Bug#328183: #328183: no-tls-direct-seg-refs

[Samuel Thibault <samuel.thibault@ens-lyon.org>]

Petr Salinger, le Thu 05 Jan 2006 09:22:29 +0100, a écrit :
> > > > will add a very little extra cost to single thread-specific variable
> > > > accesses, but it shouldn't be noticeable at all.
> > > 
> > > Well, to be correct, switch -mno-tls-direct-seg-refs
> > > add extra cost to ALL thread-specific variables accesses.
> > 
> > No, only to isolated ones. The compiler can cache gs:0 (and it will do
> > it anyway when -mno-tls-direct-seg-refs is not provided.
> 
> This is still more then single variable ...
> 
> Anyway, look at configure flags for basic libc in debian/sysdeps/i386.mk:
> 
> libc_extra_config_options = $(extra_config_options) --with-tls --without-__thread
> 
> It doesn't use TLS internally, there is no .tbss ot .tdata segment in libc.so.

There is:

$ objdump -T libc-2.3.5.so | grep -e "\(tbss\|tdata\)"
001315e0 l    d  .tdata 00000000              .tdata
001315e8 l    d  .tbss  00000000              .tbss
00000008 g    D  .tbss  00000004  GLIBC_PRIVATE errno
0000001c g    D  .tbss  00000004  GLIBC_PRIVATE h_errno
00000004 g    D  .tdata 00000004  GLIBC_PRIVATE __resp

Please also look at how syscall() sets errno:
0x000cd863 <syscall+67>:        mov    %edx,%gs:(%ecx)

Regards,
Samuel