Re: string word-at-a-time and atomic.h FAQ on twitter

[Rich Felker <dalias@libc.org>]

On Wed, Jan 13, 2016 at 06:30:50PM +0100, Szabolcs Nagy wrote:
> i tried to do it with -fno-lto
> 
> but it seems gcc-6 miscompiles musl with -flto anyway:
> 
> lto incorrectly dead code eliminates _dlstart_c.
> (the libc entry point, _dlstart, is defined in toplevel
> inline asm in ldso/dlstart.c and it jumps to _dlstart_c)

I think the files that receive "crt" treatment (-DCRT?) should be
hard-coded as no-LTO. Even if it weren't for this issue, there are
also potential issues with optimizations that might move code across
relocations and initialization. At the very least there should be a
full asm barrier between such init code and the rest of the program.

> lto breaks symbol binding for environ, _environ, ___environ.
> (they should be weak, without that environ in a main binary
> has different address than in libc.so)
> 
> libc.so built with -flto:
> $ readelf --dyn-syms -W libc.so |grep envi
>     22: 000000000028eb90     8 OBJECT  GLOBAL DEFAULT   15 __environ
>    398: 000000000028eb90     8 OBJECT  GLOBAL PROTECTED   15 ___environ
>   1034: 000000000028eb90     8 OBJECT  GLOBAL PROTECTED   15 _environ
>   1107: 000000000028eb90     8 OBJECT  GLOBAL DEFAULT   15 environ
> 
> libc.so without -flto:
> $ readelf --dyn-syms -W libc.so |grep envi
>     22: 000000000028d2d8     8 OBJECT  GLOBAL DEFAULT   15 __environ
>    398: 000000000028d2d8     8 OBJECT  WEAK   PROTECTED   15 ___environ
>   1034: 000000000028d2d8     8 OBJECT  WEAK   PROTECTED   15 _environ
>   1107: 000000000028d2d8     8 OBJECT  WEAK   DEFAULT   15 environ

This is purely a gcc bug and indicates LTO is not really ready for use
with libc, which means I don't see much sense in trying to support it
yet. Of course that doesn't mean I'm disinterested in fixing actual C
language usage errors (aliasing, etc.) that might break with LTO.
Rather I'm disinterested in LTO-specific hacks for working around bugs
in GCC's LTO. OTOH your patch is nice from the standpoint of being
able to test LTO and find and report the bugs.

Rich