Re: [PATCH][RTL-ifcvt] Improve conditional select ops on immediates

[Uros Bizjak <ubizjak at gmail dot com>]

On Mon, Aug 3, 2015 at 7:20 PM, Kyrill Tkachov <kyrylo.tkachov@arm.com> wrote:

>>>>> Looking at the x86 movcc expansion code (ix86_expand_int_movcc) I
>>>>> don't think this is a good idea. In the expander, there is already
>>>>> quite some target-dependent code that goes great length to utilize sbb
>>>>> insn as much as possible, before cmove is used.
>>>>>
>>>>> IMO, as far as x86 is concerned, the best solution would be to revert
>>>>> the change. ix86_expand_int_movcc already does some tricks from your
>>>>> patch in a target-efficient way. Generic change that was introduced by
>>>>> your patch now interferes with this expansion.
>>>>
>>>> Well, technically the transformation was already there, it was just
>>>> never
>>>> reached for an x86 compilation because noce_try_cmove was tried in front
>>>> of
>>>> it
>>>> and used a target-specific expansion.
>>>> In any case, how's this proposal?
>>>> The transformation noce_try_store_flag_constants
>>>>         /* if (test) x = a; else x = b;
>>>>        =>   x = (-(test != 0) & (b - a)) + a;  */
>>>>
>>>> Is a catch-all-immediates transformation in
>>>> noce_try_store_flag_constants.
>>>> What if we moved it to noce_try_cmove and performed it only if the
>>>> target-specific
>>>> conditional move expansion there failed?
>>>>
>>>> That way we can try the x86_64-specific sequence first and still give
>>>> the
>>>> opportunity
>>>> to noce_try_store_flag_constants to perform the transformations that can
>>>> benefit targets
>>>> that don't have highly specific conditional move expanders.
>>>
>>> Yes, let's try this approach. As was found out, some targets (e.g.
>>> x86) hide lots of different target-dependent expansion strategies into
>>> movcc expander. Perhaps this fact should be documented in the comment
>>> in the generic code?
>>
>> Ok, I'll work on that approach and add a comment.
>
>
> I'm testing a patch that fix the testcases on x86_64 and does not
> harm codegen on aarch64. Feel free to file a PR and assign it to me.

PR67103 [1]

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67103

Thanks,
Uros.