[LLVMdev] Integer divide by zero

[Cameron McInally]

Hey Jeffrey,

Thanks for the suggestion. A few comments...

On Sun, Apr 7, 2013 at 12:31 PM, Jeffrey Yasskin <jyasskin at googlers.com>wrote:
...
>
> If you can find a way to implement -fsanitize=undefined to use the FP
> trap instead of a branch when checking floating division by 0, I
> suspect such a patch would stand a good chance of being accepted.
> (Although I'm not intimately familiar with the implementation, so
> there could be some subtlety that would prevent it.)


This would work well for the constant expression division case, since the
division by zero expression would be folded away leaving only a call to
trap. The performance penalty for using -fsanitize on other non-constant
division expressions is still unclear to me; and concerning.

Although, I've been contemplating x86-64's behaviour for this case when
floating point traps are disabled. Ideally, the compiler should preserve
that behaviour, which might make this software implementation messy.
Especially if different processors have different implementations. The
simplest solution... let the hardware behave as it should.


> You might need to
> do this in the processor-specific backend to avoid other
> undefined-behavior-based optimizations—that is, recognize "if (x == 0)
> goto err_handler; else y/x;" and replace it with
> "register-pc-in-fp-handler-map(); turn-on-fp-traps(); y/x;".
>

I believe that the constant folder would remove the constant division by
zero and conditional before the backend could have its say. We would be
left with only the jump to the error handler. That may complicate things.

Please correct me if I am misunderstanding.

Thanks again,
Cameron
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