J3/04-220

Date:      2004-01-21
To:        J3
From:      fortran.com
Subject:   LOG2 intrinsic function
Reference: Pub-112


This was submitted by Van Snyder  van.snyder@jpl.nasa.gov

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Number:

Title: LOG2 intrinsic function

Submitted by: J3

Status: For Consideration

References: Fortran 95, Section 13

Basic Functionality: Integer part of base-2 logarithm of (the absolute value
of?) an integer.


Rationale: One occasionally needs to know the integer part of the base-2
logarithm of an integer, that is, the largest exponent of a
power of 2 that is less than or equal to that number.  It's
not difficult to do this with a loop that starts with 1 and
doubles, but some processors have instructions that do it in
a single cycle.  Even without such an instruction, it's
likely that an intrinsic function would be more efficient than
even an optimized version of what one might write in a
program.


Estimated Impact: Trivial.


Detailed Specification: Provide an intrinsic function to compute the integer part
of the base-2 logarithm of an integer.  Maybe specify that
it computes the base-2 logarithm of the absolute value of
its argument, since that's more likely to be useful than an
error message.

The usual desire for this function is for the integer part
of the logarithm, not for a floating-point approximation to
the logarithm.  Providing the latter would just get in the
way of what's really wanted, and would be less efficient.
If one really wants the logarithm (not its integer part),
one can get it about as efficiently as possible using
log(real(I))/log(2.0)

It ought to be generic for all kinds of integers, not
just specific for default integers.

It's possible that a processor could do a better job of the
base-2 logarithm of a floating-point number in an intrinsic
function than one can do with log(x)/log(2.0), but it's so
rarely wanted that there's probably no point to providing
it for floating point -- but that would be a trivial future
extension if it's ever desired.


History: Submitted as Pub-112