Re: [PATCH] add definition of max_align_t to stddef.h

["Paweł Dziepak" <pdziepak@quarnos.org>]

2014-05-07 5:13 GMT+02:00 Rich Felker <dalias@libc.org>:
> On Tue, May 06, 2014 at 12:35:55PM +0200, Paweł Dziepak wrote:
>> >> would be a good thing to mach the definition gcc and clang use, i.e.
>> >> something like that:
>> >>
>> >> union max_align_t {
>> >>     alignas(long long) long long _ll;
>> >>     alignas(long double) long double _ld;
>> >> };
>> >
>> > This should not give results different from omitting the "alignas".
>> > The only reason it does give different results is a bug in GCC, so we
>> > should not be copying this confusing mess that's a no-op for a correct
>> > compiler. (Applying alignas(T) to type T is always a no-op.)
>>
>> I should have checked whether GCC 4.9 has changed before sending that.
>> As I said earlier, alignof in 4.9 seems to be fixed and on i386 for
>> fundamental types values <=4 are returned. alignof(max_align_t)
>> remains 8, though.
>
> Then GCC still has a bug. The above definition should give an
> alignment of 4, not 8. Neither alignas(long long) nor alignas(long
> double) should impose 8-byte alignment.

To clarify: GCC defines max_align_t so its alignment is 8. My original
definition (without alignas) makes max_align_t 4-byte-aligned (both
GCC 4.8.2 and 4.9). My second definition (with alignas) results in
8-byte-aligned max_align_t on GCC 4.8.2 and bug in GCC 4.9
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61053

GCC uses its own __alignof__ to define max_align_t. __alignof__
returns the recommended alignment (as oposed to minimal in case of
alignof), which in case of long long is 8.

>> However, while 4, undobtedly, is the expected value of
>> alignof(max_align_t) I don't think that 8 is really wrong (well, from
>> the C11 point of view). The standard is not very specific about what
>> max_align_t really should be and if the compiler supports larger
>> alignment in all contexts there is no reason that alignof(max_align_t)
>> cannot be larger than alignof() of the type with the strictest
>> alignment requirements.
>> Obviously, since max_align_t is the part of ABI it is not like the
>> implementation can set alignof(max_align_t) to any value or it would
>> risk compatibility problems with binaries compiled with different
>> max_align_t. Since both GCC and Clang already define max_align_t so
>> that its alignment is 8 on i386 I think that Musl should do the same.
>
> If we want to achieve an alignment of 8, the above definition is
> wrong; it will no longer have alignment 8 once the bug is fixed.
> However I'm not convinced it's the right thing to do. Defining it as 8
> is tightening malloc's contract to always return 8-byte-aligned memory
> (note that it presently returns at least 16-byte alignment anyway, but
> this is an implementation detail that's not meant to be observable,
> not part of the interface contract).

I've mentioned earlier that it seems that the only option is to use
GCC extensions (i.e. __alignof__) to match their definition of
max_align_t, just like it is done in this patch:
http://www.openwall.com/lists/musl/2014/04/28/3
It is not nice that GCC forces malloc to support "extended" alignment
but I don't think there is much that can be done about it.

Paweł