Simplify X / X, 0 / X and X % X

[Richard Biener]

On Sat, Nov 5, 2016 at 3:30 AM, Jeff Law <law@redhat.com> wrote:
> On 11/04/2016 02:07 PM, Marc Glisse wrote:
>>
>> Hello,
>>
>> since we were discussing this recently...
>>
>> The condition is copied from the existing 0 % X case, visible in the
>> context of the diff.
>>
>> As far as I understand, the main case where we do not want to optimize
>> is during constexpr evaluation in the C++ front-end (it wants to detect
>> the undefined behavior), and with late folding I think this means we
>> only need to care about an explicit 0/0, not about X/X where X would
>> become 0 after the simplification.
>>
>> And later, if we do have something like X/0, we could handle it the same
>> way as we currently handle *(char*)0, insert a trap after that
>> instruction and clear the following code, which likely gives better code
>> than replacing 0/0 with 1.
>
> Yup.  I'd prefer to insert a trap if we ultimately expose a division by zero
> -- including cases where that division occurs as a result of a PHI arg being
> zero and the PHI result being used as a denominator in a division
> expression.
>
> It ought to be extremely easy to detect & transform that case (and probably
> warn for it too).

We have gimple-ssa-isolate-paths.c for that, right?

Richard.

>
>
> I'm leaving the actual review to Richi.
> jeff
>