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

[Kyrill Tkachov]

On 03/08/15 16:53, Kyrill Tkachov wrote:
> On 03/08/15 16:45, Uros Bizjak wrote:
>> On Mon, Aug 3, 2015 at 5:37 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.

Thanks,
Kyrill

>
> Thanks,
> Kyrill
>
>
>> Uros.
>>