[PATCH] D136582: [InstCombine] fold sub pattern to and

[chenglin.bi via Phabricator via llvm-commits]

bcl5980 added a comment.

In D136582#3886568 <https://reviews.llvm.org/D136582#3886568>, @spatel wrote:

> In D136582#3884899 <https://reviews.llvm.org/D136582#3884899>, @bcl5980 wrote:
>
>> In D136582#3882587 <https://reviews.llvm.org/D136582#3882587>, @spatel wrote:
>>
>>> In D136582#3882497 <https://reviews.llvm.org/D136582#3882497>, @bcl5980 wrote:
>>>
>>>> In D136582#3882251 <https://reviews.llvm.org/D136582#3882251>, @spatel wrote:
>>>>
>>>>> In D136582#3880779 <https://reviews.llvm.org/D136582#3880779>, @bcl5980 wrote:
>>>>>
>>>>>> In D136582#3880098 <https://reviews.llvm.org/D136582#3880098>, @spatel wrote:
>>>>>>
>>>>>>> I don't know if it would have any effect on this patch, but we seem to be missing a constant-shrinking (DemandedBits) opportunity for sub nuw:
>>>>>>> https://alive2.llvm.org/ce/z/7Lb_Dy
>>>>>>>
>>>>>>> We can clear high-bits of the mask constant based on the highest bit set in the subtract constant.
>>>>>>
>>>>>> Yeah, I also find the shrinking. And that part has no effect on this patch. Condition 1/3 don't care any high-bits for C2.
>>>>>
>>>>> This patch still seems too specific. 
>>>>> Can we generalize this as a mask-of-subtract canonicalization instead - https://alive2.llvm.org/ce/z/qz_KmH ?
>>>>> If we do that, an existing fold for subtract should reduce the motivating example.
>>>>
>>>> Thanks for the idea. It is a more clean and simplier way. 
>>>> I only have a little concern about the subtract's combination after the canonicalization. For example:
>>>> https://alive2.llvm.org/ce/z/i5Qk9-
>>>> It will break exist `and` combination. Generally I think subtract's combination should be less than `and`.
>>>
>>> Good point. It seems we are missing some family of canonicalizations with subtract-from-constant and bitwise logic. There may be some common pre-condition with a low-bit mask and any logic op?
>>> https://alive2.llvm.org/ce/z/qrsyWe
>>
>> The pattern is still complicate:
>> https://alive2.llvm.org/ce/z/Mx5tNC
>
> I think this can be reduced to low-bit mask constraints:
> https://alive2.llvm.org/ce/z/dJqQHN
> I also noticed a deficiency in demanded bits for sub. I don't know if that would cause trouble for this patch, but we probably want to do it to be consistent with `add`:
> D136788 <https://reviews.llvm.org/D136788>

Yeah, only consider sub without nuw can be this pattern. But if we consider nuw, we still need to use the condition I send before I think.


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