AC_CHECK_ALIGNOF fails when cross-compiling with gcc >= 4.5

[Michael Olbrich]

Hi,

when cross-compiling with gcc >= 4.5 (I think), AC_CHECK_ALIGNOF fails to
detect the alignment. This happens because the compiler fails with "error:
storage size of ‘test_array’ isn’t constant" instead of succeeding.
Using __builtin_offsetof helps. Patch attached. I'm not quite sure when
__builtin_offsetof should be used, but it is used in the Linux kernel for
gcc 4.x, so I think this is correct.

Regards,
Michael

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From: Michael Olbrich <m.olbr...@pengutronix.de>
Date: Wed, 25 Apr 2012 10:16:58 +0200
Subject: [PATCH] fix AC_CHECK_ALIGNOF when cross-compiling with gcc >= 4.5

Signed-off-by: Michael Olbrich <m.olbr...@pengutronix.de>
---
 lib/autoconf/types.m4 |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/lib/autoconf/types.m4 b/lib/autoconf/types.m4
index 9f482da..15fa603 100644
--- a/lib/autoconf/types.m4
+++ b/lib/autoconf/types.m4
@@ -800,8 +800,12 @@ m4_define([_AC_CHECK_ALIGNOF],
 _AC_CACHE_CHECK_INT([alignment of $1], [AS_TR_SH([ac_cv_alignof_$3])],
   [(long int) offsetof (ac__type_alignof_, y)],
   [AC_INCLUDES_DEFAULT([$2])
-#ifndef offsetof
-# define offsetof(type, member) ((char *) &((type *) 0)->member - (char *) 0)
+#if __GNUC_PREREQ(4,0)
+# define offsetof(type, member) __builtin_offsetof(type, member)
+#else
+# ifndef offsetof
+#  define offsetof(type, member) ((char *) &((type *) 0)->member - (char *) 0)
+# endif
 #endif
 typedef struct { char x; $1 y; } ac__type_alignof_;],
   [if test "$AS_TR_SH([ac_cv_type_$3])" = yes; then

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