[PATCH] D86262: [LoopIdiomRecognizePass] Options to disable part or the entire Loop Idiom Recognize Pass

[Florian Hahn via Phabricator via llvm-commits]

fhahn added a comment.

In D86262#2228123 <https://reviews.llvm.org/D86262#2228123>, @nemanjai wrote:

> I agree that the justification provided seems somewhat inadequate - using `memset/memcpy` as a canonical form and teaching DA to handle it seems quite reasonable. However, there are libraries that are highly tuned wrt. when to call these functions and when not to. So it seems to me perfectly reasonable to provide an option to disable this for such uses. For example, a function may perform a memcpy/memset operation on data that is known to only be called only from range-limited sites. The compiler (without PGO+LTO which are often not an option) cannot know that it should expand such calls in the back end.

The issue you raise seems to be a cost-modeling issue, right? Ideally the compiler would know whether it is faster to use a loop or a memcpy. IIUC you are worried about the cases where the compiler replaces a loop with a memset, but the loop would be faster? Is there a way to improve the cost-modeling? IMO adding options like this potentially leads to papering over cost-modeling issues, rather than addressing the underlying issue. Also, such options tend to now work well together with LTO.


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https://reviews.llvm.org/D86262