Author: Ed Schröder
Date: 05:42:09 08/27/05
Go up one level in this thread
On August 27, 2005 at 05:36:51, Gerd Isenberg wrote: >On August 27, 2005 at 04:34:03, Ed Schröder wrote: > >>On August 27, 2005 at 00:43:29, Tony Werten wrote: >> >>>On August 26, 2005 at 18:12:30, Ed Schröder wrote: >>> >>>>I am no longer up-to-date regarding the newest processors (such as the AMD-64) >>>>and the internal working concerning speed, hence my question: >>>> >>>>Which (similar) code is faster? >>>> >>>> test byte ptr xxx,1 | test byte ptr xxx,1 >>>> je label | mov AL,[ECX] >>>> mov AL,[ECX] | je label >>>> mov BL,[EDX] | mov BL,[EDX] >>>> ... ........ | ... ........ >>>> ... ........ | ... ........ >>>>label: | label: >>>> >>>>Thanks in advance, >>> >>>Hi Ed, >> >>Hey Tony, >> >> >>>probably not what you wanted to know, but the code is quite different from each >>>other. >>> >>>If the jump condition is met 50% of the time, then the left code will execute >>>the 2 moves 50% of the time for an average of 1 move per loop and the right side >>>100%+50% is 1.5 moves per loop on average. >>> >>>Did you mean something else ? >> >>Yep :) >> >>The background of my question is the processor's capability to do 2 instructions >>at the same time. Following this logic the code on the right (in principle) is >>supposed to be faster. >> >> >> >>>2 BTW's: >>> >>>1 Depending on what you do with AL and BL, you might want to use the full >>>registers by doing movzx eax,[ecx] and movzx ebx,[edx] (No penalty on new >>>processors) >> >>That's good to know, thank you. > > >Yes, reading bytes to partial registers is expensive anyway, 4 cycles latency. > >MOV reg8, mem8 8Ah mm-xxx-xxx DirectPath 4 >MOV AL, mem8 A0h DirectPath 4 Are you saying Gerd that: mov EAX, mem32 is faster than mov AL,mem8 ? >Tony is right - zero extending to ax,eax,rax is also 4 cycles. > >MOVZX reg16/32/64, mem8 0Fh B6h mm-xxx-xxx DirectPath 4 This is clear, not much has changed. >If you have some "global", very often used array[eg. 64], it might be worth to >waste some memory (eg. four cachelines instead of one) and switch to native >32-bit int size: > >MOV reg16/32/64, mreg16/32/64 8Bh 11-xxx-xxx DirectPath 1 > >Also, avoid the shorter but redundant EAX-Move encoding: > >MOV AX/EAX/RAX, mem16/32/64 A1h DirectPath 4/3/3 Right, never us it. Ed
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