[Bug c/52023] New: _Alignof (double) yields wrong value on x86

[eggert at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023

             Bug #: 52023
           Summary: _Alignof (double) yields wrong value on x86
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: egg...@gnu.org
              Host: x86-64
            Target: x86
             Build: x86-64


Created attachment 26482
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=26482
gcc -v output

On x86, alignof (double) should be 4, since that's the alignment
requirement used for 'double' when laying out structures.  But with
GCC alignof incorrectly returns 8.  This causes some gnulib tests to fail
and (more importantly) it may cause problems with programs such as
conservative garbage collectors that use alignof to determine which
addresses might be of valid objects.

The problem also occurs with other fundamental types, such as 'long long'.

Here's a simple test case:

  #include <stddef.h>
  #include <stdalign.h>
  int a = alignof (double);
  int b = offsetof (struct { char c; double d; }, d);
  int main (void) { return a == b; }

Here's the preprocessed output:

  int a = _Alignof (double);
  int b = __builtin_offsetof (struct { char c; double d; }, d);
  int main (void) { return a == b; }

'main' should return 1, but it returns 0 due to the bug, which causes
'a' to be 8 instead of 4 as it should be.

I'm using GCC on the trunk (r183603), built on Fedora 16 x86-64 with
no special flags, and I'm compiling with 'gcc -v -m32 test.c' and
running with './a.out'.  I'm attaching the output of 'gcc -v'.