[PATCH] Linker: Replace overridden subprograms

[David Blaikie]

On Tue, Dec 16, 2014 at 10:06 AM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:
>
>
> > On 2014-Dec-16, at 10:00, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >
> >
> > On Tue, Dec 16, 2014 at 9:48 AM, Rafael Espíndola <
> rafael.espindola at gmail.com> wrote:
> > > It's possible to put DWARF subprogram definitions for linkonce_odr
> functions
> > > into comdat groups so they are dropped along with the linkonce_odr
> function,
> > > but Clang (& I don't think GCC by default) does that (there's some
> overhead
> > > to that representational choice, etc). But yes, it can be done. I think
> > > today, for both GCC and Clang, you'd end up with two subprogram
> definitions
> > > (one in each DWARF compile unit) each pointing to the same
> high_pc/low_pc to
> > > describe the function.
> >
> > I thought this was part of a special case for dwarf in linkers (and
> > the missing feature why lld produced binaries are much larger in -g
> > builds).
> >
> > To the best of my knowledge, at least the usual Linux linkers (gold and
> binutils-ld) don't have any special cases for DWARF (well, gold has some
> magic to generate a GDB debug info index, maybe), it's just sections and
> relocations like any other data in an object file. /maybe/ dsymutil on
> MacOS does, but I've not heard about it. (& possibly dwz - a debug
> info-aware compression tool could do that trick, it's designed to eliminate
> redundancy in debug info)
> >
> > A basic test case shows roughly what I described. (attached dwarfdump,
> if you're curious - you can see what the source would've looked like from
> the DWARF there (inline function in a header, two translation units that
> instantiate the inline function, etc))
> >
> >
> > > (but yeah, vaguely sounds like what you're suggesting would make sense
> - I
> > > haven't looked at the initial proposed patch yet, though)
> >
> > Cheers,
> > Rafael
> > <dump.txt>
>
> Even if other linkers don't do it, is it reasonable to delete subprograms
> from
> compile units if the canonical one is from another compile unit (and has a
> subprogram there)?
>

I think so - more importantly, I assume that's what we'll end up doing
anyway. (check the actual DWARF output - since we build a map of
subprograms, I don't think we end up emitting the subprogram to both CUs
anyway - we just ignore all but the first one we find - that's my bet
anyway)


>
> If so, I can just do that.
>
> If not, is the original patch okay (as a stop-gap)?


 Probably, though the code seems somewhat complicated & I haven't quite
managed to internalize it all.
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