probably we reach the quota of branches for this loop. If so // stop unswitching. if (!BranchesInfo.countLoop( currentLoop, getAnalysis<TargetTransformInfoWrapperPass>().getTTI( *currentLoop->getHeader()->getParent()), AC)) 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))) { ++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; } /// isTrivialLoopExitBlock - 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; } /// isTrivialLoopExitBlock - 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; } /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is /// trivial: that is, that the condition controls whether or not the loop does /// anything at all. If this is a trivial condition, unswitching produces no /// code duplications (equivalently, it produces a simpler loop and a new empty /// loop, which gets deleted). /// /// If this is a trivial condition, return true, otherwise return false. When /// returning true, this sets Cond and Val to the condition that controls the /// trivial condition: when Cond dynamically equals Val, the loop is known to /// exit. Finally, this sets LoopExit to the BB that the loop exits to when /// Cond == Val. /// bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val, BasicBlock **LoopExit) { BasicBlock *Header = currentLoop->getHeader(); TerminatorInst *HeaderTerm = Header->getTerminator(); LLVMContext &Context = Header->getContext(); BasicBlock *LoopExitBB = nullptr; if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) { // If the header block doesn't end with a conditional branch on Cond, we // can't handle it. if (!BI->isConditional() || BI->getCondition() != Cond) 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)))) { if (Val) *Val = ConstantInt::getTrue(Context); } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop, BI->getSuccessor(1)))) { if (Val) *Val = ConstantInt::getFalse(Context); } } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) { // If this isn't a switch on Cond, we can't handle it. if (SI->getCondition() != Cond) 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; if (Val) *Val = 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. if (LoopExit) *LoopExit = LoopExitBB; // We already know that nothing uses any scalar values defined inside of this // loop. As such, we just have to check to see 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. We already checked the // tail, check the header now. for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I) if (I->mayHaveSideEffects()) return false; return true; } /// UnswitchIfProfitable - 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) { Function *F = loopHeader->getParent(); Constant *CondVal = nullptr; BasicBlock *ExitBlock = nullptr; if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) { // If the condition is trivial, always unswitch. There is no code growth // for this case. UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock); return true; } // Check to see if it would be profitable to unswitch current loop. // Do not do non-trivial unswitch while optimizing for size. if (OptimizeForSize || F->hasFnAttribute(Attribute::OptimizeForSize)) return false; UnswitchNontrivialCondition(LoopCond, Val, currentLoop); return true; } /// CloneLoop - 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 = new Loop(); LPM->insertLoop(New, 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; } /// EmitPreheaderBranchOnCondition - 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) { // 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; 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); // Insert the new branch. BranchInst *BI = BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt); // 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); } /// UnswitchTrivialCondition - 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) { 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->begin(), 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()); 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; } /// SplitExitEdges - 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", /*AliasAnalysis*/ nullptr, DT, LI, /*PreserveLCSSA*/ true); } } /// UnswitchNontrivialCondition - 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) { 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 (ScalarEvolution *SE = getAnalysisIfAvailable<ScalarEvolution>()) SE->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, F->getBasicBlockList(), NewBlocks[0], 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); 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); } /// RemoveFromWorklist - 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()); } /// ReplaceUsesOfWith - 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; } // RewriteLoopBodyWithConditionConstant - 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. if (DT) DT->addNewBlock(Abort, NewSISucc); } SimplifyCode(Worklist, L); } /// SimplifyCode - Okay, 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) { 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)) 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, 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; } } }