From: Akim Demaille <akim@epita.fr>
Date: Sat, 9 Jun 2001 18:39:12 +0000 (+0000)
Subject: * doc/autoconf.texi (Limitations of Builtins): Complete the
X-Git-Tag: AUTOCONF-2.50a~55
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=d4bfb5fa660400964003071ad6c47452bbed4cd9;p=thirdparty%2Fautoconf.git

* doc/autoconf.texi (Limitations of Builtins): Complete the
description of the here-docs penalties with Alexandre Oliva's
explanations.
---

diff --git a/ChangeLog b/ChangeLog
index 8920a597e..b9292a2c7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2001-06-09  Akim Demaille  <akim@epita.fr>
+
+	* doc/autoconf.texi (Limitations of Builtins): Complete the
+	description of the here-docs penalties with Alexandre Oliva's
+	explanations.
+
 2001-06-01  Paul Eggert  <eggert@twinsun.com>
 
 	* doc/autoconf.texi: Talk about here documents and speedups.
diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 49a3ba6a2..e696841a8 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -5975,6 +5975,39 @@ inefficiently.  Users can generally speed things up by using a faster
 shell, e.g., by using the command @samp{bash ./configure} rather than
 plain @samp{./configure}.
 
+Some shells can be extremely inefficient when there are a lot of
+here-documents inside a single statement.  For instance if your
+@file{configure.ac} includes something like:
+
+@example
+@group
+if <cross_compiling>; then
+  assume this and that
+else
+  check this
+  check that
+  check something else
+  on and on forever
+fi
+@end group
+@end example
+
+A shell parses the whole @code{if}/@code{fi} construct, creating
+temporary files for each here document in it.  Some shells create links
+for such here-documents on every @code{fork ()}, so that the clean-up
+code they had installed correctly removes them.  It is creating the
+links that the shell can take forever.
+
+Moving the tests out of the @code{if}/@code{fi}, or creating multiple
+@code{if}/@code{fi} constructs, would improve the performance
+significantly.  Anyway, this kind of construct is not exactly the
+typical use of Autoconf.  In fact, it's even not recommended, because M4
+macros can't look into shell conditionals, so we may fail to expand a
+macro when it was expanded before in a conditional path, and the
+condition turned out to be false at run-time, and we end up not
+executing the macro at all.
+
+
 @item @command{exit}
 @c -----------------
 @cindex @command{exit}