[PATCH] D137934: [InstCombine] Fold extractelt with select of constants

[Thomas Symalla via Phabricator via llvm-commits]

tsymalla added a comment.

In D137934#3936962 <https://reviews.llvm.org/D137934#3936962>, @spatel wrote:

> In D137934#3928765 <https://reviews.llvm.org/D137934#3928765>, @tsymalla wrote:
>
>> @spatel Thanks for adding these tests. These work in general, except the case where the second operand of the ExtractElement instruction is not a constant. The reason is that currently `constantFoldOperationIntoSelectOperand` expects both operands to be either the original select or a constant, but not an additional select, so a new extractelement instruction can be created to extract the actual value. If one of the operands is another select, it could be obvious that the sequence shows OOB behavior:
>>
>>   %e = select i1 %c, i32 3, i32 4
>>   %s = select i1 %c, <3 x i32> <i32 2, i32 3, i32 4>, <3 x i32> <i32 5, i32 6, i32 7>
>>   %r = extractelement <3 x i32> %s, i32 %e
>>   ret i32 %r
>>
>> Even if I know how to make the regular case work, I'll wrap my head around that particular issue.
>
> Just bail out if the extract index value is not an immediate constant? The case where the index value is a select-of-constants that can be reduced might be another patch, but that doesn't seem like a common pattern.

Yeah, I have that working, but it's not going to optimize such cases like in the `shufflevector` tests for now. I'll go with bailing out for now.


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