[committed] libstdc++: Make std::assume_aligned a constexpr function [PR 97132]
Jonathan Wakely
jwakely@redhat.com
Mon Sep 21 13:30:33 GMT 2020
The cast from void* to T* in std::assume_aligned is not valid in a
constexpr function. The optimization hint is redundant during constant
evaluation anyway (the compiler can see the object and knows its
alignment). Simply return the original pointer without applying the
__builtin_assume_aligned hint to it when doing constant evaluation.
This change also removes the preprocessor branch that works around
uintptr_t not being available. We already assume that type is present
elsewhere in the library.
libstdc++-v3/ChangeLog:
PR libstdc++/97132
* include/bits/align.h (align) [!_GLIBCXX_USE_C99_STDINT_TR1]:
Remove unused code.
(assume_aligned): Do not use __builtin_assume_aligned during
constant evaluation.
* testsuite/20_util/assume_aligned/1.cc: Improve test.
* testsuite/20_util/assume_aligned/97132.cc: New test.
Tested powerpc64le-linux. Committed to trunk.
This should be backported to gcc-9 and gcc-10 too.
-------------- next part --------------
commit f10ed928e2f8ecc2c859abff8f2f9296b11b8d95
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Mon Sep 21 14:28:58 2020
libstdc++: Make std::assume_aligned a constexpr function [PR 97132]
The cast from void* to T* in std::assume_aligned is not valid in a
constexpr function. The optimization hint is redundant during constant
evaluation anyway (the compiler can see the object and knows its
alignment). Simply return the original pointer without applying the
__builtin_assume_aligned hint to it when doing constant evaluation.
This change also removes the preprocessor branch that works around
uintptr_t not being available. We already assume that type is present
elsewhere in the library.
libstdc++-v3/ChangeLog:
PR libstdc++/97132
* include/bits/align.h (align) [!_GLIBCXX_USE_C99_STDINT_TR1]:
Remove unused code.
(assume_aligned): Do not use __builtin_assume_aligned during
constant evaluation.
* testsuite/20_util/assume_aligned/1.cc: Improve test.
* testsuite/20_util/assume_aligned/97132.cc: New test.
diff --git a/libstdc++-v3/include/bits/align.h b/libstdc++-v3/include/bits/align.h
index c3267f22934..faa92bec2f8 100644
--- a/libstdc++-v3/include/bits/align.h
+++ b/libstdc++-v3/include/bits/align.h
@@ -41,7 +41,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
/**
* @brief Fit aligned storage in buffer.
- * @ingroup memory
*
* This function tries to fit @a __size bytes of storage with alignment
* @a __align into the buffer @a __ptr of size @a __space bytes. If such
@@ -56,18 +55,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
* @param __space Size of the buffer pointed to by @a __ptr.
* @return the updated pointer if the aligned storage fits, otherwise nullptr.
*
+ * @ingroup memory
*/
inline void*
align(size_t __align, size_t __size, void*& __ptr, size_t& __space) noexcept
{
-#ifdef _GLIBCXX_USE_C99_STDINT_TR1
const auto __intptr = reinterpret_cast<uintptr_t>(__ptr);
-#else
- // Cannot use std::uintptr_t so assume that std::size_t can be used instead.
- static_assert(sizeof(size_t) >= sizeof(void*),
- "std::size_t must be a suitable substitute for std::uintptr_t");
- const auto __intptr = reinterpret_cast<unsigned long long>(__ptr);
-#endif
const auto __aligned = (__intptr - 1u + __align) & -__align;
const auto __diff = __aligned - __intptr;
if ((__size + __diff) > __space)
@@ -86,15 +79,26 @@ align(size_t __align, size_t __size, void*& __ptr, size_t& __space) noexcept
* @tparam _Align An alignment value (i.e. a power of two)
* @tparam _Tp An object type
* @param __ptr A pointer that is aligned to _Align
+ *
+ * C++20 20.10.6 [ptr.align]
+ *
* @ingroup memory
*/
template<size_t _Align, class _Tp>
[[nodiscard,__gnu__::__always_inline__]]
- constexpr _Tp* assume_aligned(_Tp* __ptr)
+ constexpr _Tp*
+ assume_aligned(_Tp* __ptr) noexcept
{
static_assert(std::has_single_bit(_Align));
- _GLIBCXX_DEBUG_ASSERT((std::uintptr_t)__ptr % _Align == 0);
- return static_cast<_Tp*>(__builtin_assume_aligned(__ptr, _Align));
+ if (std::is_constant_evaluated())
+ return __ptr;
+ else
+ {
+ // This function is expected to be used in hot code, where
+ // __glibcxx_assert would add unwanted overhead.
+ _GLIBCXX_DEBUG_ASSERT((uintptr_t)__ptr % _Align == 0);
+ return static_cast<_Tp*>(__builtin_assume_aligned(__ptr, _Align));
+ }
}
#endif // C++2a
diff --git a/libstdc++-v3/testsuite/20_util/assume_aligned/1.cc b/libstdc++-v3/testsuite/20_util/assume_aligned/1.cc
index 1a34cc4bc63..16bf22caefe 100644
--- a/libstdc++-v3/testsuite/20_util/assume_aligned/1.cc
+++ b/libstdc++-v3/testsuite/20_util/assume_aligned/1.cc
@@ -15,7 +15,7 @@
// with this library; see the file COPYING3. If not see
// <http://www.gnu.org/licenses/>.
-// { dg-options "-std=gnu++2a" }
+// { dg-options "-std=gnu++2a -O2" }
// { dg-do run { target c++2a } }
#include <memory>
@@ -29,7 +29,28 @@ test01()
VERIFY( p == &i );
}
+[[gnu::noipa,gnu::noinline]]
+int*
+create_aligned(std::size_t alignment, void* p, std::size_t n)
+{
+ return ::new(std::align(alignment, sizeof(int), p, n)) int(42);
+}
+
+extern "C" void undefined(); // call to this should be optimized away
+
+void
+test02()
+{
+ unsigned char buf[sizeof(int) * 128];
+ int* p = create_aligned(64, buf + 1, sizeof(buf) - 1);
+ int* q = std::assume_aligned<64>(p);
+ if ((std::uintptr_t)q % 64)
+ undefined();
+ VERIFY( p == q );
+}
+
int main()
{
test01();
+ test02();
}
diff --git a/libstdc++-v3/testsuite/20_util/assume_aligned/97132.cc b/libstdc++-v3/testsuite/20_util/assume_aligned/97132.cc
new file mode 100644
index 00000000000..887e3470651
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/assume_aligned/97132.cc
@@ -0,0 +1,34 @@
+// Copyright (C) 2020 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3. If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <memory>
+
+// PR libstdc++/97132 - assume_aligned is not constexpr
+
+constexpr bool test01()
+{
+ struct alignas(32) S { int i; };
+ S s{42};
+ int* p = std::assume_aligned<32>(&s.i);
+ *p = 48;
+ return s.i == 48;
+}
+
+static_assert( test01() );
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