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

[Teresa Johnson via Phabricator via llvm-commits]

tejohnson added a comment.

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.


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https://reviews.llvm.org/D75790