1 //===- InstructionCombining.cpp - Combine multiple instructions -------------=//
3 // InstructionCombining - Combine instructions to form fewer, simple
4 // instructions. This pass does not modify the CFG, and has a tendancy to
5 // make instructions dead, so a subsequent DIE pass is useful.
7 // This pass combines things like:
13 // This is a simple worklist driven algorithm.
15 //===----------------------------------------------------------------------===//
17 #include "llvm/Transforms/Scalar/InstructionCombining.h"
18 #include "llvm/ConstantHandling.h"
19 #include "llvm/iMemory.h"
20 #include "llvm/iOther.h"
21 #include "llvm/iOperators.h"
22 #include "llvm/Pass.h"
23 #include "llvm/Support/InstIterator.h"
24 #include "llvm/Support/InstVisitor.h"
25 #include "../TransformInternals.h"
29 class InstCombiner : public FunctionPass,
30 public InstVisitor<InstCombiner, Instruction*> {
31 // Worklist of all of the instructions that need to be simplified.
32 std::vector<Instruction*> WorkList;
34 void AddUsesToWorkList(Instruction *I) {
35 // The instruction was simplified, add all users of the instruction to
36 // the work lists because they might get more simplified now...
38 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
40 WorkList.push_back(cast<Instruction>(*UI));
44 const char *getPassName() const { return "Instruction Combining"; }
46 virtual bool runOnFunction(Function *F);
48 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
52 // Visitation implementation - Implement instruction combining for different
53 // instruction types. The semantics are as follows:
55 // null - No change was made
56 // I - Change was made, I is still valid
57 // otherwise - Change was made, replace I with returned instruction
60 Instruction *visitAdd(BinaryOperator *I);
61 Instruction *visitSub(BinaryOperator *I);
62 Instruction *visitMul(BinaryOperator *I);
63 Instruction *visitCastInst(CastInst *CI);
64 Instruction *visitGetElementPtrInst(GetElementPtrInst *GEP);
65 Instruction *visitMemAccessInst(MemAccessInst *MAI);
67 // visitInstruction - Specify what to return for unhandled instructions...
68 Instruction *visitInstruction(Instruction *I) { return 0; }
74 // Make sure that this instruction has a constant on the right hand side if it
75 // has any constant arguments. If not, fix it an return true.
77 static bool SimplifyBinOp(BinaryOperator *I) {
78 if (isa<Constant>(I->getOperand(0)) && !isa<Constant>(I->getOperand(1)))
79 return !I->swapOperands();
83 Instruction *InstCombiner::visitAdd(BinaryOperator *I) {
84 if (I->use_empty()) return 0; // Don't fix dead add instructions...
85 bool Changed = SimplifyBinOp(I);
86 Value *Op1 = I->getOperand(0);
88 // Simplify add instructions with a constant RHS...
89 if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(1))) {
90 // Eliminate 'add int %X, 0'
91 if (I->getType()->isIntegral() && Op2->isNullValue()) {
92 AddUsesToWorkList(I); // Add all modified instrs to worklist
93 I->replaceAllUsesWith(Op1);
97 if (BinaryOperator *IOp1 = dyn_cast<BinaryOperator>(Op1)) {
98 Changed |= SimplifyBinOp(IOp1);
100 if (IOp1->getOpcode() == Instruction::Add &&
101 isa<Constant>(IOp1->getOperand(1))) {
103 // %Y = add int %X, 1
104 // %Z = add int %Y, 1
106 // %Z = add int %X, 2
108 if (Constant *Val = *Op2 + *cast<Constant>(IOp1->getOperand(1))) {
109 I->setOperand(0, IOp1->getOperand(0));
110 I->setOperand(1, Val);
117 return Changed ? I : 0;
120 Instruction *InstCombiner::visitSub(BinaryOperator *I) {
121 if (I->use_empty()) return 0; // Don't fix dead add instructions...
122 bool Changed = SimplifyBinOp(I);
124 // If this is a subtract instruction with a constant RHS, convert it to an add
125 // instruction of a negative constant
127 if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(1)))
129 if (Constant *RHS = *Constant::getNullValue(I->getType()) - *Op2) {
130 return BinaryOperator::create(Instruction::Add, I->getOperand(0), RHS,
134 return Changed ? I : 0;
137 Instruction *InstCombiner::visitMul(BinaryOperator *I) {
138 if (I->use_empty()) return 0; // Don't fix dead add instructions...
139 bool Changed = SimplifyBinOp(I);
140 Value *Op1 = I->getOperand(0);
142 // Simplify add instructions with a constant RHS...
143 if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(1))) {
144 if (I->getType()->isIntegral() && cast<ConstantInt>(Op2)->equalsInt(1)){
145 // Eliminate 'mul int %X, 1'
146 AddUsesToWorkList(I); // Add all modified instrs to worklist
147 I->replaceAllUsesWith(Op1);
150 } else if (I->getType()->isIntegral() &&
151 cast<ConstantInt>(Op2)->equalsInt(2)) {
152 // Convert 'mul int %X, 2' to 'add int %X, %X'
153 return BinaryOperator::create(Instruction::Add, Op1, Op1, I->getName());
155 } else if (Op2->isNullValue()) {
156 // Eliminate 'mul int %X, 0'
157 AddUsesToWorkList(I); // Add all modified instrs to worklist
158 I->replaceAllUsesWith(Op2); // Set this value to zero directly
163 return Changed ? I : 0;
167 // isEliminableCastOfCast - Return true if it is valid to eliminate the CI
170 static inline bool isEliminableCastOfCast(const CastInst *CI,
171 const CastInst *CSrc) {
172 assert(CI->getOperand(0) == CSrc);
173 const Type *SrcTy = CSrc->getOperand(0)->getType();
174 const Type *MidTy = CSrc->getType();
175 const Type *DstTy = CI->getType();
177 // It is legal to eliminate the instruction if casting A->B->A
178 if (SrcTy == DstTy) return true;
180 // Allow free casting and conversion of sizes as long as the sign doesn't
182 if (SrcTy->isSigned() == MidTy->isSigned() &&
183 MidTy->isSigned() == DstTy->isSigned())
186 // Otherwise, we cannot succeed. Specifically we do not want to allow things
187 // like: short -> ushort -> uint, because this can create wrong results if
188 // the input short is negative!
194 // CastInst simplification
196 Instruction *InstCombiner::visitCastInst(CastInst *CI) {
197 // If the user is casting a value to the same type, eliminate this cast
199 if (CI->getType() == CI->getOperand(0)->getType() && !CI->use_empty()) {
200 AddUsesToWorkList(CI); // Add all modified instrs to worklist
201 CI->replaceAllUsesWith(CI->getOperand(0));
206 // If casting the result of another cast instruction, try to eliminate this
209 if (CastInst *CSrc = dyn_cast<CastInst>(CI->getOperand(0)))
210 if (isEliminableCastOfCast(CI, CSrc)) {
211 // This instruction now refers directly to the cast's src operand. This
212 // has a good chance of making CSrc dead.
213 CI->setOperand(0, CSrc->getOperand(0));
222 Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst *GEP) {
223 // Is it getelementptr %P, uint 0
224 // If so, elminate the noop.
225 if (GEP->getNumOperands() == 2 && !GEP->use_empty() &&
226 GEP->getOperand(1) == Constant::getNullValue(Type::UIntTy)) {
227 AddUsesToWorkList(GEP); // Add all modified instrs to worklist
228 GEP->replaceAllUsesWith(GEP->getOperand(0));
232 return visitMemAccessInst(GEP);
236 // Combine Indices - If the source pointer to this mem access instruction is a
237 // getelementptr instruction, combine the indices of the GEP into this
240 Instruction *InstCombiner::visitMemAccessInst(MemAccessInst *MAI) {
241 GetElementPtrInst *Src =
242 dyn_cast<GetElementPtrInst>(MAI->getPointerOperand());
245 std::vector<Value *> Indices;
247 // Only special case we have to watch out for is pointer arithmetic on the
249 unsigned FirstIdx = MAI->getFirstIndexOperandNumber();
250 if (FirstIdx == MAI->getNumOperands() ||
251 (FirstIdx == MAI->getNumOperands()-1 &&
252 MAI->getOperand(FirstIdx) == ConstantUInt::get(Type::UIntTy, 0))) {
253 // Replace the index list on this MAI with the index on the getelementptr
254 Indices.insert(Indices.end(), Src->idx_begin(), Src->idx_end());
255 } else if (*MAI->idx_begin() == ConstantUInt::get(Type::UIntTy, 0)) {
256 // Otherwise we can do the fold if the first index of the GEP is a zero
257 Indices.insert(Indices.end(), Src->idx_begin(), Src->idx_end());
258 Indices.insert(Indices.end(), MAI->idx_begin()+1, MAI->idx_end());
261 if (Indices.empty()) return 0; // Can't do the fold?
263 switch (MAI->getOpcode()) {
264 case Instruction::GetElementPtr:
265 return new GetElementPtrInst(Src->getOperand(0), Indices, MAI->getName());
266 case Instruction::Load:
267 return new LoadInst(Src->getOperand(0), Indices, MAI->getName());
268 case Instruction::Store:
269 return new StoreInst(MAI->getOperand(0), Src->getOperand(0), Indices);
271 assert(0 && "Unknown memaccessinst!");
279 bool InstCombiner::runOnFunction(Function *F) {
280 bool Changed = false;
282 WorkList.insert(WorkList.end(), inst_begin(F), inst_end(F));
284 while (!WorkList.empty()) {
285 Instruction *I = WorkList.back(); // Get an instruction from the worklist
288 // Now that we have an instruction, try combining it to simplify it...
289 Instruction *Result = visit(I);
291 // Should we replace the old instruction with a new one?
293 ReplaceInstWithInst(I, Result);
295 WorkList.push_back(Result);
296 AddUsesToWorkList(Result);
304 Pass *createInstructionCombiningPass() {
305 return new InstCombiner();