554 lines · cpp
1//===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//2//3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.4// See https://llvm.org/LICENSE.txt for license information.5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception6//7//===----------------------------------------------------------------------===//8//9// This pass transforms loops by placing phi nodes at the end of the loops for10// all values that are live across the loop boundary. For example, it turns11// the left into the right code:12//13// for (...) for (...)14// if (c) if (c)15// X1 = ... X1 = ...16// else else17// X2 = ... X2 = ...18// X3 = phi(X1, X2) X3 = phi(X1, X2)19// ... = X3 + 4 X4 = phi(X3)20// ... = X4 + 421//22// This is still valid LLVM; the extra phi nodes are purely redundant, and will23// be trivially eliminated by InstCombine. The major benefit of this24// transformation is that it makes many other loop optimizations, such as25// LoopUnswitching, simpler.26//27//===----------------------------------------------------------------------===//28 29#include "llvm/Transforms/Utils/LCSSA.h"30#include "llvm/ADT/STLExtras.h"31#include "llvm/ADT/Statistic.h"32#include "llvm/Analysis/AliasAnalysis.h"33#include "llvm/Analysis/BasicAliasAnalysis.h"34#include "llvm/Analysis/BranchProbabilityInfo.h"35#include "llvm/Analysis/GlobalsModRef.h"36#include "llvm/Analysis/LoopInfo.h"37#include "llvm/Analysis/LoopPass.h"38#include "llvm/Analysis/MemorySSA.h"39#include "llvm/Analysis/ScalarEvolution.h"40#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"41#include "llvm/IR/DebugInfo.h"42#include "llvm/IR/Dominators.h"43#include "llvm/IR/Instructions.h"44#include "llvm/IR/PredIteratorCache.h"45#include "llvm/InitializePasses.h"46#include "llvm/Pass.h"47#include "llvm/Support/CommandLine.h"48#include "llvm/Transforms/Utils.h"49#include "llvm/Transforms/Utils/LoopUtils.h"50#include "llvm/Transforms/Utils/SSAUpdater.h"51using namespace llvm;52 53#define DEBUG_TYPE "lcssa"54 55STATISTIC(NumLCSSA, "Number of live out of a loop variables");56 57#ifdef EXPENSIVE_CHECKS58static bool VerifyLoopLCSSA = true;59#else60static bool VerifyLoopLCSSA = false;61#endif62static cl::opt<bool, true>63 VerifyLoopLCSSAFlag("verify-loop-lcssa", cl::location(VerifyLoopLCSSA),64 cl::Hidden,65 cl::desc("Verify loop lcssa form (time consuming)"));66 67/// Return true if the specified block is in the list.68static bool isExitBlock(BasicBlock *BB,69 const SmallVectorImpl<BasicBlock *> &ExitBlocks) {70 return is_contained(ExitBlocks, BB);71}72 73// Cache the Loop ExitBlocks computed during the analysis. We expect to get a74// lot of instructions within the same loops, computing the exit blocks is75// expensive, and we're not mutating the loop structure.76using LoopExitBlocksTy = SmallDenseMap<Loop *, SmallVector<BasicBlock *, 1>>;77 78/// For every instruction from the worklist, check to see if it has any uses79/// that are outside the current loop. If so, insert LCSSA PHI nodes and80/// rewrite the uses.81static bool82formLCSSAForInstructionsImpl(SmallVectorImpl<Instruction *> &Worklist,83 const DominatorTree &DT, const LoopInfo &LI,84 ScalarEvolution *SE,85 SmallVectorImpl<PHINode *> *PHIsToRemove,86 SmallVectorImpl<PHINode *> *InsertedPHIs,87 LoopExitBlocksTy &LoopExitBlocks) {88 SmallVector<Use *, 16> UsesToRewrite;89 SmallSetVector<PHINode *, 16> LocalPHIsToRemove;90 PredIteratorCache PredCache;91 bool Changed = false;92 93 while (!Worklist.empty()) {94 UsesToRewrite.clear();95 96 Instruction *I = Worklist.pop_back_val();97 assert(!I->getType()->isTokenTy() && "Tokens shouldn't be in the worklist");98 BasicBlock *InstBB = I->getParent();99 Loop *L = LI.getLoopFor(InstBB);100 assert(L && "Instruction belongs to a BB that's not part of a loop");101 auto [It, Inserted] = LoopExitBlocks.try_emplace(L);102 if (Inserted)103 L->getExitBlocks(It->second);104 const SmallVectorImpl<BasicBlock *> &ExitBlocks = It->second;105 106 if (ExitBlocks.empty())107 continue;108 109 for (Use &U : make_early_inc_range(I->uses())) {110 Instruction *User = cast<Instruction>(U.getUser());111 BasicBlock *UserBB = User->getParent();112 113 // Skip uses in unreachable blocks.114 if (!DT.isReachableFromEntry(UserBB)) {115 U.set(PoisonValue::get(I->getType()));116 continue;117 }118 119 // For practical purposes, we consider that the use in a PHI120 // occurs in the respective predecessor block. For more info,121 // see the `phi` doc in LangRef and the LCSSA doc.122 if (auto *PN = dyn_cast<PHINode>(User))123 UserBB = PN->getIncomingBlock(U);124 125 if (InstBB != UserBB && !L->contains(UserBB))126 UsesToRewrite.push_back(&U);127 }128 129 // If there are no uses outside the loop, exit with no change.130 if (UsesToRewrite.empty())131 continue;132 133 ++NumLCSSA; // We are applying the transformation134 135 // Invoke instructions are special in that their result value is not136 // available along their unwind edge. The code below tests to see whether137 // DomBB dominates the value, so adjust DomBB to the normal destination138 // block, which is effectively where the value is first usable.139 BasicBlock *DomBB = InstBB;140 if (auto *Inv = dyn_cast<InvokeInst>(I))141 DomBB = Inv->getNormalDest();142 143 const DomTreeNode *DomNode = DT.getNode(DomBB);144 145 SmallVector<PHINode *, 16> AddedPHIs;146 SmallVector<PHINode *, 8> PostProcessPHIs;147 148 SmallVector<PHINode *, 4> LocalInsertedPHIs;149 SSAUpdater SSAUpdate(&LocalInsertedPHIs);150 SSAUpdate.Initialize(I->getType(), I->getName());151 152 // Insert the LCSSA phi's into all of the exit blocks dominated by the153 // value, and add them to the Phi's map.154 bool HasSCEV = SE && SE->isSCEVable(I->getType()) &&155 SE->getExistingSCEV(I) != nullptr;156 for (BasicBlock *ExitBB : ExitBlocks) {157 if (!DT.dominates(DomNode, DT.getNode(ExitBB)))158 continue;159 160 // If we already inserted something for this BB, don't reprocess it.161 if (SSAUpdate.HasValueForBlock(ExitBB))162 continue;163 PHINode *PN = PHINode::Create(I->getType(), PredCache.size(ExitBB),164 I->getName() + ".lcssa");165 PN->insertBefore(ExitBB->begin());166 if (InsertedPHIs)167 InsertedPHIs->push_back(PN);168 // Get the debug location from the original instruction.169 PN->setDebugLoc(I->getDebugLoc());170 171 // Add inputs from inside the loop for this PHI. This is valid172 // because `I` dominates `ExitBB` (checked above). This implies173 // that every incoming block/edge is dominated by `I` as well,174 // i.e. we can add uses of `I` to those incoming edges/append to the incoming175 // blocks without violating the SSA dominance property.176 for (BasicBlock *Pred : PredCache.get(ExitBB)) {177 PN->addIncoming(I, Pred);178 179 // If the exit block has a predecessor not within the loop, arrange for180 // the incoming value use corresponding to that predecessor to be181 // rewritten in terms of a different LCSSA PHI.182 if (!L->contains(Pred))183 UsesToRewrite.push_back(184 &PN->getOperandUse(PN->getOperandNumForIncomingValue(185 PN->getNumIncomingValues() - 1)));186 }187 188 AddedPHIs.push_back(PN);189 190 // Remember that this phi makes the value alive in this block.191 SSAUpdate.AddAvailableValue(ExitBB, PN);192 193 // LoopSimplify might fail to simplify some loops (e.g. when indirect194 // branches are involved). In such situations, it might happen that an195 // exit for Loop L1 is the header of a disjoint Loop L2. Thus, when we196 // create PHIs in such an exit block, we are also inserting PHIs into L2's197 // header. This could break LCSSA form for L2 because these inserted PHIs198 // can also have uses outside of L2. Remember all PHIs in such situation199 // as to revisit than later on. FIXME: Remove this if indirectbr support200 // into LoopSimplify gets improved.201 if (auto *OtherLoop = LI.getLoopFor(ExitBB))202 if (!L->contains(OtherLoop))203 PostProcessPHIs.push_back(PN);204 205 // If we have a cached SCEV for the original instruction, make sure the206 // new LCSSA phi node is also cached. This makes sures that BECounts207 // based on it will be invalidated when the LCSSA phi node is invalidated,208 // which some passes rely on.209 if (HasSCEV)210 SE->getSCEV(PN);211 }212 213 // Rewrite all uses outside the loop in terms of the new PHIs we just214 // inserted.215 for (Use *UseToRewrite : UsesToRewrite) {216 Instruction *User = cast<Instruction>(UseToRewrite->getUser());217 BasicBlock *UserBB = User->getParent();218 219 // For practical purposes, we consider that the use in a PHI220 // occurs in the respective predecessor block. For more info,221 // see the `phi` doc in LangRef and the LCSSA doc.222 if (auto *PN = dyn_cast<PHINode>(User))223 UserBB = PN->getIncomingBlock(*UseToRewrite);224 225 // If this use is in an exit block, rewrite to use the newly inserted PHI.226 // This is required for correctness because SSAUpdate doesn't handle uses227 // in the same block. It assumes the PHI we inserted is at the end of the228 // block.229 if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {230 UseToRewrite->set(&UserBB->front());231 continue;232 }233 234 // If we added a single PHI, it must dominate all uses and we can directly235 // rename it.236 if (AddedPHIs.size() == 1) {237 UseToRewrite->set(AddedPHIs[0]);238 continue;239 }240 241 // Otherwise, do full PHI insertion.242 SSAUpdate.RewriteUse(*UseToRewrite);243 }244 245 SmallVector<DbgVariableRecord *, 4> DbgVariableRecords;246 llvm::findDbgValues(I, DbgVariableRecords);247 248 // Update pre-existing debug value uses that reside outside the loop.249 for (DbgVariableRecord *DVR : DbgVariableRecords) {250 BasicBlock *UserBB = DVR->getMarker()->getParent();251 if (InstBB == UserBB || L->contains(UserBB))252 continue;253 // We currently only handle debug values residing in blocks that were254 // traversed while rewriting the uses. If we inserted just a single PHI,255 // we will handle all relevant debug values.256 Value *V = AddedPHIs.size() == 1 ? AddedPHIs[0]257 : SSAUpdate.FindValueForBlock(UserBB);258 if (V)259 DVR->replaceVariableLocationOp(I, V);260 }261 262 // SSAUpdater might have inserted phi-nodes inside other loops. We'll need263 // to post-process them to keep LCSSA form.264 for (PHINode *InsertedPN : LocalInsertedPHIs) {265 if (auto *OtherLoop = LI.getLoopFor(InsertedPN->getParent()))266 if (!L->contains(OtherLoop))267 PostProcessPHIs.push_back(InsertedPN);268 if (InsertedPHIs)269 InsertedPHIs->push_back(InsertedPN);270 }271 272 // Post process PHI instructions that were inserted into another disjoint273 // loop and update their exits properly.274 for (auto *PostProcessPN : PostProcessPHIs)275 if (!PostProcessPN->use_empty())276 Worklist.push_back(PostProcessPN);277 278 // Keep track of PHI nodes that we want to remove because they did not have279 // any uses rewritten.280 for (PHINode *PN : AddedPHIs)281 if (PN->use_empty())282 LocalPHIsToRemove.insert(PN);283 284 Changed = true;285 }286 287 // Remove PHI nodes that did not have any uses rewritten or add them to288 // PHIsToRemove, so the caller can remove them after some additional cleanup.289 // We need to redo the use_empty() check here, because even if the PHI node290 // wasn't used when added to LocalPHIsToRemove, later added PHI nodes can be291 // using it. This cleanup is not guaranteed to handle trees/cycles of PHI292 // nodes that only are used by each other. Such situations has only been293 // noticed when the input IR contains unreachable code, and leaving some extra294 // redundant PHI nodes in such situations is considered a minor problem.295 if (PHIsToRemove) {296 PHIsToRemove->append(LocalPHIsToRemove.begin(), LocalPHIsToRemove.end());297 } else {298 for (PHINode *PN : LocalPHIsToRemove)299 if (PN->use_empty())300 PN->eraseFromParent();301 }302 return Changed;303}304 305/// For every instruction from the worklist, check to see if it has any uses306/// that are outside the current loop. If so, insert LCSSA PHI nodes and307/// rewrite the uses.308bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,309 const DominatorTree &DT, const LoopInfo &LI,310 ScalarEvolution *SE,311 SmallVectorImpl<PHINode *> *PHIsToRemove,312 SmallVectorImpl<PHINode *> *InsertedPHIs) {313 LoopExitBlocksTy LoopExitBlocks;314 315 return formLCSSAForInstructionsImpl(Worklist, DT, LI, SE, PHIsToRemove,316 InsertedPHIs, LoopExitBlocks);317}318 319// Compute the set of BasicBlocks in the loop `L` dominating at least one exit.320static void computeBlocksDominatingExits(321 Loop &L, const DominatorTree &DT, ArrayRef<BasicBlock *> ExitBlocks,322 SmallSetVector<BasicBlock *, 8> &BlocksDominatingExits) {323 // We start from the exit blocks, as every block trivially dominates itself324 // (not strictly).325 SmallVector<BasicBlock *, 8> BBWorklist(ExitBlocks);326 327 while (!BBWorklist.empty()) {328 BasicBlock *BB = BBWorklist.pop_back_val();329 330 // Check if this is a loop header. If this is the case, we're done.331 if (L.getHeader() == BB)332 continue;333 334 // Otherwise, add its immediate predecessor in the dominator tree to the335 // worklist, unless we visited it already.336 BasicBlock *IDomBB = DT.getNode(BB)->getIDom()->getBlock();337 338 // Exit blocks can have an immediate dominator not belonging to the339 // loop. For an exit block to be immediately dominated by another block340 // outside the loop, it implies not all paths from that dominator, to the341 // exit block, go through the loop.342 // Example:343 //344 // |---- A345 // | |346 // | B<--347 // | | |348 // |---> C --349 // |350 // D351 //352 // C is the exit block of the loop and it's immediately dominated by A,353 // which doesn't belong to the loop.354 if (!L.contains(IDomBB))355 continue;356 357 if (BlocksDominatingExits.insert(IDomBB))358 BBWorklist.push_back(IDomBB);359 }360}361 362static bool formLCSSAImpl(Loop &L, const DominatorTree &DT, const LoopInfo *LI,363 ScalarEvolution *SE,364 LoopExitBlocksTy &LoopExitBlocks) {365 bool Changed = false;366 367#ifdef EXPENSIVE_CHECKS368 // Verify all sub-loops are in LCSSA form already.369 for (Loop *SubLoop: L) {370 (void)SubLoop; // Silence unused variable warning.371 assert(SubLoop->isRecursivelyLCSSAForm(DT, *LI) && "Subloop not in LCSSA!");372 }373#endif374 375 auto [It, Inserted] = LoopExitBlocks.try_emplace(&L);376 if (Inserted)377 L.getExitBlocks(It->second);378 const SmallVectorImpl<BasicBlock *> &ExitBlocks = It->second;379 if (ExitBlocks.empty())380 return false;381 382 SmallSetVector<BasicBlock *, 8> BlocksDominatingExits;383 384 // We want to avoid use-scanning leveraging dominance informations.385 // If a block doesn't dominate any of the loop exits, the none of the values386 // defined in the loop can be used outside.387 // We compute the set of blocks fullfilling the conditions in advance388 // walking the dominator tree upwards until we hit a loop header.389 computeBlocksDominatingExits(L, DT, ExitBlocks, BlocksDominatingExits);390 391 SmallVector<Instruction *, 8> Worklist;392 393 // Look at all the instructions in the loop, checking to see if they have uses394 // outside the loop. If so, put them into the worklist to rewrite those uses.395 for (BasicBlock *BB : BlocksDominatingExits) {396 // Skip blocks that are part of any sub-loops, they must be in LCSSA397 // already.398 if (LI->getLoopFor(BB) != &L)399 continue;400 for (Instruction &I : *BB) {401 // Reject two common cases fast: instructions with no uses (like stores)402 // and instructions with one use that is in the same block as this.403 if (I.use_empty() ||404 (I.hasOneUse() && I.user_back()->getParent() == BB &&405 !isa<PHINode>(I.user_back())))406 continue;407 408 // Tokens cannot be used in PHI nodes, so we skip over them.409 // We can run into tokens which are live out of a loop with catchswitch410 // instructions in Windows EH if the catchswitch has one catchpad which411 // is inside the loop and another which is not.412 if (I.getType()->isTokenTy())413 continue;414 415 Worklist.push_back(&I);416 }417 }418 419 Changed = formLCSSAForInstructionsImpl(Worklist, DT, *LI, SE, nullptr,420 nullptr, LoopExitBlocks);421 422 assert(L.isLCSSAForm(DT));423 424 return Changed;425}426 427bool llvm::formLCSSA(Loop &L, const DominatorTree &DT, const LoopInfo *LI,428 ScalarEvolution *SE) {429 LoopExitBlocksTy LoopExitBlocks;430 431 return formLCSSAImpl(L, DT, LI, SE, LoopExitBlocks);432}433 434/// Process a loop nest depth first.435static bool formLCSSARecursivelyImpl(Loop &L, const DominatorTree &DT,436 const LoopInfo *LI, ScalarEvolution *SE,437 LoopExitBlocksTy &LoopExitBlocks) {438 bool Changed = false;439 440 // Recurse depth-first through inner loops.441 for (Loop *SubLoop : L.getSubLoops())442 Changed |= formLCSSARecursivelyImpl(*SubLoop, DT, LI, SE, LoopExitBlocks);443 444 Changed |= formLCSSAImpl(L, DT, LI, SE, LoopExitBlocks);445 return Changed;446}447 448/// Process a loop nest depth first.449bool llvm::formLCSSARecursively(Loop &L, const DominatorTree &DT,450 const LoopInfo *LI, ScalarEvolution *SE) {451 LoopExitBlocksTy LoopExitBlocks;452 453 return formLCSSARecursivelyImpl(L, DT, LI, SE, LoopExitBlocks);454}455 456/// Process all loops in the function, inner-most out.457static bool formLCSSAOnAllLoops(const LoopInfo *LI, const DominatorTree &DT,458 ScalarEvolution *SE) {459 bool Changed = false;460 for (const auto &L : *LI)461 Changed |= formLCSSARecursively(*L, DT, LI, SE);462 return Changed;463}464 465namespace {466struct LCSSAWrapperPass : public FunctionPass {467 static char ID; // Pass identification, replacement for typeid468 LCSSAWrapperPass() : FunctionPass(ID) {469 initializeLCSSAWrapperPassPass(*PassRegistry::getPassRegistry());470 }471 472 // Cached analysis information for the current function.473 DominatorTree *DT;474 LoopInfo *LI;475 ScalarEvolution *SE;476 477 bool runOnFunction(Function &F) override;478 void verifyAnalysis() const override {479 // This check is very expensive. On the loop intensive compiles it may cause480 // up to 10x slowdown. Currently it's disabled by default. LPPassManager481 // always does limited form of the LCSSA verification. Similar reasoning482 // was used for the LoopInfo verifier.483 if (VerifyLoopLCSSA) {484 assert(all_of(*LI,485 [&](Loop *L) {486 return L->isRecursivelyLCSSAForm(*DT, *LI);487 }) &&488 "LCSSA form is broken!");489 }490 };491 492 /// This transformation requires natural loop information & requires that493 /// loop preheaders be inserted into the CFG. It maintains both of these,494 /// as well as the CFG. It also requires dominator information.495 void getAnalysisUsage(AnalysisUsage &AU) const override {496 AU.setPreservesCFG();497 498 AU.addRequired<DominatorTreeWrapperPass>();499 AU.addRequired<LoopInfoWrapperPass>();500 AU.addPreservedID(LoopSimplifyID);501 AU.addPreserved<AAResultsWrapperPass>();502 AU.addPreserved<BasicAAWrapperPass>();503 AU.addPreserved<GlobalsAAWrapperPass>();504 AU.addPreserved<ScalarEvolutionWrapperPass>();505 AU.addPreserved<SCEVAAWrapperPass>();506 AU.addPreserved<BranchProbabilityInfoWrapperPass>();507 AU.addPreserved<MemorySSAWrapperPass>();508 509 // This is needed to perform LCSSA verification inside LPPassManager510 AU.addRequired<LCSSAVerificationPass>();511 AU.addPreserved<LCSSAVerificationPass>();512 }513};514}515 516char LCSSAWrapperPass::ID = 0;517INITIALIZE_PASS_BEGIN(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",518 false, false)519INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)520INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)521INITIALIZE_PASS_DEPENDENCY(LCSSAVerificationPass)522INITIALIZE_PASS_END(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",523 false, false)524 525Pass *llvm::createLCSSAPass() { return new LCSSAWrapperPass(); }526char &llvm::LCSSAID = LCSSAWrapperPass::ID;527 528/// Transform \p F into loop-closed SSA form.529bool LCSSAWrapperPass::runOnFunction(Function &F) {530 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();531 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();532 auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();533 SE = SEWP ? &SEWP->getSE() : nullptr;534 535 return formLCSSAOnAllLoops(LI, *DT, SE);536}537 538PreservedAnalyses LCSSAPass::run(Function &F, FunctionAnalysisManager &AM) {539 auto &LI = AM.getResult<LoopAnalysis>(F);540 auto &DT = AM.getResult<DominatorTreeAnalysis>(F);541 auto *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F);542 if (!formLCSSAOnAllLoops(&LI, DT, SE))543 return PreservedAnalyses::all();544 545 PreservedAnalyses PA;546 PA.preserveSet<CFGAnalyses>();547 PA.preserve<ScalarEvolutionAnalysis>();548 // BPI maps terminators to probabilities, since we don't modify the CFG, no549 // updates are needed to preserve it.550 PA.preserve<BranchProbabilityAnalysis>();551 PA.preserve<MemorySSAAnalysis>();552 return PA;553}554