1 //===- MergedLoadStoreMotion.cpp - merge and hoist/sink load/stores -------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
11 //! \brief This pass performs merges of loads and stores on both sides of a
12 // diamond (hammock). It hoists the loads and sinks the stores.
14 // The algorithm iteratively hoists two loads to the same address out of a
15 // diamond (hammock) and merges them into a single load in the header. Similar
16 // it sinks and merges two stores to the tail block (footer). The algorithm
17 // iterates over the instructions of one side of the diamond and attempts to
18 // find a matching load/store on the other side. It hoists / sinks when it
19 // thinks it safe to do so. This optimization helps with eg. hiding load
20 // latencies, triggering if-conversion, and reducing static code size.
22 //===----------------------------------------------------------------------===//
26 // Diamond shaped code before merge:
29 // br %cond, label %if.then, label %if.else
34 // %lt = load %addr_l %le = load %addr_l
35 // <use %lt> <use %le>
37 // store %st, %addr_s store %se, %addr_s
38 // br label %if.end br label %if.end
45 // Diamond shaped code after merge:
49 // br %cond, label %if.then, label %if.else
56 // br label %if.end br label %if.end
61 // %s.sink = phi [%st, if.then], [%se, if.else]
63 // store %s.sink, %addr_s
67 //===----------------------- TODO -----------------------------------------===//
69 // 1) Generalize to regions other than diamonds
70 // 2) Be more aggressive merging memory operations
71 // Note that both changes require register pressure control
73 //===----------------------------------------------------------------------===//
75 #include "llvm/Transforms/Scalar.h"
76 #include "llvm/ADT/SetVector.h"
77 #include "llvm/ADT/SmallPtrSet.h"
78 #include "llvm/ADT/Statistic.h"
79 #include "llvm/Analysis/AliasAnalysis.h"
80 #include "llvm/Analysis/CFG.h"
81 #include "llvm/Analysis/Loads.h"
82 #include "llvm/Analysis/MemoryBuiltins.h"
83 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
84 #include "llvm/IR/Metadata.h"
85 #include "llvm/IR/PatternMatch.h"
86 #include "llvm/Support/Allocator.h"
87 #include "llvm/Support/CommandLine.h"
88 #include "llvm/Support/Debug.h"
89 #include "llvm/Target/TargetLibraryInfo.h"
90 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
91 #include "llvm/Transforms/Utils/SSAUpdater.h"
95 #define DEBUG_TYPE "mldst-motion"
97 //===----------------------------------------------------------------------===//
98 // MergedLoadStoreMotion Pass
99 //===----------------------------------------------------------------------===//
102 class MergedLoadStoreMotion : public FunctionPass {
104 MemoryDependenceAnalysis *MD;
107 static char ID; // Pass identification, replacement for typeid
108 explicit MergedLoadStoreMotion(void)
109 : FunctionPass(ID), MD(nullptr), MagicCompileTimeControl(250) {
110 initializeMergedLoadStoreMotionPass(*PassRegistry::getPassRegistry());
113 bool runOnFunction(Function &F) override;
116 // This transformation requires dominator postdominator info
117 void getAnalysisUsage(AnalysisUsage &AU) const override {
118 AU.addRequired<TargetLibraryInfo>();
119 AU.addRequired<MemoryDependenceAnalysis>();
120 AU.addRequired<AliasAnalysis>();
121 AU.addPreserved<AliasAnalysis>();
127 /// \brief Remove instruction from parent and update memory dependence
130 void removeInstruction(Instruction *Inst);
131 BasicBlock *getDiamondTail(BasicBlock *BB);
132 bool isDiamondHead(BasicBlock *BB);
133 // Routines for hoisting loads
134 bool isLoadHoistBarrierInRange(const Instruction& Start,
135 const Instruction& End,
137 LoadInst *canHoistFromBlock(BasicBlock *BB, LoadInst *LI);
138 void hoistInstruction(BasicBlock *BB, Instruction *HoistCand,
139 Instruction *ElseInst);
140 bool isSafeToHoist(Instruction *I) const;
141 bool hoistLoad(BasicBlock *BB, LoadInst *HoistCand, LoadInst *ElseInst);
142 bool mergeLoads(BasicBlock *BB);
143 // Routines for sinking stores
144 StoreInst *canSinkFromBlock(BasicBlock *BB, StoreInst *SI);
145 PHINode *getPHIOperand(BasicBlock *BB, StoreInst *S0, StoreInst *S1);
146 bool isStoreSinkBarrier(Instruction *Inst);
147 bool sinkStore(BasicBlock *BB, StoreInst *SinkCand, StoreInst *ElseInst);
148 bool mergeStores(BasicBlock *BB);
149 // The mergeLoad/Store algorithms could have Size0 * Size1 complexity,
150 // where Size0 and Size1 are the #instructions on the two sides of
151 // the diamond. The constant chosen here is arbitrary. Compiler Time
152 // Control is enforced by the check Size0 * Size1 < MagicCompileTimeControl.
153 const int MagicCompileTimeControl;
156 char MergedLoadStoreMotion::ID = 0;
160 /// \brief createMergedLoadStoreMotionPass - The public interface to this file.
162 FunctionPass *llvm::createMergedLoadStoreMotionPass() {
163 return new MergedLoadStoreMotion();
166 INITIALIZE_PASS_BEGIN(MergedLoadStoreMotion, "mldst-motion",
167 "MergedLoadStoreMotion", false, false)
168 INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
169 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
170 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
171 INITIALIZE_PASS_END(MergedLoadStoreMotion, "mldst-motion",
172 "MergedLoadStoreMotion", false, false)
175 /// \brief Remove instruction from parent and update memory dependence analysis.
177 void MergedLoadStoreMotion::removeInstruction(Instruction *Inst) {
178 // Notify the memory dependence analysis.
180 MD->removeInstruction(Inst);
181 if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
182 MD->invalidateCachedPointerInfo(LI->getPointerOperand());
183 if (Inst->getType()->getScalarType()->isPointerTy()) {
184 MD->invalidateCachedPointerInfo(Inst);
187 Inst->eraseFromParent();
191 /// \brief Return tail block of a diamond.
193 BasicBlock *MergedLoadStoreMotion::getDiamondTail(BasicBlock *BB) {
194 assert(isDiamondHead(BB) && "Basic block is not head of a diamond");
195 BranchInst *BI = (BranchInst *)(BB->getTerminator());
196 BasicBlock *Succ0 = BI->getSuccessor(0);
197 BasicBlock *Tail = Succ0->getTerminator()->getSuccessor(0);
202 /// \brief True when BB is the head of a diamond (hammock)
204 bool MergedLoadStoreMotion::isDiamondHead(BasicBlock *BB) {
207 if (!isa<BranchInst>(BB->getTerminator()))
209 if (BB->getTerminator()->getNumSuccessors() != 2)
212 BranchInst *BI = (BranchInst *)(BB->getTerminator());
213 BasicBlock *Succ0 = BI->getSuccessor(0);
214 BasicBlock *Succ1 = BI->getSuccessor(1);
216 if (!Succ0->getSinglePredecessor() ||
217 Succ0->getTerminator()->getNumSuccessors() != 1)
219 if (!Succ1->getSinglePredecessor() ||
220 Succ1->getTerminator()->getNumSuccessors() != 1)
223 BasicBlock *Tail = Succ0->getTerminator()->getSuccessor(0);
225 if (Succ1->getTerminator()->getSuccessor(0) != Tail)
231 /// \brief True when instruction is a hoist barrier for a load
233 /// Whenever an instruction could possibly modify the value
234 /// being loaded or protect against the load from happening
235 /// it is considered a hoist barrier.
238 bool MergedLoadStoreMotion::isLoadHoistBarrierInRange(const Instruction& Start,
239 const Instruction& End,
241 AliasAnalysis::Location Loc = AA->getLocation(LI);
242 return AA->canInstructionRangeModify(Start, End, Loc);
246 /// \brief Decide if a load can be hoisted
248 /// When there is a load in \p BB to the same address as \p LI
249 /// and it can be hoisted from \p BB, return that load.
250 /// Otherwise return Null.
252 LoadInst *MergedLoadStoreMotion::canHoistFromBlock(BasicBlock *BB1,
255 for (BasicBlock::iterator BBI = BB1->begin(), BBE = BB1->end(); BBI != BBE;
257 Instruction *Inst = BBI;
259 // Only merge and hoist loads when their result in used only in BB
260 if (!isa<LoadInst>(Inst) || Inst->isUsedOutsideOfBlock(BB1))
263 LoadInst *Load1 = dyn_cast<LoadInst>(Inst);
264 BasicBlock *BB0 = Load0->getParent();
266 AliasAnalysis::Location Loc0 = AA->getLocation(Load0);
267 AliasAnalysis::Location Loc1 = AA->getLocation(Load1);
268 if (AA->isMustAlias(Loc0, Loc1) && Load0->isSameOperationAs(Load1) &&
269 !isLoadHoistBarrierInRange(BB1->front(), *Load1, Load1) &&
270 !isLoadHoistBarrierInRange(BB0->front(), *Load0, Load0)) {
278 /// \brief Merge two equivalent instructions \p HoistCand and \p ElseInst into
281 /// BB is the head of a diamond
283 void MergedLoadStoreMotion::hoistInstruction(BasicBlock *BB,
284 Instruction *HoistCand,
285 Instruction *ElseInst) {
286 DEBUG(dbgs() << " Hoist Instruction into BB \n"; BB->dump();
287 dbgs() << "Instruction Left\n"; HoistCand->dump(); dbgs() << "\n";
288 dbgs() << "Instruction Right\n"; ElseInst->dump(); dbgs() << "\n");
289 // Hoist the instruction.
290 assert(HoistCand->getParent() != BB);
292 // Intersect optional metadata.
293 HoistCand->intersectOptionalDataWith(ElseInst);
294 HoistCand->dropUnknownMetadata();
296 // Prepend point for instruction insert
297 Instruction *HoistPt = BB->getTerminator();
299 // Merged instruction
300 Instruction *HoistedInst = HoistCand->clone();
302 // Notify AA of the new value.
303 if (isa<LoadInst>(HoistCand))
304 AA->copyValue(HoistCand, HoistedInst);
306 // Hoist instruction.
307 HoistedInst->insertBefore(HoistPt);
309 HoistCand->replaceAllUsesWith(HoistedInst);
310 removeInstruction(HoistCand);
311 // Replace the else block instruction.
312 ElseInst->replaceAllUsesWith(HoistedInst);
313 removeInstruction(ElseInst);
317 /// \brief Return true if no operand of \p I is defined in I's parent block
319 bool MergedLoadStoreMotion::isSafeToHoist(Instruction *I) const {
320 BasicBlock *Parent = I->getParent();
321 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
322 Instruction *Instr = dyn_cast<Instruction>(I->getOperand(i));
323 if (Instr && Instr->getParent() == Parent)
330 /// \brief Merge two equivalent loads and GEPs and hoist into diamond head
332 bool MergedLoadStoreMotion::hoistLoad(BasicBlock *BB, LoadInst *L0,
334 // Only one definition?
335 Instruction *A0 = dyn_cast<Instruction>(L0->getPointerOperand());
336 Instruction *A1 = dyn_cast<Instruction>(L1->getPointerOperand());
337 if (A0 && A1 && A0->isIdenticalTo(A1) && isSafeToHoist(A0) &&
338 A0->hasOneUse() && (A0->getParent() == L0->getParent()) &&
339 A1->hasOneUse() && (A1->getParent() == L1->getParent()) &&
340 isa<GetElementPtrInst>(A0)) {
341 DEBUG(dbgs() << "Hoist Instruction into BB \n"; BB->dump();
342 dbgs() << "Instruction Left\n"; L0->dump(); dbgs() << "\n";
343 dbgs() << "Instruction Right\n"; L1->dump(); dbgs() << "\n");
344 hoistInstruction(BB, A0, A1);
345 hoistInstruction(BB, L0, L1);
352 /// \brief Try to hoist two loads to same address into diamond header
354 /// Starting from a diamond head block, iterate over the instructions in one
355 /// successor block and try to match a load in the second successor.
357 bool MergedLoadStoreMotion::mergeLoads(BasicBlock *BB) {
358 bool MergedLoads = false;
359 assert(isDiamondHead(BB));
360 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
361 BasicBlock *Succ0 = BI->getSuccessor(0);
362 BasicBlock *Succ1 = BI->getSuccessor(1);
363 // #Instructions in Succ1 for Compile Time Control
364 int Size1 = Succ1->size();
366 for (BasicBlock::iterator BBI = Succ0->begin(), BBE = Succ0->end();
369 Instruction *I = BBI;
372 // Only move non-simple (atomic, volatile) loads.
373 LoadInst *L0 = dyn_cast<LoadInst>(I);
374 if (!L0 || !L0->isSimple() || L0->isUsedOutsideOfBlock(Succ0))
378 if (NLoads * Size1 >= MagicCompileTimeControl)
380 if (LoadInst *L1 = canHoistFromBlock(Succ1, L0)) {
381 bool Res = hoistLoad(BB, L0, L1);
383 // Don't attempt to hoist above loads that had not been hoisted.
392 /// \brief True when instruction is sink barrier for a store
394 bool MergedLoadStoreMotion::isStoreSinkBarrier(Instruction *Inst) {
395 // FIXME: Conservatively let a load instruction block the store.
396 // Use alias analysis instead.
397 if (isa<LoadInst>(Inst))
399 if (isa<CallInst>(Inst))
401 if (isa<TerminatorInst>(Inst) && !isa<BranchInst>(Inst))
403 // Note: mayHaveSideEffects covers all instructions that could
404 // trigger a change to state. Eg. in-flight stores have to be executed
405 // before ordered loads or fences, calls could invoke functions that store
406 // data to memory etc.
407 if (!isa<StoreInst>(Inst) && Inst->mayHaveSideEffects()) {
410 DEBUG(dbgs() << "No Sink Barrier\n");
415 /// \brief Check if \p BB contains a store to the same address as \p SI
417 /// \return The store in \p when it is safe to sink. Otherwise return Null.
419 StoreInst *MergedLoadStoreMotion::canSinkFromBlock(BasicBlock *BB,
422 DEBUG(dbgs() << "can Sink? : "; SI->dump(); dbgs() << "\n");
423 for (BasicBlock::reverse_iterator RBI = BB->rbegin(), RBE = BB->rend();
425 Instruction *Inst = &*RBI;
427 // Only move loads if they are used in the block.
428 if (isStoreSinkBarrier(Inst))
430 if (isa<StoreInst>(Inst)) {
431 AliasAnalysis::Location LocSI = AA->getLocation(SI);
432 AliasAnalysis::Location LocInst = AA->getLocation((StoreInst *)Inst);
433 if (AA->isMustAlias(LocSI, LocInst)) {
434 I = (StoreInst *)Inst;
443 /// \brief Create a PHI node in BB for the operands of S0 and S1
445 PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0,
447 // Create a phi if the values mismatch.
449 Value *Opd1 = S0->getValueOperand();
450 Value *Opd2 = S1->getValueOperand();
452 NewPN = PHINode::Create(Opd1->getType(), 2, Opd2->getName() + ".sink",
454 NewPN->addIncoming(Opd1, S0->getParent());
455 NewPN->addIncoming(Opd2, S1->getParent());
456 if (NewPN->getType()->getScalarType()->isPointerTy()) {
457 // Notify AA of the new value.
458 AA->copyValue(Opd1, NewPN);
459 AA->copyValue(Opd2, NewPN);
460 // AA needs to be informed when a PHI-use of the pointer value is added
461 for (unsigned I = 0, E = NewPN->getNumIncomingValues(); I != E; ++I) {
462 unsigned J = PHINode::getOperandNumForIncomingValue(I);
463 AA->addEscapingUse(NewPN->getOperandUse(J));
466 MD->invalidateCachedPointerInfo(NewPN);
473 /// \brief Merge two stores to same address and sink into \p BB
475 /// Also sinks GEP instruction computing the store address
477 bool MergedLoadStoreMotion::sinkStore(BasicBlock *BB, StoreInst *S0,
479 // Only one definition?
480 Instruction *A0 = dyn_cast<Instruction>(S0->getPointerOperand());
481 Instruction *A1 = dyn_cast<Instruction>(S1->getPointerOperand());
482 if (A0 && A1 && A0->isIdenticalTo(A1) && A0->hasOneUse() &&
483 (A0->getParent() == S0->getParent()) && A1->hasOneUse() &&
484 (A1->getParent() == S1->getParent()) && isa<GetElementPtrInst>(A0)) {
485 DEBUG(dbgs() << "Sink Instruction into BB \n"; BB->dump();
486 dbgs() << "Instruction Left\n"; S0->dump(); dbgs() << "\n";
487 dbgs() << "Instruction Right\n"; S1->dump(); dbgs() << "\n");
488 // Hoist the instruction.
489 BasicBlock::iterator InsertPt = BB->getFirstInsertionPt();
490 // Intersect optional metadata.
491 S0->intersectOptionalDataWith(S1);
492 S0->dropUnknownMetadata();
494 // Create the new store to be inserted at the join point.
495 StoreInst *SNew = (StoreInst *)(S0->clone());
496 Instruction *ANew = A0->clone();
497 AA->copyValue(S0, SNew);
498 SNew->insertBefore(InsertPt);
499 ANew->insertBefore(SNew);
501 assert(S0->getParent() == A0->getParent());
502 assert(S1->getParent() == A1->getParent());
504 PHINode *NewPN = getPHIOperand(BB, S0, S1);
505 // New PHI operand? Use it.
507 SNew->setOperand(0, NewPN);
508 removeInstruction(S0);
509 removeInstruction(S1);
510 A0->replaceAllUsesWith(ANew);
511 removeInstruction(A0);
512 A1->replaceAllUsesWith(ANew);
513 removeInstruction(A1);
520 /// \brief True when two stores are equivalent and can sink into the footer
522 /// Starting from a diamond tail block, iterate over the instructions in one
523 /// predecessor block and try to match a store in the second predecessor.
525 bool MergedLoadStoreMotion::mergeStores(BasicBlock *T) {
527 bool MergedStores = false;
528 assert(T && "Footer of a diamond cannot be empty");
530 pred_iterator PI = pred_begin(T), E = pred_end(T);
532 BasicBlock *Pred0 = *PI;
534 BasicBlock *Pred1 = *PI;
536 // tail block of a diamond/hammock?
540 return false; // No. More than 2 predecessors.
542 // #Instructions in Succ1 for Compile Time Control
543 int Size1 = Pred1->size();
546 for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend();
549 Instruction *I = &*RBI;
551 if (isStoreSinkBarrier(I))
553 // Sink move non-simple (atomic, volatile) stores
554 if (!isa<StoreInst>(I))
556 StoreInst *S0 = (StoreInst *)I;
561 if (NStores * Size1 >= MagicCompileTimeControl)
563 if (StoreInst *S1 = canSinkFromBlock(Pred1, S0)) {
564 bool Res = sinkStore(T, S0, S1);
566 // Don't attempt to sink below stores that had to stick around
567 // But after removal of a store and some of its feeding
568 // instruction search again from the beginning since the iterator
569 // is likely stale at this point.
573 RBI = Pred0->rbegin();
575 DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump());
582 /// \brief Run the transformation for each function
584 bool MergedLoadStoreMotion::runOnFunction(Function &F) {
585 MD = &getAnalysis<MemoryDependenceAnalysis>();
586 AA = &getAnalysis<AliasAnalysis>();
588 bool Changed = false;
589 DEBUG(dbgs() << "Instruction Merger\n");
591 // Merge unconditional branches, allowing PRE to catch more
592 // optimization opportunities.
593 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE;) {
594 BasicBlock *BB = FI++;
596 // Hoist equivalent loads and sink stores
597 // outside diamonds when possible
598 if (isDiamondHead(BB)) {
599 Changed |= mergeLoads(BB);
600 Changed |= mergeStores(getDiamondTail(BB));