[PATCH] Linker: Replace overridden subprograms

[Duncan P. N. Exon Smith]

> On 2014 Dec 16, at 10:27, David Blaikie <dblaikie at gmail.com> wrote:
> 
> 
> 
> 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.

How does this look?

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