[PATCH 1/5] std::latch: reduce internal implementation from ptrdiff_t to int

[Jonathan Wakely]

On 01/03/21 09:56 +0100, Richard Biener via Libstdc++ wrote:
>On Sun, Feb 28, 2021 at 10:53 PM Hans-Peter Nilsson <hp@bitrange.com> wrote:
>>
>>
>>
>> On Fri, 26 Feb 2021, Thiago Macieira via Gcc-patches wrote:
>>
>> > On Friday, 26 February 2021 11:31:00 PST Andreas Schwab wrote:
>> > > On Feb 26 2021, Thiago Macieira wrote:
>> > > > On Friday, 26 February 2021 10:14:42 PST Andreas Schwab wrote:
>> > > >> On Feb 26 2021, Thiago Macieira via Gcc-patches wrote:
>> > > >> > -    alignas(__alignof__(ptrdiff_t)) ptrdiff_t _M_a;
>> > > >> > +    alignas(__alignof__(int)) int _M_a;
>> > > >>
>> > > >> Futexes must be aligned to 4 bytes.
>> > > >
>> > > > Agreed, but doesn't this accomplish that?
>> > >
>> > > No.  It uses whatever alignment the type already has, and is an
>> > > elaborate no-op.
>> >
>> > I thought so too when I read the original line. But I expected it was written
>> > like that for a reason, especially since the same pattern appears in other
>> > places.
>> >
>> > I can change to "alignas(4)" (which is a GCC extension, I believe). Is that
>> > the correct solution?
>>
>> IMNSHO make use of the corresponding atomic type.  Then there'd
>> be no need for separate what's-the-right-align-curse games.

That won't work though, because we need direct access to the integer
object, not to a std::atomic<int> which contains it.

>Or align as the corresponding atomic type (in case using an actual
>std::atomic<int> is undesirable).  OTOH the proposed code isn't
>any more bogus than the previous ...

The previous code accounts for the fact that ptrdiff_t is a typedef
for an unspecified type, and that some ABIs allow struct members to have
weaker alignment than they would have otherwise.

e.g. __alignof__(long long) != alignof(long long) on x86.

Yes, I know ptrdiff_t isn't long long on x86, but since it's a typedef
for some target-specific type, it's still possible that
alignof != __alignof__. So alignas(__alignof__(T)) is not necessarily
a no-op. So not bogus.

For int, there shouldn't be any need to force the alignment. I don't
think any ABI supported by GCC allows int members to be aligned to
less than __alignof__(int). Users could break it by compiling with
#pragma pack, but I have no sympathy for such silliness.