[llvm-dev] large slowdown in DAGCombiner::MergeConsecutiveStores

Jeff Bezanson via llvm-dev llvm-dev at lists.llvm.org
Fri Mar 20 11:58:12 PDT 2020

Thanks for those references. I tried this with a ~1 week old master and
indeed the situation is much better. It's now about 2x slower than 6.0.1.
The extra time is still in MergeConsecutiveStores. Do you think there's any
scope for further limiting the work done there? The performance is now at
least tolerable though.

Many thanks to everyone who has looked into these issues; this makes a big
difference for some interesting Julia use cases.


On Thu, Mar 19, 2020 at 4:37 PM Florian Hahn <florian_hahn at apple.com> wrote:

> Hi,
> > On Mar 19, 2020, at 20:29, Jeff Bezanson via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Hello all,
> >
> > We are seeing a large compiler performance regression in moving from
> LLVM 6.0.1 to 8.0.1. We have a long function (~50000 instructions) that
> used to compile in about a minute but now takes at least an hour. All the
> time is in MergeConsecutiveStores, I believe due to super-linear behavior
> in analyzing very long chains of stores. For example, this change makes the
> problem go away:
> >
> Are you seeing the same performance regression on master?
> Over the last year, at least 2 patches landed to improve compile time in
> similar cases: https://reviews.llvm.org/D62633
> https://reviews.llvm.org/D65174
> Cheers
> Florian
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