Re: OCaml flambda optimizations causing a compilation slow down

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On Fri, May 03, 2024 at 10:54:02AM -0600, Jerry James wrote:
> On Fri, May 3, 2024 at 6:22 AM Richard W.M. Jones <rjones@xxxxxxxxxx> wrote:
> > In Fedora we enable flambda in our OCaml package.  Debian does not.
> 
> As the person who pushed to get flambda enabled in Fedora, I feel kind
> of responsible for this.

I'm not really sure it's a problem.  Annoying if you're compiling
coccinelle for sure, but it doesn't break anything.

> > We recently found that one package (coccinelle) takes much, much
> > longer to compile when this is enabled.  As in one particular file
> > goes from seconds -> 30 minutes to compile.  I investigated and the
> > difference is entirely explained by enabling flambda, and goes away
> > when disabled.  Apart from this being annoying, it doesn't seem to be
> > a problem in any other way (for example, the final program doesn't
> > noticable run faster or slower).  Upstream coccinelle seem to be using
> > Debian and therefore haven't seen the problem.
> >
> > Thread about all that:
> > https://lore.kernel.org/cocci/20240502085433.GA30267@xxxxxxxxxx/
> >
> > 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.
> 
> Wow, that's an amazing blow up in compile time.  I note that the file
> in question is generated by menhir.  I remember seeing reports that
> some menhir features generate code that cause compile times to
> explode.  I can't seem to find a usable link to the menhir mailing
> list archives at the moment.  I can experiment with the different code
> generation options from menhir and see if anything helps.
> 
> Also, I was going to contact you about this today, but OCaml 5.2.0RC1
> was tagged yesterday.  I started doing test builds here:
> 
> https://copr.fedorainfracloud.org/coprs/jjames/OCaml5.2/

I was watching the bug and planning to ignore it until 5.2 was
released.  However it's great that you're doing test builds, thanks.

> I need to build a few more packages before coccinelle can be
> attempted, but it will be interesting to see if the build time
> improves.

I tested the tip of the 5.1 branch upstream and that still had the
issue.  I didn't test 5.2.

Rich.

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