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 // The simple and common case.
211 if (!isa<PHINode>(Inst) && !isa<LandingPadInst>(Inst))
214 // We can't insert directly before a phi node or landing pad. Insert before
215 // the terminator of the incoming or dominating block.
216 assert(Entry != Inst->getParent() && "PHI or landing pad in entry block!");
217 if (Idx != ~0U && isa<PHINode>(Inst))
218 return cast<PHINode>(Inst)->getIncomingBlock(Idx)->getTerminator();
220 BasicBlock *IDom = DT->getNode(Inst->getParent())->getIDom()->getBlock();
221 return IDom->getTerminator();
224 /// \brief Find an insertion point that dominates all uses.
225 Instruction *ConstantHoisting::
226 findConstantInsertionPoint(const ConstantInfo &ConstInfo) const {
227 assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry.");
228 // Collect all basic blocks.
229 SmallPtrSet<BasicBlock *, 8> BBs;
230 for (auto const &RCI : ConstInfo.RebasedConstants)
231 for (auto const &U : RCI.Uses)
232 BBs.insert(U.Inst->getParent());
234 if (BBs.count(Entry))
235 return &Entry->front();
237 while (BBs.size() >= 2) {
238 BasicBlock *BB, *BB1, *BB2;
240 BB2 = *std::next(BBs.begin());
241 BB = DT->findNearestCommonDominator(BB1, BB2);
243 return &Entry->front();
248 assert((BBs.size() == 1) && "Expected only one element.");
249 Instruction &FirstInst = (*BBs.begin())->front();
250 return findMatInsertPt(&FirstInst);
254 /// \brief Record constant integer ConstInt for instruction Inst at operand
257 /// The operand at index Idx is not necessarily the constant integer itself. It
258 /// could also be a cast instruction or a constant expression that uses the
260 void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap,
263 ConstantInt *ConstInt) {
265 // Ask the target about the cost of materializing the constant for the given
266 // instruction and operand index.
267 if (auto IntrInst = dyn_cast<IntrinsicInst>(Inst))
268 Cost = TTI->getIntImmCost(IntrInst->getIntrinsicID(), Idx,
269 ConstInt->getValue(), ConstInt->getType());
271 Cost = TTI->getIntImmCost(Inst->getOpcode(), Idx, ConstInt->getValue(),
272 ConstInt->getType());
274 // Ignore cheap integer constants.
275 if (Cost > TargetTransformInfo::TCC_Basic) {
276 ConstCandMapType::iterator Itr;
278 std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(ConstInt, 0));
280 ConstCandVec.push_back(ConstantCandidate(ConstInt));
281 Itr->second = ConstCandVec.size() - 1;
283 ConstCandVec[Itr->second].addUser(Inst, Idx, Cost);
284 DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx)))
285 dbgs() << "Collect constant " << *ConstInt << " from " << *Inst
286 << " with cost " << Cost << '\n';
288 dbgs() << "Collect constant " << *ConstInt << " indirectly from "
289 << *Inst << " via " << *Inst->getOperand(Idx) << " with cost "
295 /// \brief Scan the instruction for expensive integer constants and record them
296 /// in the constant candidate vector.
297 void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap,
299 // Skip all cast instructions. They are visited indirectly later on.
303 // Can't handle inline asm. Skip it.
304 if (auto Call = dyn_cast<CallInst>(Inst))
305 if (isa<InlineAsm>(Call->getCalledValue()))
308 // Scan all operands.
309 for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) {
310 Value *Opnd = Inst->getOperand(Idx);
312 // Visit constant integers.
313 if (auto ConstInt = dyn_cast<ConstantInt>(Opnd)) {
314 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
318 // Visit cast instructions that have constant integers.
319 if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
320 // Only visit cast instructions, which have been skipped. All other
321 // instructions should have already been visited.
322 if (!CastInst->isCast())
325 if (auto *ConstInt = dyn_cast<ConstantInt>(CastInst->getOperand(0))) {
326 // Pretend the constant is directly used by the instruction and ignore
327 // the cast instruction.
328 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
333 // Visit constant expressions that have constant integers.
334 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
335 // Only visit constant cast expressions.
336 if (!ConstExpr->isCast())
339 if (auto ConstInt = dyn_cast<ConstantInt>(ConstExpr->getOperand(0))) {
340 // Pretend the constant is directly used by the instruction and ignore
341 // the constant expression.
342 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
346 } // end of for all operands
349 /// \brief Collect all integer constants in the function that cannot be folded
350 /// into an instruction itself.
351 void ConstantHoisting::collectConstantCandidates(Function &Fn) {
352 ConstCandMapType ConstCandMap;
353 for (Function::iterator BB : Fn)
354 for (BasicBlock::iterator Inst : *BB)
355 collectConstantCandidates(ConstCandMap, Inst);
358 /// \brief Find the base constant within the given range and rebase all other
359 /// constants with respect to the base constant.
360 void ConstantHoisting::findAndMakeBaseConstant(ConstCandVecType::iterator S,
361 ConstCandVecType::iterator E) {
363 unsigned NumUses = 0;
364 // Use the constant that has the maximum cost as base constant.
365 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
366 NumUses += ConstCand->Uses.size();
367 if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost)
368 MaxCostItr = ConstCand;
371 // Don't hoist constants that have only one use.
375 ConstantInfo ConstInfo;
376 ConstInfo.BaseConstant = MaxCostItr->ConstInt;
377 Type *Ty = ConstInfo.BaseConstant->getType();
379 // Rebase the constants with respect to the base constant.
380 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
381 APInt Diff = ConstCand->ConstInt->getValue() -
382 ConstInfo.BaseConstant->getValue();
383 Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, Diff);
384 ConstInfo.RebasedConstants.push_back(
385 RebasedConstantInfo(std::move(ConstCand->Uses), Offset));
387 ConstantVec.push_back(ConstInfo);
390 /// \brief Finds and combines constant candidates that can be easily
391 /// rematerialized with an add from a common base constant.
392 void ConstantHoisting::findBaseConstants() {
393 // Sort the constants by value and type. This invalidates the mapping!
394 std::sort(ConstCandVec.begin(), ConstCandVec.end(),
395 [](const ConstantCandidate &LHS, const ConstantCandidate &RHS) {
396 if (LHS.ConstInt->getType() != RHS.ConstInt->getType())
397 return LHS.ConstInt->getType()->getBitWidth() <
398 RHS.ConstInt->getType()->getBitWidth();
399 return LHS.ConstInt->getValue().ult(RHS.ConstInt->getValue());
402 // Simple linear scan through the sorted constant candidate vector for viable
404 auto MinValItr = ConstCandVec.begin();
405 for (auto CC = std::next(ConstCandVec.begin()), E = ConstCandVec.end();
407 if (MinValItr->ConstInt->getType() == CC->ConstInt->getType()) {
408 // Check if the constant is in range of an add with immediate.
409 APInt Diff = CC->ConstInt->getValue() - MinValItr->ConstInt->getValue();
410 if ((Diff.getBitWidth() <= 64) &&
411 TTI->isLegalAddImmediate(Diff.getSExtValue()))
414 // We either have now a different constant type or the constant is not in
415 // range of an add with immediate anymore.
416 findAndMakeBaseConstant(MinValItr, CC);
417 // Start a new base constant search.
420 // Finalize the last base constant search.
421 findAndMakeBaseConstant(MinValItr, ConstCandVec.end());
424 /// \brief Updates the operand at Idx in instruction Inst with the result of
425 /// instruction Mat. If the instruction is a PHI node then special
426 /// handling for duplicate values form the same incomming basic block is
428 /// \return The update will always succeed, but the return value indicated if
429 /// Mat was used for the update or not.
430 static bool updateOperand(Instruction *Inst, unsigned Idx, Instruction *Mat) {
431 if (auto PHI = dyn_cast<PHINode>(Inst)) {
432 // Check if any previous operand of the PHI node has the same incoming basic
433 // block. This is a very odd case that happens when the incoming basic block
434 // has a switch statement. In this case use the same value as the previous
435 // operand(s), otherwise we will fail verification due to different values.
436 // The values are actually the same, but the variable names are different
437 // and the verifier doesn't like that.
438 BasicBlock *IncomingBB = PHI->getIncomingBlock(Idx);
439 for (unsigned i = 0; i < Idx; ++i) {
440 if (PHI->getIncomingBlock(i) == IncomingBB) {
441 Value *IncomingVal = PHI->getIncomingValue(i);
442 Inst->setOperand(Idx, IncomingVal);
448 Inst->setOperand(Idx, Mat);
452 /// \brief Emit materialization code for all rebased constants and update their
454 void ConstantHoisting::emitBaseConstants(Instruction *Base, Constant *Offset,
455 const ConstantUser &ConstUser) {
456 Instruction *Mat = Base;
458 Instruction *InsertionPt = findMatInsertPt(ConstUser.Inst,
460 Mat = BinaryOperator::Create(Instruction::Add, Base, Offset,
461 "const_mat", InsertionPt);
463 DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)
464 << " + " << *Offset << ") in BB "
465 << Mat->getParent()->getName() << '\n' << *Mat << '\n');
466 Mat->setDebugLoc(ConstUser.Inst->getDebugLoc());
468 Value *Opnd = ConstUser.Inst->getOperand(ConstUser.OpndIdx);
470 // Visit constant integer.
471 if (isa<ConstantInt>(Opnd)) {
472 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
473 if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, Mat) && Offset)
474 Mat->eraseFromParent();
475 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
479 // Visit cast instruction.
480 if (auto CastInst = dyn_cast<Instruction>(Opnd)) {
481 assert(CastInst->isCast() && "Expected an cast instruction!");
482 // Check if we already have visited this cast instruction before to avoid
483 // unnecessary cloning.
484 Instruction *&ClonedCastInst = ClonedCastMap[CastInst];
485 if (!ClonedCastInst) {
486 ClonedCastInst = CastInst->clone();
487 ClonedCastInst->setOperand(0, Mat);
488 ClonedCastInst->insertAfter(CastInst);
489 // Use the same debug location as the original cast instruction.
490 ClonedCastInst->setDebugLoc(CastInst->getDebugLoc());
491 DEBUG(dbgs() << "Clone instruction: " << *ClonedCastInst << '\n'
492 << "To : " << *CastInst << '\n');
495 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
496 updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ClonedCastInst);
497 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
501 // Visit constant expression.
502 if (auto ConstExpr = dyn_cast<ConstantExpr>(Opnd)) {
503 Instruction *ConstExprInst = ConstExpr->getAsInstruction();
504 ConstExprInst->setOperand(0, Mat);
505 ConstExprInst->insertBefore(findMatInsertPt(ConstUser.Inst,
508 // Use the same debug location as the instruction we are about to update.
509 ConstExprInst->setDebugLoc(ConstUser.Inst->getDebugLoc());
511 DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n'
512 << "From : " << *ConstExpr << '\n');
513 DEBUG(dbgs() << "Update: " << *ConstUser.Inst << '\n');
514 if (!updateOperand(ConstUser.Inst, ConstUser.OpndIdx, ConstExprInst)) {
515 ConstExprInst->eraseFromParent();
517 Mat->eraseFromParent();
519 DEBUG(dbgs() << "To : " << *ConstUser.Inst << '\n');
524 /// \brief Hoist and hide the base constant behind a bitcast and emit
525 /// materialization code for derived constants.
526 bool ConstantHoisting::emitBaseConstants() {
527 bool MadeChange = false;
528 for (auto const &ConstInfo : ConstantVec) {
529 // Hoist and hide the base constant behind a bitcast.
530 Instruction *IP = findConstantInsertionPoint(ConstInfo);
531 IntegerType *Ty = ConstInfo.BaseConstant->getType();
533 new BitCastInst(ConstInfo.BaseConstant, Ty, "const", IP);
534 DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseConstant << ") to BB "
535 << IP->getParent()->getName() << '\n' << *Base << '\n');
536 NumConstantsHoisted++;
538 // Emit materialization code for all rebased constants.
539 for (auto const &RCI : ConstInfo.RebasedConstants) {
540 NumConstantsRebased++;
541 for (auto const &U : RCI.Uses)
542 emitBaseConstants(Base, RCI.Offset, U);
545 // Use the same debug location as the last user of the constant.
546 assert(!Base->use_empty() && "The use list is empty!?");
547 assert(isa<Instruction>(Base->user_back()) &&
548 "All uses should be instructions.");
549 Base->setDebugLoc(cast<Instruction>(Base->user_back())->getDebugLoc());
551 // Correct for base constant, which we counted above too.
552 NumConstantsRebased--;
558 /// \brief Check all cast instructions we made a copy of and remove them if they
559 /// have no more users.
560 void ConstantHoisting::deleteDeadCastInst() const {
561 for (auto const &I : ClonedCastMap)
562 if (I.first->use_empty())
563 I.first->eraseFromParent();
566 /// \brief Optimize expensive integer constants in the given function.
567 bool ConstantHoisting::optimizeConstants(Function &Fn) {
568 // Collect all constant candidates.
569 collectConstantCandidates(Fn);
571 // There are no constant candidates to worry about.
572 if (ConstCandVec.empty())
575 // Combine constants that can be easily materialized with an add from a common
579 // There are no constants to emit.
580 if (ConstantVec.empty())
583 // Finally hoist the base constant and emit materialization code for dependent
585 bool MadeChange = emitBaseConstants();
587 // Cleanup dead instructions.
588 deleteDeadCastInst();