Re: Range lists, zero-length functions, linker gc

David Blaikie
2020-05-31T22:36:14Z

On Sun, May 31, 2020 at 3:30 PM Mark Wielaard <mark@klomp.org> wrote:
>
> Hi,
>
> On Sun, May 31, 2020 at 01:49:12PM -0700, David Blaikie wrote:
> > On Sun, May 31, 2020 at 1:41 PM Mark Wielaard <mark@klomp.org> wrote:
> > > On Sun, May 31, 2020 at 11:55:06AM -0700, Fangrui Song via Elfutils-devel wrote:
> > > > I am eager to know what you think
> > > > of the ideas from binutils/gdb/elfutils's perspective.
> > >
> > > I think this is a producer problem. If a (code) section can be totally
> > > dropped then the associated (.debug) sections should have been
> > > generated together with that (code) section in a COMDAT group. That
> > > way when the linker drops that section, all the associated sections in
> > > that COMDAT group will get dropped with it. If you don't do that, then
> > > the DWARF is malformed and there is not much a consumer can do about
> > > it.
> > >
> > > Said otherwise, I don't think it is correct for the linker (with
> > > --gc-sections) to drop any sections that have references to it
> > > (through relocation symbols) from other (.debug) sections.
> >
> > That's probably not practical for at least some users - the
> > easiest/most thorough counter-example is Split DWARF - the DWARF is in
> > another file the linker can't see. All the linker sees is a list of
> > addresses (debug_addr).
>
> I might be missing something, but I think this works fine with Split
> DWARF. As long as you make sure that the .dwo files/sections are
> separated along the same lines as the ELF section groups are. That
> means each section group either gets its own .dwo file, or you
> generate the .dwo sections in the same section group in the same
> object file using the SHF_EXCLUDED trick. That way each .debug.dwo
> uses their own index into the separate .debug_addr tables. If that
> group, with the .debug_addr table, gets discarded, then the reference
> to the .dwo also disappears and it simply won't be used.

Oh, a whole separate .dwo file per function? That would be pretty
extreme/difficult to implement (now the compiler's producing a
variable number of output files? using some naming scheme so the build
system could find them again for building a .dwp if needed, etc).
Certainly Bazel (& the internal Google version used to build most
Google software) can't handle an unbounded/unknown number of output
files from a build action.

Multiple CUs in a single .dwo file is not really supported, which
would be another challenge (we had to compromise debug info quality a
little because of this limitation when doing ThinLTO - unable to emit
multiple CUs into each thin-linked .o file) - at which point maybe the
compiler'd need to produce an intermediate .dwp file of sorts... but
there wouldn't be any great way for the debugger to find those
intermediate .dwp files (since it's basically "either find the .dwo
file that's written in the DWARF, or find the .dwp file relative to
the executable name)? Not sure.

& again the overhead of all those separate contributions, headers,
etc, turns out to be not very desirable in any case.

- Dave