[Bug target/81664] __attribute__((target("movbe"))) does not work

["ubizjak at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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