[PATCH] D75790: [PowerPC] Fix compile time issue in recursive CTR analysis code

Tue Mar 10 14:46:24 PDT 2020

MaskRay added a comment.

In D75790#1915671 <https://reviews.llvm.org/D75790#1915671>, @tejohnson wrote:

> In D75790#1915590 <https://reviews.llvm.org/D75790#1915590>, @MaskRay wrote:
>
> > Generally looks good, but the title needs to be clarified. This piece of code is related to an optimization which uses CTR as the loop count register.
>
>
> Updated the title.
>
> > For context, D6786 <https://reviews.llvm.org/D6786> introduced `mightUseCTR`.  rL251582 <https://reviews.llvm.org/rL251582> made it recurse into the constant.
> > 
> > IIUC, with ThinLTO's ImportConstantsWithRefs optimization, a constant can be very large, and recursing into it for every instruction can make the compilation slow. Is that the case?
>
> No, the constant itself was quite simple. Importing it simply enabled some additional optimization (in IndVarSimplify) that created a huge instruction expression. It was iterating that expression, not the constant itself, that is so slow.

It'd be nice to give a (reduced) example in the description, even if testing it reliably may be unrealistic.

Repository:
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https://reviews.llvm.org/D75790