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DateWed, 18 Mar 1998 09:21:38 +0100
FromKurt Garloff <>
SubjectRe: BogoMIPS revisitied...MMX
On Tue, Mar 17, 1998 at 04:39:22PM -0800, Perry Harrington wrote:

> I know that the bogomips loop is a >small< and tight loop of 3 or
> 4 instructions.  Why is it that my P100 spits out something like
> 39.19 BogoMIPS and my P233MMX spits out 466.15 bogomips?  I know
> that the Pentium had some issue with this, being that a standard
> 486 executed *that* particular loop faster.

1:
 decl %loops
 jns 1b

In C: while (loops-- >= 0);
loops is something like -4(%esp)

> So, why such the HUGE difference?

It depends on the optimizations your processor is able to do. Branch
prediction and good parallelization of the pipelines do help this code a lot.

Some results: Pentium: 2.5 cycles/loop
              6x86   : 1
	      K6     : 0.5
	      P5 MMX : 0.5

On some machines it also depends on the alignment of the label 1: (and maybe
on the one of loops), that's why this code is no logner inlined in recent
kernels.

-- 
Kurt Garloff, Dortmund 
<K.Garloff@ping.de>
PGP key on http://student.physik.uni-dortmund.de/homepages/garloff

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