Re: building musl libc.so with gcc -flto

[Andy Lutomirski <luto@kernel.org>]

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.

--Andy