Re: [PATCH][0/n] Merge from match-and-simplify

[Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>]

On 16/10/14 21:43, Andrew Pinski wrote:
> On Thu, Oct 16, 2014 at 1:38 PM, Sebastian Pop <sebpop@gmail.com> wrote:
>> Richard Biener wrote:
>>>
>>> I have posted 5 patches as part of a larger series to merge
>>> (parts) from the match-and-simplify branch.  While I think
>>> there was overall consensus that the idea behind the project
>>> is sound there are technical questions left for how the
>>> thing should look in the end.  I've raised them in 3/n
>>> which is the only patch of the series that contains any
>>> patterns sofar.
>>>
>>> To re-iterate here (as I expect most people will only look
>>> at [0/n] patches ;)), the question is whether we are fine
>>> with making fold-const (thus fold_{unary,binary,ternary})
>>> not handle some cases it handles currently.


>>
>> I have tested on aarch64 all the code in the match-and-simplify against trunk as
>> of the last merge at r216315:
>>
>> 2014-10-16  Richard Biener  <rguenther@suse.de>
>>
>>          Merge from trunk r216235 through r216315.
>>
>> Overall, I see a lot of perf regressions (about 2/3 of the tests) than
>> improvements (1/3 of the tests).  I will try to reduce tests.

>>
>> For instance, saxpy regresses at -O3 on aarch64:
>>
>> void saxpy(double* x, double* y, double* z) {
>>      int i=0;
>>      for (i = 0 ; i < ARRAY_SIZE; i++) {
>>          z[i] = x[i] + scalar*y[i];
>>      }
>> }
>
> This looks like a scheduling issue rather than anything else.  The
> scheduler for a57 is not complete and does not model some things like
> the fusion of the compares and branch which is most likely what you
> are seeing.


Huh !! how is that related to the code generation shown by Seb ?

See the replacement of subs by cmp and sub. Folding cmp into other flag 
setting instructions is a very useful optimization on ARM and AArch64 
and that's what appears missing in fold-const. That maybe what's causing 
the slowdown. I've never known that to be caused by any scheduler 
vagaries !

regards
Ramana




>
> Thanks,
> Andrew Pinski
>
>>
>> $ diff -u base.s mas.s
>> --- base.s      2014-10-16 15:30:15.351430000 -0500
>> +++ mas.s       2014-10-16 15:30:16.183035000 -0500
>> @@ -2,12 +2,14 @@
>>          add     x1, x2, 800
>>          ldr     q0, [x0, x2]
>>          add     x3, x2, 1600
>> +       cmp     x0, 784
>>          ldr     q1, [x0, x1]
>> +       add     x1, x0, 16
>>          fmla    v0.2d, v1.2d, v2.2d
>>          str     q0, [x0, x3]
>> -       add     x0, x0, 16
>> -       cmp     x0, 800
>> +       mov     x0, x1
>>          bne     .L140
>>   .LBE179:
>> -       subs    w4, w4, #1
>> +       cmp     w4, 1
>> +       sub     w4, w4, #1
>>          bne     .L139
>>
>>
>>
>> Thanks,
>> Sebastian
>