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<div style=""><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">Hi,</span><br>
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<div><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">Do you have any data on how often LoopDistribute triggers on a larger set of programs (like llvm-test-suite + SPEC)? AFAIK the implementation is very
limited at the moment (geared towards catching the case in hmmer) and I suspect lack of generality is one of the reasons why it is not enabled by default yet.</span></div>
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<div><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">It would be good to have some fresh numbers on how often LoopDistribute triggers. From what I remember, there are a handful of cases in the test suite,
but nothing that significantly affects performance (other than hmmer, obviously).</span></div>
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<div><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">Also, there’s been an effort to improve the cost-modeling for LoopDistribute (</span><a href="https://reviews.llvm.org/D100381"><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">https://reviews.llvm.org/D100381</span></a><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">)
Should we make progress in that direction first, before enabling by default?</span></div>
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<div><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">Unfortunately, there were some problems with this effort.</span><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
First, the current implementation of LoopDistribute relies heavily on LoopAccessAnalysis, which made it difficult to adapt.</span><br>
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<div><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">More</span><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);"> importantly though, I'm not convinced
that LoopDistribute will be beneficial other than in cases where it enables more vectorization. (The memcpy detection gcc might be interesting, I didn't look at that.) It reduces both ILP and MLP, which in some cases might be made up by lower register or cache
pressure, but this is hard or impossible for the compiler to know.</span></div>
<div><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);"><br>
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<div><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">While working on this, with a more aggressive LoopDistribute across several benchmarks, I did not see any improvements that didn't turn out to be noise,
and plenty of cases where it was actively degrading performance.</span></div>
<div><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);"><br>
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<div><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">Therefore, I'm not sure this direction is worth pursuing further, and I believe the current heuristic of "distribute when it enables new vectorization"
is actually pretty reasonable, if not very general.</span></div>
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<div><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">Cheers,</span></div>
<div><span style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">Sanne</span><br>
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