[ovs-dev] [PATCH 5/6] Re: byte-order: Make hton128() and ntoh128() behave like their counterparts.

[Ben Pfaff]

On Mon, Nov 23, 2015 at 04:26:18PM -0800, Joe Stringer wrote:
> On 23 November 2015 at 12:49, Joe Stringer <joestringer at nicira.com> wrote:
> > On 23 November 2015 at 10:06, Ben Pfaff <blp at ovn.org> wrote:
> >> When Joe added these types I assumed that he used the unconventional
> >> prototypes for hton128() and ntoh128() because the return value
> >> convention was inefficient.  If GCC and Clang actually optimize the use
> >> of a return value in some kind of sensible way then I agree that the
> >> usual convention is nicer.
> >>
> >> Joe, did you have another reason?
> >
> > This was mostly done based on an assumption that this was more
> > optimal, rather than actually digging into the compiled code and
> > seeing that it was generated differently.
> >
> > Looking now, before this patch vs. after on my 64-bit system..
> >
> > GCC-4.9: hton128/ntoh128 require one less MOV with this patch, but
> > calling conventions (in format_u128) require 3 extra MOV (+4 MOV, -1
> > LEA) for format_u128().
> > Clang-3.7: hton128/ntoh128 are roughly equivalent, although with this
> > patch they use some MOVUPS/MOVAPS instructions for 128-bit moves.
> > Calling conventions seem to require as much as 6-12 (!) extra MOVs
> > however, details below.
> 
> <snip>
> 
> Woops, looks like I missed compiler optimizations. They look much
> closer together with --O2, so I think this patch makes sense to apply
> without reservation.

Thanks.

Acked-by: Ben Pfaff <blp at ovn.org>