stdalign: relax _Alignof and tighten _Alignas test

* m4/stdalign.m4 (gl_STDALIGN_H): Relax the _Alignof test,
as it was too strict: alignof must divide offsetof, but it need
not equal offsetof.  Inspired by Joseph S. Myers's comment
<http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023#c10>.
Conversely, tighten the _Alignas test a bit, as the resulting
alignment must be exactly 8.

diff --git a/ChangeLog b/ChangeLog
index 399be2e..b5fd92e 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,13 @@
+2012-01-27  Paul Eggert  <eggert@cs.ucla.edu>
+
+       stdalign: relax _Alignof and tighten _Alignas test
+       * m4/stdalign.m4 (gl_STDALIGN_H): Relax the _Alignof test,
+       as it was too strict: alignof must divide offsetof, but it need
+       not equal offsetof.  Inspired by Joseph S. Myers's comment
+       <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023#c10>.
+       Conversely, tighten the _Alignas test a bit, as the resulting
+       alignment must be exactly 8.
+
 2012-01-27  Bruno Haible  <bruno@clisp.org>
 
        stdalign: Document the last change.

diff --git a/m4/stdalign.m4 b/m4/stdalign.m4
index d78a589..6659c9c 100644 (file)
--- a/m4/stdalign.m4
+++ b/m4/stdalign.m4
@@ -25,15 +25,16 @@ AC_DEFUN([gl_STDALIGN_H],
             #else
             # define ao(type) offsetof (struct { char a; type b; }, b)
             #endif
-            char test1[_Alignof (double) == ao (double) ? 1 : -1];
-            char test2[alignof (long int) == ao (long int) ? 1 : -1];
+            char test_double[ao (double) % _Alignof (double) == 0 ? 1 : -1];
+            char test_long[ao (long int) % _Alignof (long int) == 0 ? 1 : -1];
+            char test_alignof[alignof (double) == _Alignof (double) ? 1 : -1];
 
             /* Test _Alignas only on platforms where gnulib can help.  */
             #if \
                 (__GNUC__ || __IBMC__ || __IBMCPP__ \
                  || 0x5110 <= __SUNPRO_C || 1300 <= _MSC_VER)
               int alignas (8) alignas_int = 1;
-              char test3[8 <= _Alignof (alignas_int) ? 1 : -1];
+              char test_alignas[_Alignof (alignas_int) == 8 ? 1 : -1];
             #endif
           ]])],
        [gl_cv_header_working_stdalign_h=yes],