Re: Specific support for Intel Atom architecture

[Andi Kleen]

On Tue, May 12, 2009 at 10:45:00AM -0700, Ulrich Drepper wrote:
> On Tue, May 12, 2009 at 8:04 AM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> > The problem is that you can't express the situations where
> > movbe is better than bswap (you need both and the old and the new
> > value) in inline assembler in a way that gcc decides automatically.
> 
> True.  But I was mostly thinking about loads from memory.  A quick
> search for ntoh*/hton* shows code like
> 
>         u_int16_t queue_num = ntohs(nfmsg->res_id);
> 
> If there would be a ntohs_load() macro movbe could be used.

It wouldn't surprise me if

	movbe memory,%reg

generates the same uops sequence internally as

	mov memory,%reg
	bswap %reg

I doubt there's any dedicated hardware for this in Atom (but I don't
know for sure) 

So unless you're really decoding constrained it would only
save a few bytes of code size. Probably not worth having 
incompatible modules for or adding special code to the source.

-Andi

-- 
ak@xxxxxxxxxxxxxxx -- Speaking for myself only.
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