[PATCH] D112108: [InstCombine] Fold `(a & ~b) & ~c` to `a & ~(b | c)`

[Stanislav Mekhanoshin via Phabricator via llvm-commits]

rampitec added a comment.

In D112108#3078966 <https://reviews.llvm.org/D112108#3078966>, @spatel wrote:

> In D112108#3078420 <https://reviews.llvm.org/D112108#3078420>, @rampitec wrote:
>
>>   (c & ~(a | b)) | (b & ~(a | c)) --> ~a & (b ^ c)
>>
>> We currently cannot simplify it.
>>
>>   define i32 @or_not_and(i32 %a, i32 %b, i32 %c) {
>>     %or1 = or i32 %a, %b
>>     %not1 = xor i32 %or1, -1
>>     %and1 = and i32 %not1, %c
>>     %or2 = or i32 %a, %c
>>     %not2 = xor i32 %or2, -1
>>     %and2 = and i32 %not2, %b
>>     %or3 = or i32 %and1, %and2
>>     ret i32 %or3
>>   }
>
> Hmm...so that's 2 of the patterns in this patch glued together. I don't see an intermediate fold that we can use to reduce it. Either you have to hard-code a big pattern match or add a generalized boolean logic solver (as its own pass).

Having a separate boolean logic solver might be a good idea in a long run. I.e. collect operands unused ouside of a dag solved, build truth table, generate a minimal expression. But that probably needs much more thought than this initial idea.


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