https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61572
--- Comment #10 from Richard Biener <rguenth at gcc dot gnu.org> ---
static inline __attribute__((no_instrument_function)) int
autofs4_compat_get_set_timeout(struct autofs_sb_info *sbi,
compat_ulong_t *p)
{
int rv;
unsigned long ntimeout;
if ((rv = ({ int __ret_gu; register
__typeof__(__builtin_choose_expr(sizeof(*(p)) > sizeof(0UL), 0ULL, 0UL))
__val_gu asm("%""rdx"); (void)0; might_fault(); asm volatile("call
__get_user_%P3" : "=a" (__ret_gu), "=r" (__val_gu) : "0" (p), "i"
(sizeof(*(p)))); (ntimeout) = (__typeof__(*(p))) __val_gu; __ret_gu; }))
ok, so the register variable is used to get at the return value of __get_user.
I wonder why that's not using proper constraints instead ...
Probably macro-expansion of GET_USER.
Creepy.
#define get_user(x, ptr) \
({ \
int __ret_gu; \
register __inttype(*(ptr)) __val_gu asm("%"_ASM_DX); \
__chk_user_ptr(ptr); \
might_fault(); \
asm volatile("call __get_user_%P3" \
: "=a" (__ret_gu), "=r" (__val_gu) \
: "0" (ptr), "i" (sizeof(*(ptr)))); \
(x) = (__typeof__(*(ptr))) __val_gu; \
__ret_gu; \
})
Probably because this uses "proper" registers dependent on the size of *ptr
which may end up using a %ecx:%edx register pair.
What a hack to not use a compile-time conditional on sizeof (*ptr).
