1 //===- ConstantHoisting.cpp - Prepare code for expensive constants --------===//
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 //===----------------------------------------------------------------------===//
10 // This pass identifies expensive constants to hoist and coalesces them to
11 // better prepare it for SelectionDAG-based code generation. This works around
12 // the limitations of the basic-block-at-a-time approach.
14 // First it scans all instructions for integer constants and calculates its
15 // cost. If the constant can be folded into the instruction (the cost is
16 // TCC_Free) or the cost is just a simple operation (TCC_BASIC), then we don't
17 // consider it expensive and leave it alone. This is the default behavior and
18 // the default implementation of getIntImmCost will always return TCC_Free.
20 // If the cost is more than TCC_BASIC, then the integer constant can't be folded
21 // into the instruction and it might be beneficial to hoist the constant.
22 // Similar constants are coalesced to reduce register pressure and
23 // materialization code.
25 // When a constant is hoisted, it is also hidden behind a bitcast to force it to
26 // be live-out of the basic block. Otherwise the constant would be just
27 // duplicated and each basic block would have its own copy in the SelectionDAG.
28 // The SelectionDAG recognizes such constants as opaque and doesn't perform
29 // certain transformations on them, which would create a new expensive constant.
31 // This optimization is only applied to integer constants in instructions and
32 // simple (this means not nested) constant cast expressions. For example:
33 // %0 = load i64* inttoptr (i64 big_constant to i64*)
34 //===----------------------------------------------------------------------===//
36 #define DEBUG_TYPE "consthoist"
37 #include "llvm/Transforms/Scalar.h"
38 #include "llvm/ADT/SmallSet.h"
39 #include "llvm/ADT/SmallVector.h"
40 #include "llvm/ADT/Statistic.h"
41 #include "llvm/Analysis/TargetTransformInfo.h"
42 #include "llvm/IR/Constants.h"
43 #include "llvm/IR/Dominators.h"
44 #include "llvm/IR/IntrinsicInst.h"
45 #include "llvm/Pass.h"
46 #include "llvm/Support/Debug.h"
50 STATISTIC(NumConstantsHoisted, "Number of constants hoisted");
51 STATISTIC(NumConstantsRebased, "Number of constants rebased");
55 struct RebasedConstantInfo;
57 typedef SmallVector<ConstantUser, 8> ConstantUseListType;
58 typedef SmallVector<RebasedConstantInfo, 4> RebasedConstantListType;
60 /// \brief Keeps track of the user of a constant and the operand index where the
66 ConstantUser(Instruction *Inst, unsigned Idx) : Inst(Inst), OpndIdx(Idx) { }
69 /// \brief Keeps track of a constant candidate and its uses.
70 struct ConstantCandidate {
71 ConstantUseListType Uses;
72 ConstantInt *ConstInt;
73 unsigned CumulativeCost;
75 ConstantCandidate(ConstantInt *ConstInt)
76 : ConstInt(ConstInt), CumulativeCost(0) { }
78 /// \brief Add the user to the use list and update the cost.
79 void addUser(Instruction *Inst, unsigned Idx, unsigned Cost) {
80 CumulativeCost += Cost;
81 Uses.push_back(ConstantUser(Inst, Idx));
85 /// \brief This represents a constant that has been rebased with respect to a
86 /// base constant. The difference to the base constant is recorded in Offset.
87 struct RebasedConstantInfo {
88 ConstantUseListType Uses;
90 mutable BasicBlock *IDom;
92 RebasedConstantInfo(ConstantUseListType &&Uses, Constant *Offset)
93 : Uses(Uses), Offset(Offset), IDom(nullptr) { }
96 /// \brief A base constant and all its rebased constants.
98 ConstantInt *BaseConstant;
99 RebasedConstantListType RebasedConstants;
102 /// \brief The constant hoisting pass.
103 class ConstantHoisting : public FunctionPass {
104 typedef DenseMap<ConstantInt *, unsigned> ConstCandMapType;
105 typedef std::vector<ConstantCandidate> ConstCandVecType;
107 const TargetTransformInfo *TTI;
111 /// Keeps track of constant candidates found in the function.
112 ConstCandVecType ConstCandVec;
114 /// Keep track of cast instructions we already cloned.
115 SmallDenseMap<Instruction *, Instruction *> ClonedCastMap;
117 /// These are the final constants we decided to hoist.
118 SmallVector<ConstantInfo, 8> ConstantVec;
120 static char ID; // Pass identification, replacement for typeid
121 ConstantHoisting() : FunctionPass(ID), TTI(0), DT(0), Entry(0) {
122 initializeConstantHoistingPass(*PassRegistry::getPassRegistry());
125 bool runOnFunction(Function &Fn) override;
127 const char *getPassName() const override { return "Constant Hoisting"; }
129 void getAnalysisUsage(AnalysisUsage &AU) const override {
130 AU.setPreservesCFG();
131 AU.addRequired<DominatorTreeWrapperPass>();
132 AU.addRequired<TargetTransformInfo>();
136 /// \brief Initialize the pass.
137 void setup(Function &Fn) {
138 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
139 TTI = &getAnalysis<TargetTransformInfo>();
140 Entry = &Fn.getEntryBlock();
146 ClonedCastMap.clear();
147 ConstCandVec.clear();
154 /// \brief Find the common dominator of all uses and cache the result for
156 BasicBlock *getIDom(const RebasedConstantInfo &RCI) const {
159 RCI.IDom = findIDomOfAllUses(RCI.Uses);
160 assert(RCI.IDom && "Invalid IDom.");
164 BasicBlock *findIDomOfAllUses(const ConstantUseListType &Uses) const;
165 Instruction *findMatInsertPt(Instruction *Inst, unsigned Idx = ~0U) const;
166 Instruction *findConstantInsertionPoint(const ConstantInfo &ConstInfo) const;
167 void collectConstantCandidates(ConstCandMapType &ConstCandMap,
168 Instruction *Inst, unsigned Idx,
169 ConstantInt *ConstInt);
170 void collectConstantCandidates(ConstCandMapType &ConstCandMap,
172 void collectConstantCandidates(Function &Fn);
173 void findAndMakeBaseConstant(ConstCandVecType::iterator S,
174 ConstCandVecType::iterator E);
175 void findBaseConstants();
176 void emitBaseConstants(Instruction *Base, Constant *Offset,
177 const ConstantUser &ConstUser);
178 bool emitBaseConstants();
179 void deleteDeadCastInst() const;
180 bool optimizeConstants(Function &Fn);
184 char ConstantHoisting::ID = 0;
185 INITIALIZE_PASS_BEGIN(ConstantHoisting, "consthoist", "Constant Hoisting",
187 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
188 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
189 INITIALIZE_PASS_END(ConstantHoisting, "consthoist", "Constant Hoisting",
192 FunctionPass *llvm::createConstantHoistingPass() {
193 return new ConstantHoisting();
196 /// \brief Perform the constant hoisting optimization for the given function.
197 bool ConstantHoisting::runOnFunction(Function &Fn) {
198 DEBUG(dbgs() << "********** Begin Constant Hoisting **********\n");
199 DEBUG(dbgs() << "********** Function: " << Fn.getName() << '\n');
203 bool MadeChange = optimizeConstants(Fn);
206 DEBUG(dbgs() << "********** Function after Constant Hoisting: "
207 << Fn.getName() << '\n');
210 DEBUG(dbgs() << "********** End Constant Hoisting **********\n");
217 /// \brief Find nearest common dominator of all uses.
218 /// FIXME: Replace this with NearestCommonDominator once it is in common code.
220 ConstantHoisting::findIDomOfAllUses(const ConstantUseListType &Uses) const {
221 // Collect all basic blocks.
222 SmallPtrSet<BasicBlock *, 8> BBs;
223 for (auto const &U : Uses)
224 BBs.insert(findMatInsertPt(U.Inst, U.OpndIdx)->getParent());
226 if (BBs.count(Entry))
229 while (BBs.size() >= 2) {
230 BasicBlock *BB, *BB1, *BB2;
232 BB2 = *std::next(BBs.begin());
233 BB = DT->findNearestCommonDominator(BB1, BB2);
240 assert((BBs.size() == 1) && "Expected only one element.");
244 /// \brief Find the constant materialization insertion point.
245 Instruction *ConstantHoisting::findMatInsertPt(Instruction *Inst,
246 unsigned Idx) const {
247 // The simple and common case.
248 if (!isa<PHINode>(Inst) && !isa<LandingPadInst>(Inst))
251 // We can't insert directly before a phi node or landing pad. Insert before
252 // the terminator of the incoming or dominating block.
253 assert(Entry != Inst->getParent() && "PHI or landing pad in entry block!");
254 if (Idx != ~0U && isa<PHINode>(Inst))
255 return cast<PHINode>(Inst)->getIncomingBlock(Idx)->getTerminator();
257 BasicBlock *IDom = DT->getNode(Inst->getParent())->getIDom()->getBlock();
258 return IDom->getTerminator();
261 /// \brief Find an insertion point that dominates all uses.
262 Instruction *ConstantHoisting::
263 findConstantInsertionPoint(const ConstantInfo &ConstInfo) const {
264 assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry.");
265 // Collect all IDoms.
266 SmallPtrSet<BasicBlock *, 8> BBs;
267 for (auto const &RCI : ConstInfo.RebasedConstants)
268 BBs.insert(getIDom(RCI));
270 assert(!BBs.empty() && "No dominators!?");
272 if (BBs.count(Entry))
273 return &Entry->front();
275 while (BBs.size() >= 2) {
276 BasicBlock *BB, *BB1, *BB2;
278 BB2 = *std::next(BBs.begin());
279 BB = DT->findNearestCommonDominator(BB1, BB2);
281 return &Entry->front();
286 assert((BBs.size() == 1) && "Expected only one element.");
287 Instruction &FirstInst = (*BBs.begin())->front();
288 return findMatInsertPt(&FirstInst);
292 /// \brief Record constant integer ConstInt for instruction Inst at operand
295 /// The operand at index Idx is not necessarily the constant integer itself. It
296 /// could also be a cast instruction or a constant expression that uses the
298 void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap,
301 ConstantInt *ConstInt) {
303 // Ask the target about the cost of materializing the constant for the given
304 // instruction and operand index.
305 if (auto IntrInst = dyn_cast<IntrinsicInst>(Inst))
306 Cost = TTI->getIntImmCost(IntrInst->getIntrinsicID(), Idx,
307 ConstInt->getValue(), ConstInt->getType());
309 Cost = TTI->getIntImmCost(Inst->getOpcode(), Idx, ConstInt->getValue(),
310 ConstInt->getType());
312 // Ignore cheap integer constants.
313 if (Cost > TargetTransformInfo::TCC_Basic) {
314 ConstCandMapType::iterator Itr;
316 std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(ConstInt, 0));
318 ConstCandVec.push_back(ConstantCandidate(ConstInt));
319 Itr->second = ConstCandVec.size() - 1;
321 ConstCandVec[Itr->second].addUser(Inst, Idx, Cost);
322 DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx)))
323 dbgs() << "Collect constant " << *ConstInt << " from " << *Inst
324 << " with cost " << Cost << '\n';
326 dbgs() << "Collect constant " << *ConstInt << " indirectly from "
327 << *Inst << " via " << *Inst->getOperand(Idx) << " with cost "
333 /// \brief Scan the instruction for expensive integer constants and record them
334 /// in the constant candidate vector.
335 void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap,
337 // Skip all cast instructions. They are visited indirectly later on.
341 // Can't handle inline asm. Skip it.
342 if (auto Call = dyn_cast<CallInst>(Inst))
343 if (isa<InlineAsm>(Call->getCalledValue()))
346 // Scan all operands.
347 for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) {
348 Value *Opnd = Inst->getOperand(Idx);
350 // Visit constant integers.
351 if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) {
352 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
356 // Visit cast instructions that have constant integers.
357 if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
358 // Only visit cast instructions, which have been skipped. All other
359 // instructions should have already been visited.
360 if (!CastInst->isCast())
363 if (auto *ConstInt = dyn_cast<ConstantInt>(CastInst->getOperand(0))) {
364 // Pretend the constant is directly used by the instruction and ignore
365 // the cast instruction.
366 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
371 // Visit constant expressions that have constant integers.
372 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
373 // Only visit constant cast expressions.
374 if (!ConstExpr->isCast())
377 if (auto ConstInt = dyn_cast<ConstantInt>(ConstExpr->getOperand(0))) {
378 // Pretend the constant is directly used by the instruction and ignore
379 // the constant expression.
380 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
384 } // end of for all operands
387 /// \brief Collect all integer constants in the function that cannot be folded
388 /// into an instruction itself.
389 void ConstantHoisting::collectConstantCandidates(Function &Fn) {
390 ConstCandMapType ConstCandMap;
391 for (Function::iterator BB : Fn)
392 for (BasicBlock::iterator Inst : *BB)
393 collectConstantCandidates(ConstCandMap, Inst);
396 /// \brief Find the base constant within the given range and rebase all other
397 /// constants with respect to the base constant.
398 void ConstantHoisting::findAndMakeBaseConstant(ConstCandVecType::iterator S,
399 ConstCandVecType::iterator E) {
401 unsigned NumUses = 0;
402 // Use the constant that has the maximum cost as base constant.
403 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
404 NumUses += ConstCand->Uses.size();
405 if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost)
406 MaxCostItr = ConstCand;
409 // Don't hoist constants that have only one use.
413 ConstantInfo ConstInfo;
414 ConstInfo.BaseConstant = MaxCostItr->ConstInt;
415 Type *Ty = ConstInfo.BaseConstant->getType();
417 // Rebase the constants with respect to the base constant.
418 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
419 APInt Diff = ConstCand->ConstInt->getValue() -
420 ConstInfo.BaseConstant->getValue();
421 Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff);
422 ConstInfo.RebasedConstants.push_back(
423 RebasedConstantInfo(std::move(ConstCand->Uses), Offset));
425 ConstantVec.push_back(ConstInfo);
428 /// \brief Finds and combines constant candidates that can be easily
429 /// rematerialized with an add from a common base constant.
430 void ConstantHoisting::findBaseConstants() {
431 // Sort the constants by value and type. This invalidates the mapping!
432 std::sort(ConstCandVec.begin(), ConstCandVec.end(),
433 [](const ConstantCandidate &LHS, const ConstantCandidate &RHS) {
434 if (LHS.ConstInt->getType() != RHS.ConstInt->getType())
435 return LHS.ConstInt->getType()->getBitWidth() <
436 RHS.ConstInt->getType()->getBitWidth();
437 return LHS.ConstInt->getValue().ult(RHS.ConstInt->getValue());
440 // Simple linear scan through the sorted constant candidate vector for viable
442 auto MinValItr = ConstCandVec.begin();
443 for (auto CC = std::next(ConstCandVec.begin()), E = ConstCandVec.end();
445 if (MinValItr->ConstInt->getType() == CC->ConstInt->getType()) {
446 // Check if the constant is in range of an add with immediate.
447 APInt Diff = CC->ConstInt->getValue() - MinValItr->ConstInt->getValue();
448 if ((Diff.getBitWidth() <= 64) &&
449 TTI->isLegalAddImmediate(Diff.getSExtValue()))
452 // We either have now a different constant type or the constant is not in
453 // range of an add with immediate anymore.
454 findAndMakeBaseConstant(MinValItr, CC);
455 // Start a new base constant search.
458 // Finalize the last base constant search.
459 findAndMakeBaseConstant(MinValItr, ConstCandVec.end());
462 /// \brief Updates the operand at Idx in instruction Inst with the result of
463 /// instruction Mat. If the instruction is a PHI node then special
464 /// handling for duplicate values form the same incomming basic block is
466 /// \return The update will always succeed, but the return value indicated if
467 /// Mat was used for the update or not.
468 static bool updateOperand(Instruction *Inst, unsigned Idx, Instruction *Mat) {
469 if (auto PHI = dyn_cast<PHINode>(Inst)) {
470 // Check if any previous operand of the PHI node has the same incoming basic
471 // block. This is a very odd case that happens when the incoming basic block
472 // has a switch statement. In this case use the same value as the previous
473 // operand(s), otherwise we will fail verification due to different values.
474 // The values are actually the same, but the variable names are different
475 // and the verifier doesn't like that.
476 BasicBlock *IncomingBB = PHI->getIncomingBlock(Idx);
477 for (unsigned i = 0; i < Idx; ++i) {
478 if (PHI->getIncomingBlock(i) == IncomingBB) {
479 Value *IncomingVal = PHI->getIncomingValue(i);
480 Inst->setOperand(Idx, IncomingVal);
486 Inst->setOperand(Idx, Mat);
490 /// \brief Emit materialization code for all rebased constants and update their
492 void ConstantHoisting::emitBaseConstants(Instruction *Base, Constant *Offset,
493 const ConstantUser &ConstUser) {
494 Instruction *Mat = Base;
496 Instruction *InsertionPt = findMatInsertPt(ConstUser.Inst,
498 Mat = BinaryOperator::Create(Instruction::Add, Base, Offset,
499 "const_mat", InsertionPt);
501 DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)
502 << " + " << *Offset << ") in BB "
503 << Mat->getParent()->getName() << '\n' << *Mat << '\n');
504 Mat->setDebugLoc(ConstUser.Inst->getDebugLoc());
506 Value *Opnd = ConstUser.Inst->getOperand(ConstUser.OpndIdx);
508 // Visit constant integer.
509 if (isa<ConstantInt>(Opnd)) {
510 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
511 if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat) && Offset)
512 Mat->eraseFromParent();
513 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
517 // Visit cast instruction.
518 if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
519 assert(CastInst->isCast() && "Expected an cast instruction!");
520 // Check if we already have visited this cast instruction before to avoid
521 // unnecessary cloning.
522 Instruction *&ClonedCastInst = ClonedCastMap[CastInst];
523 if (!ClonedCastInst) {
524 ClonedCastInst = CastInst->clone();
525 ClonedCastInst->setOperand(0, Mat);
526 ClonedCastInst->insertAfter(CastInst);
527 // Use the same debug location as the original cast instruction.
528 ClonedCastInst->setDebugLoc(CastInst->getDebugLoc());
529 DEBUG(dbgs() << "Clone instruction: " << *ClonedCastInst << '\n'
530 << "To : " << *CastInst << '\n');
533 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
534 updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ClonedCastInst);
535 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
539 // Visit constant expression.
540 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
541 Instruction *ConstExprInst = ConstExpr->getAsInstruction();
542 ConstExprInst->setOperand(0, Mat);
543 ConstExprInst->insertBefore(findMatInsertPt(ConstUser.Inst,
546 // Use the same debug location as the instruction we are about to update.
547 ConstExprInst->setDebugLoc(ConstUser.Inst->getDebugLoc());
549 DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n'
550 << "From : " << *ConstExpr << '\n');
551 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
552 if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ConstExprInst)) {
553 ConstExprInst->eraseFromParent();
555 Mat->eraseFromParent();
557 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
562 /// \brief Hoist and hide the base constant behind a bitcast and emit
563 /// materialization code for derived constants.
564 bool ConstantHoisting::emitBaseConstants() {
565 bool MadeChange = false;
566 for (auto const &ConstInfo : ConstantVec) {
567 // Hoist and hide the base constant behind a bitcast.
568 Instruction *IP = findConstantInsertionPoint(ConstInfo);
569 IntegerType *Ty = ConstInfo.BaseConstant->getType();
571 new BitCastInst(ConstInfo.BaseConstant, Ty, "const", IP);
572 DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseConstant << ") to BB "
573 << IP->getParent()->getName() << '\n' << *Base << '\n');
574 NumConstantsHoisted++;
576 // Emit materialization code for all rebased constants.
577 for (auto const &RCI : ConstInfo.RebasedConstants) {
578 NumConstantsRebased++;
579 for (auto const &U : RCI.Uses)
580 emitBaseConstants(Base, RCI.Offset, U);
583 // Use the same debug location as the last user of the constant.
584 assert(!Base->use_empty() && "The use list is empty!?");
585 assert(isa<Instruction>(Base->user_back()) &&
586 "All uses should be instructions.");
587 Base->setDebugLoc(cast<Instruction>(Base->user_back())->getDebugLoc());
589 // Correct for base constant, which we counted above too.
590 NumConstantsRebased--;
596 /// \brief Check all cast instructions we made a copy of and remove them if they
597 /// have no more users.
598 void ConstantHoisting::deleteDeadCastInst() const {
599 for (auto const &I : ClonedCastMap)
600 if (I.first->use_empty())
601 I.first->eraseFromParent();
604 /// \brief Optimize expensive integer constants in the given function.
605 bool ConstantHoisting::optimizeConstants(Function &Fn) {
606 // Collect all constant candidates.
607 collectConstantCandidates(Fn);
609 // There are no constant candidates to worry about.
610 if (ConstCandVec.empty())
613 // Combine constants that can be easily materialized with an add from a common
617 // There are no constants to emit.
618 if (ConstantVec.empty())
621 // Finally hoist the base constant and emit materialization code for dependent
623 bool MadeChange = emitBaseConstants();
625 // Cleanup dead instructions.
626 deleteDeadCastInst();