Change 13347 by jhi@alpha on 2001/11/29 01:23:08
Subject: [PATCH Exporter.pm] Explain dangers & workaround for AUTOLOADed
constant subs
From: Barrie Slaymaker <[EMAIL PROTECTED]>
Date: Wed, 28 Nov 2001 14:10:01 -0500
Message-ID: <[EMAIL PROTECTED]>
Affected files ...
.... //depot/perl/lib/Exporter.pm#17 edit
Differences ...
==== //depot/perl/lib/Exporter.pm#17 (text) ====
Index: perl/lib/Exporter.pm
--- perl/lib/Exporter.pm.~1~ Wed Nov 28 18:30:06 2001
+++ perl/lib/Exporter.pm Wed Nov 28 18:30:06 2001
@@ -307,4 +307,34 @@
names being silently added to @EXPORT or @EXPORT_OK. Future versions
may make this a fatal error.
+=head2 C<AUTOLOAD>ed Constants
+
+Many modules make use of C<AUTOLOAD>ing for constant subroutines to avoid
+having to compile and waste memory on rarely used values (see L<perlsub> for
+details on constant subroutines). Calls to such constant subroutines are not
+optimized away at compile time because they can't be checked at compile time
+for constancy.
+
+Even if a prototype is available at compile time, the body of the subroutine is
+not (it hasn't been C<AUTOLOAD>ed yet). perl needs to examine both the C<()>
+prototype and the body of a subroutine at compile time to detect that it can
+safely replace calls to that subroutine with the constant value.
+
+A workaround for this is to call the constants once in a C<BEGIN> block:
+
+ package My ;
+
+ use Socket ;
+
+ foo( SO_LINGER ); ## SO_LINGER NOT optimized away; called at runtime
+ BEGIN { SO_LINGER }
+ foo( SO_LINGER ); ## SO_LINGER optimized away at compile time.
+
+This forces the C<AUTOLOAD> for C<SOLINGER> to take place before SO_LINGER is
+encountered later in C<My> package.
+
+If you are writing a package that C<AUTOLOAD>s, consider forcing an C<AUTOLOAD>
+for any constants explicitly imported by other packages or which are usually
+used when your package is C<use>d.
+
=cut
End of Patch.
