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

Wed Oct 26 02:06:28 PDT 2022

bcl5980 added a comment.

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

And to avoid potential regression I prefer this pattern:
https://alive2.llvm.org/ce/z/TdYIkn

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