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/MapVector.h"
39 #include "llvm/ADT/SmallSet.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/CommandLine.h"
47 #include "llvm/Support/Debug.h"
51 STATISTIC(NumConstantsHoisted, "Number of constants hoisted");
52 STATISTIC(NumConstantsRebased, "Number of constants rebased");
56 typedef SmallVector<User *, 4> ConstantUseListType;
57 struct ConstantCandidate {
58 unsigned CumulativeCost;
59 ConstantUseListType Uses;
63 ConstantInt *BaseConstant;
64 struct RebasedConstantInfo {
65 ConstantInt *OriginalConstant;
67 ConstantUseListType Uses;
69 typedef SmallVector<RebasedConstantInfo, 4> RebasedConstantListType;
70 RebasedConstantListType RebasedConstants;
73 class ConstantHoisting : public FunctionPass {
74 const TargetTransformInfo *TTI;
77 /// Keeps track of expensive constants found in the function.
78 typedef MapVector<ConstantInt *, ConstantCandidate> ConstantMapType;
79 ConstantMapType ConstantMap;
81 /// These are the final constants we decided to hoist.
82 SmallVector<ConstantInfo, 4> Constants;
84 static char ID; // Pass identification, replacement for typeid
85 ConstantHoisting() : FunctionPass(ID), TTI(0) {
86 initializeConstantHoistingPass(*PassRegistry::getPassRegistry());
89 bool runOnFunction(Function &F) override;
91 const char *getPassName() const override { return "Constant Hoisting"; }
93 void getAnalysisUsage(AnalysisUsage &AU) const override {
95 AU.addRequired<DominatorTreeWrapperPass>();
96 AU.addRequired<TargetTransformInfo>();
100 void CollectConstant(User *U, unsigned Opcode, Intrinsic::ID IID,
102 void CollectConstants(Instruction *I);
103 void CollectConstants(Function &F);
104 void FindAndMakeBaseConstant(ConstantMapType::iterator S,
105 ConstantMapType::iterator E);
106 void FindBaseConstants();
107 Instruction *FindConstantInsertionPoint(Function &F,
108 const ConstantInfo &CI) const;
109 void EmitBaseConstants(Function &F, User *U, Instruction *Base,
110 Constant *Offset, ConstantInt *OriginalConstant);
111 bool EmitBaseConstants(Function &F);
112 bool OptimizeConstants(Function &F);
116 char ConstantHoisting::ID = 0;
117 INITIALIZE_PASS_BEGIN(ConstantHoisting, "consthoist", "Constant Hoisting",
119 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
120 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
121 INITIALIZE_PASS_END(ConstantHoisting, "consthoist", "Constant Hoisting",
124 FunctionPass *llvm::createConstantHoistingPass() {
125 return new ConstantHoisting();
128 /// \brief Perform the constant hoisting optimization for the given function.
129 bool ConstantHoisting::runOnFunction(Function &F) {
130 DEBUG(dbgs() << "********** Constant Hoisting **********\n");
131 DEBUG(dbgs() << "********** Function: " << F.getName() << '\n');
133 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
134 TTI = &getAnalysis<TargetTransformInfo>();
136 return OptimizeConstants(F);
139 void ConstantHoisting::CollectConstant(User * U, unsigned Opcode,
140 Intrinsic::ID IID, ConstantInt *C) {
143 Cost = TTI->getIntImmCost(Opcode, C->getValue(), C->getType());
145 Cost = TTI->getIntImmCost(IID, C->getValue(), C->getType());
147 if (Cost > TargetTransformInfo::TCC_Basic) {
148 ConstantCandidate &CC = ConstantMap[C];
149 CC.CumulativeCost += Cost;
150 CC.Uses.push_back(U);
151 DEBUG(dbgs() << "Collect constant " << *C << " with cost " << Cost
152 << " from " << *U << '\n');
156 /// \brief Scan the instruction or constant expression for expensive integer
157 /// constants and record them in the constant map.
158 void ConstantHoisting::CollectConstants(Instruction *I) {
160 Intrinsic::ID IID = Intrinsic::not_intrinsic;
161 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
162 IID = II->getIntrinsicID();
164 Opcode = I->getOpcode();
166 // Scan all operands.
167 for (User::op_iterator O = I->op_begin(), E = I->op_end(); O != E; ++O) {
168 if (ConstantInt *C = dyn_cast<ConstantInt>(O)) {
169 CollectConstant(I, Opcode, IID, C);
172 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(O)) {
173 // We only handle constant cast expressions.
177 if (ConstantInt *C = dyn_cast<ConstantInt>(CE->getOperand(0))) {
178 // Ignore the cast expression and use the opcode of the instruction.
179 CollectConstant(CE, Opcode, IID, C);
186 /// \brief Collect all integer constants in the function that cannot be folded
187 /// into an instruction itself.
188 void ConstantHoisting::CollectConstants(Function &F) {
189 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
190 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
194 /// \brief Find the base constant within the given range and rebase all other
195 /// constants with respect to the base constant.
196 void ConstantHoisting::FindAndMakeBaseConstant(ConstantMapType::iterator S,
197 ConstantMapType::iterator E) {
198 ConstantMapType::iterator MaxCostItr = S;
199 unsigned NumUses = 0;
200 // Use the constant that has the maximum cost as base constant.
201 for (ConstantMapType::iterator I = S; I != E; ++I) {
202 NumUses += I->second.Uses.size();
203 if (I->second.CumulativeCost > MaxCostItr->second.CumulativeCost)
207 // Don't hoist constants that have only one use.
212 CI.BaseConstant = MaxCostItr->first;
213 Type *Ty = CI.BaseConstant->getType();
214 // Rebase the constants with respect to the base constant.
215 for (ConstantMapType::iterator I = S; I != E; ++I) {
216 APInt Diff = I->first->getValue() - CI.BaseConstant->getValue();
217 ConstantInfo::RebasedConstantInfo RCI;
218 RCI.OriginalConstant = I->first;
219 RCI.Offset = ConstantInt::get(Ty, Diff);
220 RCI.Uses = std::move(I->second.Uses);
221 CI.RebasedConstants.push_back(RCI);
223 Constants.push_back(CI);
226 /// \brief Finds and combines constants that can be easily rematerialized with
227 /// an add from a common base constant.
228 void ConstantHoisting::FindBaseConstants() {
229 // Sort the constants by value and type. This invalidates the mapping.
230 std::sort(ConstantMap.begin(), ConstantMap.end(),
231 [](const std::pair<ConstantInt *, ConstantCandidate> &LHS,
232 const std::pair<ConstantInt *, ConstantCandidate> &RHS) {
233 if (LHS.first->getType() != RHS.first->getType())
234 return LHS.first->getType()->getBitWidth() <
235 RHS.first->getType()->getBitWidth();
236 return LHS.first->getValue().ult(RHS.first->getValue());
239 // Simple linear scan through the sorted constant map for viable merge
241 ConstantMapType::iterator MinValItr = ConstantMap.begin();
242 for (ConstantMapType::iterator I = std::next(ConstantMap.begin()),
243 E = ConstantMap.end(); I != E; ++I) {
244 if (MinValItr->first->getType() == I->first->getType()) {
245 // Check if the constant is in range of an add with immediate.
246 APInt Diff = I->first->getValue() - MinValItr->first->getValue();
247 if ((Diff.getBitWidth() <= 64) &&
248 TTI->isLegalAddImmediate(Diff.getSExtValue()))
251 // We either have now a different constant type or the constant is not in
252 // range of an add with immediate anymore.
253 FindAndMakeBaseConstant(MinValItr, I);
254 // Start a new base constant search.
257 // Finalize the last base constant search.
258 FindAndMakeBaseConstant(MinValItr, ConstantMap.end());
261 /// \brief Records the basic block of the instruction or all basic blocks of the
262 /// users of the constant expression.
263 static void CollectBasicBlocks(SmallPtrSet<BasicBlock *, 4> &BBs, Function &F,
265 if (Instruction *I = dyn_cast<Instruction>(U))
266 BBs.insert(I->getParent());
267 else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U))
268 // Find all users of this constant expression.
269 for (User *UU : CE->users())
270 // Only record users that are instructions. We don't want to go down a
271 // nested constant expression chain. Also check if the instruction is even
272 // in the current function.
273 if (Instruction *I = dyn_cast<Instruction>(UU))
274 if(I->getParent()->getParent() == &F)
275 BBs.insert(I->getParent());
278 /// \brief Find the instruction we should insert the constant materialization
280 static Instruction *getMatInsertPt(Instruction *I, const DominatorTree *DT) {
281 if (!isa<PHINode>(I) && !isa<LandingPadInst>(I)) // Simple case.
284 // We can't insert directly before a phi node or landing pad. Insert before
285 // the terminator of the dominating block.
286 assert(&I->getParent()->getParent()->getEntryBlock() != I->getParent() &&
287 "PHI or landing pad in entry block!");
288 BasicBlock *IDom = DT->getNode(I->getParent())->getIDom()->getBlock();
289 return IDom->getTerminator();
292 /// \brief Find an insertion point that dominates all uses.
293 Instruction *ConstantHoisting::
294 FindConstantInsertionPoint(Function &F, const ConstantInfo &CI) const {
295 BasicBlock *Entry = &F.getEntryBlock();
297 // Collect all basic blocks.
298 SmallPtrSet<BasicBlock *, 4> BBs;
299 ConstantInfo::RebasedConstantListType::const_iterator RCI, RCE;
300 for (RCI = CI.RebasedConstants.begin(), RCE = CI.RebasedConstants.end();
302 for (SmallVectorImpl<User *>::const_iterator U = RCI->Uses.begin(),
303 E = RCI->Uses.end(); U != E; ++U)
304 CollectBasicBlocks(BBs, F, *U);
306 if (BBs.count(Entry))
307 return getMatInsertPt(&Entry->front(), DT);
309 while (BBs.size() >= 2) {
310 BasicBlock *BB, *BB1, *BB2;
312 BB2 = *std::next(BBs.begin());
313 BB = DT->findNearestCommonDominator(BB1, BB2);
315 return getMatInsertPt(&Entry->front(), DT);
320 assert((BBs.size() == 1) && "Expected only one element.");
321 Instruction &FirstInst = (*BBs.begin())->front();
322 return getMatInsertPt(&FirstInst, DT);
325 /// \brief Emit materialization code for all rebased constants and update their
327 void ConstantHoisting::EmitBaseConstants(Function &F, User *U,
328 Instruction *Base, Constant *Offset,
329 ConstantInt *OriginalConstant) {
330 if (Instruction *I = dyn_cast<Instruction>(U)) {
331 Instruction *Mat = Base;
332 if (!Offset->isNullValue()) {
333 Mat = BinaryOperator::Create(Instruction::Add, Base, Offset,
334 "const_mat", getMatInsertPt(I, DT));
336 // Use the same debug location as the instruction we are about to update.
337 Mat->setDebugLoc(I->getDebugLoc());
339 DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)
340 << " + " << *Offset << ") in BB "
341 << I->getParent()->getName() << '\n' << *Mat << '\n');
343 DEBUG(dbgs() << "Update: " << *I << '\n');
344 I->replaceUsesOfWith(OriginalConstant, Mat);
345 DEBUG(dbgs() << "To: " << *I << '\n');
348 assert(isa<ConstantExpr>(U) && "Expected a ConstantExpr.");
349 ConstantExpr *CE = cast<ConstantExpr>(U);
350 SmallVector<std::pair<Instruction *, Instruction *>, 8> WorkList;
351 DEBUG(dbgs() << "Visit ConstantExpr " << *CE << '\n');
352 for (User *UU : CE->users()) {
353 DEBUG(dbgs() << "Check user "; UU->print(dbgs()); dbgs() << '\n');
354 // We only handel instructions here and won't walk down a ConstantExpr chain
355 // to replace all ConstExpr with instructions.
356 if (Instruction *I = dyn_cast<Instruction>(UU)) {
357 // Only update constant expressions in the current function.
358 if (I->getParent()->getParent() != &F) {
359 DEBUG(dbgs() << "Not in the same function - skip.\n");
363 Instruction *Mat = Base;
364 Instruction *InsertBefore = getMatInsertPt(I, DT);
365 if (!Offset->isNullValue()) {
366 Mat = BinaryOperator::Create(Instruction::Add, Base, Offset,
367 "const_mat", InsertBefore);
369 // Use the same debug location as the instruction we are about to
371 Mat->setDebugLoc(I->getDebugLoc());
373 DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)
374 << " + " << *Offset << ") in BB "
375 << I->getParent()->getName() << '\n' << *Mat << '\n');
377 Instruction *ICE = CE->getAsInstruction();
378 ICE->replaceUsesOfWith(OriginalConstant, Mat);
379 ICE->insertBefore(InsertBefore);
381 // Use the same debug location as the instruction we are about to update.
382 ICE->setDebugLoc(I->getDebugLoc());
384 WorkList.push_back(std::make_pair(I, ICE));
386 DEBUG(dbgs() << "Not an instruction - skip.\n");
389 SmallVectorImpl<std::pair<Instruction *, Instruction *> >::iterator I, E;
390 for (I = WorkList.begin(), E = WorkList.end(); I != E; ++I) {
391 DEBUG(dbgs() << "Create instruction: " << *I->second << '\n');
392 DEBUG(dbgs() << "Update: " << *I->first << '\n');
393 I->first->replaceUsesOfWith(CE, I->second);
394 DEBUG(dbgs() << "To: " << *I->first << '\n');
398 /// \brief Hoist and hide the base constant behind a bitcast and emit
399 /// materialization code for derived constants.
400 bool ConstantHoisting::EmitBaseConstants(Function &F) {
401 bool MadeChange = false;
402 SmallVectorImpl<ConstantInfo>::iterator CI, CE;
403 for (CI = Constants.begin(), CE = Constants.end(); CI != CE; ++CI) {
404 // Hoist and hide the base constant behind a bitcast.
405 Instruction *IP = FindConstantInsertionPoint(F, *CI);
406 IntegerType *Ty = CI->BaseConstant->getType();
407 Instruction *Base = new BitCastInst(CI->BaseConstant, Ty, "const", IP);
408 DEBUG(dbgs() << "Hoist constant (" << *CI->BaseConstant << ") to BB "
409 << IP->getParent()->getName() << '\n');
410 NumConstantsHoisted++;
412 // Emit materialization code for all rebased constants.
413 ConstantInfo::RebasedConstantListType::iterator RCI, RCE;
414 for (RCI = CI->RebasedConstants.begin(), RCE = CI->RebasedConstants.end();
416 NumConstantsRebased++;
417 for (SmallVectorImpl<User *>::iterator U = RCI->Uses.begin(),
418 E = RCI->Uses.end(); U != E; ++U)
419 EmitBaseConstants(F, *U, Base, RCI->Offset, RCI->OriginalConstant);
422 // Use the same debug location as the last user of the constant.
423 assert(!Base->use_empty() && "The use list is empty!?");
424 assert(isa<Instruction>(Base->user_back()) &&
425 "All uses should be instructions.");
426 Base->setDebugLoc(cast<Instruction>(Base->user_back())->getDebugLoc());
428 // Correct for base constant, which we counted above too.
429 NumConstantsRebased--;
435 /// \brief Optimize expensive integer constants in the given function.
436 bool ConstantHoisting::OptimizeConstants(Function &F) {
437 bool MadeChange = false;
439 // Collect all constant candidates.
442 // There are no constants to worry about.
443 if (ConstantMap.empty())
446 // Combine constants that can be easily materialized with an add from a common
450 // Finally hoist the base constant and emit materializating code for dependent
452 MadeChange |= EmitBaseConstants(F);