Re: [PATCH] Present But Cannot Be Compiled

[Eric Blake]

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

> 
> As a first step on which we all agree, I'll apply this patch in a few
> hours (to give Americans some time to wake up) which changes the
> behavior of _AC_CHECK_HEADERS_MONGREL to use the compiler's result.

How about this as the second step, which makes AC_CHECK_HEADERS_ONCE silently 
prefer the compiler result by supplying the fourth argument?  This macro is 
newer than when we announced the transition period, so I don't think we even 
need to cater to the possibility of present but cannot be compiled (developers 
using it should already be aware of the issue).

I'm also basing the idea that newer macros should unilaterally use newer 
semantics from have this comment on AC_CHECK_TYPES:

# TYPES is an m4 list.  There are no ambiguities here, we mean the newer
# AC_CHECK_TYPE.

which goes on to call only _AC_CHECK_TYPE_NEW (AC_CHECK_TYPE is another one of 
those mongrel macros).

If I don't hear anything, I'll apply this in 24 hours.

From: Eric Blake <address@hidden>
Date: Thu, 6 Nov 2008 14:40:47 -0700
Subject: [PATCH] Skip preprocessor check in AC_CHECK_HEADERS_ONCE.

* lib/autoconf/headers.m4 (_AC_HEADERS_EXPANSION): Provide fourth
argument to speed up check.

Signed-off-by: Eric Blake <address@hidden>
---
 ChangeLog               |    6 ++++++
 lib/autoconf/headers.m4 |    2 +-
 2 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 37e69c7..a627ae3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2008-11-06  Eric Blake  <address@hidden>
+
+       Skip preprocessor check in AC_CHECK_HEADERS_ONCE.
+       * lib/autoconf/headers.m4 (_AC_HEADERS_EXPANSION): Provide fourth
+       argument to speed up check.
+
 2008-11-06  Paolo Bonzini  <address@hidden>
 
        Remove three forks per _AC_RUN_LOG_STDERR in the common case.
diff --git a/lib/autoconf/headers.m4 b/lib/autoconf/headers.m4
index cb1cb9c..6802c38 100644
--- a/lib/autoconf/headers.m4
+++ b/lib/autoconf/headers.m4
@@ -265,7 +265,7 @@ AC_DEFUN([AC_CHECK_HEADERS_ONCE],
 m4_define([_AC_HEADERS_EXPANSION],
 [
   m4_divert_text([DEFAULTS], [ac_header_list=])
-  AC_CHECK_HEADERS([$ac_header_list])
+  AC_CHECK_HEADERS([$ac_header_list], [], [], [AC_INCLUDES_DEFAULT])
   m4_define([_AC_HEADERS_EXPANSION], [])
 ])
 
-- 
1.6.0.2