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);
91 const char *getPassName() const { return "Constant Hoisting"; }
93 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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);
154 /// \brief Scan the instruction or constant expression for expensive integer
155 /// constants and record them in the constant map.
156 void ConstantHoisting::CollectConstants(Instruction *I) {
158 Intrinsic::ID IID = Intrinsic::not_intrinsic;
159 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
160 IID = II->getIntrinsicID();
162 Opcode = I->getOpcode();
164 // Scan all operands.
165 for (User::op_iterator O = I->op_begin(), E = I->op_end(); O != E; ++O) {
166 if (ConstantInt *C = dyn_cast<ConstantInt>(O)) {
167 CollectConstant(I, Opcode, IID, C);
170 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(O)) {
171 // We only handle constant cast expressions.
175 if (ConstantInt *C = dyn_cast<ConstantInt>(CE->getOperand(0))) {
176 // Ignore the cast expression and use the opcode of the instruction.
177 CollectConstant(CE, Opcode, IID, C);
184 /// \brief Collect all integer constants in the function that cannot be folded
185 /// into an instruction itself.
186 void ConstantHoisting::CollectConstants(Function &F) {
187 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
188 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
192 /// \brief Compare function for sorting integer constants by type and by value
193 /// within a type in ConstantMaps.
195 ConstantMapLessThan(const std::pair<ConstantInt *, ConstantCandidate> &LHS,
196 const std::pair<ConstantInt *, ConstantCandidate> &RHS) {
197 if (LHS.first->getType() == RHS.first->getType())
198 return LHS.first->getValue().ult(RHS.first->getValue());
200 return LHS.first->getType()->getBitWidth() <
201 RHS.first->getType()->getBitWidth();
204 /// \brief Find the base constant within the given range and rebase all other
205 /// constants with respect to the base constant.
206 void ConstantHoisting::FindAndMakeBaseConstant(ConstantMapType::iterator S,
207 ConstantMapType::iterator E) {
208 ConstantMapType::iterator MaxCostItr = S;
209 unsigned NumUses = 0;
210 // Use the constant that has the maximum cost as base constant.
211 for (ConstantMapType::iterator I = S; I != E; ++I) {
212 NumUses += I->second.Uses.size();
213 if (I->second.CumulativeCost > MaxCostItr->second.CumulativeCost)
217 // Don't hoist constants that have only one use.
222 CI.BaseConstant = MaxCostItr->first;
223 Type *Ty = CI.BaseConstant->getType();
224 // Rebase the constants with respect to the base constant.
225 for (ConstantMapType::iterator I = S; I != E; ++I) {
226 APInt Diff = I->first->getValue() - CI.BaseConstant->getValue();
227 ConstantInfo::RebasedConstantInfo RCI;
228 RCI.OriginalConstant = I->first;
229 RCI.Offset = ConstantInt::get(Ty, Diff);
230 RCI.Uses = llvm_move(I->second.Uses);
231 CI.RebasedConstants.push_back(RCI);
233 Constants.push_back(CI);
236 /// \brief Finds and combines constants that can be easily rematerialized with
237 /// an add from a common base constant.
238 void ConstantHoisting::FindBaseConstants() {
239 // Sort the constants by value and type. This invalidates the mapping.
240 std::sort(ConstantMap.begin(), ConstantMap.end(), ConstantMapLessThan);
242 // Simple linear scan through the sorted constant map for viable merge
244 ConstantMapType::iterator MinValItr = ConstantMap.begin();
245 for (ConstantMapType::iterator I = llvm::next(ConstantMap.begin()),
246 E = ConstantMap.end(); I != E; ++I) {
247 if (MinValItr->first->getType() == I->first->getType()) {
248 // Check if the constant is in range of an add with immediate.
249 APInt Diff = I->first->getValue() - MinValItr->first->getValue();
250 if ((Diff.getBitWidth() <= 64) &&
251 TTI->isLegalAddImmediate(Diff.getSExtValue()))
254 // We either have now a different constant type or the constant is not in
255 // range of an add with immediate anymore.
256 FindAndMakeBaseConstant(MinValItr, I);
257 // Start a new base constant search.
260 // Finalize the last base constant search.
261 FindAndMakeBaseConstant(MinValItr, ConstantMap.end());
264 /// \brief Records the basic block of the instruction or all basic blocks of the
265 /// users of the constant expression.
266 static void CollectBasicBlocks(SmallPtrSet<BasicBlock *, 4> &BBs, Function &F,
268 if (Instruction *I = dyn_cast<Instruction>(U))
269 BBs.insert(I->getParent());
270 else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U))
271 // Find all users of this constant expression.
272 for (Value::use_iterator UU = CE->use_begin(), E = CE->use_end();
274 // Only record users that are instructions. We don't want to go down a
275 // nested constant expression chain. Also check if the instruction is even
276 // in the current function.
277 if (Instruction *I = dyn_cast<Instruction>(*UU))
278 if(I->getParent()->getParent() == &F)
279 BBs.insert(I->getParent());
282 /// \brief Find an insertion point that dominates all uses.
283 Instruction *ConstantHoisting::
284 FindConstantInsertionPoint(Function &F, const ConstantInfo &CI) const {
285 BasicBlock *Entry = &F.getEntryBlock();
287 // Collect all basic blocks.
288 SmallPtrSet<BasicBlock *, 4> BBs;
289 ConstantInfo::RebasedConstantListType::const_iterator RCI, RCE;
290 for (RCI = CI.RebasedConstants.begin(), RCE = CI.RebasedConstants.end();
292 for (SmallVectorImpl<User *>::const_iterator U = RCI->Uses.begin(),
293 E = RCI->Uses.end(); U != E; ++U)
294 CollectBasicBlocks(BBs, F, *U);
296 if (BBs.count(Entry))
297 return Entry->getFirstInsertionPt();
299 while (BBs.size() >= 2) {
300 BasicBlock *BB, *BB1, *BB2;
302 BB2 = *llvm::next(BBs.begin());
303 BB = DT->findNearestCommonDominator(BB1, BB2);
305 return Entry->getFirstInsertionPt();
310 assert((BBs.size() == 1) && "Expected only one element.");
311 return (*BBs.begin())->getFirstInsertionPt();
314 /// \brief Emit materialization code for all rebased constants and update their
316 void ConstantHoisting::EmitBaseConstants(Function &F, User *U,
317 Instruction *Base, Constant *Offset,
318 ConstantInt *OriginalConstant) {
319 if (Instruction *I = dyn_cast<Instruction>(U)) {
320 Instruction *Mat = Base;
321 if (!Offset->isNullValue()) {
322 Mat = BinaryOperator::Create(Instruction::Add, Base, Offset,
325 // Use the same debug location as the instruction we are about to update.
326 Mat->setDebugLoc(I->getDebugLoc());
328 DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)
329 << " + " << *Offset << ") in BB "
330 << I->getParent()->getName() << '\n' << *Mat << '\n');
332 DEBUG(dbgs() << "Update: " << *I << '\n');
333 I->replaceUsesOfWith(OriginalConstant, Mat);
334 DEBUG(dbgs() << "To: " << *I << '\n');
337 assert(isa<ConstantExpr>(U) && "Expected a ConstantExpr.");
338 ConstantExpr *CE = cast<ConstantExpr>(U);
339 for (Value::use_iterator UU = CE->use_begin(), E = CE->use_end();
341 // We only handel instructions here and won't walk down a ConstantExpr chain
342 // to replace all ConstExpr with instructions.
343 if (Instruction *I = dyn_cast<Instruction>(*UU)) {
344 // Only update constant expressions in the current function.
345 if (I->getParent()->getParent() != &F)
348 Instruction *Mat = Base;
349 if (!Offset->isNullValue()) {
350 Mat = BinaryOperator::Create(Instruction::Add, Base, Offset,
353 // Use the same debug location as the instruction we are about to
355 Mat->setDebugLoc(I->getDebugLoc());
357 DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)
358 << " + " << *Offset << ") in BB "
359 << I->getParent()->getName() << '\n' << *Mat << '\n');
361 Instruction *ICE = CE->getAsInstruction();
362 ICE->replaceUsesOfWith(OriginalConstant, Mat);
363 ICE->insertBefore(I);
365 // Use the same debug location as the instruction we are about to update.
366 ICE->setDebugLoc(I->getDebugLoc());
368 DEBUG(dbgs() << "Create instruction: " << *ICE << '\n');
369 DEBUG(dbgs() << "Update: " << *I << '\n');
370 I->replaceUsesOfWith(CE, ICE);
371 DEBUG(dbgs() << "To: " << *I << '\n');
376 /// \brief Hoist and hide the base constant behind a bitcast and emit
377 /// materialization code for derived constants.
378 bool ConstantHoisting::EmitBaseConstants(Function &F) {
379 bool MadeChange = false;
380 SmallVectorImpl<ConstantInfo>::iterator CI, CE;
381 for (CI = Constants.begin(), CE = Constants.end(); CI != CE; ++CI) {
382 // Hoist and hide the base constant behind a bitcast.
383 Instruction *IP = FindConstantInsertionPoint(F, *CI);
384 IntegerType *Ty = CI->BaseConstant->getType();
385 Instruction *Base = new BitCastInst(CI->BaseConstant, Ty, "const", IP);
386 DEBUG(dbgs() << "Hoist constant (" << *CI->BaseConstant << ") to BB "
387 << IP->getParent()->getName() << '\n');
388 NumConstantsHoisted++;
390 // Emit materialization code for all rebased constants.
391 ConstantInfo::RebasedConstantListType::iterator RCI, RCE;
392 for (RCI = CI->RebasedConstants.begin(), RCE = CI->RebasedConstants.end();
394 NumConstantsRebased++;
395 for (SmallVectorImpl<User *>::iterator U = RCI->Uses.begin(),
396 E = RCI->Uses.end(); U != E; ++U)
397 EmitBaseConstants(F, *U, Base, RCI->Offset, RCI->OriginalConstant);
400 // Use the same debug location as the last user of the constant.
401 assert(!Base->use_empty() && "The use list is empty!?");
402 assert(isa<Instruction>(Base->use_back()) &&
403 "All uses should be instructions.");
404 Base->setDebugLoc(cast<Instruction>(Base->use_back())->getDebugLoc());
406 // Correct for base constant, which we counted above too.
407 NumConstantsRebased--;
413 /// \brief Optimize expensive integer constants in the given function.
414 bool ConstantHoisting::OptimizeConstants(Function &F) {
415 bool MadeChange = false;
417 // Collect all constant candidates.
420 // There are no constants to worry about.
421 if (ConstantMap.empty())
424 // Combine constants that can be easily materialized with an add from a common
428 // Finaly hoist the base constant and emit materializating code for dependent
430 MadeChange |= EmitBaseConstants(F);