[2.3 BRANCH]: gstabs issue

Kelly O'Hair kelly.ohair at oracle.com
Wed Aug 8 21:31:22 PDT 2012

Interesting. Thanks for the information.


On Aug 8, 2012, at 11:48 AM, Mark Wielaard wrote:

> On Wed, 2012-08-08 at 11:11 -0700, Kelly O'Hair wrote:
>> I'm just lurking on this alias, but I have a dbx developer history, and in particular debug formats.
>> I suspect the reason for using stabs is that they aren't necessarily smaller in the .o files than Dwarf, but when the
>> .so file is created, the Dwarf debug info is very significantly larger than stabs due to the stabs size optimizations
>> that happens at link time and NO Dwarf size optimizations at link time.
>> I'm not 100% up on what Linux/gcc/ld is doing at link time, but when Dwarf came on the scene
>> many years ago, the plan was to have the linker process the debug information and do some folding of the
>> graph a bit so the size would be significantly reduced, similar to what has always happened with stabs.
>> I have no idea what the status of that is, it never happened in the Solaris world, just not sure about Linux.
> There are a couple of efforts to do something like you suggest. I assume
> the linker could easily "optimize" STABS since it was basically all
> encoded as symbol table entries, so duplicates could just be handled as
> normal symbols in the linker. But DWARF is more expressive and cannot
> easily be expressed as just a fancy symbol (it has a tree structure).
> Having the linker do even more work seems not popular. The linker is
> already a bottleneck.
> But there is a way to encode part of the type DIE tree in elf COMDAT
> sections, which the linker could merge. This isn't enabled everywhere
> though, since there are a couple of corner cases that seem
> underspecified. But most tools have been updated to work
> with .debug_types now if they are there (at least elfutils/systemtap,
> valgrind and gdb).  See http://gcc.gnu.org/wiki/DwarfSeparateTypeInfo
> and appendix E in the DWARF4 standard.
> To ease the linker pain, there are also some ways in newer DWARF to just
> have less relocations to process in the .debug sections (newer DWARF
> allows you to express more with offsets and less with addresses).
> There is a DWARF compressor that works post-linker, dwz, which picks out
> common DIE subtrees. This is being experimented with in Fedora for the
> next release. See
> https://fedoraproject.org/wiki/Features/DwarfCompressor
> And there is an effort to keep a lot of the .debug sections outside of
> the linker itself and store some parts completely separately from the
> (linked) object files (kind of what is done now with separate debuginfo
> after the link phase, but so that the linker can just leave it behind).
> See http://gcc.gnu.org/wiki/DebugFission
> Cheers,
> Mark

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