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/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 *visitMemAccessInst(MemAccessInst *MAI);
66 // visitInstruction - Specify what to return for unhandled instructions...
67 Instruction *visitInstruction(Instruction *I) { return 0; }
73 // Make sure that this instruction has a constant on the right hand side if it
74 // has any constant arguments. If not, fix it an return true.
76 static bool SimplifyBinOp(BinaryOperator *I) {
77 if (isa<Constant>(I->getOperand(0)) && !isa<Constant>(I->getOperand(1)))
78 if (!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);
152 return Changed ? I : 0;
156 // CastInst simplification - If the user is casting a value to the same type,
157 // eliminate this cast instruction...
159 Instruction *InstCombiner::visitCastInst(CastInst *CI) {
160 if (CI->getType() == CI->getOperand(0)->getType() && !CI->use_empty()) {
161 AddUsesToWorkList(CI); // Add all modified instrs to worklist
162 CI->replaceAllUsesWith(CI->getOperand(0));
168 // Combine Indices - If the source pointer to this mem access instruction is a
169 // getelementptr instruction, combine the indices of the GEP into this
172 Instruction *InstCombiner::visitMemAccessInst(MemAccessInst *MAI) {
173 GetElementPtrInst *Src =
174 dyn_cast<GetElementPtrInst>(MAI->getPointerOperand());
177 std::vector<Value *> Indices;
179 // Only special case we have to watch out for is pointer arithmetic on the
181 unsigned FirstIdx = MAI->getFirstIndexOperandNumber();
182 if (FirstIdx == MAI->getNumOperands() ||
183 (FirstIdx == MAI->getNumOperands()-1 &&
184 MAI->getOperand(FirstIdx) == ConstantUInt::get(Type::UIntTy, 0))) {
185 // Replace the index list on this MAI with the index on the getelementptr
186 Indices.insert(Indices.end(), Src->idx_begin(), Src->idx_end());
187 } else if (*MAI->idx_begin() == ConstantUInt::get(Type::UIntTy, 0)) {
188 // Otherwise we can do the fold if the first index of the GEP is a zero
189 Indices.insert(Indices.end(), Src->idx_begin(), Src->idx_end());
190 Indices.insert(Indices.end(), MAI->idx_begin()+1, MAI->idx_end());
193 if (Indices.empty()) return 0; // Can't do the fold?
195 switch (MAI->getOpcode()) {
196 case Instruction::GetElementPtr:
197 return new GetElementPtrInst(Src->getOperand(0), Indices, MAI->getName());
198 case Instruction::Load:
199 return new LoadInst(Src->getOperand(0), Indices, MAI->getName());
200 case Instruction::Store:
201 return new StoreInst(MAI->getOperand(0), Src->getOperand(0), Indices);
203 assert(0 && "Unknown memaccessinst!");
211 bool InstCombiner::runOnFunction(Function *F) {
212 bool Changed = false;
214 WorkList.insert(WorkList.end(), inst_begin(F), inst_end(F));
216 while (!WorkList.empty()) {
217 Instruction *I = WorkList.back(); // Get an instruction from the worklist
220 // Now that we have an instruction, try combining it to simplify it...
221 Instruction *Result = visit(I);
223 // Should we replace the old instruction with a new one?
225 ReplaceInstWithInst(I, Result);
227 WorkList.push_back(Result);
228 AddUsesToWorkList(Result);
236 Pass *createInstructionCombiningPass() {
237 return new InstCombiner();