Re: [PATCH][RFA][PR tree-optimization/79095] Improve overflow test optimization and avoid invalid warnings

[Richard Biener <richard.guenther@gmail.com>]

On January 27, 2017 7:30:07 PM GMT+01:00, Jeff Law <law@redhat.com> wrote:
>On 01/27/2017 05:08 AM, Richard Biener wrote:
>> On Fri, Jan 27, 2017 at 10:02 AM, Marc Glisse <marc.glisse@inria.fr>
>wrote:
>>> On Thu, 26 Jan 2017, Jeff Law wrote:
>>>
>>>>> I assume this causes a regression for code like
>>>>>
>>>>> unsigned f(unsigned a){
>>>>>   unsigned b=a+1;
>>>>>   if(b<a)return 42;
>>>>>   return b;
>>>>> }
>>>>
>>>> Yes.  The transformation ruins the conversion into ADD_OVERFLOW for
>the +-
>>>> 1 case.  However, ISTM that we could potentially recover the
>ADD_OVERFLOW in
>>>> phi-opt.  It's a very simple pattern that would be presented to
>phi-opt, so
>>>> it might not be terrible to recover -- which has the advantage that
>if a
>>>> user wrote an optimized overflow test we'd be able to recover
>ADD_OVERFLOW
>>>> for it.
>>>
>>>
>>> phi-opt is a bit surprising at first glance because there can be
>overflow
>>> checking without condition/PHI, but if it is convenient to catch
>many
>>> cases...
>>
>> Yeah, and it's still on my TODO to add some helpers exercising
>> match.pd COND_EXPR
>> patterns from PHI nodes and their controlling condition.
>It turns out to be better to fix the existing machinery to detect 
>ADD_OVERFLOW in the transformed case than to add new detection to
>phi-opt.
>
>The problem with improving the detection of ADD_OVERFLOW is that the 
>transformed test may allow the ADD/SUB to be sunk.  So by the time we 
>run the pass to detect ADD_OVERFLOW, the test and arithmetic may be in 
>different blocks -- ugh.
>
>The more I keep thinking about this the more I wonder if transforming 
>the conditional is just more of a headache than its worth -- the main 
>need here is to drive propagation of known constants into the THEN/ELSE
>
>clauses.  Transforming the conditional makes that easy for VRP & DOM to
>
>discover those constant and the transform is easy to write in match.pd.
>
>But we could just go back to discovering the case in VRP or DOM via 
>open-coding detection, then propagating the known constants without 
>transforming the conditional.

Indeed we can do that.  And in fact with named patterns in match.pd you could even avoid the open-coding.

Richard.

>Jeff