Fix a performance regression in QDataStream

Commit e176dd7 replaced a `new
char[n]` with a std::make_unique<char[]>(n), probably on this author's
insistence.

But the two are not equivalent: make_unique() value-initializes, even
arrays, even of built-in type, which means that each buffer resize
writes each byte twice: first nulling out the whole buffer as part of
value-initialization, then in the memcpy() and any following read()s.

For buffers of several MiB, or even GiB in size, this is very costly.

Fix by adding and using a backport of C++20
make_unique_for_overwrite(), which performs the equivalent of the old
code (ie. default-, not value-initialization).

Also add q20::is_(un)bounded_array, which are needed for the
implementation of q20::make_unique_for_overwrite().

Amends e176dd7.

Pick-to: 6.8 6.5
Change-Id: I8865c7369e522ec475df122e3d00d6aba3b24561
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 1a9f8cc)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/global/q20memory.h

@@ -9,7 +9,7 @@
 
 #include <QtCore/q17memory.h>
 
-#include <type_traits>
+#include <QtCore/q20type_traits.h>
 #include <utility>
 
 //
@@ -46,6 +46,32 @@ T *construct_at(T *ptr, Args && ... args)
 #endif // __cpp_lib_constexpr_dynamic_alloc
 } // namespace q20
 
+// like std::make_unique_for_overwrite (excl. C++23-added constexpr)
+namespace q20 {
+#ifdef __cpp_lib_smart_ptr_for_overwrite
+using std::make_unique_for_overwrite;
+#else
+// https://eel.is/c++draft/unique.ptr.create#6
+template <typename T>
+std::enable_if_t<!std::is_array_v<T>, std::unique_ptr<T>>
+make_unique_for_overwrite()
+// https://eel.is/c++draft/unique.ptr.create#7
+{ return std::unique_ptr<T>(new T); }
+
+// https://eel.is/c++draft/unique.ptr.create#8
+template <typename T>
+std::enable_if_t<q20::is_unbounded_array_v<T>, std::unique_ptr<T>>
+make_unique_for_overwrite(std::size_t n)
+// https://eel.is/c++draft/unique.ptr.create#9
+{ return std::unique_ptr<T>(new std::remove_extent_t<T>[n]); }
+
+// https://eel.is/c++draft/unique.ptr.create#10
+template <typename T, typename...Args>
+std::enable_if_t<q20::is_bounded_array_v<T>>
+make_unique_for_overwrite(Args&&...) = delete;
+
+#endif // __cpp_lib_smart_ptr_for_overwrite
+} // namespace q20
 
 namespace q20 {
 // like std::to_address

src/corelib/global/q20type_traits.h

@@ -27,6 +27,23 @@
 
 QT_BEGIN_NAMESPACE
 
+namespace q20 {
+// like std::is_(un)bounded_array
+#ifdef __cpp_lib_bounded_array_traits
+using std::is_bounded_array;
+using std::is_bounded_array_v;
+using std::is_unbounded_array;
+using std::is_unbounded_array_v;
+#else
+template <typename T> struct is_bounded_array : std::false_type {};
+template <typename T, std::size_t N> struct is_bounded_array<T[N]> : std::true_type {};
+template <typename T> struct is_unbounded_array : std::false_type {};
+template <typename T> struct is_unbounded_array<T[]> : std::true_type {};
+template <typename T> constexpr inline bool is_bounded_array_v = q20::is_bounded_array<T>::value;
+template <typename T> constexpr inline bool is_unbounded_array_v = q20::is_unbounded_array<T>::value;
+#endif
+}
+
 namespace q20 {
 // like std::is_constant_evaluated
 #ifdef __cpp_lib_is_constant_evaluated

src/corelib/serialization/qdatastream.cpp

@@ -13,6 +13,8 @@
 #include <stdlib.h>
 #include "qendian.h"
 
+#include <QtCore/q20memory.h>
+
 QT_BEGIN_NAMESPACE
 
 constexpr quint32 QDataStream::NullCode;
@@ -1095,7 +1097,7 @@ QDataStream &QDataStream::readBytes(char *&s, qint64 &l)
     do {
         qsizetype blockSize = qMin(step, len - allocated);
         const qsizetype n = allocated + blockSize + 1;
-        if (const auto prevBuf = std::exchange(curBuf, std::make_unique<char[]>(n)))
+        if (const auto prevBuf = std::exchange(curBuf, q20::make_unique_for_overwrite<char[]>(n)))
             memcpy(curBuf.get(), prevBuf.get(), allocated);
         if (readBlock(curBuf.get() + allocated, blockSize) != blockSize)
             return *this;