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

Chris Berry cberry at acorn.com
Mon Jan 4 13:18:08 EST 1999


Marc Sissom wrote:
> 
> So there is a minimum 5 instruction penalty for procs
> that don't support a native arbitrary rotate, 2 SHIFTs,
> 2 ANDs and an OR. Sometimes a multi-bit shift is not
> supported in a single cycle so the cost is even greater...
> 

Provided the shift produced 0s in the "empty" bits (which
is (afaik) always the case with a left shift, and is also
the case with a logical (as opposed to arithmetic) right
shifts) you don't need the ANDs.

So on the majority of processors, you're looking at 2 shifts
and an OR, assuming a multi-bit shift is available.


chrisb.

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