Re: Patch ping (was Re: [PATCH] libstdc++: Clear padding bits in atomic compare_exchange)
On Fri, 9 Sept 2022 at 20:01, Thomas Rodgers wrote:
>
> s/__weak/__is_weak/g perhaps?
Yes, that'll do. Fixed by the attached, with a test to avoid it happening again.
Tested x86_64-linux, pushed to trunk.
>
> On Fri, Sep 9, 2022 at 11:46 AM Iain Sandoe via Libstdc++
> wrote:
>>
>>
>>
>> > On 9 Sep 2022, at 19:36, Rainer Orth wrote:
>> >
>>
>> >> Here's a complete patch that combines the various incremental patches
>> >> that have been going around. I'm testing this now.
>> >>
>> >> Please take a look.
>> >
>> > unfortunately, this patch broke macOS bootstrap (seen on
>> > x86_64-apple-darwin11.4.2):
>> >
>> > In file included from
>> > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/shared_ptr_atomic.h:33,
>> > from
>> > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/memory:78,
>> > from
>> > /vol/gcc/src/hg/master/darwin/libstdc++-v3/include/precompiled/stdc++.h:82:
>> > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/atomic_base.h:
>> > In function 'bool std::__atomic_impl::__compare_exchange(_Tp&,
>> > _Val<_Tp>&, _Val<_Tp>&, bool, std::memory_order, std::memory_order)':
>> > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/atomic_base.h:1008:49:
>> > error: expected primary-expression before ',' token
>> > 1008 | __weak, int(__s),
>> > int(__f)))
>> > | ^
>> > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/atomic_base.h:1017:50:
>> > error: expected primary-expression before ',' token
>> > 1017 |__weak, int(__s),
>> > int(__f));
>> > | ^
>> >
>> > Darwin gcc predefines __weak= in gcc/config/darwin-c.cc
>> > (darwin_cpp_builtins).
>>
>> yes, __weak and __strong are Objective C things (in principle, applicable to
>> non-Darwin targets
>> using NeXT runtime - there is at least one such target).
>>
>> Iain
>>
commit 007680f946eaffa3c6321624129e1ec18e673091
Author: Jonathan Wakely
Date: Fri Sep 9 21:03:58 2022
libstdc++: Rename parameter to avoid darwin __weak qualifier
libstdc++-v3/ChangeLog:
* include/bits/atomic_base.h (__atomic_impl::__compare_exchange):
Rename __weak to __is_weak.
* testsuite/17_intro/names.cc: Add __weak and __strong.
diff --git a/libstdc++-v3/include/bits/atomic_base.h
b/libstdc++-v3/include/bits/atomic_base.h
index 29315547aab..6ea3268fdf0 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -990,7 +990,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template
_GLIBCXX_ALWAYS_INLINE bool
__compare_exchange(_Tp& __val, _Val<_Tp>& __e, _Val<_Tp>& __i,
-bool __weak, memory_order __s, memory_order __f)
noexcept
+bool __is_weak,
+memory_order __s, memory_order __f) noexcept
{
__glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
@@ -1005,7 +1006,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
__atomic_impl::__clear_padding(*__exp);
if (__atomic_compare_exchange(std::__addressof(__val), __exp,
__atomic_impl::__clear_padding(__i),
- __weak, int(__s), int(__f)))
+ __is_weak, int(__s), int(__f)))
return true;
__builtin_memcpy(std::__addressof(__e), __exp, sizeof(_Vp));
return false;
@@ -1014,7 +1015,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
return __atomic_compare_exchange(std::__addressof(__val),
std::__addressof(__e),
std::__addressof(__i),
- __weak, int(__s), int(__f));
+ __is_weak, int(__s), int(__f));
}
} // namespace __atomic_impl
diff --git a/libstdc++-v3/testsuite/17_intro/names.cc
b/libstdc++-v3/testsuite/17_intro/names.cc
index ede2fe8caa7..86fb8f8999b 100644
--- a/libstdc++-v3/testsuite/17_intro/names.cc
+++ b/libstdc++-v3/testsuite/17_intro/names.cc
@@ -129,6 +129,10 @@
// This clashes with newlib so don't use it.
# define __lockablecannot be used as an identifier
+#ifndef __APPLE__
+#define __weak predefined qualifier on darwin
+#define __strong predefined qualifier on darwin
+#endif
// Common template parameter names
#define OutputIterator OutputIterator is not a reserved name
Re: Patch ping (was Re: [PATCH] libstdc++: Clear padding bits in atomic compare_exchange)
s/__weak/__is_weak/g perhaps? On Fri, Sep 9, 2022 at 11:46 AM Iain Sandoe via Libstdc++ < libstd...@gcc.gnu.org> wrote: > > > > On 9 Sep 2022, at 19:36, Rainer Orth > wrote: > > > > >> Here's a complete patch that combines the various incremental patches > >> that have been going around. I'm testing this now. > >> > >> Please take a look. > > > > unfortunately, this patch broke macOS bootstrap (seen on > > x86_64-apple-darwin11.4.2): > > > > In file included from > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/shared_ptr_atomic.h:33, > > from > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/memory:78, > > from > /vol/gcc/src/hg/master/darwin/libstdc++-v3/include/precompiled/stdc++.h:82: > > > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/atomic_base.h: > In function 'bool std::__atomic_impl::__compare_exchange(_Tp&, _Val<_Tp>&, > _Val<_Tp>&, bool, std::memory_order, std::memory_order)': > > > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/atomic_base.h:1008:49: > error: expected primary-expression before ',' token > > 1008 | __weak, int(__s), > int(__f))) > > | ^ > > > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/atomic_base.h:1017:50: > error: expected primary-expression before ',' token > > 1017 |__weak, int(__s), > int(__f)); > > | ^ > > > > Darwin gcc predefines __weak= in gcc/config/darwin-c.cc > (darwin_cpp_builtins). > > yes, __weak and __strong are Objective C things (in principle, applicable > to non-Darwin targets > using NeXT runtime - there is at least one such target). > > Iain > >
Re: Patch ping (was Re: [PATCH] libstdc++: Clear padding bits in atomic compare_exchange)
> On 9 Sep 2022, at 19:36, Rainer Orth wrote: > >> Here's a complete patch that combines the various incremental patches >> that have been going around. I'm testing this now. >> >> Please take a look. > > unfortunately, this patch broke macOS bootstrap (seen on > x86_64-apple-darwin11.4.2): > > In file included from > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/shared_ptr_atomic.h:33, > from > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/memory:78, > from > /vol/gcc/src/hg/master/darwin/libstdc++-v3/include/precompiled/stdc++.h:82: > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/atomic_base.h: > In function 'bool std::__atomic_impl::__compare_exchange(_Tp&, _Val<_Tp>&, > _Val<_Tp>&, bool, std::memory_order, std::memory_order)': > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/atomic_base.h:1008:49: > error: expected primary-expression before ',' token > 1008 | __weak, int(__s), int(__f))) > | ^ > /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/atomic_base.h:1017:50: > error: expected primary-expression before ',' token > 1017 |__weak, int(__s), int(__f)); > | ^ > > Darwin gcc predefines __weak= in gcc/config/darwin-c.cc (darwin_cpp_builtins). yes, __weak and __strong are Objective C things (in principle, applicable to non-Darwin targets using NeXT runtime - there is at least one such target). Iain
Re: Patch ping (was Re: [PATCH] libstdc++: Clear padding bits in atomic compare_exchange)
Hi Jonathan, > Here's a complete patch that combines the various incremental patches > that have been going around. I'm testing this now. > > Please take a look. unfortunately, this patch broke macOS bootstrap (seen on x86_64-apple-darwin11.4.2): In file included from /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/shared_ptr_atomic.h:33, from /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/memory:78, from /vol/gcc/src/hg/master/darwin/libstdc++-v3/include/precompiled/stdc++.h:82: /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/atomic_base.h: In function 'bool std::__atomic_impl::__compare_exchange(_Tp&, _Val<_Tp>&, _Val<_Tp>&, bool, std::memory_order, std::memory_order)': /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/atomic_base.h:1008:49: error: expected primary-expression before ',' token 1008 | __weak, int(__s), int(__f))) | ^ /var/gcc/regression/master/10.7-gcc/build/x86_64-apple-darwin11.4.2/libstdc++-v3/include/bits/atomic_base.h:1017:50: error: expected primary-expression before ',' token 1017 |__weak, int(__s), int(__f)); | ^ Darwin gcc predefines __weak= in gcc/config/darwin-c.cc (darwin_cpp_builtins). Rainer -- - Rainer Orth, Center for Biotechnology, Bielefeld University
Re: Patch ping (was Re: [PATCH] libstdc++: Clear padding bits in atomic compare_exchange)
Looks good to me. Tom. On Wed, Sep 7, 2022 at 4:56 AM Jonathan Wakely wrote: > Here's a complete patch that combines the various incremental patches > that have been going around. I'm testing this now. > > Please take a look. >
Re: Patch ping (was Re: [PATCH] libstdc++: Clear padding bits in atomic compare_exchange)
Here's a complete patch that combines the various incremental patches
that have been going around. I'm testing this now.
Please take a look.
commit 4a0a8ec5bc2a890a1568f99eace666e9f72d
Author: Thomas Rodgers
Date: Thu Aug 25 11:11:40 2022
libstdc++: Clear padding bits in atomic compare_exchange
This change implements P0528 which requires that padding bits not
participate in atomic compare exchange operations. All arguments to the
generic template are 'sanitized' by the __builtin_clear_padding intrinsic
before they are used in comparisons. This requires that any stores
also sanitize the incoming value.
Co-authored-by: Jakub Jelinek
Co-authored-by: Jonathan Wakely
Signed-off-by: Thomas Rodgers
libstdc++-v3/ChangeLog:
* include/bits/atomic_base.h (__atomic_impl::__maybe_has_padding):
New function.
(__atomic_impl::clear_padding): Likewise.
(__atomic_impl::__compare_exchange): Likewise.
(__atomic_impl::compare_exchange_weak): Delegate to
__compare_exchange.
(__atomic_impl::compare_exchange_strong): Likewise.
* include/std/atomic (atomic::atomic(T)): Clear padding when
possible in a constexpr function.
(atomic::store): Clear padding.
(atomic::exchange): Likewise.
(atomic::compare_exchange_weak): Use __compare_exchange.
(atomic::compare_exchange_strong): Likewise.
* testsuite/29_atomics/atomic/compare_exchange_padding.cc: New
test.
* testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc:
New test.
diff --git a/libstdc++-v3/include/bits/atomic_base.h
b/libstdc++-v3/include/bits/atomic_base.h
index d29e4434177..29315547aab 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -33,6 +33,7 @@
#pragma GCC system_header
#include
+#include // For placement new
#include
#include
#include
@@ -952,19 +953,76 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ return __atomic_fetch_sub(&_M_p, _M_type_size(__d), int(__m)); }
};
- /// @endcond
+ namespace __atomic_impl
+ {
+// Implementation details of atomic padding handling
+
+template
+ constexpr bool
+ __maybe_has_padding()
+ {
+#if ! __has_builtin(__builtin_clear_padding)
+ return false;
+#elif __has_builtin(__has_unique_object_representations)
+ return !__has_unique_object_representations(_Tp)
+ && !is_same<_Tp, float>::value && !is_same<_Tp, double>::value;
+#else
+ return true;
+#endif
+ }
+
+template
+ _GLIBCXX_ALWAYS_INLINE _Tp*
+ __clear_padding(_Tp& __val) noexcept
+ {
+ auto* __ptr = std::__addressof(__val);
+#if __has_builtin(__builtin_clear_padding)
+ if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Tp>())
+ __builtin_clear_padding(__ptr);
+#endif
+ return __ptr;
+ }
+
+// Remove volatile and create a non-deduced context for value arguments.
+template
+ using _Val = typename remove_volatile<_Tp>::type;
+
+template
+ _GLIBCXX_ALWAYS_INLINE bool
+ __compare_exchange(_Tp& __val, _Val<_Tp>& __e, _Val<_Tp>& __i,
+bool __weak, memory_order __s, memory_order __f)
noexcept
+ {
+ __glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
+
+ using _Vp = _Val<_Tp>;
+
+ if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Vp>())
+ {
+ // We must not modify __e on success, so cannot clear its padding.
+ // Copy into a buffer and clear that, then copy back on failure.
+ alignas(_Vp) unsigned char __buf[sizeof(_Vp)];
+ _Vp* __exp = ::new((void*)__buf) _Vp(__e);
+ __atomic_impl::__clear_padding(*__exp);
+ if (__atomic_compare_exchange(std::__addressof(__val), __exp,
+ __atomic_impl::__clear_padding(__i),
+ __weak, int(__s), int(__f)))
+ return true;
+ __builtin_memcpy(std::__addressof(__e), __exp, sizeof(_Vp));
+ return false;
+ }
+ else
+ return __atomic_compare_exchange(std::__addressof(__val),
+ std::__addressof(__e),
+ std::__addressof(__i),
+ __weak, int(__s), int(__f));
+ }
+ } // namespace __atomic_impl
#if __cplusplus > 201703L
- /// @cond undocumented
-
// Implementation details of atomic_ref and atomic.
namespace __atomic_impl
{
-// Remove volatile and create a non-deduced context for value arguments.
-template
- using _Val = remove_volatile_t<_Tp>;
-
-// As above, but for difference_type arguments.
+// Like _Val above, but for difference_type arguments.
template
Re: Patch ping (was Re: [PATCH] libstdc++: Clear padding bits in atomic compare_exchange)
Sorry for the delay in getting to this.
I am currently working on moving the bulk of the atomic wait implementation
into the .so. I'd like to get that work to a stable state before revisiting
this patch, but obviously if we want this to make it into GCC13, it needs
to happen sooner rather than later.
On Thu, Aug 25, 2022 at 3:11 AM Jakub Jelinek wrote:
> On Tue, Jan 18, 2022 at 09:48:19PM +, Jonathan Wakely via Gcc-patches
> wrote:
> > On Tue, 2 Nov 2021 at 01:26, Thomas Rodgers wrote:
> >
> > > This should address Jonathan's feedback and adds support for
> atomic_ref
> > >
> >
> >
> > >This change implements P0528 which requires that padding bits not
> > >participate in atomic compare exchange operations. All arguments to the
> > >generic template are 'sanitized' by the __builtin_clearpadding intrisic
> >
> > The name of the intrinsic and the word "instrinsic" have typos.
>
> I'd like to ping this patch.
> To make some progress, I've tried to incorporate some of Jonathan's
> review comments below, but it is incomplete.
>
> ChangeLog + wording above it fixed.
>
> > >
> > > explicit
> > > __atomic_ref(_Tp& __t) : _M_ptr(std::__addressof(__t))
> > >- { __glibcxx_assert(((uintptr_t)_M_ptr % required_alignment) ==
> 0); }
> > >+ {
> > >+ __glibcxx_assert(((uintptr_t)_M_ptr % required_alignment) == 0);
> > >+#if __cplusplus > 201402L && __has_builtin(__builtin_clear_padding)
> > >+ __builtin_clear_padding(_M_ptr);
> > >+#endif
> > >+ }
> >
> > Is this safe to do?
> >
> > What if multiple threads all create a std::atomic_ref round the same
> object
> > at once, they'll all try to clear padding, and so race, won't they?
> > I don't think we can clear padding on atomic_ref construction, only on
> > store and RMW operations.
>
> Didn't touch the above.
> >
> >
> > >--- a/libstdc++-v3/include/std/atomic
> > >+++ b/libstdc++-v3/include/std/atomic
>
> The patch against this file doesn't apply it all.
>
> > >--- /dev/null
> > >+++
> >
> b/libstdc++-v3/testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc
> > >@@ -0,0 +1,43 @@
> > >+// { dg-options "-std=gnu++2a" }
> > >+// { dg-do run { target c++2a } }
> >
> > This new test is using "2a" not "20".
>
> Fixed thus, but the other testcase wasn't in the patch at all.
>
> Here it is:
>
> libstdc++: Clear padding bits in atomic compare_exchange
>
> This change implements P0528 which requires that padding bits not
> participate in atomic compare exchange operations. All arguments to the
> generic template are 'sanitized' by the __builtin_clear_padding intrinsic
> before they are used in comparisons. This requires that any stores
> also sanitize the incoming value.
>
> Signed-off-by: Thomas Rodgers
>
> libstdc++-v3/ChangeLog:
>
> * include/std/atomic (atomic::atomic(_Tp)): Clear padding for
> __cplusplus > 201703L.
> (atomic::store()): Clear padding.
> (atomic::exchange()): Likewise.
> (atomic::compare_exchange_weak()): Likewise.
> (atomic::compare_exchange_strong()): Likewise.
> * include/bits/atomic_base.h (__atomic_impl::__clear_padding()):
> New function.
> (__atomic_impl::__maybe_has_padding()): Likewise.
> (__atomic_impl::__compare_exchange()): Likewise.
> (__atomic_impl::compare_exchange_weak()): Delegate to
> __compare_exchange().
> (__atomic_impl::compare_exchange_strong()): Likewise.
> * testsuite/29_atomics/atomic/compare_exchange_padding.cc: New
> test.
> * testsuite/28_atomics/atomic_ref/compare_exchange_padding.cc:
> Likewise.
>
> --- a/libstdc++-v3/include/bits/atomic_base.h.jj2022-05-16
> 09:46:02.361059682 +0200
> +++ b/libstdc++-v3/include/bits/atomic_base.h 2022-08-25
> 12:06:13.758883172 +0200
> @@ -954,6 +954,87 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
>/// @endcond
>
> + // Implementation details of atomic padding handling
> + namespace __atomic_impl
> + {
> +template
> + _GLIBCXX_ALWAYS_INLINE _Tp*
> + __clear_padding(_Tp& __val) noexcept
> + {
> + auto* __ptr = std::__addressof(__val);
> +#if __has_builtin(__builtin_clear_padding)
> + __builtin_clear_padding(std::__addressof(__val));
> +#endif
> + return __ptr;
> + }
> +
> +template
> + constexpr bool
> + __maybe_has_padding()
> + {
> +#if ! __has_builtin(__builtin_clear_padding)
> + return false;
> +#elif __has_builtin(__has_unique_object_representations)
> + return !__has_unique_object_representations(_Tp)
> + && !is_floating_point<_Tp>::value;
> +#else
> + return true;
> +#endif
> + }
> +
> +template
> + _GLIBCXX_ALWAYS_INLINE bool
> + __compare_exchange(_Tp& __val, _Tp& __e, _Tp& __i, bool __weak,
> +memory_order __s, memory_order __f) noexcept
> + {
> + __glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
> +
> + if _GLIBCXX17_CONSTEXPR
Patch ping (was Re: [PATCH] libstdc++: Clear padding bits in atomic compare_exchange)
On Tue, Jan 18, 2022 at 09:48:19PM +, Jonathan Wakely via Gcc-patches wrote:
> On Tue, 2 Nov 2021 at 01:26, Thomas Rodgers wrote:
>
> > This should address Jonathan's feedback and adds support for atomic_ref
> >
>
>
> >This change implements P0528 which requires that padding bits not
> >participate in atomic compare exchange operations. All arguments to the
> >generic template are 'sanitized' by the __builtin_clearpadding intrisic
>
> The name of the intrinsic and the word "instrinsic" have typos.
I'd like to ping this patch.
To make some progress, I've tried to incorporate some of Jonathan's
review comments below, but it is incomplete.
ChangeLog + wording above it fixed.
> >
> > explicit
> > __atomic_ref(_Tp& __t) : _M_ptr(std::__addressof(__t))
> >- { __glibcxx_assert(((uintptr_t)_M_ptr % required_alignment) == 0); }
> >+ {
> >+ __glibcxx_assert(((uintptr_t)_M_ptr % required_alignment) == 0);
> >+#if __cplusplus > 201402L && __has_builtin(__builtin_clear_padding)
> >+ __builtin_clear_padding(_M_ptr);
> >+#endif
> >+ }
>
> Is this safe to do?
>
> What if multiple threads all create a std::atomic_ref round the same object
> at once, they'll all try to clear padding, and so race, won't they?
> I don't think we can clear padding on atomic_ref construction, only on
> store and RMW operations.
Didn't touch the above.
>
>
> >--- a/libstdc++-v3/include/std/atomic
> >+++ b/libstdc++-v3/include/std/atomic
The patch against this file doesn't apply it all.
> >--- /dev/null
> >+++
> b/libstdc++-v3/testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc
> >@@ -0,0 +1,43 @@
> >+// { dg-options "-std=gnu++2a" }
> >+// { dg-do run { target c++2a } }
>
> This new test is using "2a" not "20".
Fixed thus, but the other testcase wasn't in the patch at all.
Here it is:
libstdc++: Clear padding bits in atomic compare_exchange
This change implements P0528 which requires that padding bits not
participate in atomic compare exchange operations. All arguments to the
generic template are 'sanitized' by the __builtin_clear_padding intrinsic
before they are used in comparisons. This requires that any stores
also sanitize the incoming value.
Signed-off-by: Thomas Rodgers
libstdc++-v3/ChangeLog:
* include/std/atomic (atomic::atomic(_Tp)): Clear padding for
__cplusplus > 201703L.
(atomic::store()): Clear padding.
(atomic::exchange()): Likewise.
(atomic::compare_exchange_weak()): Likewise.
(atomic::compare_exchange_strong()): Likewise.
* include/bits/atomic_base.h (__atomic_impl::__clear_padding()):
New function.
(__atomic_impl::__maybe_has_padding()): Likewise.
(__atomic_impl::__compare_exchange()): Likewise.
(__atomic_impl::compare_exchange_weak()): Delegate to
__compare_exchange().
(__atomic_impl::compare_exchange_strong()): Likewise.
* testsuite/29_atomics/atomic/compare_exchange_padding.cc: New
test.
* testsuite/28_atomics/atomic_ref/compare_exchange_padding.cc:
Likewise.
--- a/libstdc++-v3/include/bits/atomic_base.h.jj2022-05-16
09:46:02.361059682 +0200
+++ b/libstdc++-v3/include/bits/atomic_base.h 2022-08-25 12:06:13.758883172
+0200
@@ -954,6 +954,87 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
/// @endcond
+ // Implementation details of atomic padding handling
+ namespace __atomic_impl
+ {
+template
+ _GLIBCXX_ALWAYS_INLINE _Tp*
+ __clear_padding(_Tp& __val) noexcept
+ {
+ auto* __ptr = std::__addressof(__val);
+#if __has_builtin(__builtin_clear_padding)
+ __builtin_clear_padding(std::__addressof(__val));
+#endif
+ return __ptr;
+ }
+
+template
+ constexpr bool
+ __maybe_has_padding()
+ {
+#if ! __has_builtin(__builtin_clear_padding)
+ return false;
+#elif __has_builtin(__has_unique_object_representations)
+ return !__has_unique_object_representations(_Tp)
+ && !is_floating_point<_Tp>::value;
+#else
+ return true;
+#endif
+ }
+
+template
+ _GLIBCXX_ALWAYS_INLINE bool
+ __compare_exchange(_Tp& __val, _Tp& __e, _Tp& __i, bool __weak,
+memory_order __s, memory_order __f) noexcept
+ {
+ __glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
+
+ if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Tp>())
+ {
+ alignas(_Tp) unsigned char __buf[sizeof(_Tp)];
+ _Tp* __exp = ::new((void*)__buf) _Tp(__e);
+ __exp = __atomic_impl::__clear_padding(*__exp);
+ auto* __des = __atomic_impl::__clear_padding(__i);
+ if (__atomic_compare_exchange(std::__addressof(__val), __exp,
__des, __weak,
+ int(__s), int(__f)))
+ return true;
+ __builtin_memcpy(std::addressof(__e), __exp, sizeof(_Tp));
+ return false;
+ }
+ else
+ return
