[PATCH] D48342: [libcxx] Optimize vectors construction of trivial types from an iterator range with const-ness mismatch.

[Volodymyr Sapsai via Phabricator via cfe-commits]

vsapsai created this revision.
vsapsai added reviewers: mclow.lists, EricWF.
Herald added a subscriber: christof.

We already have a specialization that will use memcpy for construction
of trivial types from an iterator range like

  std::vector<int>(int *, int *);

But if we have const-ness mismatch like

  std::vector<int>(const int *, const int *);

we would use a slow path that copies each element individually. This change
enables the optimal specialization for const-ness mismatch. Fixes PR37574.

rdar://problem/40485845


https://reviews.llvm.org/D48342

Files:
  libcxx/include/memory
  
libcxx/test/std/containers/sequences/vector/vector.cons/construct_iter_iter_different_value_type.pass.cpp


Index: libcxx/test/std/containers/sequences/vector/vector.cons/construct_iter_iter_different_value_type.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/containers/sequences/vector/vector.cons/construct_iter_iter_different_value_type.pass.cpp
@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <vector>
+
+// template <class InputIter> vector(InputIter first, InputIter last);
+
+// Initialize a vector with a different value type. Make sure initialization
+// is performed with each element value, not with a memory blob.
+
+#include <vector>
+#include <cassert>
+#include <cfloat>
+#include <cmath>
+
+int main()
+{
+    int array[3] = {0, 1, 2};
+    std::vector<float> v(array, array + 3);
+    assert(std::fabs(v[0] - 0.0f) < FLT_EPSILON);
+    assert(std::fabs(v[1] - 1.0f) < FLT_EPSILON);
+    assert(std::fabs(v[2] - 2.0f) < FLT_EPSILON);
+}
Index: libcxx/include/memory
===================================================================
--- libcxx/include/memory
+++ libcxx/include/memory
@@ -1399,6 +1399,13 @@
 {
 };
 
+template <class _Alloc, class _Pointer, class ..._Args>
+struct __has_construct_missing
+    : integral_constant<bool,
+        !__has_construct<_Alloc, _Pointer, _Args...>::value>
+{
+};
+
 template <class _Alloc, class _Pointer>
 auto
 __has_destroy_test(_Alloc&& __a, _Pointer&& __p)
@@ -1467,14 +1474,26 @@
 {
 };
 
+template <class _Alloc, class _Pointer, class ..._Args>
+struct __has_construct_missing
+    : false_type
+{
+};
+
 #else  // _LIBCPP_HAS_NO_VARIADICS
 
 template <class _Alloc, class _Pointer, class _Args>
 struct __has_construct
     : false_type
 {
 };
 
+template <class _Alloc, class _Pointer, class _Args>
+struct __has_construct_missing
+    : false_type
+{
+};
+
 #endif  // _LIBCPP_HAS_NO_VARIADICS
 
 template <class _Alloc, class _Pointer>
@@ -1622,7 +1641,7 @@
         typename enable_if
         <
             (is_same<allocator_type, allocator<_Tp> >::value
-                || !__has_construct<allocator_type, _Tp*, _Tp>::value) &&
+                || __has_construct_missing<allocator_type, _Tp*, _Tp>::value) &&
              is_trivially_move_constructible<_Tp>::value,
             void
         >::type
@@ -1646,23 +1665,27 @@
                 construct(__a, _VSTD::__to_raw_pointer(__begin2), *__begin1);
         }
 
-    template <class _Tp>
+    template <class _SourceTp, class _DestTp>
         _LIBCPP_INLINE_VISIBILITY
         static
         typename enable_if
         <
-            (is_same<allocator_type, allocator<_Tp> >::value
-                || !__has_construct<allocator_type, _Tp*, _Tp>::value) &&
-             is_trivially_move_constructible<_Tp>::value,
+            (is_same
+             <
+                typename _VSTD::remove_const<typename allocator_type::value_type>::type,
+                typename _VSTD::remove_const<_SourceTp>::type
+             >::value
+                || __has_construct_missing<allocator_type, _DestTp*, _SourceTp>::value) &&
+             is_trivially_move_constructible<_DestTp>::value,
             void
         >::type
-        __construct_range_forward(allocator_type&, _Tp* __begin1, _Tp* __end1, _Tp*& __begin2)
+        __construct_range_forward(allocator_type&, _SourceTp* __begin1, _SourceTp* __end1, _DestTp*& __begin2)
         {
-            typedef typename remove_const<_Tp>::type _Vp;
+            typedef typename remove_const<_DestTp>::type _Vp;
             ptrdiff_t _Np = __end1 - __begin1;
             if (_Np > 0)
             {
-                _VSTD::memcpy(const_cast<_Vp*>(__begin2), __begin1, _Np * sizeof(_Tp));
+                _VSTD::memcpy(const_cast<_Vp*>(__begin2), __begin1, _Np * sizeof(_DestTp));
                 __begin2 += _Np;
             }
         }
@@ -1686,7 +1709,7 @@
         typename enable_if
         <
             (is_same<allocator_type, allocator<_Tp> >::value
-                || !__has_construct<allocator_type, _Tp*, _Tp>::value) &&
+                || __has_construct_missing<allocator_type, _Tp*, _Tp>::value) &&
              is_trivially_move_constructible<_Tp>::value,
             void
         >::type

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