[PATCH] D110076: [AMDGPU][GlobalISel] Code quality: Combine V_RSQ

Thu Nov 4 05:29:51 PDT 2021

matejam added a comment.

In D110076#3108478 <https://reviews.llvm.org/D110076#3108478>, @foad wrote:

>> Added implementation for all possible cases which should be combined into rsq (rcp(sqrt(x)), sqrt(rcp(x)), 1/sqrt(x), sqrt(1/x)).
>
> I thought this would be two separate combines:
>
> 1. (1.0 / x) -> (rcp x)
> 2. (sqrt (rcp x)) or (rcp (sqrt x)) -> (rsq x)
>
> Is there some reason we don't implement the first combine, e.g. because of the precision of the rcp instruction is not good enough? What does SelectionDAG do?

If we run an .ll test which has (1.0 / x), by the time it gets to the amdgpu-postlegalizer-combiner it will be combined into rcp, just like SDAG.
This is a 'fake' case of a .mir test, where we put the (1.0 / x) in the test and let the combiner take care of that.

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  https://reviews.llvm.org/D110076/new/

https://reviews.llvm.org/D110076