Re: abs()

[Peter Strazdins <peter@bullet.anu.edu.au>]

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
  >>