Utils: Add SmallString

SmallString is a very simple utf8 string class. It's purpose is performance.
It uses a short string opimization which provides 31 bytes of heap free
memory to save a 30 bytes long string plus null terminator. If the string
gets larger heap is allocated. The grow strategy is 1.5 to improve reuse
of allocated memory.

It uses optionally constexpr to provide string literals.

Change-Id: I7757fb51abfeca200d074cbfce2f1d99edc0ecb0
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/libs/utils/smallstringlayout.h

@@ -0,0 +1,140 @@
+/****************************************************************************
+**
+** Copyright (C) 2016 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#pragma once
+
+#include <QtGlobal>
+
+#include <cstdint>
+
+#pragma push_macro("constexpr")
+#ifndef __cpp_constexpr
+#define constexpr
+#endif
+
+#pragma push_macro("noexcept")
+#ifndef __cpp_noexcept
+#define noexcept
+#endif
+
+#ifdef __cpp_alignas
+#define ALIGNAS_16 alignas(16)
+#else
+#define ALIGNAS_16
+#endif
+
+namespace Utils {
+
+namespace Internal {
+
+using size_type = std::size_t;
+
+static const int smallStringLayoutByteSize = 32;
+static const int maximumShortStringDataAreaSize = smallStringLayoutByteSize - 1;
+
+struct AllocatedLayout {
+    struct Data {
+        char *pointer;
+        size_type size;
+        size_type capacity;
+    } data;
+    char dummy[maximumShortStringDataAreaSize - sizeof(Data)];
+    std::uint8_t isReference : 1;
+    std::uint8_t shortStringSize: 6;
+    std::uint8_t hasAllocated : 1;
+};
+
+struct ReferenceLayout {
+    struct Data {
+        const char *pointer;
+        size_type size;
+        size_type capacity;
+    } data;
+    char dummy[maximumShortStringDataAreaSize - sizeof(Data)];
+    std::uint8_t shortStringSize: 6;
+    std::uint8_t isReference : 1;
+    std::uint8_t hasAllocated : 1;
+};
+
+struct ShortStringLayout {
+    char string[maximumShortStringDataAreaSize];
+    std::uint8_t shortStringSize: 6;
+    std::uint8_t isReference : 1;
+    std::uint8_t hasAllocated : 1;
+};
+
+struct ALIGNAS_16 StringDataLayout {
+    StringDataLayout() noexcept = default;
+
+    constexpr StringDataLayout(const char *string,
+                               size_type size) noexcept
+       : reference({{string, size, 0}, {}, 0, true, true})
+    {
+    }
+
+    template<size_type Size>
+    constexpr StringDataLayout(const char(&string)[Size]) noexcept
+#if __cpp_constexpr < 201304
+        : reference({{string, Size - 1, 0}, {}, 0, true, true})
+#endif
+    {
+#if __cpp_constexpr >= 201304
+        if (Size <= maximumShortStringDataAreaSize) {
+            for (size_type i = 0; i < Size; ++i)
+                shortString.string[i] = string[i];
+
+#if defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Woverflow"
+            shortString.shortStringSize = std::uint8_t(Size) - 1;
+#pragma GCC diagnostic pop
+#endif
+            shortString.hasAllocated = false;
+            shortString.isReference = false;
+        } else {
+            reference.data.pointer = string;
+            reference.data.size = Size - 1;
+            reference.data.capacity = 0;
+            reference.shortStringSize = 0;
+            reference.hasAllocated = true;
+            reference.isReference = true;
+        }
+#endif
+    }
+
+
+
+    union {
+        AllocatedLayout allocated;
+        ReferenceLayout reference;
+        ShortStringLayout shortString = {};
+    };
+};
+
+} // namespace Internal
+}  // namespace Utils
+
+#pragma pop_macro("noexcept")
+#pragma pop_macro("constexpr")