[llvm-dev] Issue with the function hash used as PGO index

[Xinliang David Li via llvm-dev]

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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200515/ca26d9e9/attachment.html>