Author: Bo Persson
Date: 06:34:58 07/07/03
Go up one level in this thread
On July 07, 2003 at 08:48:39, Gerd Isenberg wrote:
>at least one colleague has the same strange effect than Dieter:
>
>Gerd
>
>
>Gerd P4 2.4GHz:
> nothing 3951541892 13.390
> abs() 1713113360 13.141
> simple_abs() 1713113360 19.562
> omid_abs() 1713113360 13.672
> sbb_abs() 1713113360 17.969
> cdq_abs() 1713113360 17.625
> fish_abs() 1713113360 21.750
> sar_abs() 1713113360 16.984
> cmovl_abs() 1713113360 16.782
> cmovs_abs() 1713113360 16.781
>
This isn't as strange as it might seem. We are trying to time an *extremely*
small piece of code. The instructions selected by the compiler actually executes
att a different speed on different processors.
I have MSVC 7.1 where do_nothing results in:
; 304 : for (i = 0; i < MAX_ITERATIONS; ++i) {
; 305 :
; 306 : // subtract so we get both positive and negative numbers
; 307 : int a = rand() - 16384;
00020 e8 00 00 00 00 call _rand
00025 4f dec edi
; 308 :
; 309 : sum += a;
00026 8d b4 06 00 c0
ff ff lea esi, DWORD PTR [esi+eax-16384]
0002d 75 f1 jne SHORT $L10491
; 310 : }
Here an LEA is used to compute sum + a - 16384 in a single instruction!
while test_abs is just slightly different:
; 25 : for (i = 0; i < MAX_ITERATIONS; ++i) {
; 26 :
; 27 : // subtract so we get both positive and negative numbers
; 28 : int a = rand() - 16384;
00020 e8 00 00 00 00 call _rand
00025 2d 00 40 00 00 sub eax, 16384 ; 00004000H
; 29 :
; 30 : sum += abs(a);
0002a 99 cdq
0002b 33 c2 xor eax, edx
0002d 2b c2 sub eax, edx
0002f 03 f0 add esi, eax
00031 4f dec edi
00032 75 ec jne SHORT $L10356
On a P4 the LEA instruction is broken up into several (but unspecified)
micro-ops. It is not fast - in fact Intel says that it is no longer an
optimization to use it! On the PIII, of course, it has dedicated hardware...
Except for the CDQ, all the other instructions are in the core RISC set, that
executes at up to 3 instructions per clock on a P4.
So doing something fast *can* be quicker than doing nothing slowly. :-)
Bo Persson
bop2@telia.com
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