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

[Peter Collingbourne via llvm-dev]

On Wed, Apr 6, 2016 at 1:33 PM, Teresa Johnson <tejohnson at google.com> wrote:

>
>
> 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).
>
> 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).
>

I see. I suppose that if we did form promotion names using external name
hashes, we could soundly compile parts of the object file to native code at
compile time, if we could somehow determine ahead of time that such
compilation would be safe. I'm working on a proposal along those lines that
I hope to share soon.

Peter
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