[llvm-dev] [cfe-dev] Just a quick heads up -- removing BBVectorize from LLVM (and Clang)

Davide Italiano via llvm-dev llvm-dev at lists.llvm.org
Fri Jun 30 20:18:17 PDT 2017

On Fri, Jun 30, 2017 at 7:01 PM, C Bergström via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> I don't know how others feel, but it seems this was removed more by feeling
> and less by actual data points. It doesn't mean it wasn't the correct
> choice, but just because nobody spoke up could mean that they silently enjoy
> the benefits and maybe some end user far away isn't subscribed. With this in
> mind - I hope others would consider turning optimizations into an Assert,
> Warning or Error if they will be removed - at least for a release. That way
> there's some visible warning that users had a chance to see and potentially
> relay the feedback. The middle ground could be disabling the transformation
> and just warning on the analysis pass. "Foo optimization has been removed
> and it may impact your code performance"...
> Too much work?

FWIW, people have independently found issues/miscompiles in the BB
Vectorizer. I picked up this only recently, but some of them have been
reported on bugzilla and been there forever without anybody looking at
them (see Zhendong's reports).
I don't think resurrecting would be hard if anybody really wants to.
It requires some amount of effort nobody is apparently able to put at
the moment. We removed loadcombine for a similar reason recently.


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