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239 lines
7.7 KiB
C++
239 lines
7.7 KiB
C++
//===- InstructionCombining.cpp - Combine multiple instructions -------------=//
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//
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// InstructionCombining - Combine instructions to form fewer, simple
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// instructions. This pass does not modify the CFG, and has a tendancy to
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// make instructions dead, so a subsequent DCE pass is useful.
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//
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// This pass combines things like:
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// %Y = add int 1, %X
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// %Z = add int 1, %Y
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// into:
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// %Z = add int 2, %X
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//
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// This is a simple worklist driven algorithm.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar/InstructionCombining.h"
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#include "llvm/ConstantHandling.h"
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#include "llvm/iMemory.h"
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#include "llvm/iOther.h"
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#include "llvm/iOperators.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/InstIterator.h"
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#include "llvm/Support/InstVisitor.h"
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#include "../TransformInternals.h"
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namespace {
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class InstCombiner : public FunctionPass,
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public InstVisitor<InstCombiner, Instruction*> {
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// Worklist of all of the instructions that need to be simplified.
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std::vector<Instruction*> WorkList;
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void AddUsesToWorkList(Instruction *I) {
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// The instruction was simplified, add all users of the instruction to
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// the work lists because they might get more simplified now...
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//
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for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
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UI != UE; ++UI)
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WorkList.push_back(cast<Instruction>(*UI));
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}
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public:
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const char *getPassName() const { return "Instruction Combining"; }
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virtual bool runOnFunction(Function *F);
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.preservesCFG();
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}
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// Visitation implementation - Implement instruction combining for different
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// instruction types. The semantics are as follows:
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// Return Value:
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// null - No change was made
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// I - Change was made, I is still valid
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// otherwise - Change was made, replace I with returned instruction
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//
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Instruction *visitAdd(BinaryOperator *I);
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Instruction *visitSub(BinaryOperator *I);
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Instruction *visitMul(BinaryOperator *I);
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Instruction *visitCastInst(CastInst *CI);
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Instruction *visitMemAccessInst(MemAccessInst *MAI);
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// visitInstruction - Specify what to return for unhandled instructions...
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Instruction *visitInstruction(Instruction *I) { return 0; }
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};
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}
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// Make sure that this instruction has a constant on the right hand side if it
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// has any constant arguments. If not, fix it an return true.
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//
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static bool SimplifyBinOp(BinaryOperator *I) {
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if (isa<Constant>(I->getOperand(0)) && !isa<Constant>(I->getOperand(1)))
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if (!I->swapOperands())
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return true;
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return false;
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}
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Instruction *InstCombiner::visitAdd(BinaryOperator *I) {
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if (I->use_empty()) return 0; // Don't fix dead add instructions...
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bool Changed = SimplifyBinOp(I);
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Value *Op1 = I->getOperand(0);
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// Simplify add instructions with a constant RHS...
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if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(1))) {
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// Eliminate 'add int %X, 0'
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if (I->getType()->isIntegral() && Op2->isNullValue()) {
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AddUsesToWorkList(I); // Add all modified instrs to worklist
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I->replaceAllUsesWith(Op1);
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return I;
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}
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if (BinaryOperator *IOp1 = dyn_cast<BinaryOperator>(Op1)) {
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Changed |= SimplifyBinOp(IOp1);
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if (IOp1->getOpcode() == Instruction::Add &&
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isa<Constant>(IOp1->getOperand(1))) {
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// Fold:
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// %Y = add int %X, 1
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// %Z = add int %Y, 1
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// into:
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// %Z = add int %X, 2
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//
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if (Constant *Val = *Op2 + *cast<Constant>(IOp1->getOperand(1))) {
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I->setOperand(0, IOp1->getOperand(0));
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I->setOperand(1, Val);
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return I;
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}
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}
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}
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}
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return Changed ? I : 0;
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}
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Instruction *InstCombiner::visitSub(BinaryOperator *I) {
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if (I->use_empty()) return 0; // Don't fix dead add instructions...
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bool Changed = SimplifyBinOp(I);
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// If this is a subtract instruction with a constant RHS, convert it to an add
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// instruction of a negative constant
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//
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if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(1)))
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// Calculate 0 - RHS
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if (Constant *RHS = *Constant::getNullValue(I->getType()) - *Op2) {
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return BinaryOperator::create(Instruction::Add, I->getOperand(0), RHS,
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I->getName());
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}
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return Changed ? I : 0;
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}
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Instruction *InstCombiner::visitMul(BinaryOperator *I) {
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if (I->use_empty()) return 0; // Don't fix dead add instructions...
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bool Changed = SimplifyBinOp(I);
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Value *Op1 = I->getOperand(0);
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// Simplify add instructions with a constant RHS...
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if (Constant *Op2 = dyn_cast<Constant>(I->getOperand(1))) {
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if (I->getType()->isIntegral() && cast<ConstantInt>(Op2)->equalsInt(1)){
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// Eliminate 'mul int %X, 1'
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AddUsesToWorkList(I); // Add all modified instrs to worklist
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I->replaceAllUsesWith(Op1);
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return I;
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}
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}
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return Changed ? I : 0;
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}
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// CastInst simplification - If the user is casting a value to the same type,
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// eliminate this cast instruction...
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//
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Instruction *InstCombiner::visitCastInst(CastInst *CI) {
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if (CI->getType() == CI->getOperand(0)->getType() && !CI->use_empty()) {
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AddUsesToWorkList(CI); // Add all modified instrs to worklist
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CI->replaceAllUsesWith(CI->getOperand(0));
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return CI;
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}
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return 0;
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}
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// Combine Indices - If the source pointer to this mem access instruction is a
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// getelementptr instruction, combine the indices of the GEP into this
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// instruction
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//
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Instruction *InstCombiner::visitMemAccessInst(MemAccessInst *MAI) {
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GetElementPtrInst *Src =
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dyn_cast<GetElementPtrInst>(MAI->getPointerOperand());
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if (!Src) return 0;
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std::vector<Value *> Indices;
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// Only special case we have to watch out for is pointer arithmetic on the
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// 0th index of MAI.
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unsigned FirstIdx = MAI->getFirstIndexOperandNumber();
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if (FirstIdx == MAI->getNumOperands() ||
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(FirstIdx == MAI->getNumOperands()-1 &&
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MAI->getOperand(FirstIdx) == ConstantUInt::get(Type::UIntTy, 0))) {
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// Replace the index list on this MAI with the index on the getelementptr
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Indices.insert(Indices.end(), Src->idx_begin(), Src->idx_end());
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} else if (*MAI->idx_begin() == ConstantUInt::get(Type::UIntTy, 0)) {
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// Otherwise we can do the fold if the first index of the GEP is a zero
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Indices.insert(Indices.end(), Src->idx_begin(), Src->idx_end());
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Indices.insert(Indices.end(), MAI->idx_begin()+1, MAI->idx_end());
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}
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if (Indices.empty()) return 0; // Can't do the fold?
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switch (MAI->getOpcode()) {
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case Instruction::GetElementPtr:
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return new GetElementPtrInst(Src->getOperand(0), Indices, MAI->getName());
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case Instruction::Load:
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return new LoadInst(Src->getOperand(0), Indices, MAI->getName());
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case Instruction::Store:
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return new StoreInst(MAI->getOperand(0), Src->getOperand(0), Indices);
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default:
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assert(0 && "Unknown memaccessinst!");
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break;
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}
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abort();
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return 0;
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}
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bool InstCombiner::runOnFunction(Function *F) {
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bool Changed = false;
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WorkList.insert(WorkList.end(), inst_begin(F), inst_end(F));
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while (!WorkList.empty()) {
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Instruction *I = WorkList.back(); // Get an instruction from the worklist
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WorkList.pop_back();
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// Now that we have an instruction, try combining it to simplify it...
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Instruction *Result = visit(I);
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if (Result) {
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// Should we replace the old instruction with a new one?
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if (Result != I)
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ReplaceInstWithInst(I, Result);
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WorkList.push_back(Result);
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AddUsesToWorkList(Result);
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Changed = true;
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}
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}
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return Changed;
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}
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Pass *createInstructionCombiningPass() {
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return new InstCombiner();
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}
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