Re: [PATCH 1/2] x86: eliminate TS_XSAVE

Avi Kivity
2010-05-05T07:30:35Z

On 05/04/2010 09:24 PM, H. Peter Anvin wrote:
>
> I would like to request one change, however. I would like to see the
> alternatives code to be:
>
> movb $0,reg
> movb $1,reg
>
> ... instead of using xor (which has to be padded with NOPs, which is of
> course pointless since the slot is a fixed size.)

Right.

> I would suggest using
> a byte-sized variable instead of a dword-size variable to save a few
> bytes, too.
>

I used a bool, and the code already compiles to a byte mov. Though it
could be argued that a word instruction is better since it avoids a
false dependency, and allows a preceding instruction that modifies %reg
to be executed after the mov instruction.

> Once the jump label framework is integrated and has matured, I think we
> should consider using it to save the mov/test/jump.
>

IIRC that has an implied unlikely() which isn't suitable here?

Perhaps the immediate values patches.

--
error compiling committee.c: too many arguments to function