http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54188
--- Comment #3 from Keith Thompson <Keith.S.Thompson at gmail dot com>
2012-08-06 19:28:37 UTC ---
The results of the _Alignof operator (new in the 2011 ISO C standard)
are the same as for the __alignof__ operator (not surprisingly).
N1370 (C11 draft) 6.5.3.4 paragraph 3 says:
The _Alignof operator yields the alignment requirement of its
operand type.
Richard Guenther: You say it's "Because the ABI says so". Do you have
a reference to the ABI, particularly to a statement that a structure
should have a smaller alignment than its member?
You also say __alignof__ "does not return the minimum but the
recommended alignment". That seems inconsistent with the use of the
word "required" in C11.
I just grabbed a copy of http://www.uclibc.org/docs/psABI-i386.pdf;
is that the ABI you're referring to? Figure 3-1 covers alignment
for scalar types. It says 8-byte floating-point has an alignment
of 4 bytes, but it doesn't mention 8-byte integers. Furthermore,
the following page says:
Aggregates (structures and arrays) and unions assume the alignment
of their most strictly aligned component.
That seems inconsistent with the behavior of the following program:
#include <stdio.h>
int main(void) {
printf("_Alignof(long long) = %d\n",
(int)_Alignof(long long));
printf("_Alignof(struct {long long x;}) = %d\n",
(int)_Alignof(struct {long long x;}));
return 0;
}
whose output on my system, with
gcc -std=c11 -pedantic c.c -o c && ./c
is:
_Alignof(long long) = 8
_Alignof(struct {long long x;}) = 4
