llvm dev - May 2020 - Issue with the function hash used as PGO index

Hi Folks,

This review

    https://reviews.llvm.org/D79961 

fixes a relatively serious bug in the computation of the function hash used to
compute index in profile files in clang. Basically the bug was allowing trivial
hash collision of two function that only differ by their last statement(s). This
could lead to segfaukts like this one [1].

I'm happy with the patch (if you want to review it, please jump in) but
I'm also
worried that it changes the hash of *most* functions, thus invalidating all
profile files generated with previous clang revision.

So if it lands, people will find themselves with a lot of obsolete profile data
that need to be regenerated... I think I'd at least state that clearly on
the
mlist, due to the potential large impact.

- Serge


[1] https://bugzilla.redhat.com/show_bug.cgi?id=1827282

FYI, there is also IR PGO that computes function hash using CFG. To turn it
on, use -fprofile-generate and -fprofile-use options.

David

On Thu, May 14, 2020 at 11:59 PM Serge Guelton via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi Folks,
>
> This review
>
>     https://reviews.llvm.org/D79961
>
> fixes a relatively serious bug in the computation of the function hash
> used to
> compute index in profile files in clang. Basically the bug was allowing
> trivial
> hash collision of two function that only differ by their last
> statement(s). This
> could lead to segfaukts like this one [1].
>
> I'm happy with the patch (if you want to review it, please jump in) but
> I'm also
> worried that it changes the hash of *most* functions, thus invalidating all
> profile files generated with previous clang revision.
>
> So if it lands, people will find themselves with a lot of obsolete profile
> data
> that need to be regenerated... I think I'd at least state that clearly
on
> the
> mlist, due to the potential large impact.
>
> - Serge
>
>
> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1827282
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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