FLEXALIGNOF too pessimistic

[Bruno Haible]

Hi Paul,

The FLEXALIGNOF macro, from flexmember.h, in the pre-C11 or C++ case,
returns unreasonably large values. When we have a

  struct { int a[64]; char c[]; }

and use FLEXALIGNOF, it will return 256. Which means that if one wants
to allocate such a struct with 4 elements of c[], one will allocate
512 bytes, when 260 bytes would be sufficient.

How about making this value smaller?

I see two approaches?
  a) Cap this alignment at max_align_t.
  b) Use <stdalign.h>, as in the patch below (untested).

What do you think?

Bruno


diff --git a/lib/flexmember.h b/lib/flexmember.h
index c71ea65..be8e955 100644
--- a/lib/flexmember.h
+++ b/lib/flexmember.h
@@ -17,18 +17,11 @@
 
    Written by Paul Eggert.  */
 
+#include <stdalign.h>
 #include <stddef.h>
 
-/* Nonzero multiple of alignment of TYPE, suitable for FLEXSIZEOF below.
-   On older platforms without _Alignof, use a pessimistic bound that is
-   safe in practice even if FLEXIBLE_ARRAY_MEMBER is 1.
-   On newer platforms, use _Alignof to get a tighter bound.  */
-
-#if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112
-# define FLEXALIGNOF(type) (sizeof (type) & ~ (sizeof (type) - 1))
-#else
-# define FLEXALIGNOF(type) _Alignof (type)
-#endif
+/* Nonzero multiple of alignment of TYPE, suitable for FLEXSIZEOF below.  */
+#define FLEXALIGNOF(type) _Alignof (type)
 
 /* Upper bound on the size of a struct of type TYPE with a flexible
    array member named MEMBER that is followed by N bytes of other data.
diff --git a/modules/flexmember b/modules/flexmember
index 5107676..cbc259e 100644
--- a/modules/flexmember
+++ b/modules/flexmember
@@ -6,6 +6,7 @@ lib/flexmember.h
 m4/flexmember.m4
 
 Depends-on:
+stdalign
 
 configure.ac:
 AC_C_FLEXIBLE_ARRAY_MEMBER