[PATCH] D155218: [InstCombine] Optimize addition/subtraction operations of splats of vscale multiplied by a constant

Tue Aug 29 07:28:52 PDT 2023

nikic added a comment.

In D155218#4587835 <https://reviews.llvm.org/D155218#4587835>, @igor.kirillov wrote:

>> Please make sure to pre-commit test coverage (https://llvm.org/docs/TestingGuide.html#precommit-workflow-for-tests). This is also missing some tests, in particular multi-use and negative tests.
>
> @nikic Added a test with `use`. Could you clarify what you mean by negative tests in this context?
> I am aware of pre-commits. I just wasn't sure we wanted this patch at all. That's why I will pre-commit them just before landing the patch.

Negative test = test where no transform happens. Generally speaking, you want one negative test for every condition in the transform, such that each test fails exactly one condition.

In D155218#4576394 <https://reviews.llvm.org/D155218#4576394>, @paulwalker-arm wrote:

> I guess I'm making a generalisation here based on scalable vectors needing to calculate addresses and the like based on vscale and so am assuming specialised add/sub instructions will always exist.  My eyes are not tuned in to reading rvv asm, can @craig.topper  or @reames help clarify whether this combine also makes sense from a RISCV point of view.

To make the question more precise: If you have `spat(vscale * C1) + splat(vscale * C2)`, does it make sense to transform it into `splat(vscale * (C1+C2))` **if neither `splat(vscale *  C1)` nor `splat(vscale * C2)` are going away**?

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