[Lldb-commits] [PATCH] D51208: [DWARF] Fix dwarf5-index-is-used.cpp

David Blaikie via Phabricator via lldb-commits lldb-commits at lists.llvm.org
Fri Aug 24 13:53:14 PDT 2018

dblaikie added a comment.
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In https://reviews.llvm.org/D51208#1212320, @labath wrote:

> As far as the strict intention of this test goes, the change is fine, as it is meant to check that the accelerator tables get used *when* they are generated. How do you achieve them being generated is not important.
> However, I am not sure now under what circumstances will the accelerator tables be emitted in the first place. David, does this mean that we will now not emit .debug-names even if one specifies `-glldb`? Was that intentional?

Not especially intentional - but I clearly didn't give it quite enough thought. Mostly I was modelling the behavior of GCC (no pubnames by default - you can opt-in to them (& split-dwarf opts in by default, but one thing GCC didn't do was allow them to be turned off again - which is what I wanted)).

As for the default behavior for DWARFv5 - have you run much in the way of performance numbers on how much debug_names speeds things up? From what I could see with a gdb-index (sort of like debug_names - but linker generated, so it's a single table for the whole program) it didn't seem to take GDB long to parse/build up its own index compared to using the one in the file. So it seemed to me like the extra link time, object size, etc, wasn't worth it in the normal case. The really bad case for me is split-DWARF (worse with a distributed filesystem) - where the debugger has to read all the .dwo files & might have a high latency filesystem for each file it reads... super slow. But if the debug info was either in the executable (not split) or in a DWP (split then packaged), it seemed pretty brief.

But if LLDB has different performance characteristics, or the default should be different for other reasons - I'm fine with that. I think I left it on for Apple so as not to mess with their stuff because of the MachO/dsym sort of thing that's a bit different from the environments I'm looking at.



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