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

ints can be used as futex on Linux. ptrdiff_t on 64-bit Linux can't.
---
 libstdc++-v3/include/std/latch | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

On Feb 26 2021, Thiago Macieira via Gcc-patches wrote:

> @@ -85,7 +85,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      }
>  
>    private:
> -    alignas(__alignof__(ptrdiff_t)) ptrdiff_t _M_a;
> +    alignas(__alignof__(int)) int _M_a;

Futexes must be aligned to 4 bytes.

Andreas.

On Friday, 26 February 2021 10:14:42 PST Andreas Schwab wrote:
> On Feb 26 2021, Thiago Macieira via Gcc-patches wrote:
> > @@ -85,7 +85,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > }
> > 
> > private:
> > -    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?

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:
>> > @@ -85,7 +85,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>> > }
>> > 
>> > private:
>> > -    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.

Andreas.

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?

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.

brgds, H-P

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.

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 ...

Richard.

> brgds, H-P

On 2021-02-28 13:31, Hans-Peter Nilsson 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.

There is no predicate wait on atomic<T>.

> brgds, H-P

On 26/02/21 07:59 -0800, Thiago Macieira via Libstdc++ wrote:
>ints can be used as futex on Linux. ptrdiff_t on 64-bit Linux can't.

Yes, we should do this for GCC 11.


> libstdc++-v3/include/std/latch | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
>diff --git a/libstdc++-v3/include/std/latch b/libstdc++-v3/include/std/latch
>index ef8c301e5e9..156aea5c5e5 100644
>--- a/libstdc++-v3/include/std/latch
>+++ b/libstdc++-v3/include/std/latch
>@@ -48,7 +48,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>   public:
>     static constexpr ptrdiff_t
>     max() noexcept
>-    { return __gnu_cxx::__int_traits<ptrdiff_t>::__max; }
>+    { return __gnu_cxx::__int_traits<int>::__max; }
>
>     constexpr explicit latch(ptrdiff_t __expected) noexcept
>       : _M_a(__expected) { }
>@@ -85,7 +85,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>     }
>
>   private:
>-    alignas(__alignof__(ptrdiff_t)) ptrdiff_t _M_a;
>+    alignas(__alignof__(int)) int _M_a;
>   };
> _GLIBCXX_END_NAMESPACE_VERSION
> } // namespace
>-- 
>2.30.1
>

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.

On Mär 03 2021, Jonathan Wakely wrote:

> 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).

There is no requirement that __alignof__(int) is big enough.

Andreas.

On 03/03/21 14:56 +0000, Jonathan Wakely wrote:
>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?

It's not a GCC extensions. You're thinking of alignas(obj) which is a
GCC extension.

>>>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.

Jakub said on IRC that m68k might have alignof(int) == 2, so we'd need
to increase that alignment to use it as a futex.

N.B. std::atomic and std::atomic_ref don't use alignas(__alignof__(T))
they do this instead:

       static_assert(is_trivially_copyable_v<_Tp>);

       // 1/2/4/8/16-byte types must be aligned to at least their size.
       static constexpr int _S_min_alignment
	= (sizeof(_Tp) & (sizeof(_Tp) - 1)) || sizeof(_Tp) > 16
	? 0 : sizeof(_Tp);

     public:

       static constexpr size_t required_alignment
	= _S_min_alignment > alignof(_Tp) ? _S_min_alignment : alignof(_Tp);

Using std::atomic_ref<T>::required_alignment would give that value.

For something being used as a futex we should just use alignas(4)
though, since that's what the kernel requires.

On Wed, 3 Mar 2021, Jonathan Wakely wrote:
> 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).

(sizeof(int) last)

The CRIS ABI does as in default packed, and ISTR there was some
other gcc port too, but I don't recall whether that (too) had no
(current) Linux port.

brgds, H-P

On Wednesday, 3 March 2021 06:34:26 PST Jonathan Wakely wrote:
> On 26/02/21 07:59 -0800, Thiago Macieira via Libstdc++ wrote:
> >ints can be used as futex on Linux. ptrdiff_t on 64-bit Linux can't.
> 
> Yes, we should do this for GCC 11.

Want me to re-submit this one alone, with the "alignas(4)" with a commend 
indicating it's what the kernel requires?

On 03/03/21 09:14 -0800, Thiago Macieira via Libstdc++ wrote:
>On Wednesday, 3 March 2021 06:34:26 PST Jonathan Wakely wrote:
>> On 26/02/21 07:59 -0800, Thiago Macieira via Libstdc++ wrote:
>> >ints can be used as futex on Linux. ptrdiff_t on 64-bit Linux can't.
>>
>> Yes, we should do this for GCC 11.
>
>Want me to re-submit this one alone, with the "alignas(4)" with a commend
>indicating it's what the kernel requires?

Yes please.