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 DEBUG(dbgs() << "********** Begin Constant Hoisting **********\n");
190 DEBUG(dbgs() << "********** Function: " << Fn.getName() << '\n');
194 bool MadeChange = optimizeConstants(Fn);
197 DEBUG(dbgs() << "********** Function after Constant Hoisting: "
198 << Fn.getName() << '\n');
201 DEBUG(dbgs() << "********** End Constant Hoisting **********\n");
209 /// \brief Find the constant materialization insertion point.
210 Instruction *ConstantHoisting::findMatInsertPt(Instruction *Inst,
211 unsigned Idx) const {
212 // If the operand is a cast instruction, then we have to materialize the
213 // constant before the cast instruction.
215 Value *Opnd = Inst->getOperand(Idx);
216 if (auto CastInst = dyn_cast<Instruction>(Opnd))
217 if (CastInst->isCast())
221 // The simple and common case. This also includes constant expressions.
222 if (!isa<PHINode>(Inst) && !isa<LandingPadInst>(Inst))
225 // We can't insert directly before a phi node or landing pad. Insert before
226 // the terminator of the incoming or dominating block.
227 assert(Entry != Inst->getParent() && "PHI or landing pad in entry block!");
228 if (Idx != ~0U && isa<PHINode>(Inst))
229 return cast<PHINode>(Inst)->getIncomingBlock(Idx)->getTerminator();
231 BasicBlock *IDom = DT->getNode(Inst->getParent())->getIDom()->getBlock();
232 return IDom->getTerminator();
235 /// \brief Find an insertion point that dominates all uses.
236 Instruction *ConstantHoisting::
237 findConstantInsertionPoint(const ConstantInfo &ConstInfo) const {
238 assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry.");
239 // Collect all basic blocks.
240 SmallPtrSet<BasicBlock *, 8> BBs;
241 for (auto const &RCI : ConstInfo.RebasedConstants)
242 for (auto const &U : RCI.Uses)
243 BBs.insert(findMatInsertPt(U.Inst, U.OpndIdx)->getParent());
245 if (BBs.count(Entry))
246 return &Entry->front();
248 while (BBs.size() >= 2) {
249 BasicBlock *BB, *BB1, *BB2;
251 BB2 = *std::next(BBs.begin());
252 BB = DT->findNearestCommonDominator(BB1, BB2);
254 return &Entry->front();
259 assert((BBs.size() == 1) && "Expected only one element.");
260 Instruction &FirstInst = (*BBs.begin())->front();
261 return findMatInsertPt(&FirstInst);
265 /// \brief Record constant integer ConstInt for instruction Inst at operand
268 /// The operand at index Idx is not necessarily the constant integer itself. It
269 /// could also be a cast instruction or a constant expression that uses the
271 void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap,
274 ConstantInt *ConstInt) {
276 // Ask the target about the cost of materializing the constant for the given
277 // instruction and operand index.
278 if (auto IntrInst = dyn_cast<IntrinsicInst>(Inst))
279 Cost = TTI->getIntImmCost(IntrInst->getIntrinsicID(), Idx,
280 ConstInt->getValue(), ConstInt->getType());
282 Cost = TTI->getIntImmCost(Inst->getOpcode(), Idx, ConstInt->getValue(),
283 ConstInt->getType());
285 // Ignore cheap integer constants.
286 if (Cost > TargetTransformInfo::TCC_Basic) {
287 ConstCandMapType::iterator Itr;
289 std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(ConstInt, 0));
291 ConstCandVec.push_back(ConstantCandidate(ConstInt));
292 Itr->second = ConstCandVec.size() - 1;
294 ConstCandVec[Itr->second].addUser(Inst, Idx, Cost);
295 DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx)))
296 dbgs() << "Collect constant " << *ConstInt << " from " << *Inst
297 << " with cost " << Cost << '\n';
299 dbgs() << "Collect constant " << *ConstInt << " indirectly from "
300 << *Inst << " via " << *Inst->getOperand(Idx) << " with cost "
306 /// \brief Scan the instruction for expensive integer constants and record them
307 /// in the constant candidate vector.
308 void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap,
310 // Skip all cast instructions. They are visited indirectly later on.
314 // Can't handle inline asm. Skip it.
315 if (auto Call = dyn_cast<CallInst>(Inst))
316 if (isa<InlineAsm>(Call->getCalledValue()))
319 // Scan all operands.
320 for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) {
321 Value *Opnd = Inst->getOperand(Idx);
323 // Visit constant integers.
324 if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) {
325 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
329 // Visit cast instructions that have constant integers.
330 if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
331 // Only visit cast instructions, which have been skipped. All other
332 // instructions should have already been visited.
333 if (!CastInst->isCast())
336 if (auto *ConstInt = dyn_cast<ConstantInt>(CastInst->getOperand(0))) {
337 // Pretend the constant is directly used by the instruction and ignore
338 // the cast instruction.
339 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
344 // Visit constant expressions that have constant integers.
345 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
346 // Only visit constant cast expressions.
347 if (!ConstExpr->isCast())
350 if (auto ConstInt = dyn_cast<ConstantInt>(ConstExpr->getOperand(0))) {
351 // Pretend the constant is directly used by the instruction and ignore
352 // the constant expression.
353 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
357 } // end of for all operands
360 /// \brief Collect all integer constants in the function that cannot be folded
361 /// into an instruction itself.
362 void ConstantHoisting::collectConstantCandidates(Function &Fn) {
363 ConstCandMapType ConstCandMap;
364 for (Function::iterator BB : Fn)
365 for (BasicBlock::iterator Inst : *BB)
366 collectConstantCandidates(ConstCandMap, Inst);
369 /// \brief Find the base constant within the given range and rebase all other
370 /// constants with respect to the base constant.
371 void ConstantHoisting::findAndMakeBaseConstant(ConstCandVecType::iterator S,
372 ConstCandVecType::iterator E) {
374 unsigned NumUses = 0;
375 // Use the constant that has the maximum cost as base constant.
376 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
377 NumUses += ConstCand->Uses.size();
378 if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost)
379 MaxCostItr = ConstCand;
382 // Don't hoist constants that have only one use.
386 ConstantInfo ConstInfo;
387 ConstInfo.BaseConstant = MaxCostItr->ConstInt;
388 Type *Ty = ConstInfo.BaseConstant->getType();
390 // Rebase the constants with respect to the base constant.
391 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
392 APInt Diff = ConstCand->ConstInt->getValue() -
393 ConstInfo.BaseConstant->getValue();
394 Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff);
395 ConstInfo.RebasedConstants.push_back(
396 RebasedConstantInfo(std::move(ConstCand->Uses), Offset));
398 ConstantVec.push_back(std::move(ConstInfo));
401 /// \brief Finds and combines constant candidates that can be easily
402 /// rematerialized with an add from a common base constant.
403 void ConstantHoisting::findBaseConstants() {
404 // Sort the constants by value and type. This invalidates the mapping!
405 std::sort(ConstCandVec.begin(), ConstCandVec.end(),
406 [](const ConstantCandidate &LHS, const ConstantCandidate &RHS) {
407 if (LHS.ConstInt->getType() != RHS.ConstInt->getType())
408 return LHS.ConstInt->getType()->getBitWidth() <
409 RHS.ConstInt->getType()->getBitWidth();
410 return LHS.ConstInt->getValue().ult(RHS.ConstInt->getValue());
413 // Simple linear scan through the sorted constant candidate vector for viable
415 auto MinValItr = ConstCandVec.begin();
416 for (auto CC = std::next(ConstCandVec.begin()), E = ConstCandVec.end();
418 if (MinValItr->ConstInt->getType() == CC->ConstInt->getType()) {
419 // Check if the constant is in range of an add with immediate.
420 APInt Diff = CC->ConstInt->getValue() - MinValItr->ConstInt->getValue();
421 if ((Diff.getBitWidth() <= 64) &&
422 TTI->isLegalAddImmediate(Diff.getSExtValue()))
425 // We either have now a different constant type or the constant is not in
426 // range of an add with immediate anymore.
427 findAndMakeBaseConstant(MinValItr, CC);
428 // Start a new base constant search.
431 // Finalize the last base constant search.
432 findAndMakeBaseConstant(MinValItr, ConstCandVec.end());
435 /// \brief Updates the operand at Idx in instruction Inst with the result of
436 /// instruction Mat. If the instruction is a PHI node then special
437 /// handling for duplicate values form the same incomming basic block is
439 /// \return The update will always succeed, but the return value indicated if
440 /// Mat was used for the update or not.
441 static bool updateOperand(Instruction *Inst, unsigned Idx, Instruction *Mat) {
442 if (auto PHI = dyn_cast<PHINode>(Inst)) {
443 // Check if any previous operand of the PHI node has the same incoming basic
444 // block. This is a very odd case that happens when the incoming basic block
445 // has a switch statement. In this case use the same value as the previous
446 // operand(s), otherwise we will fail verification due to different values.
447 // The values are actually the same, but the variable names are different
448 // and the verifier doesn't like that.
449 BasicBlock *IncomingBB = PHI->getIncomingBlock(Idx);
450 for (unsigned i = 0; i < Idx; ++i) {
451 if (PHI->getIncomingBlock(i) == IncomingBB) {
452 Value *IncomingVal = PHI->getIncomingValue(i);
453 Inst->setOperand(Idx, IncomingVal);
459 Inst->setOperand(Idx, Mat);
463 /// \brief Emit materialization code for all rebased constants and update their
465 void ConstantHoisting::emitBaseConstants(Instruction *Base, Constant *Offset,
466 const ConstantUser &ConstUser) {
467 Instruction *Mat = Base;
469 Instruction *InsertionPt = findMatInsertPt(ConstUser.Inst,
471 Mat = BinaryOperator::Create(Instruction::Add, Base, Offset,
472 "const_mat", InsertionPt);
474 DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)
475 << " + " << *Offset << ") in BB "
476 << Mat->getParent()->getName() << '\n' << *Mat << '\n');
477 Mat->setDebugLoc(ConstUser.Inst->getDebugLoc());
479 Value *Opnd = ConstUser.Inst->getOperand(ConstUser.OpndIdx);
481 // Visit constant integer.
482 if (isa<ConstantInt>(Opnd)) {
483 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
484 if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat) && Offset)
485 Mat->eraseFromParent();
486 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
490 // Visit cast instruction.
491 if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
492 assert(CastInst->isCast() && "Expected an cast instruction!");
493 // Check if we already have visited this cast instruction before to avoid
494 // unnecessary cloning.
495 Instruction *&ClonedCastInst = ClonedCastMap[CastInst];
496 if (!ClonedCastInst) {
497 ClonedCastInst = CastInst->clone();
498 ClonedCastInst->setOperand(0, Mat);
499 ClonedCastInst->insertAfter(CastInst);
500 // Use the same debug location as the original cast instruction.
501 ClonedCastInst->setDebugLoc(CastInst->getDebugLoc());
502 DEBUG(dbgs() << "Clone instruction: " << *CastInst << '\n'
503 << "To : " << *ClonedCastInst << '\n');
506 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
507 updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ClonedCastInst);
508 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
512 // Visit constant expression.
513 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
514 Instruction *ConstExprInst = ConstExpr->getAsInstruction();
515 ConstExprInst->setOperand(0, Mat);
516 ConstExprInst->insertBefore(findMatInsertPt(ConstUser.Inst,
519 // Use the same debug location as the instruction we are about to update.
520 ConstExprInst->setDebugLoc(ConstUser.Inst->getDebugLoc());
522 DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n'
523 << "From : " << *ConstExpr << '\n');
524 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
525 if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ConstExprInst)) {
526 ConstExprInst->eraseFromParent();
528 Mat->eraseFromParent();
530 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
535 /// \brief Hoist and hide the base constant behind a bitcast and emit
536 /// materialization code for derived constants.
537 bool ConstantHoisting::emitBaseConstants() {
538 bool MadeChange = false;
539 for (auto const &ConstInfo : ConstantVec) {
540 // Hoist and hide the base constant behind a bitcast.
541 Instruction *IP = findConstantInsertionPoint(ConstInfo);
542 IntegerType *Ty = ConstInfo.BaseConstant->getType();
544 new BitCastInst(ConstInfo.BaseConstant, Ty, "const", IP);
545 DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseConstant << ") to BB "
546 << IP->getParent()->getName() << '\n' << *Base << '\n');
547 NumConstantsHoisted++;
549 // Emit materialization code for all rebased constants.
550 for (auto const &RCI : ConstInfo.RebasedConstants) {
551 NumConstantsRebased++;
552 for (auto const &U : RCI.Uses)
553 emitBaseConstants(Base, RCI.Offset, U);
556 // Use the same debug location as the last user of the constant.
557 assert(!Base->use_empty() && "The use list is empty!?");
558 assert(isa<Instruction>(Base->user_back()) &&
559 "All uses should be instructions.");
560 Base->setDebugLoc(cast<Instruction>(Base->user_back())->getDebugLoc());
562 // Correct for base constant, which we counted above too.
563 NumConstantsRebased--;
569 /// \brief Check all cast instructions we made a copy of and remove them if they
570 /// have no more users.
571 void ConstantHoisting::deleteDeadCastInst() const {
572 for (auto const &I : ClonedCastMap)
573 if (I.first->use_empty())
574 I.first->eraseFromParent();
577 /// \brief Optimize expensive integer constants in the given function.
578 bool ConstantHoisting::optimizeConstants(Function &Fn) {
579 // Collect all constant candidates.
580 collectConstantCandidates(Fn);
582 // There are no constant candidates to worry about.
583 if (ConstCandVec.empty())
586 // Combine constants that can be easily materialized with an add from a common
590 // There are no constants to emit.
591 if (ConstantVec.empty())
594 // Finally hoist the base constant and emit materialization code for dependent
596 bool MadeChange = emitBaseConstants();
598 // Cleanup dead instructions.
599 deleteDeadCastInst();