Path: utzoo!attcan!uunet!mailrus!ncar!noao!arizona!gudeman
From: gudeman@cs.arizona.edu (David Gudeman)
Newsgroups: comp.lang.misc
Subject: implementing frexp() (was: Compiler Costs)
Message-ID: <22838@megaron.cs.arizona.edu>
Date: 9 Jul 90 01:16:23 GMT
Organization: U of Arizona CS Dept, Tucson
Lines: 43

In article  <2324@l.cc.purdue.edu> cik@l.cc.purdue.edu (Herman Rubin) writes:
[about implementing frexp(x,&n) --  mantissa of a float]
>Now I submit it will take a real turkey machine language user to match the
>slowness of any program written in C or Fortran, USING ONLY THE OPERATIONS
>OF THOSE LANGUAGES.  Also, inlining, with n being placed in the register
>where it is to be used, is likely to cut the running time by more than
>half.

In C I would write

#include <math.h>
...
  frexp(x,&n)

If this doesn't produce code as fast as assembly language, then you
don't have an optimizing compiler.  I'd be surprised if the same
doesn't hold for FORTRAN.  Basically, your implication that C doesn't
have an operation to directly destructure floats is wrong.  True, the
operation (frexp) is called a library function instead of an operator,
but the effect is the same, given a smart enough compiler.  There are
only so many operator symbols available, and frexp is an obscure
feature at best.

Also, the definition of frexp is portable.  It would work on a machine
that didn't use the traditional representation of floats.  That's
because C is intended to be a portable language.  If you don't care
about portability, (or programming time, or maintenance time), then by
all means use assembler.  If you want a C-like language that is as
machine dependent as assembler then by all means implement it, but
don't expect that it will hold much interest for anyone who doesn't
program exclusively for your machine.

If you want a machine-independent language that lets you include
machine-specific parts for performance improvement, then use C with
the asm() feature.  It is really quite meaningless to give a challenge
as above to do something as efficient as assembly language, and rule
out asm() as an option.  asm() is provided specifically to allow you
to do such things.
-- 
					David Gudeman
Department of Computer Science
The University of Arizona        gudeman@cs.arizona.edu
Tucson, AZ 85721                 noao!arizona!gudeman