Re: [i386] Scalar DImode instructions on XMM registers

[Ilya Enkovich <enkovich dot gnu at gmail dot com>]

On 20 May 23:27, Vladimir Makarov wrote:
> 
> 
> On 20/05/15 04:17 AM, Ilya Enkovich wrote:
> >On 19 May 11:22, Vladimir Makarov wrote:
> >>On 05/18/2015 08:13 AM, Ilya Enkovich wrote:
> >>>2015-05-06 17:18 GMT+03:00 Ilya Enkovich <enkovich.gnu@gmail.com>:
> >>>Hi Vladimir,
> >>>
> >>>Could you please comment on this?
> >>>
> >>>
> >>Ilya, I think that the idea is worth to try but results might be
> >>mixed.  It is hard to say until you actually try it (as example, Jan
> >>implemented -fpmath=both and it looks a pretty good idea at least
> >>for me but when I checked SPEC2000 the results were not so good even
> >>with IRA/LRA).
> >>
> >>Long ago I did some experiments and found that spilling into SSE
> >>would benefitial for Intel CPUs but not for AMD ones.  As I remember
> >>I also found that storing several scalar values into one SSE reg and
> >>extracting it when you need to do some (fp) arithmetics would
> >>benefitial for AMD but not for Intel CPUs.   In literature more
> >>general approach is called bitwise register allocator.  Actually it
> >>would be a pretty big IRA/LRA project from which some targets might
> >>benefit.
> >I suspect such things are not trivially done in IRA/LRA and want to make it as an independent optimization because its application seems to be quite narrow.
> Yes, that is true.  The complications and implementation complexity
> will be probably very high in this project and the positive results
> are not sure.  So the project might have a small value.
> >>
> >>As for the wrong code, it is hard for me to say anything w/o RA
> >>dumps.  If you send me the dump (-fira-verbose=16), i might say more
> >>what is going on.
> >>
> >>
> >Here are some dumps from my reproducer.  The problematic register is r108.
> >
> Thanks.  For me it looks like an inheritance bug.  It is really hard
> to fix the bug w/o the source code.  Could you send me your patch in
> order I can debug RA with it to investigate more.
> 

Sure! Here is a patch and a testcase.  I applied patch to r222125.  Cmd to reproduce:

gcc -m32 -msse4.2 -O2 pr65105.c -S -march=slm -fPIE

Thanks,
Ilya

Attachment: pr65105.c
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