Author: Gerd Isenberg
Date: 09:22:29 07/05/03
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On July 05, 2003 at 10:17:38, Omid David Tabibi wrote:
>In Genesis I heavily use the abs() function, and so tried to optimize it.
>Instead of using the abs() function defined in <math.h>, I wrote the following
>fucntion:
>
>long abs(long x) {
> long y;
> y = x >> 31;
> return (x ^ y) - y;
>}
>
>Testing it using a profiler, I found out that my implementation is about twice
>slower than the math.h implementation of abs(). I haven't looked at the
>implementation in math.h, but I can't see how a more optimized version of abs()
>can be written.
>
>Any ideas?
I guess the x86 math.h implementation of abs() uses conditional mov intruction
like this one (x in eax):
mov edx, eax ; x
neg eax ; -x
cmp eax, edx ; x - (-x)
cmovl eax, edx ; x < (-x) ? -x : x
to compare your code in asm with x in eax:
mov edx, eax ; x
sar edx, 31 ; y = x >> 31
xor eax, edx ; x^y
sub eax, edx ;(x^y)-y
hmm... i wouldn't expect that the your one is so much slower - interesting.
May be like Vincent already mentioned the "slow" arithmetic shift instruction on
P4 and more dependencies. The cmov approach also needs only two
ALU-instructions (neg, cmp), whether your aproach needs three.
Gerd
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