[rc5] Why are Alpha's so much slower then wintel boxes?

Dan Sugalski sugalskd at osshe.edu
Fri Oct 31 13:36:33 EST 1997


At 03:13 PM 10/31/1997 -0600, Brian Greul (Exch) wrote:
>Does anyone have any ideas on why Alphas might be so much slower than
>wintel boxes at crunching this code?
>I would expect them to perform on par.

That, alas, turns out not to be the case. The Alpha processor doesn't have
a rotate instruction, so it's gotta be faked by the code. This, needless to
say, slows things down, so you end up with less than stellar numbers.

>Is there a way to utilize the FPU to crunch numbers?
>after all, an integer is just an even floating point number....

Um, no, they aren't. And even if they were, no FPU that I know of will
rotate the bits in a floating point number. There's no purpose in it.

>It would seem that it might be worthwhile to convert the integers to
>floating point numbers, or to just read them in on the alpha client....
>Any thoughts on this issue?
>Reasons why it won't work?

Several. The biggest is that floating point operations are, by their very
nature, imprecise. That's the last thing you want when dealing with
cryptography.

					Dan

----------------------------------------"it's like this"-------------------
Dan Sugalski                            even samurai
sugalskd at osshe.edu                      have teddy bears
                                        and even the teddy bears
                                        get drunk
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