# Barcelona optimization guide

By: EduardoS (no.delete@this.spam.com), May 13, 2007 9:30 am
>So if your car can drive at most at 100 KM/h,
>then it is an ok driving speed considering
>T-ford getting a speed at most of 40 KM/h ??

Did you know how multiply is done into hardware?
It's very complex, too hard (and expensive) to do it in less time, 5 cycles@3GHz isn't ok, it's f***ing fast.

>If core2 can retire 4 uops per cycle and barcelona can retire 3 uops a cycle i
>understand, then core2 can blow that barcelona core completely away. That's 33% faster speed.

Barcelona and K-8 can retire 3 macro-Ops per cycle...
add eax, eax = 1 K-8's macro-Ops and 1 Conroe's uOP
but
add [eax+ebx*4+76], 34 = 1 K-8's macro-Ops and 2 Conroe's uOPs
With real code and a big mix of instructions there won't be a big diference.

>No need to have a history to predict next branch:
>
>for( i = 0 ; i < 4 ; i++ ) {
>for( j = 0 ; j < 4 ; j++ ) {
>...
>}
>}
>
>If K8-barcelona can't perfectly predict that, then AMD has some work to do.
>
Not so simple...
1) There is no processor today that will understand this loop will be taken 4 times and not taken 1 before executing it.
2) The two "for" will generate extra instructions, with a loop that is limited by the number of instructions.

To make things easier... After the C compiler otimization (but withou loop unrolling) the code should look like this:

xor ecx, ecx
outerloop:
xor eax, eax
innerloop:
;...
cmp eax, 4
jng innerloop
cmp ecx, 4
jng outerloop

So there are 8 extra instructions that must be executed even if the loop can be predicted.
note: Depending on the compiler and the code inside it may unroll it completly or change the add/cmp/jng by sub/jnz.
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