libgcj and the NPTL posix threads implementation

Mon Mar 1 22:59:00 GMT 2004

On Mon, 1 Mar 2004, Anthony Green wrote:
> > But your version is more efficient
> > within a DSO.
>> Why is this? It doesn't call any functions, does it?

For:
static int __thread x;
int t(void) {
 return x;
}
compare the output compiled with -fPIC:
t:
 pushl %ebx
 call __i686.get_pc_thunk.bx
 addl $_GLOBAL_OFFSET_TABLE_, %ebx
 leal x@TLSLDM(%ebx), %eax
 call ___tls_get_addr@PLT
 popl %ebx
 movl x@DTPOFF(%eax), %eax
 ret
vs. without:
t:
 movl %gs:x@NTPOFF, %eax
 ret
The former is necessary in a DSO where the exact thread-local offset can't
be known at compile time.
Jeff