Explain dangers & workaround for AUTOLOADed constant subs

Message-ID: <20011128140957.D5236@sizzle.whoville.com>

p4raw-id: //depot/perl@13347

1 files changed, 30 insertions, 0 deletions

diff --git a/lib/Exporter.pm b/lib/Exporter.pm
index 9a361a73fa..7067464315 100644
--- a/lib/Exporter.pm
+++ b/lib/Exporter.pm
@@ -307,4 +307,34 @@ unchanged but will trigger a warning (with C<-w>) to avoid misspelt tags
 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