Re: [Qemu-devel] [PATCH 2/2] tcg-x86_64: Avoid unnecessary REX.B prefixes.

[Richard Henderson]

On 01/05/2010 04:31 PM, Richard Henderson wrote:
> A while ago Laurent pointed out that the setcc opcode emitted by
> the setcond patch had unnecessary REX prefixes.
>
> The existing P_REXB internal opcode flag unconditionally emits
> the REX prefix.  Technically it's not needed if the register in
> question is %al, %bl, %cl, %dl.

I should perhaps mention that this patch doesn't help out all
that much.  Only EBX is free for use by the code generator, so
more often than not we're using %bpl, which does require the
REX prefix.


r~