QMessageLogger: initialize QT_FATAL_xxx cooperatively between threads

The use of function-local static implied there was a guard variable to
perform a thread-safe initialization (a critical section). We don't need
that: we can just use a plain QBasicAtomicInt and cooperatively
initialize, in parallel. This way, the QMessageLogger code does not need
to check the status of the guard variable before every access to the
atomic, which would be an atomic access before the atomic access.

Change-Id: Iedfbe8e28ca5168dea43fffddf205d122c97f71c
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/global/qlogging.cpp

@@ -149,30 +149,54 @@ static int checked_var_value(const char *varname)
     return (ok && value >= 0) ? value : 1;
 }
 
-static bool is_fatal_count_down(QAtomicInt &n)
+static bool isFatalCountDown(const char *varname, QBasicAtomicInt &n)
 {
-    // it's fatal if the current value is exactly 1,
-    // otherwise decrement if it's non-zero
+    static const int Uninitialized = -1;
+    static const int NeverFatal = 0;
+    static const int ImmediatelyFatal = 1;
 
     int v = n.loadRelaxed();
-    while (v > 1 && !n.testAndSetRelaxed(v, v - 1, v))
-        qYieldCpu();
-    return v == 1; // we exited the loop, so either v == 0 or CAS succeeded to set n from v to v-1
-}
-
-static bool isFatal(QtMsgType msgType)
-{
-    if (msgType == QtFatalMsg)
-        return true;
-
-    if (msgType == QtCriticalMsg) {
-        static QAtomicInt fatalCriticals = checked_var_value("QT_FATAL_CRITICALS");
-        return is_fatal_count_down(fatalCriticals);
+    if (v == Uninitialized) {
+        // first, initialize from the environment
+        const int env = checked_var_value(varname);
+        if (env == NeverFatal) {
+            // not fatal, now or in the future, so use a fast path
+            n.storeRelaxed(NeverFatal);
+            return false;
+        } else if (env == ImmediatelyFatal) {
+            return true;
+        } else if (n.testAndSetRelaxed(Uninitialized, env - 1, v)) {
+            return false;       // not yet fatal
+        } else {
+            // some other thread initialized before we did
+        }
     }
 
-    if (msgType == QtWarningMsg || msgType == QtCriticalMsg) {
-        static QAtomicInt fatalWarnings = checked_var_value("QT_FATAL_WARNINGS");
-        return is_fatal_count_down(fatalWarnings);
+    while (v > ImmediatelyFatal && !n.testAndSetRelaxed(v, v - 1, v))
+        qYieldCpu();
+
+    // We exited the loop, so either v already was ImmediatelyFatal or we
+    // succeeded to set n from v to v-1.
+    return v == ImmediatelyFatal;
+}
+
+Q_CONSTINIT static QBasicAtomicInt fatalCriticalsCount = { -1 };
+Q_CONSTINIT static QBasicAtomicInt fatalWarningsCount = { -1 };
+static bool isFatal(QtMsgType msgType)
+{
+    switch (msgType){
+    case QtFatalMsg:
+        return true;    // always fatal
+
+    case QtCriticalMsg:
+        return isFatalCountDown("QT_FATAL_CRITICALS", fatalCriticalsCount);
+
+    case QtWarningMsg:
+        return isFatalCountDown("QT_FATAL_WARNINGS", fatalWarningsCount);
+
+    case QtDebugMsg:
+    case QtInfoMsg:
+        break;  // never fatal
     }
 
     return false;