Short live Q_CONSTINIT!

It expands to the first available of

- constinit (C++20)
- [[clang::require_constant_initialization]] (Clang)
- __constinit (GCC >= 10)

Use it around the code (on and near static QBasicAtomic; this patch
makes no attempt to find all statics in qtbase).

[ChangeLog][QtCore][QtGlobal] Added macro Q_CONSTINIT.

Fixes: QTBUG-100484
Change-Id: I11e0363a7acb3464476859d12ec7f94319d82be7
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

4 files changed, 43 insertions, 6 deletions

diff --git a/src/corelib/global/qglobal.cpp b/src/corelib/global/qglobal.cpp
index 067172efa4..1bbabed781 100644
--- a/src/corelib/global/qglobal.cpp
+++ b/src/corelib/global/qglobal.cpp
@@ -3344,7 +3344,7 @@ Q_CORE_EXPORT Q_DECL_CONST_FUNCTION unsigned int qt_int_sqrt(unsigned int n)
 
 // In the C runtime on all platforms access to the environment is not thread-safe. We
 // add thread-safety for the Qt wrappers.
-static QBasicMutex environmentMutex;
+Q_CONSTINIT static QBasicMutex environmentMutex;
 
 /*
   Wraps tzset(), which accesses the environment, so should only be called while
@@ -4115,6 +4115,33 @@ bool qunsetenv(const char *varName)
 */
 
 /*!
+    \macro Q_CONSTINIT
+    \relates <QtGlobal>
+    \since 6.4
+
+    \brief Enforces constant initialization when supported by the compiler.
+
+    If the compiler supports the C++20 \c{constinit} keyword, Clang's
+    \c{[[clang::require_constant_initialization]]} or GCC's \c{__constinit},
+    then this macro expands to the first one of these that is available,
+    otherwise it expands to nothing.
+
+    Variables marked as \c{constinit} cause a compile-error if their
+    initialization would have to be performed at runtime.
+
+    For constants, you can use \c{constexpr} since C++11, but \c{constexpr}
+    makes variables \c{const}, too, whereas \c{constinit} ensures constant
+    initialization, but doesn't make the variable \c{const}:
+
+    \table
+    \header \li Keyword       \li Added \li immutable \li constant-initialized
+    \row    \li \c{const}     \li C++98 \li yes       \li not required
+    \row    \li \c{constexpr} \li C++11 \li yes       \li required
+    \row    \li \c{constinit} \li C++20 \li no        \li required
+    \endtable
+*/
+
+/*!
     \macro QT_POINTER_SIZE
     \relates <QtGlobal>
 
diff --git a/src/corelib/global/qglobal.h b/src/corelib/global/qglobal.h
index e69e55977b..18851cd120 100644
--- a/src/corelib/global/qglobal.h
+++ b/src/corelib/global/qglobal.h
@@ -1165,6 +1165,16 @@ constexpr std::underlying_type_t<Enum> qToUnderlying(Enum e) noexcept
 #define Q_IMPLICIT
 #endif
 
+#ifdef __cpp_constinit
+# define Q_CONSTINIT constinit
+#elif defined(__has_cpp_attribute) && __has_cpp_attribute(clang::require_constant_initialization)
+# define Q_CONSTINIT [[clang::require_constant_initialization]]
+#elif defined(Q_CC_GNU) && Q_CC_GNU >= 1000
+# define Q_CONSTINIT __constinit
+#else
+# define Q_CONSTINIT
+#endif
+
 template <typename T> inline T *qGetPtrHelper(T *ptr) noexcept { return ptr; }
 template <typename Ptr> inline auto qGetPtrHelper(Ptr &ptr) noexcept -> decltype(ptr.get())
 { static_assert(noexcept(ptr.get()), "Smart d pointers for Q_DECLARE_PRIVATE must have noexcept get()"); return ptr.get(); }
diff --git a/src/corelib/global/qglobalstatic.h b/src/corelib/global/qglobalstatic.h
index aaa01262a7..beecc1f000 100644
--- a/src/corelib/global/qglobalstatic.h
+++ b/src/corelib/global/qglobalstatic.h
@@ -63,7 +63,7 @@ template <typename QGS> union Holder
     using PlainType = std::remove_cv_t<Type>;
 
     static constexpr bool ConstructionIsNoexcept = noexcept(QGS::innerFunction(nullptr));
-    static inline QBasicAtomicInteger<qint8> guard = { QtGlobalStatic::Uninitialized };
+    Q_CONSTINIT static inline QBasicAtomicInteger<qint8> guard = { QtGlobalStatic::Uninitialized };
 
     // union's sole member
     PlainType storage;
@@ -144,7 +144,7 @@ protected:
             new (pointer) QGS_Type ARGS;                                    \
         }                                                                   \
     }; }                                                                    \
-    static QGlobalStatic<QtGlobalStatic::Holder<Q_QGS_ ## NAME>> NAME;      \
+    Q_CONSTINIT static QGlobalStatic<QtGlobalStatic::Holder<Q_QGS_ ## NAME>> NAME; \
     QT_WARNING_POP
     /**/
 
diff --git a/src/corelib/global/qlogging.cpp b/src/corelib/global/qlogging.cpp
index 254894961e..6302fc9d80 100644
--- a/src/corelib/global/qlogging.cpp
+++ b/src/corelib/global/qlogging.cpp
@@ -1096,7 +1096,7 @@ struct QMessagePattern
 Q_DECLARE_TYPEINFO(QMessagePattern::BacktraceParams, Q_RELOCATABLE_TYPE);
 #endif
 
-QBasicMutex QMessagePattern::mutex;
+Q_CONSTINIT QBasicMutex QMessagePattern::mutex;
 
 QMessagePattern::QMessagePattern()
 {
@@ -1750,7 +1750,7 @@ static bool android_default_message_handler(QtMsgType type,
 static void win_outputDebugString_helper(const QString &message)
 {
     const qsizetype maxOutputStringLength = 32766;
-    static QBasicMutex m;
+    Q_CONSTINIT static QBasicMutex m;
     auto locker = qt_unique_lock(m);
     // fast path: Avoid string copies if one output is enough
     if (message.length() <= maxOutputStringLength) {
@@ -1827,7 +1827,7 @@ static void stderr_message_handler(QtMsgType type, const QMessageLogContext &con
 #ifdef Q_OS_WASM
     // Prevent thread cross-talk, which causes Emscripten to log
     // non-valid UTF-8. FIXME: remove once we upgrade to emsdk > 2.0.30
-    static QBasicMutex m;
+    Q_CONSTINIT static QBasicMutex m;
     auto locker = qt_unique_lock(m);
 #endif