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 4a0a8ec5bc2a890a1568f99eace6111166e9f72d
Author: Thomas Rodgers <trodg...@redhat.com>
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 <ja...@redhat.com>
Co-authored-by: Jonathan Wakely <jwak...@redhat.com>
Signed-off-by: Thomas Rodgers <trodg...@redhat.com>
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<T>::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 <bits/c++config.h>
+#include <new> // For placement new
#include <stdint.h>
#include <bits/atomic_lockfree_defines.h>
#include <bits/move.h>
@@ -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<typename _Tp>
+ 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<typename _Tp>
+ _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<typename _Tp>
+ using _Val = typename remove_volatile<_Tp>::type;
+
+ template<typename _Tp>
+ _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<floating-point>.
namespace __atomic_impl
{
- // Remove volatile and create a non-deduced context for value arguments.
- template<typename _Tp>
- using _Val = remove_volatile_t<_Tp>;
-
- // As above, but for difference_type arguments.
+ // Like _Val<T> above, but for difference_type arguments.
template<typename _Tp>
using _Diff = __conditional_t<is_pointer_v<_Tp>, ptrdiff_t, _Val<_Tp>>;
@@ -979,7 +1037,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
template<typename _Tp>
_GLIBCXX_ALWAYS_INLINE void
store(_Tp* __ptr, _Val<_Tp> __t, memory_order __m) noexcept
- { __atomic_store(__ptr, std::__addressof(__t), int(__m)); }
+ {
+ __atomic_store(__ptr, __atomic_impl::__clear_padding(__t), int(__m));
+ }
template<typename _Tp>
_GLIBCXX_ALWAYS_INLINE _Val<_Tp>
@@ -997,7 +1057,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
alignas(_Tp) unsigned char __buf[sizeof(_Tp)];
auto* __dest = reinterpret_cast<_Val<_Tp>*>(__buf);
- __atomic_exchange(__ptr, std::__addressof(__desired), __dest, int(__m));
+ __atomic_exchange(__ptr, __atomic_impl::__clear_padding(__desired),
+ __dest, int(__m));
return *__dest;
}
@@ -1007,11 +1068,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_Val<_Tp> __desired, memory_order __success,
memory_order __failure) noexcept
{
- __glibcxx_assert(__is_valid_cmpexch_failure_order(__failure));
-
- return __atomic_compare_exchange(__ptr, std::__addressof(__expected),
- std::__addressof(__desired), true,
- int(__success), int(__failure));
+ return __atomic_impl::__compare_exchange(*__ptr, __expected, __desired,
+ true, __success, __failure);
}
template<typename _Tp>
@@ -1020,11 +1078,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_Val<_Tp> __desired, memory_order __success,
memory_order __failure) noexcept
{
- __glibcxx_assert(__is_valid_cmpexch_failure_order(__failure));
-
- return __atomic_compare_exchange(__ptr, std::__addressof(__expected),
- std::__addressof(__desired), false,
- int(__success), int(__failure));
+ return __atomic_impl::__compare_exchange(*__ptr, __expected, __desired,
+ false, __success, __failure);
}
#if __cpp_lib_atomic_wait
@@ -1955,9 +2010,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_Tp** _M_ptr;
};
+#endif // C++2a
/// @endcond
-#endif // C++2a
/// @} group atomics
diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic
index 70055b8fa83..b913960336d 100644
--- a/libstdc++-v3/include/std/atomic
+++ b/libstdc++-v3/include/std/atomic
@@ -230,7 +230,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
atomic& operator=(const atomic&) = delete;
atomic& operator=(const atomic&) volatile = delete;
- constexpr atomic(_Tp __i) noexcept : _M_i(__i) { }
+ constexpr atomic(_Tp __i) noexcept : _M_i(__i)
+ {
+#if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
+ if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Tp>())
+ __builtin_clear_padding(std::__addressof(_M_i));
+#endif
+ }
operator _Tp() const noexcept
{ return load(); }
@@ -270,13 +276,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
void
store(_Tp __i, memory_order __m = memory_order_seq_cst) noexcept
{
- __atomic_store(std::__addressof(_M_i), std::__addressof(__i), int(__m));
+ __atomic_store(std::__addressof(_M_i),
+ __atomic_impl::__clear_padding(__i),
+ int(__m));
}
void
store(_Tp __i, memory_order __m = memory_order_seq_cst) volatile noexcept
{
- __atomic_store(std::__addressof(_M_i), std::__addressof(__i), int(__m));
+ __atomic_store(std::__addressof(_M_i),
+ __atomic_impl::__clear_padding(__i),
+ int(__m));
}
_Tp
@@ -302,7 +312,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
alignas(_Tp) unsigned char __buf[sizeof(_Tp)];
_Tp* __ptr = reinterpret_cast<_Tp*>(__buf);
- __atomic_exchange(std::__addressof(_M_i), std::__addressof(__i),
+ __atomic_exchange(std::__addressof(_M_i),
+ __atomic_impl::__clear_padding(__i),
__ptr, int(__m));
return *__ptr;
}
@@ -313,7 +324,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
alignas(_Tp) unsigned char __buf[sizeof(_Tp)];
_Tp* __ptr = reinterpret_cast<_Tp*>(__buf);
- __atomic_exchange(std::__addressof(_M_i), std::__addressof(__i),
+ __atomic_exchange(std::__addressof(_M_i),
+ __atomic_impl::__clear_padding(__i),
__ptr, int(__m));
return *__ptr;
}
@@ -322,24 +334,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
compare_exchange_weak(_Tp& __e, _Tp __i, memory_order __s,
memory_order __f) noexcept
{
- __glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
-
- return __atomic_compare_exchange(std::__addressof(_M_i),
- std::__addressof(__e),
- std::__addressof(__i),
- true, int(__s), int(__f));
+ return __atomic_impl::__compare_exchange(_M_i, __e, __i, true,
+ __s, __f);
}
bool
compare_exchange_weak(_Tp& __e, _Tp __i, memory_order __s,
memory_order __f) volatile noexcept
{
- __glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
-
- return __atomic_compare_exchange(std::__addressof(_M_i),
- std::__addressof(__e),
- std::__addressof(__i),
- true, int(__s), int(__f));
+ return __atomic_impl::__compare_exchange(_M_i, __e, __i, true,
+ __s, __f);
}
bool
@@ -358,24 +362,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
compare_exchange_strong(_Tp& __e, _Tp __i, memory_order __s,
memory_order __f) noexcept
{
- __glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
-
- return __atomic_compare_exchange(std::__addressof(_M_i),
- std::__addressof(__e),
- std::__addressof(__i),
- false, int(__s), int(__f));
+ return __atomic_impl::__compare_exchange(_M_i, __e, __i, false,
+ __s, __f);
}
bool
compare_exchange_strong(_Tp& __e, _Tp __i, memory_order __s,
memory_order __f) volatile noexcept
{
- __glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
-
- return __atomic_compare_exchange(std::__addressof(_M_i),
- std::__addressof(__e),
- std::__addressof(__i),
- false, int(__s), int(__f));
+ return __atomic_impl::__compare_exchange(_M_i, __e, __i, false,
+ __s, __f);
}
bool
diff --git
a/libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc
b/libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc
new file mode 100644
index 00000000000..c4ab876db2a
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc
@@ -0,0 +1,42 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do run { target c++20 } }
+// { dg-add-options libatomic }
+
+#include <atomic>
+
+#include <testsuite_hooks.h>
+
+struct S { char c; short s; };
+
+void __attribute__((noinline,noipa))
+fill_struct(S& s)
+{ __builtin_memset(&s, 0xff, sizeof(S)); }
+
+bool
+compare_struct(const S& a, const S& b)
+{ return __builtin_memcmp(&a, &b, sizeof(S)) == 0; }
+
+int
+main ()
+{
+ S s;
+ fill_struct(s);
+ s.c = 'a';
+ s.s = 42;
+
+ std::atomic<S> as{ s };
+ auto ts = as.load();
+ VERIFY( !compare_struct(s, ts) ); // padding cleared on construction
+ as.exchange(s);
+ auto es = as.load();
+ VERIFY( compare_struct(ts, es) ); // padding cleared on exchange
+
+ S n;
+ fill_struct(n);
+ n.c = 'b';
+ n.s = 71;
+ // padding cleared on compexchg
+ VERIFY( as.compare_exchange_weak(s, n) );
+ VERIFY( as.compare_exchange_strong(n, s) );
+ return 0;
+}
diff --git
a/libstdc++-v3/testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc
b/libstdc++-v3/testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc
new file mode 100644
index 00000000000..1b1a12dddda
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc
@@ -0,0 +1,43 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do run { target c++20 } }
+// { dg-add-options libatomic }
+
+#include <atomic>
+
+#include <testsuite_hooks.h>
+
+struct S { char c; short s; };
+
+void __attribute__((noinline,noipa))
+fill_struct(S& s)
+{ __builtin_memset(&s, 0xff, sizeof(S)); }
+
+bool
+compare_struct(const S& a, const S& b)
+{ return __builtin_memcmp(&a, &b, sizeof(S)) == 0; }
+
+int
+main ()
+{
+ S s;
+ fill_struct(s);
+ s.c = 'a';
+ s.s = 42;
+
+ S ss{ s };
+ std::atomic_ref<S> as{ s };
+ auto ts = as.load();
+ VERIFY( !compare_struct(ss, ts) ); // padding cleared on construction
+ as.exchange(ss);
+ auto es = as.load();
+ VERIFY( compare_struct(ts, es) ); // padding cleared on exchange
+
+ S n;
+ fill_struct(n);
+ n.c = 'b';
+ n.s = 71;
+ // padding cleared on compexchg
+ VERIFY( as.compare_exchange_weak(s, n) );
+ VERIFY( as.compare_exchange_strong(n, s) );
+ return 0;
+}
