[PATCH] D85504: [Reassociate] [PowerPC] stop common out mul factors if fma is preferred on target

Fri Aug 7 01:11:54 PDT 2020

shchenz added a comment.

In D85504#2202000 <https://reviews.llvm.org/D85504#2202000>, @lebedev.ri wrote:

> (can you please maybe spellcheck both the patch description and the wording within the patch? it is really hard to read)

Done

> I'm not really sure this is the way forward.
>
> 1. This is a target-agnostic pass, it we add arbitrary restrictions it will likely miss some intended transforms

Could you be more specific? I verified this change on PowerPC for some benchmarks, no degradation found.

> 2. As usual, just preventing some transform is not the solution, because you can still have "bad" input that will still be lowered badly

Agree. But if we know that one transformation has no benefit at all or it has a negative impact on some specific targets, should we stop the transformation on these targets? Reversing the transformations back in later pass will make compiler spend compiling time both on the harmful transformations and on the reversed transformations?

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https://reviews.llvm.org/D85504