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"
49 #define DEBUG_TYPE "consthoist"
51 STATISTIC(NumConstantsHoisted, "Number of constants hoisted");
52 STATISTIC(NumConstantsRebased, "Number of constants rebased");
56 struct RebasedConstantInfo;
58 typedef SmallVector<ConstantUser, 8> ConstantUseListType;
59 typedef SmallVector<RebasedConstantInfo, 4> RebasedConstantListType;
61 /// \brief Keeps track of the user of a constant and the operand index where the
67 ConstantUser(Instruction *Inst, unsigned Idx) : Inst(Inst), OpndIdx(Idx) { }
70 /// \brief Keeps track of a constant candidate and its uses.
71 struct ConstantCandidate {
72 ConstantUseListType Uses;
73 ConstantInt *ConstInt;
74 unsigned CumulativeCost;
76 ConstantCandidate(ConstantInt *ConstInt)
77 : ConstInt(ConstInt), CumulativeCost(0) { }
79 /// \brief Add the user to the use list and update the cost.
80 void addUser(Instruction *Inst, unsigned Idx, unsigned Cost) {
81 CumulativeCost += Cost;
82 Uses.push_back(ConstantUser(Inst, Idx));
86 /// \brief This represents a constant that has been rebased with respect to a
87 /// base constant. The difference to the base constant is recorded in Offset.
88 struct RebasedConstantInfo {
89 ConstantUseListType Uses;
92 RebasedConstantInfo(ConstantUseListType &&Uses, Constant *Offset)
93 : Uses(Uses), Offset(Offset) { }
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 Instruction *findMatInsertPt(Instruction *Inst, unsigned Idx = ~0U) const;
155 Instruction *findConstantInsertionPoint(const ConstantInfo &ConstInfo) const;
156 void collectConstantCandidates(ConstCandMapType &ConstCandMap,
157 Instruction *Inst, unsigned Idx,
158 ConstantInt *ConstInt);
159 void collectConstantCandidates(ConstCandMapType &ConstCandMap,
161 void collectConstantCandidates(Function &Fn);
162 void findAndMakeBaseConstant(ConstCandVecType::iterator S,
163 ConstCandVecType::iterator E);
164 void findBaseConstants();
165 void emitBaseConstants(Instruction *Base, Constant *Offset,
166 const ConstantUser &ConstUser);
167 bool emitBaseConstants();
168 void deleteDeadCastInst() const;
169 bool optimizeConstants(Function &Fn);
173 char ConstantHoisting::ID = 0;
174 INITIALIZE_PASS_BEGIN(ConstantHoisting, "consthoist", "Constant Hoisting",
176 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
177 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
178 INITIALIZE_PASS_END(ConstantHoisting, "consthoist", "Constant Hoisting",
181 FunctionPass *llvm::createConstantHoistingPass() {
182 return new ConstantHoisting();
185 /// \brief Perform the constant hoisting optimization for the given function.
186 bool ConstantHoisting::runOnFunction(Function &Fn) {
187 DEBUG(dbgs() << "********** Begin Constant Hoisting **********\n");
188 DEBUG(dbgs() << "********** Function: " << Fn.getName() << '\n');
192 bool MadeChange = optimizeConstants(Fn);
195 DEBUG(dbgs() << "********** Function after Constant Hoisting: "
196 << Fn.getName() << '\n');
199 DEBUG(dbgs() << "********** End Constant Hoisting **********\n");
207 /// \brief Find the constant materialization insertion point.
208 Instruction *ConstantHoisting::findMatInsertPt(Instruction *Inst,
209 unsigned Idx) const {
210 // If the operand is a cast instruction, then we have to materialize the
211 // constant before the cast instruction.
213 Value *Opnd = Inst->getOperand(Idx);
214 if (auto CastInst = dyn_cast<Instruction>(Opnd))
215 if (CastInst->isCast())
219 // The simple and common case. This also includes constant expressions.
220 if (!isa<PHINode>(Inst) && !isa<LandingPadInst>(Inst))
223 // We can't insert directly before a phi node or landing pad. Insert before
224 // the terminator of the incoming or dominating block.
225 assert(Entry != Inst->getParent() && "PHI or landing pad in entry block!");
226 if (Idx != ~0U && isa<PHINode>(Inst))
227 return cast<PHINode>(Inst)->getIncomingBlock(Idx)->getTerminator();
229 BasicBlock *IDom = DT->getNode(Inst->getParent())->getIDom()->getBlock();
230 return IDom->getTerminator();
233 /// \brief Find an insertion point that dominates all uses.
234 Instruction *ConstantHoisting::
235 findConstantInsertionPoint(const ConstantInfo &ConstInfo) const {
236 assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry.");
237 // Collect all basic blocks.
238 SmallPtrSet<BasicBlock *, 8> BBs;
239 for (auto const &RCI : ConstInfo.RebasedConstants)
240 for (auto const &U : RCI.Uses)
241 BBs.insert(findMatInsertPt(U.Inst, U.OpndIdx)->getParent());
243 if (BBs.count(Entry))
244 return &Entry->front();
246 while (BBs.size() >= 2) {
247 BasicBlock *BB, *BB1, *BB2;
249 BB2 = *std::next(BBs.begin());
250 BB = DT->findNearestCommonDominator(BB1, BB2);
252 return &Entry->front();
257 assert((BBs.size() == 1) && "Expected only one element.");
258 Instruction &FirstInst = (*BBs.begin())->front();
259 return findMatInsertPt(&FirstInst);
263 /// \brief Record constant integer ConstInt for instruction Inst at operand
266 /// The operand at index Idx is not necessarily the constant integer itself. It
267 /// could also be a cast instruction or a constant expression that uses the
269 void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap,
272 ConstantInt *ConstInt) {
274 // Ask the target about the cost of materializing the constant for the given
275 // instruction and operand index.
276 if (auto IntrInst = dyn_cast<IntrinsicInst>(Inst))
277 Cost = TTI->getIntImmCost(IntrInst->getIntrinsicID(), Idx,
278 ConstInt->getValue(), ConstInt->getType());
280 Cost = TTI->getIntImmCost(Inst->getOpcode(), Idx, ConstInt->getValue(),
281 ConstInt->getType());
283 // Ignore cheap integer constants.
284 if (Cost > TargetTransformInfo::TCC_Basic) {
285 ConstCandMapType::iterator Itr;
287 std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(ConstInt, 0));
289 ConstCandVec.push_back(ConstantCandidate(ConstInt));
290 Itr->second = ConstCandVec.size() - 1;
292 ConstCandVec[Itr->second].addUser(Inst, Idx, Cost);
293 DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx)))
294 dbgs() << "Collect constant " << *ConstInt << " from " << *Inst
295 << " with cost " << Cost << '\n';
297 dbgs() << "Collect constant " << *ConstInt << " indirectly from "
298 << *Inst << " via " << *Inst->getOperand(Idx) << " with cost "
304 /// \brief Scan the instruction for expensive integer constants and record them
305 /// in the constant candidate vector.
306 void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap,
308 // Skip all cast instructions. They are visited indirectly later on.
312 // Can't handle inline asm. Skip it.
313 if (auto Call = dyn_cast<CallInst>(Inst))
314 if (isa<InlineAsm>(Call->getCalledValue()))
317 // Scan all operands.
318 for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) {
319 Value *Opnd = Inst->getOperand(Idx);
321 // Visit constant integers.
322 if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) {
323 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
327 // Visit cast instructions that have constant integers.
328 if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
329 // Only visit cast instructions, which have been skipped. All other
330 // instructions should have already been visited.
331 if (!CastInst->isCast())
334 if (auto *ConstInt = dyn_cast<ConstantInt>(CastInst->getOperand(0))) {
335 // Pretend the constant is directly used by the instruction and ignore
336 // the cast instruction.
337 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
342 // Visit constant expressions that have constant integers.
343 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
344 // Only visit constant cast expressions.
345 if (!ConstExpr->isCast())
348 if (auto ConstInt = dyn_cast<ConstantInt>(ConstExpr->getOperand(0))) {
349 // Pretend the constant is directly used by the instruction and ignore
350 // the constant expression.
351 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
355 } // end of for all operands
358 /// \brief Collect all integer constants in the function that cannot be folded
359 /// into an instruction itself.
360 void ConstantHoisting::collectConstantCandidates(Function &Fn) {
361 ConstCandMapType ConstCandMap;
362 for (Function::iterator BB : Fn)
363 for (BasicBlock::iterator Inst : *BB)
364 collectConstantCandidates(ConstCandMap, Inst);
367 /// \brief Find the base constant within the given range and rebase all other
368 /// constants with respect to the base constant.
369 void ConstantHoisting::findAndMakeBaseConstant(ConstCandVecType::iterator S,
370 ConstCandVecType::iterator E) {
372 unsigned NumUses = 0;
373 // Use the constant that has the maximum cost as base constant.
374 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
375 NumUses += ConstCand->Uses.size();
376 if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost)
377 MaxCostItr = ConstCand;
380 // Don't hoist constants that have only one use.
384 ConstantInfo ConstInfo;
385 ConstInfo.BaseConstant = MaxCostItr->ConstInt;
386 Type *Ty = ConstInfo.BaseConstant->getType();
388 // Rebase the constants with respect to the base constant.
389 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
390 APInt Diff = ConstCand->ConstInt->getValue() -
391 ConstInfo.BaseConstant->getValue();
392 Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff);
393 ConstInfo.RebasedConstants.push_back(
394 RebasedConstantInfo(std::move(ConstCand->Uses), Offset));
396 ConstantVec.push_back(ConstInfo);
399 /// \brief Finds and combines constant candidates that can be easily
400 /// rematerialized with an add from a common base constant.
401 void ConstantHoisting::findBaseConstants() {
402 // Sort the constants by value and type. This invalidates the mapping!
403 std::sort(ConstCandVec.begin(), ConstCandVec.end(),
404 [](const ConstantCandidate &LHS, const ConstantCandidate &RHS) {
405 if (LHS.ConstInt->getType() != RHS.ConstInt->getType())
406 return LHS.ConstInt->getType()->getBitWidth() <
407 RHS.ConstInt->getType()->getBitWidth();
408 return LHS.ConstInt->getValue().ult(RHS.ConstInt->getValue());
411 // Simple linear scan through the sorted constant candidate vector for viable
413 auto MinValItr = ConstCandVec.begin();
414 for (auto CC = std::next(ConstCandVec.begin()), E = ConstCandVec.end();
416 if (MinValItr->ConstInt->getType() == CC->ConstInt->getType()) {
417 // Check if the constant is in range of an add with immediate.
418 APInt Diff = CC->ConstInt->getValue() - MinValItr->ConstInt->getValue();
419 if ((Diff.getBitWidth() <= 64) &&
420 TTI->isLegalAddImmediate(Diff.getSExtValue()))
423 // We either have now a different constant type or the constant is not in
424 // range of an add with immediate anymore.
425 findAndMakeBaseConstant(MinValItr, CC);
426 // Start a new base constant search.
429 // Finalize the last base constant search.
430 findAndMakeBaseConstant(MinValItr, ConstCandVec.end());
433 /// \brief Updates the operand at Idx in instruction Inst with the result of
434 /// instruction Mat. If the instruction is a PHI node then special
435 /// handling for duplicate values form the same incomming basic block is
437 /// \return The update will always succeed, but the return value indicated if
438 /// Mat was used for the update or not.
439 static bool updateOperand(Instruction *Inst, unsigned Idx, Instruction *Mat) {
440 if (auto PHI = dyn_cast<PHINode>(Inst)) {
441 // Check if any previous operand of the PHI node has the same incoming basic
442 // block. This is a very odd case that happens when the incoming basic block
443 // has a switch statement. In this case use the same value as the previous
444 // operand(s), otherwise we will fail verification due to different values.
445 // The values are actually the same, but the variable names are different
446 // and the verifier doesn't like that.
447 BasicBlock *IncomingBB = PHI->getIncomingBlock(Idx);
448 for (unsigned i = 0; i < Idx; ++i) {
449 if (PHI->getIncomingBlock(i) == IncomingBB) {
450 Value *IncomingVal = PHI->getIncomingValue(i);
451 Inst->setOperand(Idx, IncomingVal);
457 Inst->setOperand(Idx, Mat);
461 /// \brief Emit materialization code for all rebased constants and update their
463 void ConstantHoisting::emitBaseConstants(Instruction *Base, Constant *Offset,
464 const ConstantUser &ConstUser) {
465 Instruction *Mat = Base;
467 Instruction *InsertionPt = findMatInsertPt(ConstUser.Inst,
469 Mat = BinaryOperator::Create(Instruction::Add, Base, Offset,
470 "const_mat", InsertionPt);
472 DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)
473 << " + " << *Offset << ") in BB "
474 << Mat->getParent()->getName() << '\n' << *Mat << '\n');
475 Mat->setDebugLoc(ConstUser.Inst->getDebugLoc());
477 Value *Opnd = ConstUser.Inst->getOperand(ConstUser.OpndIdx);
479 // Visit constant integer.
480 if (isa<ConstantInt>(Opnd)) {
481 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
482 if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat) && Offset)
483 Mat->eraseFromParent();
484 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
488 // Visit cast instruction.
489 if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
490 assert(CastInst->isCast() && "Expected an cast instruction!");
491 // Check if we already have visited this cast instruction before to avoid
492 // unnecessary cloning.
493 Instruction *&ClonedCastInst = ClonedCastMap[CastInst];
494 if (!ClonedCastInst) {
495 ClonedCastInst = CastInst->clone();
496 ClonedCastInst->setOperand(0, Mat);
497 ClonedCastInst->insertAfter(CastInst);
498 // Use the same debug location as the original cast instruction.
499 ClonedCastInst->setDebugLoc(CastInst->getDebugLoc());
500 DEBUG(dbgs() << "Clone instruction: " << *CastInst << '\n'
501 << "To : " << *ClonedCastInst << '\n');
504 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
505 updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ClonedCastInst);
506 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
510 // Visit constant expression.
511 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
512 Instruction *ConstExprInst = ConstExpr->getAsInstruction();
513 ConstExprInst->setOperand(0, Mat);
514 ConstExprInst->insertBefore(findMatInsertPt(ConstUser.Inst,
517 // Use the same debug location as the instruction we are about to update.
518 ConstExprInst->setDebugLoc(ConstUser.Inst->getDebugLoc());
520 DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n'
521 << "From : " << *ConstExpr << '\n');
522 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
523 if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ConstExprInst)) {
524 ConstExprInst->eraseFromParent();
526 Mat->eraseFromParent();
528 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
533 /// \brief Hoist and hide the base constant behind a bitcast and emit
534 /// materialization code for derived constants.
535 bool ConstantHoisting::emitBaseConstants() {
536 bool MadeChange = false;
537 for (auto const &ConstInfo : ConstantVec) {
538 // Hoist and hide the base constant behind a bitcast.
539 Instruction *IP = findConstantInsertionPoint(ConstInfo);
540 IntegerType *Ty = ConstInfo.BaseConstant->getType();
542 new BitCastInst(ConstInfo.BaseConstant, Ty, "const", IP);
543 DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseConstant << ") to BB "
544 << IP->getParent()->getName() << '\n' << *Base << '\n');
545 NumConstantsHoisted++;
547 // Emit materialization code for all rebased constants.
548 for (auto const &RCI : ConstInfo.RebasedConstants) {
549 NumConstantsRebased++;
550 for (auto const &U : RCI.Uses)
551 emitBaseConstants(Base, RCI.Offset, U);
554 // Use the same debug location as the last user of the constant.
555 assert(!Base->use_empty() && "The use list is empty!?");
556 assert(isa<Instruction>(Base->user_back()) &&
557 "All uses should be instructions.");
558 Base->setDebugLoc(cast<Instruction>(Base->user_back())->getDebugLoc());
560 // Correct for base constant, which we counted above too.
561 NumConstantsRebased--;
567 /// \brief Check all cast instructions we made a copy of and remove them if they
568 /// have no more users.
569 void ConstantHoisting::deleteDeadCastInst() const {
570 for (auto const &I : ClonedCastMap)
571 if (I.first->use_empty())
572 I.first->eraseFromParent();
575 /// \brief Optimize expensive integer constants in the given function.
576 bool ConstantHoisting::optimizeConstants(Function &Fn) {
577 // Collect all constant candidates.
578 collectConstantCandidates(Fn);
580 // There are no constant candidates to worry about.
581 if (ConstCandVec.empty())
584 // Combine constants that can be easily materialized with an add from a common
588 // There are no constants to emit.
589 if (ConstantVec.empty())
592 // Finally hoist the base constant and emit materialization code for dependent
594 bool MadeChange = emitBaseConstants();
596 // Cleanup dead instructions.
597 deleteDeadCastInst();