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 experessions. 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 usees.
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 ConstCandMapType ConstCandMap;
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(0), DT(0), Entry(0) {
123 initializeConstantHoistingPass(*PassRegistry::getPassRegistry());
126 bool runOnFunction(Function &Fn) override;
128 const char *getPassName() const override { return "Constant Hoisting"; }
130 void getAnalysisUsage(AnalysisUsage &AU) const override {
131 AU.setPreservesCFG();
132 AU.addRequired<DominatorTreeWrapperPass>();
133 AU.addRequired<TargetTransformInfo>();
137 /// \brief Initialize the pass.
138 void setup(Function &Fn) {
139 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
140 TTI = &getAnalysis<TargetTransformInfo>();
141 Entry = &Fn.getEntryBlock();
147 ClonedCastMap.clear();
148 ConstCandVec.clear();
149 ConstCandMap.clear();
156 /// \brief Find the common dominator of all uses and cache the result for
158 BasicBlock *getIDom(const RebasedConstantInfo &RCI) const {
161 RCI.IDom = findIDomOfAllUses(RCI.Uses);
162 assert(RCI.IDom && "Invalid IDom.");
166 BasicBlock *findIDomOfAllUses(const ConstantUseListType &Uses) const;
167 Instruction *findMatInsertPt(Instruction *Inst, unsigned Idx = ~0U) const;
168 Instruction *findConstantInsertionPoint(const ConstantInfo &ConstInfo) const;
169 void collectConstantCandidates(Instruction *Inst, unsigned Idx,
170 ConstantInt *ConstInt);
171 void collectConstantCandidates(Instruction *Inst);
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 inetger itself. It
296 /// could also be a cast instruction or a constant expression that uses the
298 void ConstantHoisting::collectConstantCandidates(Instruction *Inst,
300 ConstantInt *ConstInt) {
302 // Ask the target about the cost of materializing the constant for the given
304 if (auto IntrInst = dyn_cast<IntrinsicInst>(Inst))
305 Cost = TTI->getIntImmCost(IntrInst->getIntrinsicID(),
306 ConstInt->getValue(), ConstInt->getType());
308 Cost = TTI->getIntImmCost(Inst->getOpcode(), ConstInt->getValue(),
309 ConstInt->getType());
311 // Ignore cheap integer constants.
312 if (Cost > TargetTransformInfo::TCC_Basic) {
313 ConstCandMapType::iterator Itr;
315 std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(ConstInt, 0));
317 ConstCandVec.push_back(ConstantCandidate(ConstInt));
318 Itr->second = ConstCandVec.size() - 1;
320 ConstCandVec[Itr->second].addUser(Inst, Idx, Cost);
321 DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx)))
322 dbgs() << "Collect constant " << *ConstInt << " from " << *Inst
323 << " with cost " << Cost << '\n';
325 dbgs() << "Collect constant " << *ConstInt << " indirectly from "
326 << *Inst << " via " << *Inst->getOperand(Idx) << " with cost "
332 /// \brief Scan the instruction for expensive integer constants and record them
333 /// in the constant candidate vector.
334 void ConstantHoisting::collectConstantCandidates(Instruction *Inst) {
335 // Skip all cast instructions. They are visited indirectly later on.
339 // Can't handle inline asm. Skip it.
340 if (auto Call = dyn_cast<CallInst>(Inst))
341 if (isa<InlineAsm>(Call->getCalledValue()))
344 // Scan all operands.
345 for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) {
346 Value *Opnd = Inst->getOperand(Idx);
348 // Vist constant integers.
349 if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) {
350 collectConstantCandidates(Inst, Idx, ConstInt);
354 // Visit cast instructions that have constant integers.
355 if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
356 // Only visit cast instructions, which have been skipped. All other
357 // instructions should have already been visited.
358 if (!CastInst->isCast())
361 if (auto *ConstInt = dyn_cast<ConstantInt>(CastInst->getOperand(0))) {
362 // Pretend the constant is directly used by the instruction and ignore
363 // the cast instruction.
364 collectConstantCandidates(Inst, Idx, ConstInt);
369 // Visit constant expressions that have constant integers.
370 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
371 // Only visit constant cast expressions.
372 if (!ConstExpr->isCast())
375 if (auto ConstInt = dyn_cast<ConstantInt>(ConstExpr->getOperand(0))) {
376 // Pretend the constant is directly used by the instruction and ignore
377 // the constant expression.
378 collectConstantCandidates(Inst, Idx, ConstInt);
382 } // end of for all operands
385 /// \brief Collect all integer constants in the function that cannot be folded
386 /// into an instruction itself.
387 void ConstantHoisting::collectConstantCandidates(Function &Fn) {
388 for (Function::iterator BB : Fn)
389 for (BasicBlock::iterator Inst : *BB)
390 collectConstantCandidates(Inst);
393 /// \brief Find the base constant within the given range and rebase all other
394 /// constants with respect to the base constant.
395 void ConstantHoisting::findAndMakeBaseConstant(ConstCandVecType::iterator S,
396 ConstCandVecType::iterator E) {
398 unsigned NumUses = 0;
399 // Use the constant that has the maximum cost as base constant.
400 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
401 NumUses += ConstCand->Uses.size();
402 if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost)
403 MaxCostItr = ConstCand;
406 // Don't hoist constants that have only one use.
410 ConstantInfo ConstInfo;
411 ConstInfo.BaseConstant = MaxCostItr->ConstInt;
412 Type *Ty = ConstInfo.BaseConstant->getType();
414 // Rebase the constants with respect to the base constant.
415 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
416 APInt Diff = ConstCand->ConstInt->getValue() -
417 ConstInfo.BaseConstant->getValue();
418 Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff);
419 ConstInfo.RebasedConstants.push_back(
420 RebasedConstantInfo(std::move(ConstCand->Uses), Offset));
422 ConstantVec.push_back(ConstInfo);
425 /// \brief Finds and combines constant candidates that can be easily
426 /// rematerialized with an add from a common base constant.
427 void ConstantHoisting::findBaseConstants() {
428 // Sort the constants by value and type. This invalidates the mapping!
429 std::sort(ConstCandVec.begin(), ConstCandVec.end(),
430 [](const ConstantCandidate &LHS, const ConstantCandidate &RHS) {
431 if (LHS.ConstInt->getType() != RHS.ConstInt->getType())
432 return LHS.ConstInt->getType()->getBitWidth() <
433 RHS.ConstInt->getType()->getBitWidth();
434 return LHS.ConstInt->getValue().ult(RHS.ConstInt->getValue());
437 // Simple linear scan through the sorted constant candidate vector for viable
439 auto MinValItr = ConstCandVec.begin();
440 for (auto CC = std::next(ConstCandVec.begin()), E = ConstCandVec.end();
442 if (MinValItr->ConstInt->getType() == CC->ConstInt->getType()) {
443 // Check if the constant is in range of an add with immediate.
444 APInt Diff = CC->ConstInt->getValue() - MinValItr->ConstInt->getValue();
445 if ((Diff.getBitWidth() <= 64) &&
446 TTI->isLegalAddImmediate(Diff.getSExtValue()))
449 // We either have now a different constant type or the constant is not in
450 // range of an add with immediate anymore.
451 findAndMakeBaseConstant(MinValItr, CC);
452 // Start a new base constant search.
455 // Finalize the last base constant search.
456 findAndMakeBaseConstant(MinValItr, ConstCandVec.end());
459 /// \brief Emit materialization code for all rebased constants and update their
461 void ConstantHoisting::emitBaseConstants(Instruction *Base, Constant *Offset,
462 const ConstantUser &ConstUser) {
463 Instruction *Mat = Base;
465 Instruction *InsertionPt = findMatInsertPt(ConstUser.Inst,
467 Mat = BinaryOperator::Create(Instruction::Add, Base, Offset,
468 "const_mat", InsertionPt);
470 DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)
471 << " + " << *Offset << ") in BB "
472 << Mat->getParent()->getName() << '\n' << *Mat << '\n');
473 Mat->setDebugLoc(ConstUser.Inst->getDebugLoc());
475 Value *Opnd = ConstUser.Inst->getOperand(ConstUser.OpndIdx);
477 // Visit constant integer.
478 if (isa<ConstantInt>(Opnd)) {
479 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
480 ConstUser.Inst->setOperand(ConstUser.OpndIdx, Mat);
481 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
485 // Visit cast instruction.
486 if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
487 assert(CastInst->isCast() && "Expected an cast instruction!");
488 // Check if we already have visited this cast instruction before to avoid
489 // unnecessary cloning.
490 Instruction *&ClonedCastInst = ClonedCastMap[CastInst];
491 if (!ClonedCastInst) {
492 ClonedCastInst = CastInst->clone();
493 ClonedCastInst->setOperand(0, Mat);
494 ClonedCastInst->insertAfter(CastInst);
495 // Use the same debug location as the original cast instruction.
496 ClonedCastInst->setDebugLoc(CastInst->getDebugLoc());
497 DEBUG(dbgs() << "Clone instruction: " << *ClonedCastInst << '\n'
498 << "To : " << *CastInst << '\n');
501 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
502 ConstUser.Inst->setOperand(ConstUser.OpndIdx, ClonedCastInst);
503 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
507 // Visit constant expression.
508 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
509 Instruction *ConstExprInst = ConstExpr->getAsInstruction();
510 ConstExprInst->setOperand(0, Mat);
511 ConstExprInst->insertBefore(findMatInsertPt(ConstUser.Inst,
514 // Use the same debug location as the instruction we are about to update.
515 ConstExprInst->setDebugLoc(ConstUser.Inst->getDebugLoc());
517 DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n'
518 << "From : " << *ConstExpr << '\n');
519 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
520 ConstUser.Inst->setOperand(ConstUser.OpndIdx, ConstExprInst);
521 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
526 /// \brief Hoist and hide the base constant behind a bitcast and emit
527 /// materialization code for derived constants.
528 bool ConstantHoisting::emitBaseConstants() {
529 bool MadeChange = false;
530 for (auto const &ConstInfo : ConstantVec) {
531 // Hoist and hide the base constant behind a bitcast.
532 Instruction *IP = findConstantInsertionPoint(ConstInfo);
533 IntegerType *Ty = ConstInfo.BaseConstant->getType();
535 new BitCastInst(ConstInfo.BaseConstant, Ty, "const", IP);
536 DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseConstant << ") to BB "
537 << IP->getParent()->getName() << '\n' << *Base << '\n');
538 NumConstantsHoisted++;
540 // Emit materialization code for all rebased constants.
541 for (auto const &RCI : ConstInfo.RebasedConstants) {
542 NumConstantsRebased++;
543 for (auto const &U : RCI.Uses)
544 emitBaseConstants(Base, RCI.Offset, U);
547 // Use the same debug location as the last user of the constant.
548 assert(!Base->use_empty() && "The use list is empty!?");
549 assert(isa<Instruction>(Base->user_back()) &&
550 "All uses should be instructions.");
551 Base->setDebugLoc(cast<Instruction>(Base->user_back())->getDebugLoc());
553 // Correct for base constant, which we counted above too.
554 NumConstantsRebased--;
560 /// \brief Check all cast instructions we made a copy of and remove them if they
561 /// have no more users.
562 void ConstantHoisting::deleteDeadCastInst() const {
563 for (auto const &I : ClonedCastMap)
564 if (I.first->use_empty())
565 I.first->removeFromParent();
568 /// \brief Optimize expensive integer constants in the given function.
569 bool ConstantHoisting::optimizeConstants(Function &Fn) {
570 // Collect all constant candidates.
571 collectConstantCandidates(Fn);
573 // There are no constant candidates to worry about.
574 if (ConstCandVec.empty())
577 // Combine constants that can be easily materialized with an add from a common
581 // There are no constants to emit.
582 if (ConstantVec.empty())
585 // Finally hoist the base constant and emit materializating code for dependent
587 bool MadeChange = emitBaseConstants();
589 // Cleanup dead instructions.
590 deleteDeadCastInst();