[PATCH] D46283: [AArch64] Set vectorizer-maximize-bandwidth as default true

[Adhemerval Zanella via Phabricator via llvm-commits]

zatrazz added a comment.

In https://reviews.llvm.org/D46283#1083350, @rengolin wrote:

> This seems like an improvement, but we have to be careful with wide variations and little gain. The geomean is almost null but the standard deviation is higher than 75% of the results.
>
> This is not a good sign and is a reflection of what I have complained about exchanging numbers and not tracking which benchmarks get worse/better, then risking hurting more one side than another in the long term.
>
> The patch sets the default to *all* AArch64 cores, which is a wide range and you have only tested on one.
>
> To enable such flag by default, we'd need to run on more cores (preferably including A53) as well as other vendors (like Apple, Samsung, Qualcomm), which this patch is likely to impact.
>
> @t.p.northover @evandro @mcrosier please have a look.


My understanding is the 'vectorizer-maximize-bandwidth' was previously set and reset as default for all targets. This history is more that it has cause some functionality issues than performance one [1]. I couldn't find any follow up on this thread, so I took a more conservative approach and just enable it on AArch64. I will try to check on more recent workloads (speccpu2017) to see any gains or regressions and also rerun it on master. But I do agree that this kind of change should take with care.

[1] [llvm] r305960 - Enable vectorizer-maximize-bandwidth by default. (Dehao)
[llvm] r305990 - Revert "Enable vectorizer-maximize-bandwidth by default." (Diana Picus)
[llvm] r306336 - Enable vectorizer-maximize-bandwidth by default. (Dehao)
[llvm] r306344 - revert r306336 for breaking ppc test. (Dehao)
[llvm] r306473 - re-commit r306336: Enable vectorizer-maximize-bandwidth by default. (Dehao)
[llvm] r306792 - Revert "r306473 - re-commit r306336: Enable vectorizer-maximize-bandwidth by default." (Daniel)
[llvm] r306933 - Enable vectorizer-maximize-bandwidth by default.
[llvm] r306934 - revert r306336 for breaking ppc test.
[llvm] r306935 - re-commit r306336: Enable vectorizer-maximize-bandwidth by default.
[llvm] r306936 - Revert "r306473 - re-commit r306336: Enable vectorizer-maximize-bandwidth by default."


Repository:
  rL LLVM

https://reviews.llvm.org/D46283