[LLVMdev] Recent changes in -gmlt break sample profiling

David Blaikie dblaikie at gmail.com
Fri Oct 24 15:21:13 PDT 2014

On Fri, Oct 24, 2014 at 3:16 PM, Diego Novillo <dnovillo at google.com> wrote:

> On Fri Oct 24 2014 at 6:11:21 PM David Blaikie <dblaikie at gmail.com> wrote:
>> On Fri, Oct 24, 2014 at 2:48 PM, Diego Novillo <dnovillo at google.com>
>> wrote:
>>> I'm not sure if this was intended, but it's going to be a problem for
>>> sample profiles.
>>> When we compile with -gmlt, the profiler expects to find the line number
>>> for all the function headers so that it can compute relative line locations
>>> for the profile.
>>> The tool that reads the ELF binary is not finding them, so it writes out
>>> absolute line numbers, which are impossible to match during the profile-use
>>> phase.
>>> The problem seems to be that we are missing DW_TAG_subprogram for all
>>> the functions in the file.
>>> Attached are the dwarf dumps of the same program. One compiled with my
>>> system's clang 3.4 and the other with today's trunk. In both compiles, I
>>> used -gline-tables-only.
>>> The trunk version is missing all the subprogram tags for the functions
>>> in the file. This breaks the sample profiler.
>>> Should I file a bug, or is -gmlt going to be like this from now on? The
>>> latter would be a problem for us.
>> Open to negotiation, but this change is intentional ( for details, see
>> the commit: http://llvm.org/viewvc/llvm-project?rev=218129&view=rev ).
> Well, this breaks us. Very hard. It absolutely blocks us from using
> SamplePGO with LLVM.

Sorry about that - I knew ASan needed gmlt data of a certain form and
worked carefully to ensure llvm-symbolizer could still symbolize with these
changes, but wasn't aware of this particular dependence from PGO (just
assumed it used the line table directly).

> The alternative would be to make the compiler use absolute line numbers,
> but in the experience we've collected with GCC, this makes the profiles
> very brittle wrt source changes.

It'd be interesting to see the data - I guess you throw out profiles that
don't match on a per-function basis? So adding a few lines to one function
will invalidate the profile for that function, but not all the subsequent
ones in the file?

> I don't have a better idea atm. Would there be any other way to generate
> relative line numbers?


>   Perhaps we could use the first line number we find with samples?

Probably - I guess you use the ELF symbol table to compute the address
range of each function? Given that, you could find the smallest line number
in that address range in the line table, and make everything else relative
to that.

> The problem here is that this line will shift, depending on how the
> profile was obtained.

Not sure what you're referring to here ^

> Would it be possible to add these line numbers in some other way?
> Thanks.  Diego.
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