[Bug inline-asm/66631] inability to clobber segment regs makes tls problematic

["stsp at users dot sourceforge.net" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66631

--- Comment #11 from Stas Sergeev <stsp at users dot sourceforge.net> ---
(In reply to Andrew Pinski from comment #9)
> Clang is not GCC.  Really you should not be using the fs segment register
> for your own use as it is part of the ABI that is the TLS segment.  Any
> other use it of it is outside of the ABI and will break everything else. 
> Why can't you not use fs segment?
Alien code in place (think of wine/dosemu).

> Also why can't you write your segfault handler in assembly when you fixup
> the fs segment and then call the C functions?
In principle - yes, but in practice - looking into a source -
too much work. It is large, and it also uses a syscalls:
sys_prctl(ARCH_SET_FS, eflags_fs_gs.fsbase);
Do syscalls in asm too? :(
Of course this all is possible, but...

Should I really resort to always using -O1 till that also break? :(
Let me ask you from the other side: why not simply to
allow clobber also segregs? Improvement for everyone IMHO.
clang did that for chromium it seems - likely a jit either.
IMHO gcc should allow writing the specific things like
signal handlers, interrupt handlers in embedded systems,
etc - they all have suprises and deviations from ABI, yet
gcc usually has an extensions to cover all these needs. Why
to suddenly change that good practice?