[PATCH] D109889: AMDGPU: Lower one copy from SCC early for SelectionDAG

Tue Sep 21 08:32:53 PDT 2021

arsenm added a comment.

In D109889#3012452 <https://reviews.llvm.org/D109889#3012452>, @foad wrote:

>> In SelectionDAG path, we are currently using -1/0 when copying from SCC.
>> But in GlobalISel path, we are requesting 1/0 when copying from SCC.
>
> Is there a good reason for that difference? Is it related to TargetLowering::setBooleanContents? We always set ZeroOrOneBooleanContent for GCN subtargets.

The problem is "COPY from SCC" by itself is not a semantically meaningful concept. We can make up whatever we want. I think ZeroOrOneBooleanContent is a better choice, since it's not fighting an uphill battle for optimization priority, and there really is only one bit. Places that semantically need to use -1 can emit the select directly.

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  https://reviews.llvm.org/D109889/new/

https://reviews.llvm.org/D109889