From b2960902d114cb3440ced09ab187bcb725b7f06c Mon Sep 17 00:00:00 2001 From: Thomas Rodgers Date: Tue, 5 Oct 2021 16:17:44 -0700 Subject: [PATCH] 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_clearpadding intrisic before they are used in comparisons. This alrequires 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. --- libstdc++-v3/include/bits/atomic_base.h | 113 ++++++++++++++++-- libstdc++-v3/include/std/atomic | 73 +++-------- .../atomic_ref/compare_exchange_padding.cc | 43 +++++++ 3 files changed, 160 insertions(+), 69 deletions(-) create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc diff --git a/libstdc++-v3/include/bits/atomic_base.h b/libstdc++-v3/include/bits/atomic_base.h index 71e1de078b5..beec412761f 100644 --- a/libstdc++-v3/include/bits/atomic_base.h +++ b/libstdc++-v3/include/bits/atomic_base.h @@ -936,6 +936,89 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION { return __atomic_fetch_sub(&_M_p, _M_type_size(__d), int(__m)); } }; + // 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(__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 __has_builtin(__builtin_clear_padding) + if _GLIBCXX14_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Tp>()) + { + alignas(_Tp) unsigned char __buf[sizeof(_Tp)]; + __builtin_memcpy(__buf, std::__addressof(__e), sizeof(_Tp)); + auto* __exp = __atomic_impl::__clear_padding(*reinterpret_cast<_Tp*>(__buf)); + 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 +#endif + return __atomic_compare_exchange(std::__addressof(__val), + std::__addressof(__e), + std::__addressof(__i), + __weak, int(__s), int(__f)); + } + + template + _GLIBCXX_ALWAYS_INLINE bool + __compare_exchange(_Tp volatile& __val, _Tp& __e, _Tp& __i, bool __weak, + memory_order __s, memory_order __f) noexcept + { + __glibcxx_assert(__is_valid_cmpexch_failure_order(__f)); + +#if __has_builtin(__builtin_clear_padding) + if _GLIBCXX14_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Tp>()) + { + alignas(_Tp) unsigned char __buf[sizeof(_Tp)]; + __builtin_memcpy(__buf, std::__addressof(__e), sizeof(_Tp)); + auto* __exp = __atomic_impl::__clear_padding(*reinterpret_cast<_Tp*>(__buf)); + 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 +#endif + return __atomic_compare_exchange(std::__addressof(__val), + std::__addressof(__e), + std::__addressof(__i), + __weak, int(__s), int(__f)); + } + } // namespace __atomic_impl + #if __cplusplus > 201703L // Implementation details of atomic_ref and atomic. namespace __atomic_impl @@ -959,7 +1042,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION template _GLIBCXX_ALWAYS_INLINE void store(_Tp* __ptr, _Val<_Tp> __t, memory_order __m) noexcept - { __atomic_store(__ptr, std::__addressof(__t), int(__m)); } + { +#if __has_builtin(__builtin_clear_padding) + if _GLIBCXX14_CONSTEXPR (__maybe_has_padding<_Tp>()) + __clear_padding(__t); +#endif + __atomic_store(__ptr, std::__addressof(__t), int(__m)); + } template _GLIBCXX_ALWAYS_INLINE _Val<_Tp> @@ -977,6 +1066,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION { alignas(_Tp) unsigned char __buf[sizeof(_Tp)]; auto* __dest = reinterpret_cast<_Val<_Tp>*>(__buf); + __clear_padding(__desired); __atomic_exchange(__ptr, std::__addressof(__desired), __dest, int(__m)); return *__dest; } @@ -987,11 +1077,8 @@ _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 __compare_exchange(*__ptr, __expected, __desired, true, + __success, __failure); } template @@ -1000,11 +1087,8 @@ _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 __compare_exchange(*__ptr, __expected, __desired, false, + __success, __failure); } #if __cpp_lib_atomic_wait @@ -1376,7 +1460,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION 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 + } __atomic_ref(const __atomic_ref&) noexcept = default; diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic index 4ac9ccdc1ab..9fa7c95e7e3 100644 --- a/libstdc++-v3/include/std/atomic +++ b/libstdc++-v3/include/std/atomic @@ -228,13 +228,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION atomic& operator=(const atomic&) = delete; atomic& operator=(const atomic&) volatile = delete; -#if __cplusplus > 201703L && __has_builtin(__builtin_clear_padding) constexpr atomic(_Tp __i) noexcept : _M_i(__i) - { __builtin_clear_padding(std::__addressof(_M_i)); } -#else - constexpr atomic(_Tp __i) noexcept : _M_i(__i) - { } + { +#if __cplusplus > 201402L && __has_builtin(__builtin_clear_padding) + __builtin_clear_padding(std::__addressof(_M_i)); #endif + } operator _Tp() const noexcept { return load(); } @@ -274,18 +273,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION void store(_Tp __i, memory_order __m = memory_order_seq_cst) noexcept { -#if __has_builtin(__builtin_clear_padding) - __builtin_clear_padding(std::__addressof(__i)); -#endif + __atomic_impl::__clear_padding(__i); __atomic_store(std::__addressof(_M_i), std::__addressof(__i), int(__m)); } void store(_Tp __i, memory_order __m = memory_order_seq_cst) volatile noexcept { -#if __has_builtin(__builtin_clear_padding) - __builtin_clear_padding(std::__addressof(__i)); -#endif + __atomic_impl::__clear_padding(__i); __atomic_store(std::__addressof(_M_i), std::__addressof(__i), int(__m)); } @@ -312,9 +307,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION { alignas(_Tp) unsigned char __buf[sizeof(_Tp)]; _Tp* __ptr = reinterpret_cast<_Tp*>(__buf); -#if __has_builtin(__builtin_clear_padding) - __builtin_clear_padding(std::__addressof(__i)); -#endif + __atomic_impl::__clear_padding(__i); __atomic_exchange(std::__addressof(_M_i), std::__addressof(__i), __ptr, int(__m)); return *__ptr; @@ -326,9 +319,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION { alignas(_Tp) unsigned char __buf[sizeof(_Tp)]; _Tp* __ptr = reinterpret_cast<_Tp*>(__buf); -#if __has_builtin(__builtin_clear_padding) - __builtin_clear_padding(std::__addressof(__i)); -#endif + __atomic_impl::__clear_padding(__i); __atomic_exchange(std::__addressof(_M_i), std::__addressof(__i), __ptr, int(__m)); return *__ptr; @@ -338,32 +329,16 @@ _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)); - -#if __has_builtin(__builtin_clear_padding) - __builtin_clear_padding(std::__addressof(__e)); - __builtin_clear_padding(std::__addressof(__i)); -#endif - 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)); - -#if __has_builtin(__builtin_clear_padding) - __builtin_clear_padding(std::__addressof(__e)); - __builtin_clear_padding(std::__addressof(__i)); -#endif - 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 @@ -382,32 +357,16 @@ _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)); - -#if __has_builtin(__builtin_clear_padding) - __builtin_clear_padding(std::__addressof(__e)); - __builtin_clear_padding(std::__addressof(__i)); -#endif - 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)); - -#if __has_builtin(__builtin_clear_padding) - __builtin_clear_padding(std::__addressof(__e)); - __builtin_clear_padding(std::__addressof(__i)); -#endif - 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_ref/compare_exchange_padding.cc b/libstdc++-v3/testsuite/29_atomics/atomic_ref/compare_exchange_padding.cc new file mode 100644 index 00000000000..ffd87bc1461 --- /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 } } +// { dg-add-options libatomic } + +#include + +#include + +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 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; +} -- 2.31.1