Why is 64 bit faster. RE: [Beowulf] Win64 Clusters!!!!!!!!!!!!

Andrew Shewmaker agshew at gmail.com
Mon Apr 16 18:46:27 EDT 2007

On 4/14/07, Greg Lindahl <lindahl at pbm.com> wrote:

> The two issues (extra registers and x87) aren't related. And most HPC
> people have been avoiding the x87 for a long, long time.

Aren't they related by the introduction of the x86-64 ABI?
x87 provided 8 80-bit registers, while SSE2 on x86-64
was enhanced (vs. 32-bit SSE2) to provide 16 128-bit
registers.  AMD had to include x87 for 32-bit compatibility,
but there was no way they were going to beef up its number
of registers for 64-bit mode when they had the better SSE2

Also, from what I read on Real World Tech., the x87 stack
made it more difficult for compilers to effectively use the x87
registers in comparison to the SSE registers.  Does that match
your experience?

HPC folk that only needed single precision FP could have
been avoiding x87 on x86 architectures since 1999,
which isn't that long ago, in my opinion.

However, HPC folk with dual precision FP codes running
on x86 couldn't avoid x87 until SSE2 was introduced in 2001.
I was using AMD processors and they didn't get SSE2 support
until 2003.

Andrew Shewmaker
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