http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52023
eggert at gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|RESOLVED |UNCONFIRMED
Resolution|DUPLICATE |
--- Comment #6 from eggert at gnu dot org 2012-01-27 21:44:23 UTC ---
This is not a duplicate of Bug 10360 because that was about
__alignof__, but this one is about the C11 standard _Alignof/alignof.
GCC can assign whatever meaning it likes to __alignof__ but for
_Alignof/alignof it should conform to the standard. I agree with
Bruno that the bug should be reopened and for now I'm taking the
liberty of doing that until the matter is thought through more
carefully.
In C11, alignof(T) does not mean "the smallest alignment that will
used for T in certain contexts". It means "the alignment requirement
for T", period; see C11 section 6.5.3.4 paragraph 3. This alignment
requirement applies to "every object of that type"; see section 6.2.8
paragraph 1. Members of structures are objects; see section 6.2.5
paragraph 20; and non-bitfield members have addresses and therefore
these addresses must be multiples of their types' alignments.
In short, GCC's behavior clearly conflicts with C11 here.
Andrew gave good ABI examples, where GCC uses different alignments for
T depending on context. There's nothing wrong with GCC doing that.
All that's required by the standard is that if objects of type T
sometimes have alignment X, and sometimes Y, then alignof(T) must
yield the minimum of X and Y. (Otherwise, GCC could simply return,
say, 2**20 for alignof(X) regardless of X, on the theory that in
*some* contexts the alignment is 2**20. :-)
Also, this is not simply a question of the C standard. The GCC manual
says says (about __alignof__) that it "reports the smallest alignment
that GCC will give the data type". It does not say "the smallest
alignment that GCC will give the data type outside of a struct". If
the intent is that __alignof__ is supposed to be useful only for some
contexts in a C program, and not others, then the intent conflicts
with GCC's documentation, and either the behavior or the documentation
should be fixed for __alignof__.
As a practical matter, what would be the point of having alignof yield
a value that's greater than the alignment that GCC actually uses for
objects? What kind of programs would actually make use of this
behavior? I can certainly think of programs it would *break*, e.g., a
conservative garbage collector that uses alignof to test whether a
purported pointer is actually valid. On balance, it seems that it'd
be better to conform to the standard here, instead of having a
nonstandard interpretation of alignof.
