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Re: x86-64 on i386 (was Re: Promoting i386 version over x86_64?)



On Sun, 2009-12-13 at 13:53 -0500, Tom Lane wrote:
> Paul Jakma <paul dishone st> writes:
> > On Wed, 18 Nov 2009, Roland McGrath wrote:
> >> x86 is unlike other architectures because 64-bit also has twice as 
> >> many registers as 32-bit.  So you get to trade off the benefits of 
> >> register allocation across more registers against the memory/cache 
> >> footprint of 64-bit pointers.
> 
> > For what percentage of code is that an appreciable advantage?
> 
> Pretty much everything, actually.  The x86 ISA completely sucks.

Indeed. Paul, take a look at the Intel 64 ISA and you'll see it's a very
different beast. Intel fixed a lot of the issues with the (more than 20
year old really x86 ISA) and it's not simply a doubling of memory
footprint because variable width instructions are used in the first
place, and continue to be used in the newer ISA upgrade.

Personally, I think anyone running i386 on x86_64 who isn't doing some
kind of testing under KVM or similar is completely wasting their
computing resources and should receive a free copy of the Intel
documentation for the holidays ;)

Jon.



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