[LLVMdev] Problems with eraseFromParent()

John Criswell criswell at uiuc.edu
Sun Jun 20 21:36:10 PDT 2010

Dear Alysson,

You may be invalidating the BasicBlock::iterator by removing 
instructions while iterating through the instructions in the 
BasicBlock.  One way to fix this would be to iterate through the 
BasicBlock and record in a std::vector<> all the instructions that 
should be removed.  Then you can write a loop that removes the last 
element of the std::vector, erases it, and then repeats until the 
std::vector is empty.

Also, just to note, it looks like your code is not checking whether the 
LoadInst is volatile before deleting it.  Your transform should not 
remove volatile LoadInst or StoreInst instructions, even if they appear 

-- John T.

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();
> }
> 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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