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.
fnptr-clang36.bad.dwarfdump (4.11 KB)
fnptr-clang34.good.dwarfdump (6.28 KB)