[LLVMdev] Why the fault?

[Daniel Berlin]

Note:
This should now be fixable in trunk.

I committed a patch yesterday to make eraseFromParent return the
iterator for instructions (basic blocks coming next)

So
Instead of ++I, the loop should be:

  for (BasicBlock::reverse_iterator I = BB.rbegin(), E = BB.rend(); I
!= E;  ++I) {



    Instruction& inst = *I;


    if (isInstructionTriviallyDead(&inst, TLI))

      I = inst.eraseFromParent();

  }

This should work, if everything isn't lying :)



On Tue, Mar 31, 2015 at 7:08 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
> ilist uses std::reverse_iterator.
>
> reverse iterator erase  is not that simple, because
> std::reverse_iterator does not return  element, but element-1, when it
> is dereferenced.
> (IE it lies).
>
> In particular, the standard says the relationship between
> std::reverse_iterator and the base iterator you hand it is:
>
> &*(reverse_iterator(i)) == &*(base - 1)
>
>
> If you wanted it to be unchanged, you'd need to erase std::next(I).base().
>
> If you wanted it to be advanced by one, you'd have to do:
>
> std::advance(I);
> erase( I.base() );
>
> The API doesn't give you enough control to make this happen right now.
> One option is to make eraseFromParent return the iterators that erase
> returns, and go from there.
> Another is to  just add the instructions you want to erase to a
> vector, and erase them later.
>
>
> On Tue, Mar 31, 2015 at 6:47 PM, Mark Schimmel
> <Mark.Schimmel at synopsys.com> wrote:
>>   for (BasicBlock::reverse_iterator I = BB.rbegin(), E = BB.rend(); I != E;
>> ) {
>>
>>
>>
>>     Instruction& inst = *I;
>>
>>
>>
>>     ++I; ß iterator should be advanced to the previous instruction
>>
>>
>>
>>     // Happens to be an Instruction::SExt.
>>
>>     // Works fine if I iterate forwards
>>
>>     if (isInstructionTriviallyDead(&inst, TLI))
>>
>>       inst.eraseFromParent();
>>
>>   }
>>
>>
>>
>> Sorry for the inexperienced question, but I’m confused why this works when
>> using a forward iterator, but fails in reverse. The iterator is moved off
>> the current instruction so why would erasing the current instruction cause
>> an error?
>>
>>
>>
>>
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