[PATCH] D115713: [LV] Don't apply "TinyTripCountVectorThreshold" for loops with compile time known TC.

Mon Nov 7 06:12:58 PST 2022

lebedev.ri added a comment.

In D115713#3911820 <https://reviews.llvm.org/D115713#3911820>, @ebrevnov wrote:

> In D115713#3910792 <https://reviews.llvm.org/D115713#3910792>, @dmgreen wrote:
>
>> I think @fhahn also mentioned that he had some AArch64 examples where the same is true. I'm not sure in general where this would be useful. The vectorizers handing of small trip count loops is not amazing, considering that many such loops will already have been fully unrolled.
>
> Until I'm missing something looks like currently LV comes before Unroller&SLP which makes perfect sense to me. Anyway, I wouldn't stick to any specific pass ordering as LLVM is an infrastructure for building custom compilers. LV should do its job as good as it can. If it can prove that vectorization is beneficial it should do it (until we have an infrastructure to take dependencies between different passes into account).

(Unfortunately?) full loop unrolling happens before LV.
I personally think that once LV has outer loop vectorization, that should be considered a bug that should be resolved.

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