[LLVMdev] Recent changes in -gmlt break sample profiling

David Blaikie dblaikie at gmail.com
Fri Oct 24 15:11:21 PDT 2014

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

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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