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 const char *getPassName() const { return "Instruction Combining"; }
47 virtual bool runOnFunction(Function *F);
49 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
53 // Visitation implementation - Implement instruction combining for different
54 // instruction types. The semantics are as follows:
56 // null - No change was made
57 // I - Change was made, I is still valid
58 // otherwise - Change was made, replace I with returned instruction
61 Instruction *visitAdd(BinaryOperator *I);
62 Instruction *visitSub(BinaryOperator *I);
63 Instruction *visitMul(BinaryOperator *I);
64 Instruction *visitCastInst(CastInst *CI);
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 if (!I->swapOperands())
84 Instruction *InstCombiner::visitAdd(BinaryOperator *I) {
85 if (I->use_empty()) return 0; // Don't fix dead add instructions...
86 bool Changed = SimplifyBinOp(I);
87 Value *Op1 = I->getOperand(0);
89 // Simplify add instructions with a constant RHS...
90 if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(1))) {
91 // Eliminate 'add int %X, 0'
92 if (I->getType()->isIntegral() && Op2->isNullValue()) {
93 AddUsesToWorkList(I); // Add all modified instrs to worklist
94 I->replaceAllUsesWith(Op1);
98 if (BinaryOperator *IOp1 = dyn_cast<BinaryOperator>(Op1)) {
99 Changed |= SimplifyBinOp(IOp1);
101 if (IOp1->getOpcode() == Instruction::Add &&
102 isa<Constant>(IOp1->getOperand(1))) {
104 // %Y = add int %X, 1
105 // %Z = add int %Y, 1
107 // %Z = add int %X, 2
109 if (Constant *Val = *Op2 + *cast<Constant>(IOp1->getOperand(1))) {
110 I->setOperand(0, IOp1->getOperand(0));
111 I->setOperand(1, Val);
118 return Changed ? I : 0;
121 Instruction *InstCombiner::visitSub(BinaryOperator *I) {
122 if (I->use_empty()) return 0; // Don't fix dead add instructions...
123 bool Changed = SimplifyBinOp(I);
125 // If this is a subtract instruction with a constant RHS, convert it to an add
126 // instruction of a negative constant
128 if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(1)))
130 if (Constant *RHS = *Constant::getNullValue(I->getType()) - *Op2) {
131 return BinaryOperator::create(Instruction::Add, I->getOperand(0), RHS,
135 return Changed ? I : 0;
138 Instruction *InstCombiner::visitMul(BinaryOperator *I) {
139 if (I->use_empty()) return 0; // Don't fix dead add instructions...
140 bool Changed = SimplifyBinOp(I);
141 Value *Op1 = I->getOperand(0);
143 // Simplify add instructions with a constant RHS...
144 if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(1))) {
145 if (I->getType()->isIntegral() && cast<ConstantInt>(Op2)->equalsInt(1)){
146 // Eliminate 'mul int %X, 1'
147 AddUsesToWorkList(I); // Add all modified instrs to worklist
148 I->replaceAllUsesWith(Op1);
153 return Changed ? I : 0;
157 // CastInst simplification - If the user is casting a value to the same type,
158 // eliminate this cast instruction...
160 Instruction *InstCombiner::visitCastInst(CastInst *CI) {
161 if (CI->getType() == CI->getOperand(0)->getType() && !CI->use_empty()) {
162 AddUsesToWorkList(CI); // Add all modified instrs to worklist
163 CI->replaceAllUsesWith(CI->getOperand(0));
169 // Combine Indices - If the source pointer to this mem access instruction is a
170 // getelementptr instruction, combine the indices of the GEP into this
173 Instruction *InstCombiner::visitMemAccessInst(MemAccessInst *MAI) {
174 GetElementPtrInst *Src =
175 dyn_cast<GetElementPtrInst>(MAI->getPointerOperand());
178 std::vector<Value *> Indices;
180 // Only special case we have to watch out for is pointer arithmetic on the
182 unsigned FirstIdx = MAI->getFirstIndexOperandNumber();
183 if (FirstIdx == MAI->getNumOperands() ||
184 (FirstIdx == MAI->getNumOperands()-1 &&
185 MAI->getOperand(FirstIdx) == ConstantUInt::get(Type::UIntTy, 0))) {
186 // Replace the index list on this MAI with the index on the getelementptr
187 Indices.insert(Indices.end(), Src->idx_begin(), Src->idx_end());
188 } else if (*MAI->idx_begin() == ConstantUInt::get(Type::UIntTy, 0)) {
189 // Otherwise we can do the fold if the first index of the GEP is a zero
190 Indices.insert(Indices.end(), Src->idx_begin(), Src->idx_end());
191 Indices.insert(Indices.end(), MAI->idx_begin()+1, MAI->idx_end());
194 if (Indices.empty()) return 0; // Can't do the fold?
196 switch (MAI->getOpcode()) {
197 case Instruction::GetElementPtr:
198 return new GetElementPtrInst(Src->getOperand(0), Indices, MAI->getName());
199 case Instruction::Load:
200 return new LoadInst(Src->getOperand(0), Indices, MAI->getName());
201 case Instruction::Store:
202 return new StoreInst(MAI->getOperand(0), Src->getOperand(0), Indices);
204 assert(0 && "Unknown memaccessinst!");
212 bool InstCombiner::runOnFunction(Function *F) {
213 bool Changed = false;
215 WorkList.insert(WorkList.end(), inst_begin(F), inst_end(F));
217 while (!WorkList.empty()) {
218 Instruction *I = WorkList.back(); // Get an instruction from the worklist
221 // Now that we have an instruction, try combining it to simplify it...
222 Instruction *Result = visit(I);
224 // Should we replace the old instruction with a new one?
226 ReplaceInstWithInst(I, Result);
228 WorkList.push_back(Result);
229 AddUsesToWorkList(Result);
237 Pass *createInstructionCombiningPass() {
238 return new InstCombiner();