[PATCH] D108408: [InstCombine] Transform X == 0 ? 0 : X * Y --> X * freeze(Y).

[Filipp Zhinkin via Phabricator via llvm-commits]

fzhinkin added a comment.

In D108408#2962950 <https://reviews.llvm.org/D108408#2962950>, @lebedev.ri wrote:

> In D108408#2958735 <https://reviews.llvm.org/D108408#2958735>, @fzhinkin wrote:
>
>> In D108408#2958323 <https://reviews.llvm.org/D108408#2958323>, @lebedev.ri wrote:
>>
>>> 1. what's wrong with vectors?
>>
>> There's nothing wrong, I'll support it, thanks!
>>
>>> 2. can't we preserve no-wrap flags?
>>
>> Definitely.
>>
>>> 3. for vectors, what if some element is not zero but undef? (see ` Constant::mergeUndefsWith()`)
>>
>> Not sure that I fully understand the issue.
>> If `%a` in the snippet below contains some undefs then the select may choose either `0` or  `%m` depending on particular value of `a` (which could be `0` for undef elements).
>> Probably `%a` should be frozen to ensure that both `icmp` and `mul` will see the same `a`'s value, but (if I understood it correctly) absence of `freeze` does not compromise the correctness.
>>
>>   define <2 x i4> @src(<2 x i4> %a, <2 x i4> %b) {
>>     %c = icmp eq <2 x i4> %a, zeroinitializer
>>     %m = mul <2 x i4> %a, %b
>>     %r = select <2 x i1> %c, <2 x i4> zeroinitializer, <2 x i4> %m
>>     ret <2 x i4> %r
>>   }
>>
>> Or you meant that the code could be folded even if `icmp`'s RHS is not only the all-zeros vector, but a constant-vector containing some undefs?
>
> I mean that if in either zero constant vector, a particular element is undef, then in the other constant vector, the same element can be anything:
> https://alive2.llvm.org/ce/z/-gt253
> https://alive2.llvm.org/ce/z/3PAU3E
>
> So i think you want `if(!match(Constant::mergeUndefsWith(TrueVal, CondVal.getOperand(1)), m_Zero)) return;`

Got it, thanks for explanation!


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