[PATCH] libstdc++: Clear padding bits in atomic compare_exchange

[Thomas Rodgers <rodgert@appliantology.com>]

From: Thomas Rodgers <rodgert@twrodgers.com>

Now with checks for __has_builtin(__builtin_clear_padding)

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 <trodgers@redhat.com>

libstdc++=v3/ChangeLog:

	* include/std/atomic (atomic<T>::atomic(_Tp) clear padding for
	__cplusplus > 201703L.
	(atomic<T>::store()) Clear padding.
	(atomic<T>::exchange()) Likewise.
	(atomic<T>::compare_exchange_weak()) Likewise.
	(atomic<T>::compare_exchange_strong()) Likewise.
	* testsuite/29_atomics/atomic/compare_exchange_padding.cc: New
	test.
---
 libstdc++-v3/include/std/atomic               | 41 +++++++++++++++++-
 .../atomic/compare_exchange_padding.cc        | 42 +++++++++++++++++++
 2 files changed, 81 insertions(+), 2 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc

diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic
index 936dd50ba1c..4ac9ccdc1ab 100644
--- a/libstdc++-v3/include/std/atomic
+++ b/libstdc++-v3/include/std/atomic
@@ -228,7 +228,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       atomic& operator=(const atomic&) = delete;
       atomic& operator=(const atomic&) volatile = delete;
 
-      constexpr atomic(_Tp __i) noexcept : _M_i(__i) { }
+#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)
+      { }
+#endif
 
       operator _Tp() const noexcept
       { return load(); }
@@ -268,12 +274,18 @@ _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_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_store(std::__addressof(_M_i), std::__addressof(__i), int(__m));
       }
 
@@ -300,6 +312,9 @@ _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_exchange(std::__addressof(_M_i), std::__addressof(__i),
 			  __ptr, int(__m));
 	return *__ptr;
@@ -311,6 +326,9 @@ _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_exchange(std::__addressof(_M_i), std::__addressof(__i),
 			  __ptr, int(__m));
 	return *__ptr;
@@ -322,6 +340,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	__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),
@@ -334,6 +356,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	__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),
@@ -358,6 +384,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	__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),
@@ -370,6 +400,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	__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),
@@ -392,6 +426,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     void
     wait(_Tp __old, memory_order __m = memory_order_seq_cst) const noexcept
     {
+#if __has_builtin(__builtin_clear_padding)
+      __builtin_clear_padding(std::__addressof(__old));
+#endif
       std::__atomic_wait_address_v(&_M_i, __old,
 			 [__m, this] { return this->load(__m); });
     }
@@ -407,7 +444,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { std::__atomic_notify_address(&_M_i, true); }
 #endif // __cpp_lib_atomic_wait 
 
-    };
+   };
 #undef _GLIBCXX20_INIT
 
   /// Partial specialization for pointer types.
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..0875f168097
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc
@@ -0,0 +1,42 @@
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+// { 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;
+}
-- 
2.31.1