Re: [Nasm-devel] [PATCH] Add np and similar prefixes to instructions that should have them

[H. P. A. <hp...@zy...>]

On 02/20/2013 09:23 AM, H. Peter Anvin wrote:
> On 02/19/2013 09:39 PM, Ben Rudiak-Gould wrote:
>> This adds "np" to a bunch of SSE-style instructions that should have
>> it, "norep" (which was implemented but unused) on quasi-SSE
>> instructions that use F2 and F3 as instruction extensions but 66 for
>> operand size, "nof3" (newly implemented) on a few instructions,
>> "norexw" on some instructions that have only 32-bit and 64-bit
>> versions, and one NOLONG. It also removes some incorrect "np"s,
>> changes some "f3"s to "f3i"s, and fixes the decoding of the
>> XCHG/NOP/PAUSE mess: F390 is always PAUSE even when rex.b=1 (at least
>> according to XED).
>
> It should have been REX.R not REX.B, to prevent:
>
> 	[rep] xchg r8,rax
>
> ... from being treated as NOP or PAUSE.
>

Ah, but despite the documentation it is REX.B, not REX.R.

And yes, I can confirm this applies to PAUSE but *NOT* NOP, at least on 
Sandy Bridge, i.e.:

	F3 49 90	- PAUSE	(no swap)
	49 90		- XCHG R8,RAX (registers do swap)

Odd, but that's how it works.

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.