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

Eric Gindrup gindrup at okway.okstate.edu
Mon Nov 3 09:27:01 EST 1997


     This is certainly slower on Intels.  Can you ensure that it is slower 
     on non-rotating platforms.
     An, you approximately remember correctly.  RC5-32/12/8 is using 32-bit 
     words and therefore is performing 32-bit rotates.  This still ensures 
     one valid copy starting at the 33rd least significant bit.
     
     Also, on rotating architectures with separate FP pipes, this would 
     allow an extra key or two per millisecond...
            -- Eric Gindrup ! gindrup at Okway.okstate.edu


______________________________ Reply Separator _________________________________
Subject: Re: [rc5] Why are Alpha's so much slower then wintel boxe 
Author:  <rc5 at llamas.net > at SMTP
Date:    10/31/97 4:50 PM


At 06:25 PM 10/31/1997 -0600, Eric Gindrup wrote: 
>
>     Oops.  Drop the last term from that long sum and add that the most 
>     significant byte is now useless.
>     
>     Also, I got a bit fuzzy about x.  It would have been better to say: 
>     "Let abcdefgh be some 8-bit... and let x = [abcdefgh, 8-3-1]"  I was 
>     thinking I had said this in the rest of the message.  (*sigh*)
     
Your solution's slower than what we've got now, unfortunately. (Plus, since 
we're rotating larger numbers (32 or 64 bits, IIRC) you may still run into 
FP truncation/rounding issues) Faking a left rotate takes an AND, an OR, a 
left shift, and a right one. (Not necessarily in that order :) You can do 
it with a right rotate, too, but I don't think there's one of those around 
in the Alphas either.
     
                                        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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