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Diffstat (limited to 'llvm/lib/Transforms/Utils/LCSSA.cpp')
-rw-r--r-- | llvm/lib/Transforms/Utils/LCSSA.cpp | 497 |
1 files changed, 497 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/Utils/LCSSA.cpp b/llvm/lib/Transforms/Utils/LCSSA.cpp new file mode 100644 index 000000000000..29e7c5260f46 --- /dev/null +++ b/llvm/lib/Transforms/Utils/LCSSA.cpp @@ -0,0 +1,497 @@ +//===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===// +// +// 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 by placing phi nodes at the end of the loops for +// all values that are live across the loop boundary. For example, it turns +// the left into the right code: +// +// for (...) for (...) +// if (c) if (c) +// X1 = ... X1 = ... +// else else +// X2 = ... X2 = ... +// X3 = phi(X1, X2) X3 = phi(X1, X2) +// ... = X3 + 4 X4 = phi(X3) +// ... = X4 + 4 +// +// This is still valid LLVM; the extra phi nodes are purely redundant, and will +// be trivially eliminated by InstCombine. The major benefit of this +// transformation is that it makes many other loop optimizations, such as +// LoopUnswitching, simpler. +// +//===----------------------------------------------------------------------===// + +#include "llvm/Transforms/Utils/LCSSA.h" +#include "llvm/ADT/STLExtras.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/Analysis/AliasAnalysis.h" +#include "llvm/Analysis/BasicAliasAnalysis.h" +#include "llvm/Analysis/BranchProbabilityInfo.h" +#include "llvm/Analysis/GlobalsModRef.h" +#include "llvm/Analysis/LoopPass.h" +#include "llvm/Analysis/MemorySSA.h" +#include "llvm/Analysis/ScalarEvolution.h" +#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h" +#include "llvm/IR/Constants.h" +#include "llvm/IR/Dominators.h" +#include "llvm/IR/Function.h" +#include "llvm/IR/Instructions.h" +#include "llvm/IR/IntrinsicInst.h" +#include "llvm/IR/PredIteratorCache.h" +#include "llvm/Pass.h" +#include "llvm/Transforms/Utils.h" +#include "llvm/Transforms/Utils/Local.h" +#include "llvm/Transforms/Utils/LoopUtils.h" +#include "llvm/Transforms/Utils/SSAUpdater.h" +using namespace llvm; + +#define DEBUG_TYPE "lcssa" + +STATISTIC(NumLCSSA, "Number of live out of a loop variables"); + +#ifdef EXPENSIVE_CHECKS +static bool VerifyLoopLCSSA = true; +#else +static bool VerifyLoopLCSSA = false; +#endif +static cl::opt<bool, true> + VerifyLoopLCSSAFlag("verify-loop-lcssa", cl::location(VerifyLoopLCSSA), + cl::Hidden, + cl::desc("Verify loop lcssa form (time consuming)")); + +/// Return true if the specified block is in the list. +static bool isExitBlock(BasicBlock *BB, + const SmallVectorImpl<BasicBlock *> &ExitBlocks) { + return is_contained(ExitBlocks, BB); +} + +/// For every instruction from the worklist, check to see if it has any uses +/// that are outside the current loop. If so, insert LCSSA PHI nodes and +/// rewrite the uses. +bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist, + DominatorTree &DT, LoopInfo &LI) { + SmallVector<Use *, 16> UsesToRewrite; + SmallSetVector<PHINode *, 16> PHIsToRemove; + PredIteratorCache PredCache; + bool Changed = false; + + // Cache the Loop ExitBlocks across this loop. We expect to get a lot of + // instructions within the same loops, computing the exit blocks is + // expensive, and we're not mutating the loop structure. + SmallDenseMap<Loop*, SmallVector<BasicBlock *,1>> LoopExitBlocks; + + while (!Worklist.empty()) { + UsesToRewrite.clear(); + + Instruction *I = Worklist.pop_back_val(); + assert(!I->getType()->isTokenTy() && "Tokens shouldn't be in the worklist"); + BasicBlock *InstBB = I->getParent(); + Loop *L = LI.getLoopFor(InstBB); + assert(L && "Instruction belongs to a BB that's not part of a loop"); + if (!LoopExitBlocks.count(L)) + L->getExitBlocks(LoopExitBlocks[L]); + assert(LoopExitBlocks.count(L)); + const SmallVectorImpl<BasicBlock *> &ExitBlocks = LoopExitBlocks[L]; + + if (ExitBlocks.empty()) + continue; + + for (Use &U : I->uses()) { + Instruction *User = cast<Instruction>(U.getUser()); + BasicBlock *UserBB = User->getParent(); + if (auto *PN = dyn_cast<PHINode>(User)) + UserBB = PN->getIncomingBlock(U); + + if (InstBB != UserBB && !L->contains(UserBB)) + UsesToRewrite.push_back(&U); + } + + // If there are no uses outside the loop, exit with no change. + if (UsesToRewrite.empty()) + continue; + + ++NumLCSSA; // We are applying the transformation + + // Invoke instructions are special in that their result value is not + // available along their unwind edge. The code below tests to see whether + // DomBB dominates the value, so adjust DomBB to the normal destination + // block, which is effectively where the value is first usable. + BasicBlock *DomBB = InstBB; + if (auto *Inv = dyn_cast<InvokeInst>(I)) + DomBB = Inv->getNormalDest(); + + DomTreeNode *DomNode = DT.getNode(DomBB); + + SmallVector<PHINode *, 16> AddedPHIs; + SmallVector<PHINode *, 8> PostProcessPHIs; + + SmallVector<PHINode *, 4> InsertedPHIs; + SSAUpdater SSAUpdate(&InsertedPHIs); + SSAUpdate.Initialize(I->getType(), I->getName()); + + // Insert the LCSSA phi's into all of the exit blocks dominated by the + // value, and add them to the Phi's map. + for (BasicBlock *ExitBB : ExitBlocks) { + if (!DT.dominates(DomNode, DT.getNode(ExitBB))) + continue; + + // If we already inserted something for this BB, don't reprocess it. + if (SSAUpdate.HasValueForBlock(ExitBB)) + continue; + + PHINode *PN = PHINode::Create(I->getType(), PredCache.size(ExitBB), + I->getName() + ".lcssa", &ExitBB->front()); + // Get the debug location from the original instruction. + PN->setDebugLoc(I->getDebugLoc()); + // Add inputs from inside the loop for this PHI. + for (BasicBlock *Pred : PredCache.get(ExitBB)) { + PN->addIncoming(I, Pred); + + // If the exit block has a predecessor not within the loop, arrange for + // the incoming value use corresponding to that predecessor to be + // rewritten in terms of a different LCSSA PHI. + if (!L->contains(Pred)) + UsesToRewrite.push_back( + &PN->getOperandUse(PN->getOperandNumForIncomingValue( + PN->getNumIncomingValues() - 1))); + } + + AddedPHIs.push_back(PN); + + // Remember that this phi makes the value alive in this block. + SSAUpdate.AddAvailableValue(ExitBB, PN); + + // LoopSimplify might fail to simplify some loops (e.g. when indirect + // branches are involved). In such situations, it might happen that an + // exit for Loop L1 is the header of a disjoint Loop L2. Thus, when we + // create PHIs in such an exit block, we are also inserting PHIs into L2's + // header. This could break LCSSA form for L2 because these inserted PHIs + // can also have uses outside of L2. Remember all PHIs in such situation + // as to revisit than later on. FIXME: Remove this if indirectbr support + // into LoopSimplify gets improved. + if (auto *OtherLoop = LI.getLoopFor(ExitBB)) + if (!L->contains(OtherLoop)) + PostProcessPHIs.push_back(PN); + } + + // Rewrite all uses outside the loop in terms of the new PHIs we just + // inserted. + for (Use *UseToRewrite : UsesToRewrite) { + // If this use is in an exit block, rewrite to use the newly inserted PHI. + // This is required for correctness because SSAUpdate doesn't handle uses + // in the same block. It assumes the PHI we inserted is at the end of the + // block. + Instruction *User = cast<Instruction>(UseToRewrite->getUser()); + BasicBlock *UserBB = User->getParent(); + if (auto *PN = dyn_cast<PHINode>(User)) + UserBB = PN->getIncomingBlock(*UseToRewrite); + + if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) { + // Tell the VHs that the uses changed. This updates SCEV's caches. + if (UseToRewrite->get()->hasValueHandle()) + ValueHandleBase::ValueIsRAUWd(*UseToRewrite, &UserBB->front()); + UseToRewrite->set(&UserBB->front()); + continue; + } + + // If we added a single PHI, it must dominate all uses and we can directly + // rename it. + if (AddedPHIs.size() == 1) { + // Tell the VHs that the uses changed. This updates SCEV's caches. + // We might call ValueIsRAUWd multiple times for the same value. + if (UseToRewrite->get()->hasValueHandle()) + ValueHandleBase::ValueIsRAUWd(*UseToRewrite, AddedPHIs[0]); + UseToRewrite->set(AddedPHIs[0]); + continue; + } + + // Otherwise, do full PHI insertion. + SSAUpdate.RewriteUse(*UseToRewrite); + } + + SmallVector<DbgValueInst *, 4> DbgValues; + llvm::findDbgValues(DbgValues, I); + + // Update pre-existing debug value uses that reside outside the loop. + auto &Ctx = I->getContext(); + for (auto DVI : DbgValues) { + BasicBlock *UserBB = DVI->getParent(); + if (InstBB == UserBB || L->contains(UserBB)) + continue; + // We currently only handle debug values residing in blocks that were + // traversed while rewriting the uses. If we inserted just a single PHI, + // we will handle all relevant debug values. + Value *V = AddedPHIs.size() == 1 ? AddedPHIs[0] + : SSAUpdate.FindValueForBlock(UserBB); + if (V) + DVI->setOperand(0, MetadataAsValue::get(Ctx, ValueAsMetadata::get(V))); + } + + // SSAUpdater might have inserted phi-nodes inside other loops. We'll need + // to post-process them to keep LCSSA form. + for (PHINode *InsertedPN : InsertedPHIs) { + if (auto *OtherLoop = LI.getLoopFor(InsertedPN->getParent())) + if (!L->contains(OtherLoop)) + PostProcessPHIs.push_back(InsertedPN); + } + + // Post process PHI instructions that were inserted into another disjoint + // loop and update their exits properly. + for (auto *PostProcessPN : PostProcessPHIs) + if (!PostProcessPN->use_empty()) + Worklist.push_back(PostProcessPN); + + // Keep track of PHI nodes that we want to remove because they did not have + // any uses rewritten. If the new PHI is used, store it so that we can + // try to propagate dbg.value intrinsics to it. + SmallVector<PHINode *, 2> NeedDbgValues; + for (PHINode *PN : AddedPHIs) + if (PN->use_empty()) + PHIsToRemove.insert(PN); + else + NeedDbgValues.push_back(PN); + insertDebugValuesForPHIs(InstBB, NeedDbgValues); + Changed = true; + } + // Remove PHI nodes that did not have any uses rewritten. We need to redo the + // use_empty() check here, because even if the PHI node wasn't used when added + // to PHIsToRemove, later added PHI nodes can be using it. This cleanup is + // not guaranteed to handle trees/cycles of PHI nodes that only are used by + // each other. Such situations has only been noticed when the input IR + // contains unreachable code, and leaving some extra redundant PHI nodes in + // such situations is considered a minor problem. + for (PHINode *PN : PHIsToRemove) + if (PN->use_empty()) + PN->eraseFromParent(); + return Changed; +} + +// Compute the set of BasicBlocks in the loop `L` dominating at least one exit. +static void computeBlocksDominatingExits( + Loop &L, DominatorTree &DT, SmallVector<BasicBlock *, 8> &ExitBlocks, + SmallSetVector<BasicBlock *, 8> &BlocksDominatingExits) { + SmallVector<BasicBlock *, 8> BBWorklist; + + // We start from the exit blocks, as every block trivially dominates itself + // (not strictly). + for (BasicBlock *BB : ExitBlocks) + BBWorklist.push_back(BB); + + while (!BBWorklist.empty()) { + BasicBlock *BB = BBWorklist.pop_back_val(); + + // Check if this is a loop header. If this is the case, we're done. + if (L.getHeader() == BB) + continue; + + // Otherwise, add its immediate predecessor in the dominator tree to the + // worklist, unless we visited it already. + BasicBlock *IDomBB = DT.getNode(BB)->getIDom()->getBlock(); + + // Exit blocks can have an immediate dominator not beloinging to the + // loop. For an exit block to be immediately dominated by another block + // outside the loop, it implies not all paths from that dominator, to the + // exit block, go through the loop. + // Example: + // + // |---- A + // | | + // | B<-- + // | | | + // |---> C -- + // | + // D + // + // C is the exit block of the loop and it's immediately dominated by A, + // which doesn't belong to the loop. + if (!L.contains(IDomBB)) + continue; + + if (BlocksDominatingExits.insert(IDomBB)) + BBWorklist.push_back(IDomBB); + } +} + +bool llvm::formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI, + ScalarEvolution *SE) { + bool Changed = false; + +#ifdef EXPENSIVE_CHECKS + // Verify all sub-loops are in LCSSA form already. + for (Loop *SubLoop: L) + assert(SubLoop->isRecursivelyLCSSAForm(DT, *LI) && "Subloop not in LCSSA!"); +#endif + + SmallVector<BasicBlock *, 8> ExitBlocks; + L.getExitBlocks(ExitBlocks); + if (ExitBlocks.empty()) + return false; + + SmallSetVector<BasicBlock *, 8> BlocksDominatingExits; + + // We want to avoid use-scanning leveraging dominance informations. + // If a block doesn't dominate any of the loop exits, the none of the values + // defined in the loop can be used outside. + // We compute the set of blocks fullfilling the conditions in advance + // walking the dominator tree upwards until we hit a loop header. + computeBlocksDominatingExits(L, DT, ExitBlocks, BlocksDominatingExits); + + SmallVector<Instruction *, 8> Worklist; + + // Look at all the instructions in the loop, checking to see if they have uses + // outside the loop. If so, put them into the worklist to rewrite those uses. + for (BasicBlock *BB : BlocksDominatingExits) { + // Skip blocks that are part of any sub-loops, they must be in LCSSA + // already. + if (LI->getLoopFor(BB) != &L) + continue; + for (Instruction &I : *BB) { + // Reject two common cases fast: instructions with no uses (like stores) + // and instructions with one use that is in the same block as this. + if (I.use_empty() || + (I.hasOneUse() && I.user_back()->getParent() == BB && + !isa<PHINode>(I.user_back()))) + continue; + + // Tokens cannot be used in PHI nodes, so we skip over them. + // We can run into tokens which are live out of a loop with catchswitch + // instructions in Windows EH if the catchswitch has one catchpad which + // is inside the loop and another which is not. + if (I.getType()->isTokenTy()) + continue; + + Worklist.push_back(&I); + } + } + Changed = formLCSSAForInstructions(Worklist, DT, *LI); + + // If we modified the code, remove any caches about the loop from SCEV to + // avoid dangling entries. + // FIXME: This is a big hammer, can we clear the cache more selectively? + if (SE && Changed) + SE->forgetLoop(&L); + + assert(L.isLCSSAForm(DT)); + + return Changed; +} + +/// Process a loop nest depth first. +bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT, LoopInfo *LI, + ScalarEvolution *SE) { + bool Changed = false; + + // Recurse depth-first through inner loops. + for (Loop *SubLoop : L.getSubLoops()) + Changed |= formLCSSARecursively(*SubLoop, DT, LI, SE); + + Changed |= formLCSSA(L, DT, LI, SE); + return Changed; +} + +/// Process all loops in the function, inner-most out. +static bool formLCSSAOnAllLoops(LoopInfo *LI, DominatorTree &DT, + ScalarEvolution *SE) { + bool Changed = false; + for (auto &L : *LI) + Changed |= formLCSSARecursively(*L, DT, LI, SE); + return Changed; +} + +namespace { +struct LCSSAWrapperPass : public FunctionPass { + static char ID; // Pass identification, replacement for typeid + LCSSAWrapperPass() : FunctionPass(ID) { + initializeLCSSAWrapperPassPass(*PassRegistry::getPassRegistry()); + } + + // Cached analysis information for the current function. + DominatorTree *DT; + LoopInfo *LI; + ScalarEvolution *SE; + + bool runOnFunction(Function &F) override; + void verifyAnalysis() const override { + // This check is very expensive. On the loop intensive compiles it may cause + // up to 10x slowdown. Currently it's disabled by default. LPPassManager + // always does limited form of the LCSSA verification. Similar reasoning + // was used for the LoopInfo verifier. + if (VerifyLoopLCSSA) { + assert(all_of(*LI, + [&](Loop *L) { + return L->isRecursivelyLCSSAForm(*DT, *LI); + }) && + "LCSSA form is broken!"); + } + }; + + /// This transformation requires natural loop information & requires that + /// loop preheaders be inserted into the CFG. It maintains both of these, + /// as well as the CFG. It also requires dominator information. + void getAnalysisUsage(AnalysisUsage &AU) const override { + AU.setPreservesCFG(); + + AU.addRequired<DominatorTreeWrapperPass>(); + AU.addRequired<LoopInfoWrapperPass>(); + AU.addPreservedID(LoopSimplifyID); + AU.addPreserved<AAResultsWrapperPass>(); + AU.addPreserved<BasicAAWrapperPass>(); + AU.addPreserved<GlobalsAAWrapperPass>(); + AU.addPreserved<ScalarEvolutionWrapperPass>(); + AU.addPreserved<SCEVAAWrapperPass>(); + AU.addPreserved<BranchProbabilityInfoWrapperPass>(); + AU.addPreserved<MemorySSAWrapperPass>(); + + // This is needed to perform LCSSA verification inside LPPassManager + AU.addRequired<LCSSAVerificationPass>(); + AU.addPreserved<LCSSAVerificationPass>(); + } +}; +} + +char LCSSAWrapperPass::ID = 0; +INITIALIZE_PASS_BEGIN(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass", + false, false) +INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) +INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) +INITIALIZE_PASS_DEPENDENCY(LCSSAVerificationPass) +INITIALIZE_PASS_END(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass", + false, false) + +Pass *llvm::createLCSSAPass() { return new LCSSAWrapperPass(); } +char &llvm::LCSSAID = LCSSAWrapperPass::ID; + +/// Transform \p F into loop-closed SSA form. +bool LCSSAWrapperPass::runOnFunction(Function &F) { + LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); + DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); + auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>(); + SE = SEWP ? &SEWP->getSE() : nullptr; + + return formLCSSAOnAllLoops(LI, *DT, SE); +} + +PreservedAnalyses LCSSAPass::run(Function &F, FunctionAnalysisManager &AM) { + auto &LI = AM.getResult<LoopAnalysis>(F); + auto &DT = AM.getResult<DominatorTreeAnalysis>(F); + auto *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F); + if (!formLCSSAOnAllLoops(&LI, DT, SE)) + return PreservedAnalyses::all(); + + PreservedAnalyses PA; + PA.preserveSet<CFGAnalyses>(); + PA.preserve<BasicAA>(); + PA.preserve<GlobalsAA>(); + PA.preserve<SCEVAA>(); + PA.preserve<ScalarEvolutionAnalysis>(); + // BPI maps terminators to probabilities, since we don't modify the CFG, no + // updates are needed to preserve it. + PA.preserve<BranchProbabilityAnalysis>(); + PA.preserve<MemorySSAAnalysis>(); + return PA; +} |