403 lines · cpp
1//===- MergedLoadStoreMotion.cpp - merge and hoist/sink load/stores -------===//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//! \file10//! This pass performs merges of loads and stores on both sides of a11// diamond (hammock). It hoists the loads and sinks the stores.12//13// The algorithm iteratively hoists two loads to the same address out of a14// diamond (hammock) and merges them into a single load in the header. Similar15// it sinks and merges two stores to the tail block (footer). The algorithm16// iterates over the instructions of one side of the diamond and attempts to17// find a matching load/store on the other side. New tail/footer block may be18// insterted if the tail/footer block has more predecessors (not only the two19// predecessors that are forming the diamond). It hoists / sinks when it thinks20// it safe to do so. This optimization helps with eg. hiding load latencies,21// triggering if-conversion, and reducing static code size.22//23// NOTE: This code no longer performs load hoisting, it is subsumed by GVNHoist.24//25//===----------------------------------------------------------------------===//26//27//28// Example:29// Diamond shaped code before merge:30//31// header:32// br %cond, label %if.then, label %if.else33// + +34// + +35// + +36// if.then: if.else:37// %lt = load %addr_l %le = load %addr_l38// <use %lt> <use %le>39// <...> <...>40// store %st, %addr_s store %se, %addr_s41// br label %if.end br label %if.end42// + +43// + +44// + +45// if.end ("footer"):46// <...>47//48// Diamond shaped code after merge:49//50// header:51// %l = load %addr_l52// br %cond, label %if.then, label %if.else53// + +54// + +55// + +56// if.then: if.else:57// <use %l> <use %l>58// <...> <...>59// br label %if.end br label %if.end60// + +61// + +62// + +63// if.end ("footer"):64// %s.sink = phi [%st, if.then], [%se, if.else]65// <...>66// store %s.sink, %addr_s67// <...>68//69//70//===----------------------- TODO -----------------------------------------===//71//72// 1) Generalize to regions other than diamonds73// 2) Be more aggressive merging memory operations74// Note that both changes require register pressure control75//76//===----------------------------------------------------------------------===//77 78#include "llvm/Transforms/Scalar/MergedLoadStoreMotion.h"79#include "llvm/Analysis/AliasAnalysis.h"80#include "llvm/Analysis/GlobalsModRef.h"81#include "llvm/IR/IRBuilder.h"82#include "llvm/IR/Instructions.h"83#include "llvm/Support/Debug.h"84#include "llvm/Support/raw_ostream.h"85#include "llvm/Transforms/Scalar.h"86#include "llvm/Transforms/Utils/BasicBlockUtils.h"87#include "llvm/Transforms/Utils/Local.h"88 89using namespace llvm;90 91#define DEBUG_TYPE "mldst-motion"92 93namespace {94//===----------------------------------------------------------------------===//95// MergedLoadStoreMotion Pass96//===----------------------------------------------------------------------===//97class MergedLoadStoreMotion {98 AliasAnalysis *AA = nullptr;99 100 // The mergeLoad/Store algorithms could have Size0 * Size1 complexity,101 // where Size0 and Size1 are the #instructions on the two sides of102 // the diamond. The constant chosen here is arbitrary. Compiler Time103 // Control is enforced by the check Size0 * Size1 < MagicCompileTimeControl.104 const int MagicCompileTimeControl = 250;105 106 const bool SplitFooterBB;107public:108 MergedLoadStoreMotion(bool SplitFooterBB) : SplitFooterBB(SplitFooterBB) {}109 bool run(Function &F, AliasAnalysis &AA);110 111private:112 BasicBlock *getDiamondTail(BasicBlock *BB);113 bool isDiamondHead(BasicBlock *BB);114 // Routines for sinking stores115 StoreInst *canSinkFromBlock(BasicBlock *BB, StoreInst *SI);116 PHINode *getPHIOperand(BasicBlock *BB, StoreInst *S0, StoreInst *S1);117 bool isStoreSinkBarrierInRange(const Instruction &Start,118 const Instruction &End, MemoryLocation Loc);119 bool canSinkStoresAndGEPs(StoreInst *S0, StoreInst *S1) const;120 void sinkStoresAndGEPs(BasicBlock *BB, StoreInst *SinkCand,121 StoreInst *ElseInst);122 bool mergeStores(BasicBlock *BB);123};124} // end anonymous namespace125 126///127/// Return tail block of a diamond.128///129BasicBlock *MergedLoadStoreMotion::getDiamondTail(BasicBlock *BB) {130 assert(isDiamondHead(BB) && "Basic block is not head of a diamond");131 return BB->getTerminator()->getSuccessor(0)->getSingleSuccessor();132}133 134///135/// True when BB is the head of a diamond (hammock)136///137bool MergedLoadStoreMotion::isDiamondHead(BasicBlock *BB) {138 if (!BB)139 return false;140 auto *BI = dyn_cast<BranchInst>(BB->getTerminator());141 if (!BI || !BI->isConditional())142 return false;143 144 BasicBlock *Succ0 = BI->getSuccessor(0);145 BasicBlock *Succ1 = BI->getSuccessor(1);146 147 if (!Succ0->getSinglePredecessor())148 return false;149 if (!Succ1->getSinglePredecessor())150 return false;151 152 BasicBlock *Succ0Succ = Succ0->getSingleSuccessor();153 BasicBlock *Succ1Succ = Succ1->getSingleSuccessor();154 // Ignore triangles.155 if (!Succ0Succ || !Succ1Succ || Succ0Succ != Succ1Succ)156 return false;157 return true;158}159 160 161///162/// True when instruction is a sink barrier for a store163/// located in Loc164///165/// Whenever an instruction could possibly read or modify the166/// value being stored or protect against the store from167/// happening it is considered a sink barrier.168///169bool MergedLoadStoreMotion::isStoreSinkBarrierInRange(const Instruction &Start,170 const Instruction &End,171 MemoryLocation Loc) {172 for (const Instruction &Inst :173 make_range(Start.getIterator(), End.getIterator()))174 if (Inst.mayThrow())175 return true;176 return AA->canInstructionRangeModRef(Start, End, Loc, ModRefInfo::ModRef);177}178 179///180/// Check if \p BB contains a store to the same address as \p SI181///182/// \return The store in \p when it is safe to sink. Otherwise return Null.183///184StoreInst *MergedLoadStoreMotion::canSinkFromBlock(BasicBlock *BB1,185 StoreInst *Store0) {186 LLVM_DEBUG(dbgs() << "can Sink? : "; Store0->dump(); dbgs() << "\n");187 BasicBlock *BB0 = Store0->getParent();188 for (Instruction &Inst : reverse(*BB1)) {189 auto *Store1 = dyn_cast<StoreInst>(&Inst);190 if (!Store1)191 continue;192 193 MemoryLocation Loc0 = MemoryLocation::get(Store0);194 MemoryLocation Loc1 = MemoryLocation::get(Store1);195 196 if (AA->isMustAlias(Loc0, Loc1) &&197 !isStoreSinkBarrierInRange(*Store1->getNextNode(), BB1->back(), Loc1) &&198 !isStoreSinkBarrierInRange(*Store0->getNextNode(), BB0->back(), Loc0) &&199 Store0->hasSameSpecialState(Store1) &&200 CastInst::isBitOrNoopPointerCastable(201 Store0->getValueOperand()->getType(),202 Store1->getValueOperand()->getType(),203 Store0->getDataLayout()))204 return Store1;205 }206 return nullptr;207}208 209///210/// Create a PHI node in BB for the operands of S0 and S1211///212PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0,213 StoreInst *S1) {214 // Create a phi if the values mismatch.215 Value *Opd1 = S0->getValueOperand();216 Value *Opd2 = S1->getValueOperand();217 if (Opd1 == Opd2)218 return nullptr;219 220 auto *NewPN = PHINode::Create(Opd1->getType(), 2, Opd2->getName() + ".sink");221 NewPN->insertBefore(BB->begin());222 NewPN->applyMergedLocation(S0->getDebugLoc(), S1->getDebugLoc());223 NewPN->addIncoming(Opd1, S0->getParent());224 NewPN->addIncoming(Opd2, S1->getParent());225 return NewPN;226}227 228///229/// Check if 2 stores can be sunk, optionally together with corresponding GEPs.230///231bool MergedLoadStoreMotion::canSinkStoresAndGEPs(StoreInst *S0,232 StoreInst *S1) const {233 if (S0->getPointerOperand() == S1->getPointerOperand())234 return true;235 auto *GEP0 = dyn_cast<GetElementPtrInst>(S0->getPointerOperand());236 auto *GEP1 = dyn_cast<GetElementPtrInst>(S1->getPointerOperand());237 return GEP0 && GEP1 && GEP0->isIdenticalTo(GEP1) && GEP0->hasOneUse() &&238 (GEP0->getParent() == S0->getParent()) && GEP1->hasOneUse() &&239 (GEP1->getParent() == S1->getParent());240}241 242///243/// Merge two stores to same address and sink into \p BB244///245/// Optionally also sinks GEP instruction computing the store address246///247void MergedLoadStoreMotion::sinkStoresAndGEPs(BasicBlock *BB, StoreInst *S0,248 StoreInst *S1) {249 Value *Ptr0 = S0->getPointerOperand();250 Value *Ptr1 = S1->getPointerOperand();251 // Only one definition?252 LLVM_DEBUG(dbgs() << "Sink Instruction into BB \n"; BB->dump();253 dbgs() << "Instruction Left\n"; S0->dump(); dbgs() << "\n";254 dbgs() << "Instruction Right\n"; S1->dump(); dbgs() << "\n");255 // Hoist the instruction.256 BasicBlock::iterator InsertPt = BB->getFirstInsertionPt();257 // Intersect optional metadata.258 S0->andIRFlags(S1);259 260 combineMetadataForCSE(S0, S1, true);261 S0->applyMergedLocation(S0->getDebugLoc(), S1->getDebugLoc());262 S0->mergeDIAssignID(S1);263 264 // Insert bitcast for conflicting typed stores (or just use original value if265 // same type).266 IRBuilder<> Builder(S0);267 auto Cast = Builder.CreateBitOrPointerCast(S0->getValueOperand(),268 S1->getValueOperand()->getType());269 S0->setOperand(0, Cast);270 271 // Create the new store to be inserted at the join point.272 StoreInst *SNew = cast<StoreInst>(S0->clone());273 SNew->insertBefore(InsertPt);274 // New PHI operand? Use it.275 if (PHINode *NewPN = getPHIOperand(BB, S0, S1))276 SNew->setOperand(0, NewPN);277 S0->eraseFromParent();278 S1->eraseFromParent();279 280 if (Ptr0 != Ptr1) {281 auto *GEP0 = cast<GetElementPtrInst>(Ptr0);282 auto *GEP1 = cast<GetElementPtrInst>(Ptr1);283 Instruction *GEPNew = GEP0->clone();284 GEPNew->insertBefore(SNew->getIterator());285 GEPNew->applyMergedLocation(GEP0->getDebugLoc(), GEP1->getDebugLoc());286 SNew->setOperand(1, GEPNew);287 GEP0->replaceAllUsesWith(GEPNew);288 GEP0->eraseFromParent();289 GEP1->replaceAllUsesWith(GEPNew);290 GEP1->eraseFromParent();291 }292}293 294///295/// True when two stores are equivalent and can sink into the footer296///297/// Starting from a diamond head block, iterate over the instructions in one298/// successor block and try to match a store in the second successor.299///300bool MergedLoadStoreMotion::mergeStores(BasicBlock *HeadBB) {301 302 bool MergedStores = false;303 BasicBlock *TailBB = getDiamondTail(HeadBB);304 BasicBlock *SinkBB = TailBB;305 assert(SinkBB && "Footer of a diamond cannot be empty");306 307 succ_iterator SI = succ_begin(HeadBB);308 assert(SI != succ_end(HeadBB) && "Diamond head cannot have zero successors");309 BasicBlock *Pred0 = *SI;310 ++SI;311 assert(SI != succ_end(HeadBB) && "Diamond head cannot have single successor");312 BasicBlock *Pred1 = *SI;313 // tail block of a diamond/hammock?314 if (Pred0 == Pred1)315 return false; // No.316 // bail out early if we can not merge into the footer BB317 if (!SplitFooterBB && TailBB->hasNPredecessorsOrMore(3))318 return false;319 // #Instructions in Pred1 for Compile Time Control320 auto InstsNoDbg = Pred1->instructionsWithoutDebug();321 int Size1 = std::distance(InstsNoDbg.begin(), InstsNoDbg.end());322 int NStores = 0;323 324 for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend();325 RBI != RBE;) {326 327 Instruction *I = &*RBI;328 ++RBI;329 330 // Don't sink non-simple (atomic, volatile) stores.331 auto *S0 = dyn_cast<StoreInst>(I);332 if (!S0 || !S0->isSimple())333 continue;334 335 ++NStores;336 if (NStores * Size1 >= MagicCompileTimeControl)337 break;338 if (StoreInst *S1 = canSinkFromBlock(Pred1, S0)) {339 if (!canSinkStoresAndGEPs(S0, S1))340 // Don't attempt to sink below stores that had to stick around341 // But after removal of a store and some of its feeding342 // instruction search again from the beginning since the iterator343 // is likely stale at this point.344 break;345 346 if (SinkBB == TailBB && TailBB->hasNPredecessorsOrMore(3)) {347 // We have more than 2 predecessors. Insert a new block348 // postdominating 2 predecessors we're going to sink from.349 SinkBB = SplitBlockPredecessors(TailBB, {Pred0, Pred1}, ".sink.split");350 if (!SinkBB)351 break;352 }353 354 MergedStores = true;355 sinkStoresAndGEPs(SinkBB, S0, S1);356 RBI = Pred0->rbegin();357 RBE = Pred0->rend();358 LLVM_DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump());359 }360 }361 return MergedStores;362}363 364bool MergedLoadStoreMotion::run(Function &F, AliasAnalysis &AA) {365 this->AA = &AA;366 367 bool Changed = false;368 LLVM_DEBUG(dbgs() << "Instruction Merger\n");369 370 // Merge unconditional branches, allowing PRE to catch more371 // optimization opportunities.372 // This loop doesn't care about newly inserted/split blocks 373 // since they never will be diamond heads.374 for (BasicBlock &BB : make_early_inc_range(F))375 // Hoist equivalent loads and sink stores376 // outside diamonds when possible377 if (isDiamondHead(&BB))378 Changed |= mergeStores(&BB);379 return Changed;380}381 382PreservedAnalyses383MergedLoadStoreMotionPass::run(Function &F, FunctionAnalysisManager &AM) {384 MergedLoadStoreMotion Impl(Options.SplitFooterBB);385 auto &AA = AM.getResult<AAManager>(F);386 if (!Impl.run(F, AA))387 return PreservedAnalyses::all();388 389 PreservedAnalyses PA;390 if (!Options.SplitFooterBB)391 PA.preserveSet<CFGAnalyses>();392 return PA;393}394 395void MergedLoadStoreMotionPass::printPipeline(396 raw_ostream &OS, function_ref<StringRef(StringRef)> MapClassName2PassName) {397 static_cast<PassInfoMixin<MergedLoadStoreMotionPass> *>(this)->printPipeline(398 OS, MapClassName2PassName);399 OS << '<';400 OS << (Options.SplitFooterBB ? "" : "no-") << "split-footer-bb";401 OS << '>';402}403