Re: OCaml flambda optimizations causing a compilation slow down

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On Mon, May 06, 2024 at 09:54:01AM +0100, Richard W.M. Jones wrote:
> On Mon, May 06, 2024 at 10:16:45AM +0200, Florian Weimer wrote:
> > * Richard W. M. Jones:
> > 
> > > This email is mostly to notify that this is happening.  I'm not sure
> > > if a single package slowing down compilation means we need to do
> > > anything here, but if anyone else sees similar symptoms, let us know.
> > 
> > What blocks you from profiling it and fixing the performance issue?
> 
> I did profile it down to a couple of functions in a very dark corner
> of the compiler (see the thread on cocci devel list that I posted).
> However in the time available last week I didn't have the time to
> understand what it was doing or fix it.

(https://lore.kernel.org/cocci/20240502173148.GF30267@xxxxxxxxxx/)

I was thinking about filing an upstream bug, but this comment is
interesting (not the bug itself, since the specific problem there is
different):

https://github.com/ocaml/ocaml/issues/10014#issuecomment-725270460

Some things we could take away from this:

(1) It's expected.

(2) They're not going to fix it as they're working on an even newer
optimizer.

(3) You can add -Oclassic to affected files to get the regular
optimizer.  [This isn't exactly what the option does, but using it
does fix the issue.]

My proposed fix / workaround:

https://src.fedoraproject.org/rpms/coccinelle/pull-request/4

Rich.

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