https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80881
--- Comment #83 from Julian Waters <tanksherman27 at gmail dot com> ---
Liu Hao: The registers it's using seem to be all over the place. Prior it was
using rdx for the gs:[88] load and rax for everything else, now it's either
using any register it can find, or using rdx to store the result of rdx+rax*8.
I have no idea why the resulting assembly is so different, but this could mean
the resulting program runs less efficiently
EDIT: Nevermind, it was because of rax being the return value register and the
thread local being an array
extern _Thread_local int local;
int get(void) {
return local;
}
movl _tls_index(%rip), %eax
movq %gs:88, %rdx
movq (%rdx,%rax,8), %rax
movl local@secrel32(%rax), %eax
extern _Thread_local int local[8];
int get(void) {
return local[2] + local[4];
}
movl _tls_index(%rip), %eax
movq %gs:88, %rdx
movq (%rdx,%rax,8), %rdx
movl 16+local@secrel32(%rdx), %eax
addl 8+local@secrel32(%rdx), %eax
Uros: I see, I'll try to do so. I was mainly avoiding that to break less code
(I have a habit of doing that to anything I touch). Although, the resulting
assembly (Barring the register selection) already seems to be as compact as
possible for Windows, I'm not sure how using get_thread_pointer could make it
any more optimal. This is a genuinely curious question, not placing doubt on
whether get_thread_pointer can help optimize the resulting assembly
