SPUG: Using subroutines to return constants

[Jim Flanagan]

On Sat, 4 Nov 2000, Richard Anderson wrote:

      > Subtitle: Structured coding best practice or
      > performance-crippling hack?
      > 
      > Although Perl 5.6 has the convenient constant declaration
      > to fix the value of a variable, most of us are still coding
      > without this bit of syntactic sugar.  The traditional way
      > to refer to a constant in Perl is to use a reference to
      > a subroutine that does nothing but return the constant:
      > 
      > $PI = sub { 3.1415962 };
      > 
      > Subroutine calls are much slower than variable references,
      > but how much slower?  Here's some code that tests this:
      > 
      > Any thoughts on this?

  If you prototype a subroutine with a null arglist, and the subroutine
  returns a constant value, then the compiler inlines all of the constant
  references, and no subroutine call actually takes place at runtime.

  Consider the following code:

    use Benchmark;

    my $VARIABLE = 10;
    sub SUBCONST () { 10 };
    use constant PRAGCONST => 10;

    timethese 1000000,
      { variable => sub {$v = $VARIABLE},
	pragma   => sub {$v = PRAGCONST},
	inline   => sub {$v = SUBCONST} };

  This gives:
 
    Benchmark: timing 1000000 iterations of inline, pragma, variable...
	inline:  4 secs ( 2.14 usr  0.00 sys =  2.14 cpu)
	pragma:  3 secs ( 2.14 usr  0.00 sys =  2.14 cpu)
      variable:  2 secs ( 2.19 usr  0.00 sys =  2.19 cpu)

  The pragma is doing essentially the same thing as the inlined subroutine,
  except that it is inserting the subroutine definition right into the
  namespace of the calling routine.

  Note that my numbers have to be taken with a grain of salt because my
  Benchmark.pm (RatHead Linux 5.x) seems to be wacky.

-- 
Jim Flanagan        Collective Technologies
jimfl at colltech.com  http://www.colltech.com


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