Improve AC_REPLACE_FUNCS

[Eric Blake]

I've been playing with this patch for more than a week.  It gives a reduction 
in size by using more AC_CHECK_FUNCS_ONCE instead of inline function checks, 
and can reduce forks by using a literal rather than a shell variable in more 
places; particularly when doing AC_REPLACE_FUNCS([a b]).

It doesn't impact gnulib: each project using gnulib is given an override of 
AC_LIBOBJ that fits in better with the concept of multiple libraries, and 
gnulib currently has to hijack AC_REPLACE_FUNCS because it was not using 
AC_LIBOBJ prior to this patch.  But once gnulib can assume autoconf 2.65, it 
will also reduce configure size there.  Meanwhile, we should someday figure out 
how to improve AC_LIBOBJ to fit gnulib's needs of multiple replacement object 
libraries.


From: Eric Blake <address@hidden>
Date: Wed, 9 Sep 2009 08:32:46 -0600
Subject: [PATCH] Optimize AC_REPLACE_FUNCS.

* lib/autoconf/functions.m4 (_AC_REPLACE_FUNC): New helper macro.
(AC_REPLACE_FUNCS): Use it to check for functions only once, and
to pass literal rather than shell variable to AC_LIBOBJ.

Signed-off-by: Eric Blake <address@hidden>
---
 ChangeLog                 |    7 +++++++
 lib/autoconf/functions.m4 |   17 ++++++++++++++---
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 50f6fb7..3a4d9f5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2009-09-16  Eric Blake  <address@hidden>
+
+       Optimize AC_REPLACE_FUNCS.
+       * lib/autoconf/functions.m4 (_AC_REPLACE_FUNC): New helper macro.
+       (AC_REPLACE_FUNCS): Use it to check for functions only once, and
+       to pass literal rather than shell variable to AC_LIBOBJ.
+
 2009-09-15  Peter Breitenlohner  <address@hidden>

        Implement and document Objective C++ support.
diff --git a/lib/autoconf/functions.m4 b/lib/autoconf/functions.m4
index 810a0a7..cc2d99d 100644
--- a/lib/autoconf/functions.m4
+++ b/lib/autoconf/functions.m4
@@ -116,12 +116,23 @@ m4_define([_AC_FUNCS_EXPANSION],
 ])


+# _AC_REPLACE_FUNC(FUNCTION)
+# --------------------------
+# If FUNCTION exists, define HAVE_FUNCTION; else add FUNCTION.c
+# to the list of library objects.  FUNCTION must be literal.
+m4_define([_AC_REPLACE_FUNC],
+[_AC_CHECK_FUNC_ONCE([$1])if test "x$ac_cv_func_$1" != xyes; then
+  AC_LIBOBJ([$1])
+fi])
+
 # AC_REPLACE_FUNCS(FUNCTION...)
 # -----------------------------
+# For each FUNCTION in the whitespace separated list, perform the
+# equivalent of AC_CHECK_FUNC, then call AC_LIBOBJ if the function
+# was not found.
 AC_DEFUN([AC_REPLACE_FUNCS],
-[m4_map_args_w([$1], [AC_LIBSOURCE(], [.c)])]dnl
-[AC_CHECK_FUNCS([$1], , [_AC_LIBOBJ($ac_func)])
-])
+[m4_map_args_w([$1], [_AC_REPLACE_FUNC(], [)
+])])


 # AC_TRY_LINK_FUNC(FUNC, ACTION-IF-FOUND, ACTION-IF-NOT-FOUND)
-- 
1.6.4.2