[LLVMdev] Recent changes in -gmlt break sample profiling

[Diego Novillo]

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.

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.

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

Would it be possible to add these line numbers in some other way?


Thanks.  Diego.
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