[llvm-dev] ThinLTO naming scheme for promoted local functions

Justin Bogner via llvm-dev llvm-dev at lists.llvm.org
Wed Apr 6 14:08:59 PDT 2016

Teresa Johnson via llvm-dev <llvm-dev at lists.llvm.org> writes:
> On Wed, Apr 6, 2016 at 12:18 PM, Peter Collingbourne <peter at pcc.me.uk>
> wrote:
>> Hi David,
>> We've been considering changing the naming scheme for promoted local
>> functions in ThinLTO. Currently we just prepend the file name, but that
>> isn't really sound for a number of reasons (e.g. you can have the same file
>> name in different directories). The alternative we've been thinking about
>> is to use the hash of all external names in the module, as that is
>> guaranteed to be unique within a linkage unit (otherwise the linker would
>> complain).
>> We currently (intentionally, I believe) use the same naming scheme for
>> promoting local functions as we do for PGO,
> No, we don't use this naming scheme for ThinLTO promotion. It is only used
> for computation of the MD5 hash used in the function index (so that when we
> want to import a function referenced by indirect call profile info which
> uses this MD5 name we can find its summary).

Oh good, I was worried for a second. The PGO renaming approach isn't
very robust at all and will hopefully be replaced with something less
fragile eventually.

> The promotion name is based off a unique module identifier assigned at
> Thin-link time (when the combined index is generated and all bitcode files
> are seen).
> Thanks,
> Teresa
> so we might need to change both. Do you see any back compat concerns with
>> changing the naming scheme? I guess there are various things we can do to
>> try to ensure back compat, but I wanted to get an idea of what the
>> requirements are.
>> Thanks,
>> --
>> --
>> Peter

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