[RC5] K6-2/350 vs PII 266 mobile performance

Marc Sissom marcus at teleteam.net
Mon Dec 28 10:52:08 EST 1998


John "Chris" Wren wrote:
> 
>         I have a question about performance issues.  I have a K6-2/350
> that benchmarks about 25% higher than my PII 266 in every arena,
> except for RC5 keyrates.  I'm seeing about 590kkps on the K6, and
> 700kkps on the PII.
> 
>         Both systems are selecting the correct cores, etc.  Why is the
> K6 core performing so poorly?

Sigh, this has got to be in a FAQ somewhere.

The intel x86 chips have a hardware-single-cycle-rotate
instruction. That is, bits can be rotated around in the
registers by any arbitrary number. The RC5 algorithm
depends very heavily on this kind of bit manipulating
operation. Most processors do not support this. Either
the multi-bit ROTate is microcoded(native instruction
is supported but takes more than a single cycle) or it
must be emulated in the software by multiple SHIFTs to
the left and right and then the results of those shifts
are merged with an AND. Multiple cycle operations,
whether done in the 'hardware' or in the software take
longer than single cycles.

Intel chips(and Cyrix and Power PC I think) support the
single cycle rotate. Most others do not.

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