Author: Robert Hyatt
Date: 08:38:44 06/27/02
Go up one level in this thread
On June 27, 2002 at 00:43:59, Eugene Nalimov wrote: >SHLD/SHRD x86 instuctions shift pair of 32-bit registers left/right and return >high/low 32 bits. > >So for shift by constant amount you need exactly 2 instructions -- for left >shift, shld+shl. For right shift, shrd+shr. Shift by variable amount is more >complicated if you don't know that shift amount is less than 32, but those >instructions still help. > OK for the PC. But not all 32 bit machines do a double-register shift. Which causes an extra instruction or two to be tossed into the mix... >Eugene > >On June 27, 2002 at 00:03:06, Robert Hyatt wrote: > >>On June 25, 2002 at 20:00:47, Dieter Buerssner wrote: >> >>>On June 25, 2002 at 19:46:20, Robert Hyatt wrote: >>> >>>>The issue is this: how many instructions to do task x? For bitboards, the >>>>instruction count on a 32 bit machine jumps by a factor of two unless shifts >>>>are being done where it goes up even further. >>> >>>Why should it go up further for shifts? >>> >>>Dieter >> >> >>You can't just do two 32 bit operations. You shift one end. You have to >>save the part you shift out. You shift the other end. Then fold in the >>part you saved from the first shift...
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