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[committed] libstdc++: Fix trivial relocation for constexpr std::vector

Message ID 20211126230843.1843602-1-jwakely@redhat.com
State Committed
Commit 33adfd0d42e54c809b0c53212abe66e8c874b2f8
Headers show
Series [committed] libstdc++: Fix trivial relocation for constexpr std::vector | expand

Commit Message

Jonathan Wakely Nov. 26, 2021, 11:08 p.m. UTC
Tested x86_64-linux, pushed to trunk.


When implementing constexpr std::vector I added a check for constant
evaluation in vector::_S_use_relocate(), so that we would not try to relocate
trivial objects by using memmove. But I put it in the constexpr function
that decides whether to relocate or not, and calls to that function are
always constant evaluated. This had the effect of disabling relocation
entirely, even in non-constexpr vectors.

This removes the check in _S_use_relocate() and modifies the actual
relocation algorithm, __relocate_a_1, to use the non-trivial
implementation instead of memmove when called during constant
evaluation.

libstdc++-v3/ChangeLog:

	* include/bits/stl_uninitialized.h (__relocate_a_1): Do not use
	memmove during constant evaluation.
	* include/bits/stl_vector.h (vector::_S_use_relocate()): Do not
	check is_constant_evaluated in always-constexpr function.
---
 libstdc++-v3/include/bits/stl_uninitialized.h | 45 ++++++++++++-------
 libstdc++-v3/include/bits/stl_vector.h        |  4 --
 2 files changed, 30 insertions(+), 19 deletions(-)
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Patch

diff --git a/libstdc++-v3/include/bits/stl_uninitialized.h b/libstdc++-v3/include/bits/stl_uninitialized.h
index 673f6a07675..f2786c5a461 100644
--- a/libstdc++-v3/include/bits/stl_uninitialized.h
+++ b/libstdc++-v3/include/bits/stl_uninitialized.h
@@ -1051,6 +1051,7 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
   /// @cond undocumented
 
   template<typename _Tp, typename _Up, typename _Allocator>
+    _GLIBCXX20_CONSTEXPR
     inline void
     __relocate_object_a(_Tp* __restrict __dest, _Up* __restrict __orig,
 			_Allocator& __alloc)
@@ -1070,18 +1071,6 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct __is_bitwise_relocatable
     : is_trivial<_Tp> { };
 
-  template <typename _Tp, typename _Up>
-    _GLIBCXX20_CONSTEXPR
-    inline __enable_if_t<std::__is_bitwise_relocatable<_Tp>::value, _Tp*>
-    __relocate_a_1(_Tp* __first, _Tp* __last,
-		   _Tp* __result, allocator<_Up>&) noexcept
-    {
-      ptrdiff_t __count = __last - __first;
-      if (__count > 0)
-	__builtin_memmove(__result, __first, __count * sizeof(_Tp));
-      return __result + __count;
-    }
-
   template <typename _InputIterator, typename _ForwardIterator,
 	    typename _Allocator>
     _GLIBCXX20_CONSTEXPR
@@ -1105,6 +1094,32 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return __cur;
     }
 
+  template <typename _Tp, typename _Up>
+    _GLIBCXX20_CONSTEXPR
+    inline __enable_if_t<std::__is_bitwise_relocatable<_Tp>::value, _Tp*>
+    __relocate_a_1(_Tp* __first, _Tp* __last,
+		   _Tp* __result,
+		   [[__maybe_unused__]] allocator<_Up>& __alloc) noexcept
+    {
+      ptrdiff_t __count = __last - __first;
+      if (__count > 0)
+	{
+#ifdef __cpp_lib_is_constant_evaluated
+	  if (std::is_constant_evaluated())
+	    {
+	      // Can't use memmove. Wrap the pointer so that __relocate_a_1
+	      // resolves to the non-trivial overload above.
+	      __gnu_cxx::__normal_iterator<_Tp*, void> __out(__result);
+	      __out = std::__relocate_a_1(__first, __last, __out, __alloc);
+	      return __out.base();
+	    }
+#endif
+	  __builtin_memmove(__result, __first, __count * sizeof(_Tp));
+	}
+      return __result + __count;
+    }
+
+
   template <typename _InputIterator, typename _ForwardIterator,
 	    typename _Allocator>
     _GLIBCXX20_CONSTEXPR
@@ -1115,9 +1130,9 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
 				     std::__niter_base(__last),
 				     std::__niter_base(__result), __alloc)))
     {
-      return __relocate_a_1(std::__niter_base(__first),
-			    std::__niter_base(__last),
-			    std::__niter_base(__result), __alloc);
+      return std::__relocate_a_1(std::__niter_base(__first),
+				 std::__niter_base(__last),
+				 std::__niter_base(__result), __alloc);
     }
 
   /// @endcond
diff --git a/libstdc++-v3/include/bits/stl_vector.h b/libstdc++-v3/include/bits/stl_vector.h
index bd15b6bd8a8..4587757637e 100644
--- a/libstdc++-v3/include/bits/stl_vector.h
+++ b/libstdc++-v3/include/bits/stl_vector.h
@@ -475,10 +475,6 @@  _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       static constexpr bool
       _S_use_relocate()
       {
-#if __cplusplus >= 202002L && __has_builtin(__builtin_is_constant_evaluated)
-	if (__builtin_is_constant_evaluated())
-	  return false; // Cannot use memcpy in constant evaluation contexts.
-#endif
 	// Instantiating std::__relocate_a might cause an error outside the
 	// immediate context (in __relocate_object_a's noexcept-specifier),
 	// so only do it if we know the type can be move-inserted into *this.