QMetaTypeId: optimize qt_metatype_id()

The old code resulted in one memory fetch for the test plus another one
for the return value (at the assembler level). The new code reuses the
already-loaded value:

-       movl    _ZZN11QMetaTypeIdI12QDBusMessageE14qt_metatype_idEvE11metatype_id(%rip), %edx
-       testl   %edx, %edx
-       je      .L158
        movl    _ZZN11QMetaTypeIdI12QDBusMessageE14qt_metatype_idEvE11metatype_id(%rip), %eax
-       ret
+       testl   %eax, %eax
+       je      .L160
+       rep; ret

It also saves one load in the not-yet-initialised case:

-.L158:
+.L160:
        leaq    .LC7(%rip), %rdi
        subq    , %rsp
        .cfi_def_cfa_offset 16
        movq    himBH1, %rsi
        call    _Z17qRegisterMetaTypeI12QDBusMessageEiPKcPT_
        movl    %eax, _ZZN11QMetaTypeIdI12QDBusMessageE14qt_metatype_idEvE11metatype_id(%rip)
-       movl    _ZZN11QMetaTypeIdI12QDBusMessageE14qt_metatype_idEvE11metatype_id(%rip), %eax
        addq    , %rsp
        .cfi_def_cfa_offset 8
        ret

Change-Id: I769950449822f2b1587680e05518be0a4f3120a2
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/kernel/qmetatype.h

@@ -690,10 +690,12 @@ inline int qRegisterMetaTypeStreamOperators()
         static int qt_metatype_id()                                     \
             {                                                           \
                 static QBasicAtomicInt metatype_id = Q_BASIC_ATOMIC_INITIALIZER(0); \
-                if (!metatype_id.load())                                \
-                    metatype_id.storeRelease(qRegisterMetaType< TYPE >(#TYPE, \
-                               reinterpret_cast< TYPE *>(quintptr(-1)))); \
-                return metatype_id.loadAcquire();                       \
+                if (const int id = metatype_id.loadAcquire())           \
+                    return id;                                          \
+                const int newId = qRegisterMetaType< TYPE >(#TYPE,      \
+                              reinterpret_cast< TYPE *>(quintptr(-1))); \
+                metatype_id.storeRelease(newId);                        \
+                return newId;                                           \
             }                                                           \
     };                                                                  \
     QT_END_NAMESPACE