https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82803
nsz at gcc dot gnu.org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |nsz at gcc dot gnu.org
--- Comment #4 from nsz at gcc dot gnu.org ---
i run into the same issue:
static __thread int x;
static int *volatile p;
void f(int c)
{
while (c--)
p = &x;
}
with -xc -O2 -fPIC compiles to
pushq %rbx
leal -1(%rdi), %ebx
.L10:
leaq x@tlsld(%rip), %rdi
call __tls_get_addr@PLT
subl $1, %ebx
addq $x@dtpoff, %rax
movq %rax, p(%rip)
cmpl $-1, %ebx
jne .L10
popq %rbx
ret
note that with -funroll-loops the loop is
.L46:
leaq x@tlsld(%rip), %rdi
call __tls_get_addr@PLT
subl $8, %ebx
addq $x@dtpoff, %rax
movq %rax, p(%rip)
movq %rax, p(%rip)
movq %rax, p(%rip)
movq %rax, p(%rip)
movq %rax, p(%rip)
movq %rax, p(%rip)
movq %rax, p(%rip)
movq %rax, p(%rip)
cmpl $-1, %ebx
jne .L46
so the loop unroller knows it only needs to compute the address once, but gcc
fails to hoist it out of the loop.
if i use a simple global, then the GOT access is hoisted, if i use an
__attribute__((const)) function call then that is hoisted, only tls address
computation is broken.
the issue is not present with -m32 (i386 code gen), but it is present on e.g.
aarch64 and powerpc64 and with tlsdesc -mtls-dialect=gnu2 (then it's the
tlsdesc call that's in the loop instead of __tls_get_addr call).
