Re: abs()

[Camm Maguire <camm@enhanced.com>]

Greetings!  Just confirmed this on intel as well.  Amazing!  I don't
know why I hadn't thought of it before.  Definitely the way to go.

Take care,

Peter Strazdins <peter@bullet.anu.edu.au> writes:

> Hi Clint,
> 
> probably this is well-known, but for Sparc, gcc -O compiles a call to
> the standard maths function fabs() as a fabss instruction (this works
> for single or double precision).  Nothing could be easier :). 
> The latest Solaris compilers (f95 at least) will convert the conditional
> ( (x) >= 0.0 ? (x) : -(x) ) to an fabss too.
> 
> Regards, Peter
> 
> eg.  #define ATL_dabs(x)  fabs(x)
> 
>   >>  From owner-atlas-comm@cs.utk.edu Sun May 13 03:32:06 2001
> ...
>   >>  My guess is that there are system-dependant ways to make fabs() one cycle
>   >>  nonetheless, and I'm hoping some of you know or can easily discover them.
>   >>  Anyway, I want to ask anyone who can figure out to do fabs() without an if
>   >>  to post to the list.  The solution can be as nonportable as you want;
>   >>  I figure in-line assembler may be required, but hopefully it can be used
>   >>  with a C macro.  Here's an example macro for double precision:
>   >>  
>   >>     #define ATL_dabs(x) ( (x) >= 0.0 ? (x) : -(x) )
>   >>  
>   >>  If anyone can do it without the if, I think we can speedup quite a few
>   >>  routines . . .
>   >>  
>   >>  Any pointers appreciated,
>   >>  Clint
>   >>  
> 
> 

-- 
Camm Maguire			     			camm@enhanced.com
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