[LLVMdev] Problems with eraseFromParent()

Nick Lewycky nicholas at mxc.ca
Sun Jun 20 20:40:03 PDT 2010

Alysson wrote:
> Hi LLVMers,
>      I am working on my llvm school project optimization pass. It's
> relatively simple, and it tries to remove redundant bitcast instructions
> followed by load instructions. At first, I'm using as input a bytecode
> generated by a simple Java test program.
>      I've already found the reduntant instructions by looking at it's
> CFG, and I also could implement the code to substitute their references,
> in order to eliminate all dependencies on these instructions (the number
> of their uses are changed to zero).
>     The point is that when I finally attempt to remove the redundant
> unused instructions from the generated code, there is a seg fault error.
>     To remove them, I'm using this function:
> void EraseInst(Instruction &I) {
>              assert(I.use_empty() && "Cannot erase used instructions!");
>              I.eraseFromParent();
> }

The problem is in how you use it:

   for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; 
I++) {		
     if (I->use_empty() && dyn_cast<LoadInst>(I)) {
       errs() << "*****Instruction unused*****" << "\n";
       errs() << *I << "\n";

Once you call EraseInst, you can't use it again in the I++ part of your 
for loop. You'll have to restructure your loop so that you increment 
before erasing your instruction, and erase the one you haven't 
incremented yet.


> So please, if anyone could help me on this, i would be very grateful!
> PS: It's also following my pass and the bytecode used for tests.
> Thanks for the attention,
> Alysson
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