[RC5] Why is CSC bitslice MMX only?

Dan Oetting oetting at gldmutt.cr.usgs.gov
Thu Dec 16 14:43:31 EST 1999


At 1:51 -0700 12/16/1999, Tigger wrote:
>I haven't seen this question asked yet, so I'll be the one.  Why is the CSC
>bitslicing core limited to x86 MMX processors?  If I remember correctly from
>the DES days, the concept of bitslicing doesn't use any types of operations
>that are unique to the MMX instruction set, so that can't be why.  Does it
>have something to do with register size?  I remember someone saying that we
>can't bitslice RC5 because it would require significantly wider registers
>than current processors have.  Inquiring minds want to know...

In the classical cores, simple operations such as a 32 bit add or dynamic
rotate are a single instruction whereas the bitslice core may require over
150 instructions to perform the same operation. The advantage of bitslicing
is that each bit in the register is processing a different key so with
bigger registers more keys can be processed in the same time. For
bitslicing to be profitable the register size must be large enough to
overcome the increased cost of the calculations.

-- Dan Oetting <oetting at ghtmail.cr.usgs.gov>


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