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

Xinliang David Li via cfe-dev cfe-dev at lists.llvm.org
Fri May 15 11:14:20 PDT 2020

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


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
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