Re: [PATCH, testsuite] Fix for PR47440 - Use LCM for vzeroupper insertion

[Vladimir Yakovlev <vbyakovl23 at gmail dot com>]

Hi Uros,

Thank you for such a detailed explanation. I'll study it.

Regards,
Vladimir

2011/7/20 Uros Bizjak <ubizjak@gmail.com>:
> Hello!
>
>> > ? ? ? ?* a/gcc/gcse.c (alloc_gcse_mem): Added code to run in PRE2.
>>
>> And this is necessary because...???
>>
>> Why not just make it a separate pass in ix86-reorg that uses LCM? Look at mode switching for an example.
>
> I was also expecting that vzeroupper would be inserted in the same way
> as I387 mode switching instructions are inserted. To expand on
> Steven's suggestion, please see i386.h for OPTIMIZE_MODE_SWITCHING and
> following macros.
>
> At the moment, there are 4 separate entities that handle (four
> independent) insertions for mode switching for x87 for each mode of
> fistp or frndint instruction. Mode insertions will actually insert
> calculations of x87 control word (CW) at optimal points and push this
> new CW (together with old CW) to known stack slot to be consumed by
> fistp/frndint insn.
>
> You can add a new entitiy to enum ix86_entity (say, AVX_VZEROUPPER)
> and update OPTIMIZE_MODE_SWITCHING to perform mode insertion for
> AVX_VZEROUPPER entitiy when needed. Various modes for AVX_VZEROUPPER
> are defined in NUM_MODES_FOR_MODE_SWITCHING, mode transition in
> MODE_NEEDED and insn insertions in EMIT_MODE_SET.
>
> Please note that LCM handles all entities in parallel, so there is no
> need for extra passes. The real worker for mode switching is
> ix86_mode_needed, but don't forget that you can disable mode switching
> pass per-function when not needed through OPTIMIZE_MODE_SWITCHING
> macro.
>
> FYI: Existing x87 CW initialization insertion works this way:
> - fistp/frndint is inserted into insn stream and corresponding
> OPTIMIZE_MODE_SWITCHING flag is set.
> - inserted insn has i386_cw attribute that defines requested mode in
> which the insn operate. Based on this attribute, MODE_NEEDED handles
> mode transitions (please note that there are four independent
> entities) for each entitiy.
> - EMIT_MODE_SET emits CW initializations. These are further optimized
> by follow-up optimization passes, so two consecutive initializations
> at the same place are CSEd, etc.
>
> Uros.
>