Re: Re: Confusing optimization

["Luca Béla Palkovics" <luca dot bela dot palkovics at gmail dot com>]

Am Sonntag, den 09.05.2010, 21:49 -0700 schrieb Ian Lance Taylor:
> Some possibilities are:
> 
> 1) Measurement error.  Surprisingly often people are not measuring
>    what they think they are measuring, and you didn't provide any
>    details about how you got your timings.
function is extern "C" and I messure the time outside.

long start=GetTime(); //own function using gettimeofday (returns ms)
A();
long time=GetTime()-start;
//Output time..

So shold be correct..


> 2) Instruction cache effects, if a() and b() call other functions.
>    When both are linked together, those other functions will be at
>    different addresses, and whether they are contiguous may change,
>    all affecting the instruction cache.
> 3) Exact aligment of loop starts may shift when both are linked
>    together, affecting the processor's branch optimizers if it hasvar
>    any.  Similarly, the exact alignment of labels may shift.  You can
>    control these using gcc options like -falign-functions,
>    -falign-jumps, -falign-labels, -falign-loops.
Well I checked out now asm..
.. Ubuntu 10.04 with g++ 4.4.3

First line is function "a" alone 125ms
Second line is function "a" with "b" 130ms

...
.text:00000000004023BA                 jmp     short loc_402425
.text:00000000004023BA                 jmp     short loc_40241F
...
.text:0000000000402425                 cmp     [rsp+298h+var_118], 3
.text:000000000040241F                 cmp     [rsp+298h+var_118], 3
.text:000000000040242D                 jz      short loc_40243E
.text:0000000000402427                 jz      short loc_402438
...
LOOP START
...
.text:000000000040243E                 cmp     [rsp+298h+var_110],
0FFFFFEh
.text:0000000000402438                 cmp     [rsp+298h+var_110],
0FFFFFEh
.text:000000000040244A                 setle   al
.text:0000000000402444                 setle   al
.text:000000000040244D                 test    al, al
.text:0000000000402447                 test    al, al
.text:000000000040244F                 jnz     loc_4023BC
.text:0000000000402449                 jnz     loc_4023BC
...
.text:00000000004023C3                 cmp     [rsp+298h+var_98], 3
.text:00000000004023BC                 cmp     [rsp+298h+var_58], 3
.text:00000000004023CA                 jz      short loc_4023D9
.text:00000000004023C4                 jz      short loc_4023D3
...
.text:00000000004023D9                 mov     rax, [rsp+298h+var_110]
.text:00000000004023D3                 mov     rax, [rsp+298h+var_110]
.text:00000000004023E1                 add     [rsp+298h+var_90], rax
.text:00000000004023DB                 add     [rsp+298h+var_50], rax
.text:00000000004023E9                 cmp     [rsp+298h+var_D8], 3
.text:00000000004023E3                 cmp     [rsp+298h+var_98], 3
.text:00000000004023F1                 jz      short loc_4023FF
.text:00000000004023EB                 jz      short loc_4023F9
...
.text:00000000004023FF                 inc     [rsp+298h+var_D0]
.text:00000000004023F9                 inc     [rsp+298h+var_90]
.text:0000000000402407                 cmp     [rsp+298h+var_118], 3
.text:0000000000402401                 cmp     [rsp+298h+var_118], 3
.text:000000000040240F                 jz      short loc_40241D
.text:0000000000402409                 jz      short loc_402417
...
.text:000000000040241D                 inc     [rsp+298h+var_110]
.text:0000000000402417                 inc     [rsp+298h+var_110]
.text:0000000000402425                 cmp     [rsp+298h+var_118], 3
.text:000000000040241F                 cmp     [rsp+298h+var_118], 3
.text:000000000040242D                 jz      short loc_40243E
.text:0000000000402427                 jz      short loc_402438 
...
LOOP END
...

C++ code:
for(i=0L;i<0xFFFFFFL;i++)
{
	Temp+=i;
	Test++;
}

= ++ += ++ and < are overloaded functions..

So well .. must be the cache or the align..
Maybe I should flip my ifs .. it evertime jz.. hmmm


I hope I answered this mail in the correct way.. 
This is the 3. time using a mailing list.. :P

Luca.