[LLVMdev] problem with replacing an instruction

[Frank Winter]

Hi Jon! I guess it makes sense what you say about invalidating "I". I 
looked through the LLVM manual but could not find anything about 
'replacing in place'. Can I still iterate over the instructions in a 
basic block like

for (Instruction& I : BB) {
     if (isa<BitCastInst>(I)) {

and change 'I' in place? I said from the plus side.. How's the pseudo 
code for that?

Frank


On 06/18/2015 05:02 PM, Jonathan Roelofs wrote:
>
>
> On 6/18/15 2:40 PM, Frank Winter wrote:
>> I am trying to change this
>>
>> define void @main(float* noalias %arg0, float* noalias %arg1, float*
>> noalias %arg2) {
>> entrypoint:
>>    %0 = bitcast float* %arg1 to <4 x float>*
>>
>> intothis
>>
>> define void @main(float* noalias %arg0, float* noalias %arg1, float*
>> noalias %arg2) {
>> entrypoint:
>>    %0 = getelementptr float* %arg1, i64 0
>>    %1 = bitcast float* %0 to <4 x float>*
>>
>>
>> I must be close but the final instruction replacement results in a
>> segfault.
>>
>>
>>    for (Instruction& I : BB) {
> I think the problem might be that the iterator here
>>      if (isa<BitCastInst>(I)) {
>>        BitCastInst* BCI = dyn_cast<BitCastInst>(&I);
>>        for (Use& U : I.operands()) {
>>            Value* V = U.get();
>>            if (!dyn_cast<Instruction>(V)) {
>>              // must be a function's argument
>>              std::vector<llvm::Value*> vect_1;
>>              vect_1.push_back(Builder->getInt64(0));
>>              llvm::GetElementPtrInst* gep1 =
>> llvm::GetElementPtrInst::Create(V, ArrayRef< Value * >(vect_1) );
>>              BB.getInstList().insert(&I, gep1);
>>
>>              llvm::BitCastInst *bci = new BitCastInst (gep1,
>> BCI->getDestTy() );
>>              llvm::ReplaceInstWithInst( &I , bci );
> is being invalidated here because you're deleting the thing that it 
> points at.
>
> You can avoid deleting it if you update the bitcast instruction in 
> place instead. On the plus side, that avoids an unnecessary allocation.
>
>
> Jon
>
>>            }
>>         }
>>      }
>>   }
>>
>> What am I missing?
>>
>> Thanks,
>> Frank
>>
>>
>>
>>
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>