stddef: Override wrong max_align_t on AIX 7 with xlc in 64-bit mode

[Bruno Haible]

On AIX 7.1, 7.2, with xlc in 64-bit mode, I see this test compilation failure:

"../../gltests/test-stddef.c", line 57.1: 1506-044 (S) Expression must be a 
non-negative integer constant.
"../../gltests/test-stddef.c", line 58.1: 1506-044 (S) Expression must be a 
non-negative integer constant.
"../../gltests/test-stddef.c", line 59.1: 1506-044 (S) Expression must be a 
non-negative integer constant.
make: 1254-004 The error code from the last command is 1.

The cause is the definition of max_align_t: it is defined as 'long double',
which has alignment 4 in this case — whereas 'long int' has alignment 8.

This patch fixes it.


2021-01-03  Bruno Haible  <bruno@clisp.org>

        stddef: Override wrong max_align_t on AIX 7 with xlc in 64-bit mode.
        * m4/stddef_h.m4 (gl_STDDEF_H): Make check for good max_align_t
        stricter: Add tests for offsetof-based alignof.
        * lib/stddef.in.h (max_align_t): Override on AIX in 64-bit mode.
        * doc/posix-headers/stddef.texi: Document the AIX max_align_t bug.

diff --git a/doc/posix-headers/stddef.texi b/doc/posix-headers/stddef.texi
index 33018ef..dbc0103 100644
--- a/doc/posix-headers/stddef.texi
+++ b/doc/posix-headers/stddef.texi
@@ -13,7 +13,7 @@ NetBSD 8.0, Solaris 11.0, and others.
 
 @item
 @code{max_align_t} does not have the expected alignment on some platforms:
-NetBSD 8.0/x86.
+NetBSD 8.0/x86, AIX 7.2 with xlc in 64-bit mode.
 
 @item
 Some old platforms fail to provide @code{wchar_t}.
diff --git a/lib/stddef.in.h b/lib/stddef.in.h
index 6ef45af..77147e9 100644
--- a/lib/stddef.in.h
+++ b/lib/stddef.in.h
@@ -49,6 +49,23 @@
 
 # ifndef _@GUARD_PREFIX@_STDDEF_H
 
+/* On AIX 7.2, with xlc in 64-bit mode, <stddef.h> defines max_align_t to a
+   type with alignment 4, but 'long' has alignment 8.  */
+#  if defined _AIX && defined _ARCH_PPC64
+#   if !GNULIB_defined_max_align_t
+#    ifdef _MAX_ALIGN_T
+/* /usr/include/stddef.h has already defined max_align_t.  Override it.  */
+typedef long rpl_max_align_t;
+#     define max_align_t rpl_max_align_t
+#    else
+/* Prevent /usr/include/stddef.h from defining max_align_t.  */
+typedef long max_align_t;
+#     define _MAX_ALIGN_T
+#    endif
+#    define GNULIB_defined_max_align_t 1
+#   endif
+#  endif
+
 /* The include_next requires a split double-inclusion guard.  */
 
 #  @INCLUDE_NEXT@ @NEXT_STDDEF_H@
diff --git a/m4/stddef_h.m4 b/m4/stddef_h.m4
index 7869621..cd666c4 100644
--- a/m4/stddef_h.m4
+++ b/m4/stddef_h.m4
@@ -1,4 +1,4 @@
-# stddef_h.m4 serial 8
+# stddef_h.m4 serial 9
 dnl Copyright (C) 2009-2021 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -28,6 +28,13 @@ AC_DEFUN([gl_STDDEF_H],
             int check1[2 * (__alignof__ (double) <= __alignof__ (max_align_t)) 
- 1];
             int check2[2 * (__alignof__ (long double) <= __alignof__ 
(max_align_t)) - 1];
             #endif
+            typedef struct { char a; max_align_t b; } max_helper;
+            typedef struct { char a; long b; } long_helper;
+            typedef struct { char a; double b; } double_helper;
+            typedef struct { char a; long double b; } long_double_helper;
+            int check3[2 * (offsetof (long_helper, b) <= offsetof (max_helper, 
b)) - 1];
+            int check4[2 * (offsetof (double_helper, b) <= offsetof 
(max_helper, b)) - 1];
+            int check5[2 * (offsetof (long_double_helper, b) <= offsetof 
(max_helper, b)) - 1];
           ]])],
        [gl_cv_type_max_align_t=yes],
        [gl_cv_type_max_align_t=no])