Author: Dieter Buerssner
Date: 12:47:41 07/05/03
Go up one level in this thread
On July 05, 2003 at 12:22:29, Gerd Isenberg wrote:
>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):
It wouldn't run on Pentium then?
> 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
Or just (instead of the 2 instructions):
cdq ; edx:eax = x (eax) sign extended
But it leaves the compiler no choices in registers, while your code
would run with any register pair.
> 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
Regards,
Dieter
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