in reply to Re: Slow constants
in thread Slow constants

whereas the other two use AUTOLOAD to create the subs in their own package space and then &goto them...every time they are used.
Eh, no. The first time they are used AUTOLOAD creates them and &gotos in them. The second time, the sub is there and the sub is called.

The difference is that Errno defines all the subs at compile time - they are empty prototyped subs returning a single value - and are hence are being constant folded at compile time. If you use EAGAIN, Perl will substitute the appropriate value at compile time - which avoids all the overhead of calling a subroutine at run time.

The benchmark is biased towards heavy use of constants. A typical program will only use a few constants, and only a few times in a program. That means that if you'd export a lot of constants the way Errno does, you pay a heavier price at compile time, regardless how often, and how many constants you use. Fcntl however avoids the costs of compiling all the subs you aren't going to use - but the price of using a constant goes up.

Perl --((8:>*

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Re^3: Slow constants
by BrowserUk (Patriarch) on Nov 02, 2005 at 17:03 UTC

    Doesn't the absence of an empty prototype in the assignment *$AUTOLOAD = sub { $val } mean that these are just normal (non-constant) subs, and will therefore never be inlined, and therefore, called everytime they are used? 

    ## From Socket.pm (Fnctl.pm contains an nearly identical sub)
sub AUTOLOAD {
    my($constname);
    ($constname = $AUTOLOAD) =~ s/.*:://;
    croak "&Socket::constant not defined" if $constname eq 'constant';
    my ($error, $val) = constant($constname);
    if ($error) {
    croak $error;
    }
    *$AUTOLOAD = sub { $val };
    goto &$AUTOLOAD;
}

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      Doesn't the absence of an empty prototype in the assignment *$AUTOLOAD = sub { $val } mean that these are just normal (non-constant) subs, and will therefore never be inlined, and therefore, called everytime they are used?
      Eh, yes. And no. They will indeed by called everytime they are used - just as I said: The second time, the sub is there and the sub is called. (But a subsequent call to the same constant will not invoke AUTOLOAD again). However, it does not matter whether there's a prototype present or not - the calling code is already compiled, so it's too late for inlining. Although it might matter if there's future code to be string-evalled that uses a previous called constant.
      Perl --((8:>*
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Re^3: Slow constants
by powerman (Friar) on Nov 02, 2005 at 21:55 UTC
    Thanks for your answers!
    Now it's time to meditate a little... :)
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