Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns

[Borislav Petkov]

On Thu, Mar 26, 2015 at 10:37:32AM +0100, Ingo Molnar wrote:
> But here GAS generates the 10-byte opcode even if 'mov $0x12345678, 
> %rdi' is used, which is an unforced error.

7 bytes:

48 c7 c7 78 56 34 12    mov    $0x12345678,%rdi

which is REX + opcode C7 + ModRM + 4-byte immediate.

If it used the bX opcodes, it would have to use REX prefix too for the
%rdi register which would generate a 10-byte insn then:

               0:       48 bf 78 56 43 12 00    mov $0x12435678,%rdi
               7:       00 00 00

REX + opcode BF + 8 bytes immediate.

So I think using the REX register "rdi" forces it to use the REX prefix
and thus the longer instructions.

All IMHO of course.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
--
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/