Re: [Qemu-devel] [PATCH] Enabled building of x86_64 code on Mac OS X

[Paul Brook <paul@codesourcery.com>]

On Tuesday 27 January 2009, Jamie Lokier wrote:
> Paul Brook wrote:
> > > Out of curiousity, what are the benefits of using 64-bit apps in OS X?
> >
> > My guess is that same as most other x86 targets: The i386 legacy
> > mode is crippled by lack of registers, modern x86 hardware has a big
> > fat wide memory bus, and noone's bothered implementing an ILP32
> > 64-bit API[1], so 64-bit apps give measurably better overall
> > performance.
>
> Except they use more memory and memory bandwidth (arguably filling
> nearly half of it with zeros most of the time :-) so worse overall
> performance in some cases.

I did say overall performance.

My experience with AMD hardware is that you generally get ~10% overall 
improvement from LP64. Intel hardware (particularly the early 64-bit cores) 
less so, but it's generally still a win.

I guess ILP32 long mode would probably increase that further, and avoid the 
odd regressions. It's an awful lot of work to implement though, especially on 
a target that isn't used to having lots of incompatible variants.

Paul