llvm dev - Oct 2014 - [LLVMdev] Recent changes in -gmlt break sample profiling

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.


Thanks. Diego.
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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 ).

Even when a subprogram does have inlined subroutines, I've omited the
line/file as well, to reduce -gmlt size (actually, I imagine we could even
remove the top level subroutines entirely - just emitting the inlined
subroutines at the top level - completely garbage DWARF, but it would be
smaller & easy enough to teach symbolizers to deal with that if they
don't
already)

To be honest, optimized -gmlt is still pretty big, but these changes do
make unoptimized -gmlt essentially no cost over the line table itself,
which is nice. A longer term approach to solving this is
two-level-line-tables, but they would probably present a problem for the
algorithm you've described too.

- David

>
>
> Thanks. Diego.
>
>
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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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