Optimize debug builds when -Og is available

Enables optimizing with -Og if GCC has the option available, this
should produce faster debug binaries without compromising debugability.

Is a privateConfig to limit it to the default Qt build.

Includes two fixes for false positives of maybe_uninitialized triggered
by -Og on gcc 4.9.

Change-Id: I466d7a4070295714189024369312e6cbd36cfacf
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

config_help.txt

@@ -72,6 +72,8 @@ Build options:
   -debug ............... Build Qt with debugging turned on [no]
   -debug-and-release ... Build two versions of Qt, with and without
                          debugging turned on [yes] (Apple and Windows only)
+  -optimize-debug ...... Enable debug-friendly optimizations in debug builds
+                         [auto] (Not supported with MSVC)
   -optimized-tools ..... Build optimized host tools even in debug build [no]
   -force-debug-info .... Create symbol files for release builds [no]
   -separate-debug-info . Split off debug information to separate files [no]

configure.json

@@ -93,6 +93,7 @@
             "mp": { "type": "boolean", "name": "msvc_mp" },
             "nomake": { "type": "addString", "values": [ "examples", "tests", "tools" ] },
             "opensource": { "type": "void", "name": "commercial", "value": "no" },
+            "optimize-debug": { "type": "boolean", "name": "optimize_debug" },
             "optimized-qmake": { "type": "boolean", "name": "release_tools" },
             "optimized-tools":  { "type": "boolean", "name": "release_tools" },
             "pch": { "type": "boolean", "name": "precompile_header" },
@@ -263,6 +264,11 @@
             "type": "compilerSupportsFlag",
             "flag": "-fuse-ld=gold"
         },
+        "optimize_debug": {
+            "label": "-Og support",
+            "type": "compilerSupportsFlag",
+            "flag": "-Og"
+        },
         "enable_new_dtags": {
             "label": "new dtags support",
             "type": "linkerSupportsFlag",
@@ -479,6 +485,11 @@
             "condition": "!config.msvc && !config.integrity && tests.use_gold_linker",
             "output": [ "privateConfig", "useGoldLinker" ]
         },
+        "optimize_debug": {
+            "label": "Optimize debug build",
+            "condition": "!config.msvc && (features.debug || features.debug_and_release) && tests.optimize_debug",
+            "output": [ "privateConfig" ]
+        },
         "architecture": {
             "label": "Architecture",
             "output": [ "architecture" ]
@@ -1078,6 +1089,11 @@ Configure with '-qreal float' to create a build that is binary-compatible with 5
                     "message": "Mode",
                     "type": "buildMode"
                 },
+                {
+                    "type": "feature",
+                    "args": "optimize_debug",
+                    "condition": "!config.msvc && (features.debug || features.debug_and_release)"
+                },
                 "shared",
                 {
                     "message": "Using C++ standard",

mkspecs/common/gcc-base.conf

@@ -33,6 +33,7 @@
 
 QMAKE_CFLAGS_OPTIMIZE      = -O2
 QMAKE_CFLAGS_OPTIMIZE_FULL = -O3
+QMAKE_CFLAGS_OPTIMIZE_DEBUG = -Og
 
 QMAKE_CFLAGS               += -pipe
 QMAKE_CFLAGS_DEPS          += -M

mkspecs/features/default_post.prf

@@ -47,6 +47,11 @@ optimize_full {
     }
 }
 
+optimize_debug {
+    QMAKE_CFLAGS_DEBUG += $$QMAKE_CFLAGS_OPTIMIZE_DEBUG
+    QMAKE_CXXFLAGS_DEBUG += $$QMAKE_CFLAGS_OPTIMIZE_DEBUG
+}
+
 debug {
     QMAKE_CFLAGS += $$QMAKE_CFLAGS_DEBUG
     QMAKE_CXXFLAGS += $$QMAKE_CXXFLAGS_DEBUG

src/gui/text/qfontengine.cpp

@@ -1273,7 +1273,7 @@ const uchar *QFontEngine::getCMap(const uchar *table, uint tableSize, bool *isSy
         if (!qSafeFromBigEndian(maps + 8 * n, endPtr, &platformId))
             return 0;
 
-        quint16 platformSpecificId;
+        quint16 platformSpecificId = 0;
         if (!qSafeFromBigEndian(maps + 8 * n + 2, endPtr, &platformSpecificId))
             return 0;
 

src/plugins/platforms/eglfs/deviceintegration/eglfs_x11/qeglfsx11integration.cpp

@@ -114,7 +114,7 @@ void EventReader::run()
 {
     Qt::MouseButtons buttons;
 
-    xcb_generic_event_t *event;
+    xcb_generic_event_t *event = nullptr;
     while (running.load() && (event = xcb_wait_for_event(m_integration->connection()))) {
         uint response_type = event->response_type & ~0x80;
         switch (response_type) {