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 DCE 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/Function.h"
20 #include "llvm/iMemory.h"
21 #include "llvm/iOther.h"
22 #include "llvm/iOperators.h"
23 #include "llvm/Pass.h"
24 #include "llvm/Support/InstIterator.h"
25 #include "llvm/Support/InstVisitor.h"
26 #include "../TransformInternals.h"
30 class InstCombiner : public FunctionPass,
31 public InstVisitor<InstCombiner, Instruction*> {
32 // Worklist of all of the instructions that need to be simplified.
33 std::vector<Instruction*> WorkList;
35 void AddUsesToWorkList(Instruction *I) {
36 // The instruction was simplified, add all users of the instruction to
37 // the work lists because they might get more simplified now...
39 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
41 WorkList.push_back(cast<Instruction>(*UI));
45 virtual bool runOnFunction(Function *F);
47 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
51 // Visitation implementation - Implement instruction combining for different
52 // instruction types. The semantics are as follows:
54 // null - No change was made
55 // I - Change was made, I is still valid
56 // otherwise - Change was made, replace I with returned instruction
59 Instruction *visitAdd(BinaryOperator *I);
60 Instruction *visitSub(BinaryOperator *I);
61 Instruction *visitMul(BinaryOperator *I);
62 Instruction *visitCastInst(CastInst *CI);
63 Instruction *visitMemAccessInst(MemAccessInst *MAI);
65 // visitInstruction - Specify what to return for unhandled instructions...
66 Instruction *visitInstruction(Instruction *I) { return 0; }
72 // Make sure that this instruction has a constant on the right hand side if it
73 // has any constant arguments. If not, fix it an return true.
75 static bool SimplifyBinOp(BinaryOperator *I) {
76 if (isa<Constant>(I->getOperand(0)) && !isa<Constant>(I->getOperand(1)))
77 if (!I->swapOperands())
82 Instruction *InstCombiner::visitAdd(BinaryOperator *I) {
83 if (I->use_empty()) return 0; // Don't fix dead add instructions...
84 bool Changed = SimplifyBinOp(I);
85 Value *Op1 = I->getOperand(0);
87 // Simplify add instructions with a constant RHS...
88 if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(1))) {
89 // Eliminate 'add int %X, 0'
90 if (I->getType()->isIntegral() && Op2->isNullValue()) {
91 AddUsesToWorkList(I); // Add all modified instrs to worklist
92 I->replaceAllUsesWith(Op1);
96 if (BinaryOperator *IOp1 = dyn_cast<BinaryOperator>(Op1)) {
97 Changed |= SimplifyBinOp(IOp1);
99 if (IOp1->getOpcode() == Instruction::Add &&
100 isa<Constant>(IOp1->getOperand(1))) {
102 // %Y = add int %X, 1
103 // %Z = add int %Y, 1
105 // %Z = add int %X, 2
107 if (Constant *Val = *Op2 + *cast<Constant>(IOp1->getOperand(1))) {
108 I->setOperand(0, IOp1->getOperand(0));
109 I->setOperand(1, Val);
116 return Changed ? I : 0;
119 Instruction *InstCombiner::visitSub(BinaryOperator *I) {
120 if (I->use_empty()) return 0; // Don't fix dead add instructions...
121 bool Changed = SimplifyBinOp(I);
123 // If this is a subtract instruction with a constant RHS, convert it to an add
124 // instruction of a negative constant
126 if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(1)))
128 if (Constant *RHS = *Constant::getNullValue(I->getType()) - *Op2) {
129 return BinaryOperator::create(Instruction::Add, I->getOperand(0), RHS,
133 return Changed ? I : 0;
136 Instruction *InstCombiner::visitMul(BinaryOperator *I) {
137 if (I->use_empty()) return 0; // Don't fix dead add instructions...
138 bool Changed = SimplifyBinOp(I);
139 Value *Op1 = I->getOperand(0);
141 // Simplify add instructions with a constant RHS...
142 if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(1))) {
143 if (I->getType()->isIntegral() && cast<ConstantInt>(Op2)->equalsInt(1)){
144 // Eliminate 'mul int %X, 1'
145 AddUsesToWorkList(I); // Add all modified instrs to worklist
146 I->replaceAllUsesWith(Op1);
151 return Changed ? I : 0;
155 // CastInst simplification - If the user is casting a value to the same type,
156 // eliminate this cast instruction...
158 Instruction *InstCombiner::visitCastInst(CastInst *CI) {
159 if (CI->getType() == CI->getOperand(0)->getType() && !CI->use_empty()) {
160 AddUsesToWorkList(CI); // Add all modified instrs to worklist
161 CI->replaceAllUsesWith(CI->getOperand(0));
167 // Combine Indices - If the source pointer to this mem access instruction is a
168 // getelementptr instruction, combine the indices of the GEP into this
171 Instruction *InstCombiner::visitMemAccessInst(MemAccessInst *MAI) {
172 GetElementPtrInst *Src =
173 dyn_cast<GetElementPtrInst>(MAI->getPointerOperand());
176 std::vector<Value *> Indices;
178 // Only special case we have to watch out for is pointer arithmetic on the
180 unsigned FirstIdx = MAI->getFirstIndexOperandNumber();
181 if (FirstIdx == MAI->getNumOperands() ||
182 (FirstIdx == MAI->getNumOperands()-1 &&
183 MAI->getOperand(FirstIdx) == ConstantUInt::get(Type::UIntTy, 0))) {
184 // Replace the index list on this MAI with the index on the getelementptr
185 Indices.insert(Indices.end(), Src->idx_begin(), Src->idx_end());
186 } else if (*MAI->idx_begin() == ConstantUInt::get(Type::UIntTy, 0)) {
187 // Otherwise we can do the fold if the first index of the GEP is a zero
188 Indices.insert(Indices.end(), Src->idx_begin(), Src->idx_end());
189 Indices.insert(Indices.end(), MAI->idx_begin()+1, MAI->idx_end());
192 if (Indices.empty()) return 0; // Can't do the fold?
194 switch (MAI->getOpcode()) {
195 case Instruction::GetElementPtr:
196 return new GetElementPtrInst(Src->getOperand(0), Indices, MAI->getName());
197 case Instruction::Load:
198 return new LoadInst(Src->getOperand(0), Indices, MAI->getName());
199 case Instruction::Store:
200 return new StoreInst(MAI->getOperand(0), Src->getOperand(0), Indices);
202 assert(0 && "Unknown memaccessinst!");
210 bool InstCombiner::runOnFunction(Function *F) {
211 bool Changed = false;
213 WorkList.insert(WorkList.end(), inst_begin(F), inst_end(F));
215 while (!WorkList.empty()) {
216 Instruction *I = WorkList.back(); // Get an instruction from the worklist
219 // Now that we have an instruction, try combining it to simplify it...
220 Instruction *Result = visit(I);
222 // Should we replace the old instruction with a new one?
224 ReplaceInstWithInst(I, Result);
226 WorkList.push_back(Result);
227 AddUsesToWorkList(Result);
235 Pass *createInstructionCombiningPass() {
236 return new InstCombiner();