Re: Re: building musl libc.so with gcc -flto

[Rich Felker <dalias@libc.org>]

On Thu, Apr 30, 2015 at 01:46:21PM -0700, Andy Lutomirski wrote:
> On 04/22/2015 07:23 PM, Rich Felker wrote:
> >On Wed, Apr 22, 2015 at 03:48:52PM -0700, Andre McCurdy wrote:
> >>Hi all,
> >>
> >>Below are some observations from building musl libc.so with gcc's -flto
> >>(link time optimization) option.
> >
> >Interesting!
> >
> >>1) With today's master (afbcac68), adding -flto to CFLAGS causes the
> >>build to fail:
> >>
> >>  | `_dlstart_c' referenced in section `.text' of /tmp/cc8ceNIy.ltrans0.ltrans.o: defined in discarded section `.text' of src/ldso/dlstart.lo (symbol from plugin)
> >>  | collect2: error: ld returned 1 exit status
> >>  | make: *** [lib/libc.so] Error 1
> >>
> >>Reverting f1faa0e1 (make _dlstart_c function use hidden visibility)
> >>seems to be a workaround.
> >
> >I think the problem is that LTO is garbage collecting "unused" symbols
> >before it gets to the step of linking with asm for which there is no
> >IR code, thereby losing anything that's only referenced from asm. A
> >better workaround might be to define _dlstart_c with a different name
> >as a non-hidden function (e.g. call it __dls1) and then make
> >_dlstart_c a hidden alias for it via:
> >
> >__attribute__((__visibility__("hidden")))
> >void _dlstart_c(size_t *, size_t *);
> >
> >weak_alias(__dls1, _dlstart_c);
> >
> >If you get a chance to try that, let me know if it works. Another
> >option might be adding -Wl,-u,_dlstart_c to LDFLAGS.
> 
> Wouldn't adding __attribute__((externally_visible)) to the relevant
> symbols be more appropriate?  It's intended to solve exactly this
> problem.

I'm not clear whether it would be reliable to use this or not.
Semantically externally_visible and visibility=hidden are
contradictory. Even if we weren't trying to avoid relying on
additional GNU C features, I think it would be a bad idea to rely on
this working since the behavior under such contradictory annotations
could potentially vary widely between compilers.

Rich