Re: [PATCH 06/12] use a shell function for _AC_CHECK_HEADER_MONGREL

[Eric Blake]

Paolo Bonzini <bonzini <at> gnu.org> writes:

> 
> I split AC_CHECK_HEADER functionization in two patches.  Here is the
> first.
> 
> 2008-10-12  Paolo Bonzini  <bonzini <at> gnu.org>
> 
>       * lib/autoconf/general.m4 (_AC_CHECK_HEADER_MONGREL): Use a shell
>       function.

> +[AC_REQUIRE_SHELL_FN([ac_func_]_AC_LANG_ABBREV[_check_header_mongrel],
> +  [AS_FUNCTION_DESCRIBE([ac_func_]_AC_LANG_ABBREV[_check_header_mongrel],
> +    [LINENO FUNC INCLUDES],

I'm going to repeat my trick from AC_CHECK_FUNC in passing the cache variable 
name as an extra argument, so we don't need the AS_VAR_PUSHDEF in the function 
body.

> +    [Tests whether HEADER exists, giving a warning if it cannot be compiled
> +     using the include files in INCLUDES and setting a cache variable 
accordingly.])], [
> +AS_LINENO_PUSH([$[]1])
> +ac_header=[$]2
> +ac_includes=[$]3

Additionally, I don't like this - AC_CHECK_HEADERS invokes a shell for loop on 
the variable ac_header, which it then reads after the expansion of 
AC_CHECK_HEADER has executed; but here you are assigning into it (fortunately, 
you are reassigning the value it already had).  But what's wrong with using
$[]2 instead of $ac_header?

Meanwhile, before I actually work on this patch, I noticed a small optimization 
that I will be applying soon:

diff --git a/ChangeLog b/ChangeLog
index 61b5f33..a485066 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2008-10-24  Eric Blake  <address@hidden>
+
+       Speed up AC_CHECK_HEADER.
+       * lib/autoconf/headers.m4 (AC_CHECK_HEADER): Factor out address@hidden
+
 2008-10-24  Paolo Bonzini  <address@hidden>
        and Eric Blake  <address@hidden>
 
diff --git a/lib/autoconf/headers.m4 b/lib/autoconf/headers.m4
index 4012107..413be8c 100644
--- a/lib/autoconf/headers.m4
+++ b/lib/autoconf/headers.m4
@@ -84,11 +84,13 @@
 #
 # If INCLUDES is specified and different from `-', then use the new
 # semantics only.
+#
+# The m4_indir allows for fewer expansions of address@hidden
 AC_DEFUN([AC_CHECK_HEADER],
-[m4_case([$4],
-        [],  [_AC_CHECK_HEADER_MONGREL($@)],
-        [-], [_AC_CHECK_HEADER_OLD($@)],
-             [_AC_CHECK_HEADER_NEW($@)])
+[m4_indir(m4_case([$4],
+                 [],  [[_AC_CHECK_HEADER_MONGREL]],
+                 [-], [[_AC_CHECK_HEADER_OLD]],
+                      [[_AC_CHECK_HEADER_NEW]]), $@)
 ])# AC_CHECK_HEADER