[LLVMdev] GVN fails with bitcasts
Carl-Philip Hänsch
cphaensch at gmail.com
Sat Nov 23 12:46:05 PST 2013
I created the following pass that fixes the issue:
class bitcastCombinePass: public llvm::BasicBlockPass {
private:
JITEngine *engine;
static char ID;
public:
bitcastCombinePass(JITEngine *engine): BasicBlockPass(ID),
engine(engine) {}
virtual bool runOnBasicBlock(llvm::BasicBlock &BB) {
bool changed = false;
for(auto i = BB.begin(); i != BB.end(); i++) {
llvm::BitCastInst *bitcast =
dynamic_cast<llvm::BitCastInst*>(&*i);
if(bitcast) {
// find Twin-Bitcast
for(auto i2 = BB.begin(); i2 != i; i2++) {
llvm::BitCastInst *bitcast2 =
dynamic_cast<llvm::BitCastInst*>(&*i2);
if(bitcast2) {
// two bitcasts ... same type?
if(bitcast->getType() == bitcast2->getType()) {
llvm::GetElementPtrInst *gep1 =
dynamic_cast<llvm::GetElementPtrInst*>(bitcast->getOperand(0));
llvm::GetElementPtrInst *gep2 =
dynamic_cast<llvm::GetElementPtrInst*>(bitcast2->getOperand(0));
// both get GEP from same operand
if(gep1 && gep2 && gep1->getPointerOperand() ==
gep2->getPointerOperand()) {
llvm::APInt offset1(sizeof(int *) * 8, 0);
llvm::APInt offset2(sizeof(int *) * 8, 0);
if(gep1->accumulateConstantOffset(*engine->executionEngine->getDataLayout(),
offset1)
&&
gep2->accumulateConstantOffset(*engine->executionEngine->getDataLayout(),
offset2)
&& offset1 == offset2) {
// bitcasts point to same value =>
replace them
bitcast->replaceAllUsesWith(bitcast2);
}
}
}
}
}
}
}
return changed;
}
};
char bitcastCombinePass::ID;
This should become part of either InstCombine or GVN I think.
2013/11/23 Carl-Philip Hänsch <cphaensch at gmail.com>
> Hi,
>
> i have the following code:
> define internal %"struct.dexter::ConditionConstant"*
> @_ZN6dexter18BinaryConditionAdd8evaluateEv5(%"class.dexter::BinaryConditionAdd"*)
> {
> entry:
> %1 = getelementptr inbounds %"class.dexter::BinaryConditionAdd"* %0, i32
> 0, i32 0, i32 1
> %2 = load %"class.dexter::BaseCondition"** %1, align 8
> %3 = bitcast %"class.dexter::BaseCondition"* %2 to
> %"class.dexter::BinaryConditionAdd"*
> %4 = getelementptr inbounds %"class.dexter::BinaryConditionAdd"* %3, i32
> 0, i32 0, i32 0, i32 1, i32 0
> %5 = bitcast %union.anon* %4 to i64*
> store i64 4, i64* %5, align 8
> %6 = getelementptr inbounds %"class.dexter::BinaryConditionAdd"* %3, i32
> 0, i32 0, i32 0, i32 1
> %7 = bitcast %"struct.dexter::ConditionConstant"* %6 to i64*
> %8 = load i64* %7, align 8
> %9 = add nsw i64 %8, 2
> %10 = getelementptr inbounds %"class.dexter::BinaryConditionAdd"* %0,
> i32 0, i32 0, i32 0, i32 1, i32 0
> %11 = bitcast %union.anon* %10 to i64*
> store i64 %9, i64* %11, align 8
> %12 = getelementptr inbounds %"class.dexter::BinaryConditionAdd"* %0,
> i32 0, i32 0, i32 0, i32 1
> ret %"struct.dexter::ConditionConstant"* %12
> }
>
> %5 and %7 point to the same memory location. This is not detected because
> %4 and %6 look different. But they have the same offset and the bitcast
> creates. Do you have an idea how to fix that?
>
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