https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81664
--- Comment #4 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Daniel Fruzynski from comment #2)
> So there is another problem here: for some reason both mentioned gcc
> versions accepts -mmovbe and -mno-movbe options. If movbe is not supported,
> gcc should complain that these options are unrecognized.
-mmovbe compile flag is a different thing that "movbe" target attribute.
> I also checked generated assembly code and there are no movbe instruction
> there. However I noticed that gcc 4.8.5 on Redhat 7 generates different code
> when -mmovbe is used - it adds extra instruction marked below with ***.
Your test is flawed:
- ntohl uses uint32_t, extra instruction is zero-extend to 64bit ulong.
- movbe works only from/to memory.
Try this test:
--cut here--
#include <arpa/inet.h>
#include <inttypes.h>
extern uint32_t val;
uint32_t
__attribute__((target("movbe")))
ntohl_movbe(void)
{
return ntohl (val);
}
uint32_t
__attribute__((target("no-movbe")))
ntohl_no_movbe(void)
{
return ntohl (val);
}
--cut here--
gcc -O2:
ntohl_movbe:
movbe val(%rip), %eax
ret
.cfi_endproc
ntohl_no_movbe:
movl val(%rip), %eax
bswap %eax
ret
