diff mbox series

libstdc++: Clear padding bits in atomic compare_exchange

Message ID 20210923180837.633173-1-rodgert@appliantology.com
State New
Headers show
Series libstdc++: Clear padding bits in atomic compare_exchange | expand

Commit Message

Thomas Rodgers Sept. 23, 2021, 6:08 p.m. UTC 
From: Thomas Rodgers <rodgert@twrodgers.com>

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 atomic compare_exchange. 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               | 23 +++++++++-
 .../atomic/compare_exchange_padding.cc        | 42 +++++++++++++++++++
 2 files changed, 63 insertions(+), 2 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic/compare_exchange_padding.cc

Comments

Jakub Jelinek Sept. 23, 2021, 7:07 p.m. UTC | #1 
On Thu, Sep 23, 2021 at 11:08:37AM -0700, Thomas Rodgers wrote:
> From: Thomas Rodgers <rodgert@twrodgers.com>
> 
> This change implements P0528 which requires that padding bits not
> participate in atomic compare exchange operations. All arguments to the

Thanks for working on this.

> generic template are 'sanitized' by the __builtin_clear_padding intrinsic
> before they are used in atomic compare_exchange. This alrequires that any
> stores also sanitize the incoming value.

Not a review, just random nit.
Shouldn't the __builtin_clear_padding calls be guarded with
#if __has_builtin(__builtin_clear_padding)
or #ifdef _GLIBCXX_HAVE_BUILTIN_CLEAR_PADDING defined based on that?
I think clang doesn't support it (yet?), and it doesn't support the MSVC
__builtin_zero_non_value_bits (with very similar, but slightly different,
behavior).

> 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.

	Jakub
Thomas Rodgers Sept. 23, 2021, 8:15 p.m. UTC | #2 
Agreed, I'll revise the patch to do so.

On Thu, Sep 23, 2021 at 12:07 PM Jakub Jelinek <jakub@redhat.com> wrote:

> On Thu, Sep 23, 2021 at 11:08:37AM -0700, Thomas Rodgers wrote:
> > From: Thomas Rodgers <rodgert@twrodgers.com>
> >
> > This change implements P0528 which requires that padding bits not
> > participate in atomic compare exchange operations. All arguments to the
>
> Thanks for working on this.
>
> > generic template are 'sanitized' by the __builtin_clear_padding intrinsic
> > before they are used in atomic compare_exchange. This alrequires that any
> > stores also sanitize the incoming value.
>
> Not a review, just random nit.
> Shouldn't the __builtin_clear_padding calls be guarded with
> #if __has_builtin(__builtin_clear_padding)
> or #ifdef _GLIBCXX_HAVE_BUILTIN_CLEAR_PADDING defined based on that?
> I think clang doesn't support it (yet?), and it doesn't support the MSVC
> __builtin_zero_non_value_bits (with very similar, but slightly different,
> behavior).
>
> > 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.
>
>         Jakub
>
>
Jonathan Wakely Sept. 23, 2021, 8:15 p.m. UTC | #3 
On Thu, 23 Sep 2021, 20:07 Jakub Jelinek via Libstdc++, <
libstdc++@gcc.gnu.org> wrote:

> On Thu, Sep 23, 2021 at 11:08:37AM -0700, Thomas Rodgers wrote:
> > From: Thomas Rodgers <rodgert@twrodgers.com>
> >
> > This change implements P0528 which requires that padding bits not
> > participate in atomic compare exchange operations. All arguments to the
>
> Thanks for working on this.
>
> > generic template are 'sanitized' by the __builtin_clear_padding intrinsic
> > before they are used in atomic compare_exchange. This alrequires that any
> > stores also sanitize the incoming value.
>
> Not a review, just random nit.
> Shouldn't the __builtin_clear_padding calls be guarded with
> #if __has_builtin(__builtin_clear_padding)
> or #ifdef _GLIBCXX_HAVE_BUILTIN_CLEAR_PADDING defined based on that?
>

Yes. It can just use __has_builtin directly. All the compilers we care
about support that now.



I think clang doesn't support it (yet?), and it doesn't support the MSVC
> __builtin_zero_non_value_bits (with very similar, but slightly different,
> behavior).
>
> > 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.
>
>         Jakub
>
>
diff mbox series

Patch

diff --git a/libstdc++-v3/include/std/atomic b/libstdc++-v3/include/std/atomic
index 936dd50ba1c..51450badace 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
+      constexpr atomic(_Tp __i) noexcept : _M_i(__i)
+      { __builtin_clear_padding(std::__addressof(_M_i)); }
+#else
+      atomic(_Tp __i) noexcept : _M_i(__i)
+      { }
+#endif
 
       operator _Tp() const noexcept
       { return load(); }
@@ -268,12 +274,14 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       void
       store(_Tp __i, memory_order __m = memory_order_seq_cst) noexcept
       {
+	__builtin_clear_padding(std::__addressof(__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
       {
+	__builtin_clear_padding(std::__addressof(__i));
 	__atomic_store(std::__addressof(_M_i), std::__addressof(__i), int(__m));
       }
 
@@ -300,6 +308,7 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
         alignas(_Tp) unsigned char __buf[sizeof(_Tp)];
 	_Tp* __ptr = reinterpret_cast<_Tp*>(__buf);
+	__builtin_clear_padding(std::__addressof(__i));
 	__atomic_exchange(std::__addressof(_M_i), std::__addressof(__i),
 			  __ptr, int(__m));
 	return *__ptr;
@@ -311,6 +320,7 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
         alignas(_Tp) unsigned char __buf[sizeof(_Tp)];
 	_Tp* __ptr = reinterpret_cast<_Tp*>(__buf);
+	__builtin_clear_padding(std::__addressof(__i));
 	__atomic_exchange(std::__addressof(_M_i), std::__addressof(__i),
 			  __ptr, int(__m));
 	return *__ptr;
@@ -322,6 +332,8 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	__glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
 
+	__builtin_clear_padding(std::__addressof(__e));
+	__builtin_clear_padding(std::__addressof(__i));
 	return __atomic_compare_exchange(std::__addressof(_M_i),
 					 std::__addressof(__e),
 					 std::__addressof(__i),
@@ -334,6 +346,8 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	__glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
 
+	__builtin_clear_padding(std::__addressof(__e));
+	__builtin_clear_padding(std::__addressof(__i));
 	return __atomic_compare_exchange(std::__addressof(_M_i),
 					 std::__addressof(__e),
 					 std::__addressof(__i),
@@ -358,6 +372,8 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	__glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
 
+	__builtin_clear_padding(std::__addressof(__e));
+	__builtin_clear_padding(std::__addressof(__i));
 	return __atomic_compare_exchange(std::__addressof(_M_i),
 					 std::__addressof(__e),
 					 std::__addressof(__i),
@@ -370,6 +386,8 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	__glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
 
+	__builtin_clear_padding(std::__addressof(__e));
+	__builtin_clear_padding(std::__addressof(__i));
 	return __atomic_compare_exchange(std::__addressof(_M_i),
 					 std::__addressof(__e),
 					 std::__addressof(__i),
@@ -392,6 +410,7 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
     void
     wait(_Tp __old, memory_order __m = memory_order_seq_cst) const noexcept
     {
+      __builtin_clear_padding(std::__addressof(__old));
       std::__atomic_wait_address_v(&_M_i, __old,
 			 [__m, this] { return this->load(__m); });
     }
@@ -407,7 +426,7 @@  _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;
+}