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 #include "llvm/Transforms/Scalar.h"
37 #include "llvm/ADT/SmallSet.h"
38 #include "llvm/ADT/SmallVector.h"
39 #include "llvm/ADT/Statistic.h"
40 #include "llvm/Analysis/TargetTransformInfo.h"
41 #include "llvm/IR/Constants.h"
42 #include "llvm/IR/Dominators.h"
43 #include "llvm/IR/IntrinsicInst.h"
44 #include "llvm/Pass.h"
45 #include "llvm/Support/Debug.h"
50 #define DEBUG_TYPE "consthoist"
52 STATISTIC(NumConstantsHoisted, "Number of constants hoisted");
53 STATISTIC(NumConstantsRebased, "Number of constants rebased");
57 struct RebasedConstantInfo;
59 typedef SmallVector<ConstantUser, 8> ConstantUseListType;
60 typedef SmallVector<RebasedConstantInfo, 4> RebasedConstantListType;
62 /// \brief Keeps track of the user of a constant and the operand index where the
68 ConstantUser(Instruction *Inst, unsigned Idx) : Inst(Inst), OpndIdx(Idx) { }
71 /// \brief Keeps track of a constant candidate and its uses.
72 struct ConstantCandidate {
73 ConstantUseListType Uses;
74 ConstantInt *ConstInt;
75 unsigned CumulativeCost;
77 ConstantCandidate(ConstantInt *ConstInt)
78 : ConstInt(ConstInt), CumulativeCost(0) { }
80 /// \brief Add the user to the use list and update the cost.
81 void addUser(Instruction *Inst, unsigned Idx, unsigned Cost) {
82 CumulativeCost += Cost;
83 Uses.push_back(ConstantUser(Inst, Idx));
87 /// \brief This represents a constant that has been rebased with respect to a
88 /// base constant. The difference to the base constant is recorded in Offset.
89 struct RebasedConstantInfo {
90 ConstantUseListType Uses;
93 RebasedConstantInfo(ConstantUseListType &&Uses, Constant *Offset)
94 : Uses(std::move(Uses)), Offset(Offset) { }
97 /// \brief A base constant and all its rebased constants.
99 ConstantInt *BaseConstant;
100 RebasedConstantListType RebasedConstants;
103 /// \brief The constant hoisting pass.
104 class ConstantHoisting : public FunctionPass {
105 typedef DenseMap<ConstantInt *, unsigned> ConstCandMapType;
106 typedef std::vector<ConstantCandidate> ConstCandVecType;
108 const TargetTransformInfo *TTI;
112 /// Keeps track of constant candidates found in the function.
113 ConstCandVecType ConstCandVec;
115 /// Keep track of cast instructions we already cloned.
116 SmallDenseMap<Instruction *, Instruction *> ClonedCastMap;
118 /// These are the final constants we decided to hoist.
119 SmallVector<ConstantInfo, 8> ConstantVec;
121 static char ID; // Pass identification, replacement for typeid
122 ConstantHoisting() : FunctionPass(ID), TTI(nullptr), DT(nullptr),
124 initializeConstantHoistingPass(*PassRegistry::getPassRegistry());
127 bool runOnFunction(Function &Fn) override;
129 const char *getPassName() const override { return "Constant Hoisting"; }
131 void getAnalysisUsage(AnalysisUsage &AU) const override {
132 AU.setPreservesCFG();
133 AU.addRequired<DominatorTreeWrapperPass>();
134 AU.addRequired<TargetTransformInfoWrapperPass>();
138 /// \brief Initialize the pass.
139 void setup(Function &Fn) {
140 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
141 TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(Fn);
142 Entry = &Fn.getEntryBlock();
148 ClonedCastMap.clear();
149 ConstCandVec.clear();
156 Instruction *findMatInsertPt(Instruction *Inst, unsigned Idx = ~0U) const;
157 Instruction *findConstantInsertionPoint(const ConstantInfo &ConstInfo) const;
158 void collectConstantCandidates(ConstCandMapType &ConstCandMap,
159 Instruction *Inst, unsigned Idx,
160 ConstantInt *ConstInt);
161 void collectConstantCandidates(ConstCandMapType &ConstCandMap,
163 void collectConstantCandidates(Function &Fn);
164 void findAndMakeBaseConstant(ConstCandVecType::iterator S,
165 ConstCandVecType::iterator E);
166 void findBaseConstants();
167 void emitBaseConstants(Instruction *Base, Constant *Offset,
168 const ConstantUser &ConstUser);
169 bool emitBaseConstants();
170 void deleteDeadCastInst() const;
171 bool optimizeConstants(Function &Fn);
175 char ConstantHoisting::ID = 0;
176 INITIALIZE_PASS_BEGIN(ConstantHoisting, "consthoist", "Constant Hoisting",
178 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
179 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
180 INITIALIZE_PASS_END(ConstantHoisting, "consthoist", "Constant Hoisting",
183 FunctionPass *llvm::createConstantHoistingPass() {
184 return new ConstantHoisting();
187 /// \brief Perform the constant hoisting optimization for the given function.
188 bool ConstantHoisting::runOnFunction(Function &Fn) {
189 if (skipOptnoneFunction(Fn))
192 DEBUG(dbgs() << "********** Begin Constant Hoisting **********\n");
193 DEBUG(dbgs() << "********** Function: " << Fn.getName() << '\n');
197 bool MadeChange = optimizeConstants(Fn);
200 DEBUG(dbgs() << "********** Function after Constant Hoisting: "
201 << Fn.getName() << '\n');
204 DEBUG(dbgs() << "********** End Constant Hoisting **********\n");
212 /// \brief Find the constant materialization insertion point.
213 Instruction *ConstantHoisting::findMatInsertPt(Instruction *Inst,
214 unsigned Idx) const {
215 // If the operand is a cast instruction, then we have to materialize the
216 // constant before the cast instruction.
218 Value *Opnd = Inst->getOperand(Idx);
219 if (auto CastInst = dyn_cast<Instruction>(Opnd))
220 if (CastInst->isCast())
224 // The simple and common case. This also includes constant expressions.
225 if (!isa<PHINode>(Inst) && !isa<LandingPadInst>(Inst))
228 // We can't insert directly before a phi node or landing pad. Insert before
229 // the terminator of the incoming or dominating block.
230 assert(Entry != Inst->getParent() && "PHI or landing pad in entry block!");
231 if (Idx != ~0U && isa<PHINode>(Inst))
232 return cast<PHINode>(Inst)->getIncomingBlock(Idx)->getTerminator();
234 BasicBlock *IDom = DT->getNode(Inst->getParent())->getIDom()->getBlock();
235 return IDom->getTerminator();
238 /// \brief Find an insertion point that dominates all uses.
239 Instruction *ConstantHoisting::
240 findConstantInsertionPoint(const ConstantInfo &ConstInfo) const {
241 assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry.");
242 // Collect all basic blocks.
243 SmallPtrSet<BasicBlock *, 8> BBs;
244 for (auto const &RCI : ConstInfo.RebasedConstants)
245 for (auto const &U : RCI.Uses)
246 BBs.insert(findMatInsertPt(U.Inst, U.OpndIdx)->getParent());
248 if (BBs.count(Entry))
249 return &Entry->front();
251 while (BBs.size() >= 2) {
252 BasicBlock *BB, *BB1, *BB2;
254 BB2 = *std::next(BBs.begin());
255 BB = DT->findNearestCommonDominator(BB1, BB2);
257 return &Entry->front();
262 assert((BBs.size() == 1) && "Expected only one element.");
263 Instruction &FirstInst = (*BBs.begin())->front();
264 return findMatInsertPt(&FirstInst);
268 /// \brief Record constant integer ConstInt for instruction Inst at operand
271 /// The operand at index Idx is not necessarily the constant integer itself. It
272 /// could also be a cast instruction or a constant expression that uses the
274 void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap,
277 ConstantInt *ConstInt) {
279 // Ask the target about the cost of materializing the constant for the given
280 // instruction and operand index.
281 if (auto IntrInst = dyn_cast<IntrinsicInst>(Inst))
282 Cost = TTI->getIntImmCost(IntrInst->getIntrinsicID(), Idx,
283 ConstInt->getValue(), ConstInt->getType());
285 Cost = TTI->getIntImmCost(Inst->getOpcode(), Idx, ConstInt->getValue(),
286 ConstInt->getType());
288 // Ignore cheap integer constants.
289 if (Cost > TargetTransformInfo::TCC_Basic) {
290 ConstCandMapType::iterator Itr;
292 std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(ConstInt, 0));
294 ConstCandVec.push_back(ConstantCandidate(ConstInt));
295 Itr->second = ConstCandVec.size() - 1;
297 ConstCandVec[Itr->second].addUser(Inst, Idx, Cost);
298 DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx)))
299 dbgs() << "Collect constant " << *ConstInt << " from " << *Inst
300 << " with cost " << Cost << '\n';
302 dbgs() << "Collect constant " << *ConstInt << " indirectly from "
303 << *Inst << " via " << *Inst->getOperand(Idx) << " with cost "
309 /// \brief Scan the instruction for expensive integer constants and record them
310 /// in the constant candidate vector.
311 void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap,
313 // Skip all cast instructions. They are visited indirectly later on.
317 // Can't handle inline asm. Skip it.
318 if (auto Call = dyn_cast<CallInst>(Inst))
319 if (isa<InlineAsm>(Call->getCalledValue()))
322 // Scan all operands.
323 for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) {
324 Value *Opnd = Inst->getOperand(Idx);
326 // Visit constant integers.
327 if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) {
328 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
332 // Visit cast instructions that have constant integers.
333 if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
334 // Only visit cast instructions, which have been skipped. All other
335 // instructions should have already been visited.
336 if (!CastInst->isCast())
339 if (auto *ConstInt = dyn_cast<ConstantInt>(CastInst->getOperand(0))) {
340 // Pretend the constant is directly used by the instruction and ignore
341 // the cast instruction.
342 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
347 // Visit constant expressions that have constant integers.
348 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
349 // Only visit constant cast expressions.
350 if (!ConstExpr->isCast())
353 if (auto ConstInt = dyn_cast<ConstantInt>(ConstExpr->getOperand(0))) {
354 // Pretend the constant is directly used by the instruction and ignore
355 // the constant expression.
356 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
360 } // end of for all operands
363 /// \brief Collect all integer constants in the function that cannot be folded
364 /// into an instruction itself.
365 void ConstantHoisting::collectConstantCandidates(Function &Fn) {
366 ConstCandMapType ConstCandMap;
367 for (Function::iterator BB : Fn)
368 for (BasicBlock::iterator Inst : *BB)
369 collectConstantCandidates(ConstCandMap, Inst);
372 /// \brief Find the base constant within the given range and rebase all other
373 /// constants with respect to the base constant.
374 void ConstantHoisting::findAndMakeBaseConstant(ConstCandVecType::iterator S,
375 ConstCandVecType::iterator E) {
377 unsigned NumUses = 0;
378 // Use the constant that has the maximum cost as base constant.
379 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
380 NumUses += ConstCand->Uses.size();
381 if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost)
382 MaxCostItr = ConstCand;
385 // Don't hoist constants that have only one use.
389 ConstantInfo ConstInfo;
390 ConstInfo.BaseConstant = MaxCostItr->ConstInt;
391 Type *Ty = ConstInfo.BaseConstant->getType();
393 // Rebase the constants with respect to the base constant.
394 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
395 APInt Diff = ConstCand->ConstInt->getValue() -
396 ConstInfo.BaseConstant->getValue();
397 Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff);
398 ConstInfo.RebasedConstants.push_back(
399 RebasedConstantInfo(std::move(ConstCand->Uses), Offset));
401 ConstantVec.push_back(std::move(ConstInfo));
404 /// \brief Finds and combines constant candidates that can be easily
405 /// rematerialized with an add from a common base constant.
406 void ConstantHoisting::findBaseConstants() {
407 // Sort the constants by value and type. This invalidates the mapping!
408 std::sort(ConstCandVec.begin(), ConstCandVec.end(),
409 [](const ConstantCandidate &LHS, const ConstantCandidate &RHS) {
410 if (LHS.ConstInt->getType() != RHS.ConstInt->getType())
411 return LHS.ConstInt->getType()->getBitWidth() <
412 RHS.ConstInt->getType()->getBitWidth();
413 return LHS.ConstInt->getValue().ult(RHS.ConstInt->getValue());
416 // Simple linear scan through the sorted constant candidate vector for viable
418 auto MinValItr = ConstCandVec.begin();
419 for (auto CC = std::next(ConstCandVec.begin()), E = ConstCandVec.end();
421 if (MinValItr->ConstInt->getType() == CC->ConstInt->getType()) {
422 // Check if the constant is in range of an add with immediate.
423 APInt Diff = CC->ConstInt->getValue() - MinValItr->ConstInt->getValue();
424 if ((Diff.getBitWidth() <= 64) &&
425 TTI->isLegalAddImmediate(Diff.getSExtValue()))
428 // We either have now a different constant type or the constant is not in
429 // range of an add with immediate anymore.
430 findAndMakeBaseConstant(MinValItr, CC);
431 // Start a new base constant search.
434 // Finalize the last base constant search.
435 findAndMakeBaseConstant(MinValItr, ConstCandVec.end());
438 /// \brief Updates the operand at Idx in instruction Inst with the result of
439 /// instruction Mat. If the instruction is a PHI node then special
440 /// handling for duplicate values form the same incomming basic block is
442 /// \return The update will always succeed, but the return value indicated if
443 /// Mat was used for the update or not.
444 static bool updateOperand(Instruction *Inst, unsigned Idx, Instruction *Mat) {
445 if (auto PHI = dyn_cast<PHINode>(Inst)) {
446 // Check if any previous operand of the PHI node has the same incoming basic
447 // block. This is a very odd case that happens when the incoming basic block
448 // has a switch statement. In this case use the same value as the previous
449 // operand(s), otherwise we will fail verification due to different values.
450 // The values are actually the same, but the variable names are different
451 // and the verifier doesn't like that.
452 BasicBlock *IncomingBB = PHI->getIncomingBlock(Idx);
453 for (unsigned i = 0; i < Idx; ++i) {
454 if (PHI->getIncomingBlock(i) == IncomingBB) {
455 Value *IncomingVal = PHI->getIncomingValue(i);
456 Inst->setOperand(Idx, IncomingVal);
462 Inst->setOperand(Idx, Mat);
466 /// \brief Emit materialization code for all rebased constants and update their
468 void ConstantHoisting::emitBaseConstants(Instruction *Base, Constant *Offset,
469 const ConstantUser &ConstUser) {
470 Instruction *Mat = Base;
472 Instruction *InsertionPt = findMatInsertPt(ConstUser.Inst,
474 Mat = BinaryOperator::Create(Instruction::Add, Base, Offset,
475 "const_mat", InsertionPt);
477 DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)
478 << " + " << *Offset << ") in BB "
479 << Mat->getParent()->getName() << '\n' << *Mat << '\n');
480 Mat->setDebugLoc(ConstUser.Inst->getDebugLoc());
482 Value *Opnd = ConstUser.Inst->getOperand(ConstUser.OpndIdx);
484 // Visit constant integer.
485 if (isa<ConstantInt>(Opnd)) {
486 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
487 if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat) && Offset)
488 Mat->eraseFromParent();
489 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
493 // Visit cast instruction.
494 if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
495 assert(CastInst->isCast() && "Expected an cast instruction!");
496 // Check if we already have visited this cast instruction before to avoid
497 // unnecessary cloning.
498 Instruction *&ClonedCastInst = ClonedCastMap[CastInst];
499 if (!ClonedCastInst) {
500 ClonedCastInst = CastInst->clone();
501 ClonedCastInst->setOperand(0, Mat);
502 ClonedCastInst->insertAfter(CastInst);
503 // Use the same debug location as the original cast instruction.
504 ClonedCastInst->setDebugLoc(CastInst->getDebugLoc());
505 DEBUG(dbgs() << "Clone instruction: " << *CastInst << '\n'
506 << "To : " << *ClonedCastInst << '\n');
509 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
510 updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ClonedCastInst);
511 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
515 // Visit constant expression.
516 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
517 Instruction *ConstExprInst = ConstExpr->getAsInstruction();
518 ConstExprInst->setOperand(0, Mat);
519 ConstExprInst->insertBefore(findMatInsertPt(ConstUser.Inst,
522 // Use the same debug location as the instruction we are about to update.
523 ConstExprInst->setDebugLoc(ConstUser.Inst->getDebugLoc());
525 DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n'
526 << "From : " << *ConstExpr << '\n');
527 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
528 if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ConstExprInst)) {
529 ConstExprInst->eraseFromParent();
531 Mat->eraseFromParent();
533 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
538 /// \brief Hoist and hide the base constant behind a bitcast and emit
539 /// materialization code for derived constants.
540 bool ConstantHoisting::emitBaseConstants() {
541 bool MadeChange = false;
542 for (auto const &ConstInfo : ConstantVec) {
543 // Hoist and hide the base constant behind a bitcast.
544 Instruction *IP = findConstantInsertionPoint(ConstInfo);
545 IntegerType *Ty = ConstInfo.BaseConstant->getType();
547 new BitCastInst(ConstInfo.BaseConstant, Ty, "const", IP);
548 DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseConstant << ") to BB "
549 << IP->getParent()->getName() << '\n' << *Base << '\n');
550 NumConstantsHoisted++;
552 // Emit materialization code for all rebased constants.
553 for (auto const &RCI : ConstInfo.RebasedConstants) {
554 NumConstantsRebased++;
555 for (auto const &U : RCI.Uses)
556 emitBaseConstants(Base, RCI.Offset, U);
559 // Use the same debug location as the last user of the constant.
560 assert(!Base->use_empty() && "The use list is empty!?");
561 assert(isa<Instruction>(Base->user_back()) &&
562 "All uses should be instructions.");
563 Base->setDebugLoc(cast<Instruction>(Base->user_back())->getDebugLoc());
565 // Correct for base constant, which we counted above too.
566 NumConstantsRebased--;
572 /// \brief Check all cast instructions we made a copy of and remove them if they
573 /// have no more users.
574 void ConstantHoisting::deleteDeadCastInst() const {
575 for (auto const &I : ClonedCastMap)
576 if (I.first->use_empty())
577 I.first->eraseFromParent();
580 /// \brief Optimize expensive integer constants in the given function.
581 bool ConstantHoisting::optimizeConstants(Function &Fn) {
582 // Collect all constant candidates.
583 collectConstantCandidates(Fn);
585 // There are no constant candidates to worry about.
586 if (ConstCandVec.empty())
589 // Combine constants that can be easily materialized with an add from a common
593 // There are no constants to emit.
594 if (ConstantVec.empty())
597 // Finally hoist the base constant and emit materialization code for dependent
599 bool MadeChange = emitBaseConstants();
601 // Cleanup dead instructions.
602 deleteDeadCastInst();