wasm: make idealThreadCount() work on all threads

navigator.hardwareConcurrency can be accessed from
the main thread only. Read and cache the value on
QCoreApplication initialization.

Change-Id: I731f7f356ce106c7107977783d4b763326af06b6
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

src/corelib/kernel/qcoreapplication.cpp

@@ -119,6 +119,7 @@
 
 #ifdef Q_OS_WASM
 #include <emscripten.h>
+#include <emscripten/val.h>
 #endif
 
 #ifdef QT_BOOTSTRAPPED
@@ -800,6 +801,10 @@ void QCoreApplicationPrivate::init()
                 Module.print(err);
         });
     );
+
+#if QT_CONFIG(thread)
+    QThreadPrivate::idealThreadCount = emscripten::val::global("navigator")["hardwareConcurrency"].as<int>();
+#endif
 #endif
 
     // Store app name/version (so they're still available after QCoreApplication is destroyed)

src/corelib/thread/qthread_p.h

@@ -195,6 +195,9 @@ public:
     int waiters;
     bool terminationEnabled, terminatePending;
 #endif // Q_OS_WIN
+#ifdef Q_OS_WASM
+    static int idealThreadCount;
+#endif
     QThreadData *data;
 
     static QAbstractEventDispatcher *createEventDispatcher(QThreadData *data);

src/corelib/thread/qthread_unix.cpp

@@ -100,10 +100,6 @@
 #include <sys/neutrino.h>
 #endif
 
-#if defined(Q_OS_WASM)
-#include <emscripten/val.h>
-#endif
-
 QT_BEGIN_NAMESPACE
 
 #if QT_CONFIG(thread)
@@ -454,6 +450,10 @@ Qt::HANDLE QThread::currentThreadId() Q_DECL_NOTHROW
 #  define _SC_NPROCESSORS_ONLN 84
 #endif
 
+#ifdef Q_OS_WASM
+int QThreadPrivate::idealThreadCount = 1;
+#endif
+
 int QThread::idealThreadCount() Q_DECL_NOTHROW
 {
     int cores = 1;
@@ -503,7 +503,7 @@ int QThread::idealThreadCount() Q_DECL_NOTHROW
     cores = 1;
 #  endif
 #elif defined(Q_OS_WASM)
-    cores = emscripten::val::global("navigator")["hardwareConcurrency"].as<int>();
+    cores = QThreadPrivate::idealThreadCount;
 #else
     // the rest: Linux, Solaris, AIX, Tru64
     cores = (int)sysconf(_SC_NPROCESSORS_ONLN);