Re: standards/52571: max_align_t has smaller alignment than double

[Joerg Sonnenberger <joerg%bec.de@localhost>]

The following reply was made to PR standards/52571; it has been noted by GNATS.

From: Joerg Sonnenberger <joerg%bec.de@localhost>
To: gnats-bugs%NetBSD.org@localhost
Cc: standards-manager%netbsd.org@localhost, gnats-admin%netbsd.org@localhost,
	netbsd-bugs%netbsd.org@localhost
Subject: Re: standards/52571: max_align_t has smaller alignment than double
Date: Tue, 26 Sep 2017 10:57:18 +0200

 On Mon, Sep 25, 2017 at 04:55:00PM +0000, bruno%clisp.org@localhost wrote:
 > max_align_t is supposed to have the largest alignment among the
 > standard C types. But here, __alignof__(max_align_t) < __alignof__(double).
 
 So this one is a nice excercise in GCC extensions and why they sometimes
 don't give you what you think they do. Compare the following test case
 on i386 with and without USE_ALIGNOF:
 
 #include <stddef.h>
 
 struct foo {
   double x;
   char t;
 };
 
 int main() {
 #ifdef USE_ALIGNOF
   return __alignof__(double);
 #else
   return sizeof(struct foo) - offsetof(struct foo, t);
 #endif
 }
 
 ---
 
 For the SYSV ABI (not the bastardized version Linux is nowadays using),
 the ABI alignment of double is 32bit, but most compiler use a prefered
 alignment of 64bit. Similar for long double.
 
 Joerg