Make the use of -fdata-sections and --gc-sections more generic in Qt

Add qmake feature and configure option, which optimze the size of static
exectuable. Use for static build.
Enabled via configure --gc-binaries, or CONFIG += gc-binaries in 3rd party
projects.

Change-Id: I3c25b02caaef6a4afc6019afc9c67122dd11696d
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

config_help.txt

@@ -98,6 +98,9 @@ Build options:
   -gdb-index ........... Index the debug info to speed up GDB
                          [no; auto if -developer-build with debug info]
   -strip ............... Strip release binaries of unneeded symbols [yes]
+  -gc-binaries ......... Place each function or data item into its own section
+                         and enable linker garbage collection of unused
+                         sections. [auto for static builds, otherwise no]
   -force-asserts ....... Enable Q_ASSERT even in release builds [no]
   -developer-build ..... Compile and link Qt for developing Qt itself
                          (exports for auto-tests, extra checks, etc.) [no]

configure.json

@@ -80,6 +80,7 @@
             "force-debug-info": { "type": "boolean", "name": "force_debug_info" },
             "force-pkg-config": { "type": "void", "name": "pkg-config" },
             "framework": "boolean",
+            "gc-binaries": { "type": "boolean", "name": "gc_binaries" },
             "gdb-index": { "type": "boolean", "name": "gdb_index" },
             "gcc-sysroot": "boolean",
             "gcov": "boolean",
@@ -397,6 +398,17 @@
                 ]
             }
         },
+        "gc_binaries": {
+            "label": "support for split sections and linker garbage collection",
+            "type": "compile",
+            "test": {
+                "qmake": [
+                    "isEmpty(QMAKE_CFLAGS_SPLIT_SECTIONS): error(\"Nope\")",
+                    "isEmpty(QMAKE_CXXFLAGS_SPLIT_SECTIONS): error(\"Nope\")",
+                    "isEmpty(QMAKE_LFLAGS_GCSECTIONS): error(\"Nope\")"
+                ]
+            }
+        },
         "sse2": {
             "label": "SSE2 instructions",
             "type": "x86Simd"
@@ -592,6 +604,10 @@
         "compiler-flags": {
             "output": [ "compilerFlags" ]
         },
+        "gc_binaries": {
+            "condition": "!features.shared && tests.gc_binaries",
+            "output": [ "privateFeature" ]
+        },
         "gcc-sysroot": {
             "output": [ "gccSysroot" ],
             "condition": "input.sysroot != ''"

mkspecs/common/gcc-base.conf

@@ -50,7 +50,7 @@ QMAKE_CFLAGS_ISYSTEM        = -isystem
 QMAKE_CFLAGS_YACC          += -Wno-unused -Wno-parentheses
 QMAKE_CFLAGS_HIDESYMS      += -fvisibility=hidden
 QMAKE_CFLAGS_EXCEPTIONS_OFF += -fno-exceptions
-QMAKE_CFLAGS_SPLIT_SECTIONS += -ffunction-sections
+QMAKE_CFLAGS_SPLIT_SECTIONS += -ffunction-sections -fdata-sections
 QMAKE_CFLAGS_LTCG           = -flto -fno-fat-lto-objects
 QMAKE_CFLAGS_LTCG_FATOBJECTS = -ffat-lto-objects
 QMAKE_CFLAGS_DISABLE_LTCG   = -fno-lto

mkspecs/features/gc_binaries.prf

@@ -0,0 +1,3 @@
+QMAKE_CFLAGS += $$QMAKE_CFLAGS_SPLIT_SECTIONS
+QMAKE_CXXFLAGS += $$QMAKE_CXXFLAGS_SPLIT_SECTIONS
+QMAKE_LFLAGS += $$QMAKE_LFLAGS_GCSECTIONS

mkspecs/features/qt_app.prf

@@ -20,9 +20,6 @@ isEmpty(QMAKE_TARGET_DESCRIPTION): \
 
 isEmpty(QMAKE_INFO_PLIST): CONFIG -= app_bundle
 
-# This decreases the binary size for tools if statically linked
-QMAKE_LFLAGS += $$QMAKE_LFLAGS_GCSECTIONS
-
 host_build: QT -= gui   # no host tool will ever use gui
 host_build:force_bootstrap {
     !build_pass:qtConfig(release_tools): CONFIG += release

mkspecs/features/qt_module.prf

@@ -28,8 +28,6 @@ host_build|staticlib: CONFIG += static
 
 host_build {
     QT -= gui   # no host module will ever use gui
-    QMAKE_CFLAGS += $$QMAKE_CFLAGS_SPLIT_SECTIONS
-    QMAKE_CXXFLAGS += $$QMAKE_CXXFLAGS_SPLIT_SECTIONS
     force_bootstrap {
         !build_pass:qtConfig(release_tools): CONFIG += release
         contains(QT, core(-private)?|xml) {

mkspecs/features/uikit/gc_binaries.prf

@@ -0,0 +1,2 @@
+# bitcode (release mode) is incompatible with splitting sections.
+!bitcode|!release: load(gc_binaries)

mkspecs/linux-icc/qmake.conf

@@ -26,7 +26,7 @@ QMAKE_CFLAGS_STATIC_LIB = $$QMAKE_CFLAGS_SHLIB
 QMAKE_CFLAGS_YACC       =
 QMAKE_CFLAGS_ISYSTEM    = -isystem
 QMAKE_CFLAGS_THREAD     = -D_REENTRANT
-QMAKE_CFLAGS_SPLIT_SECTIONS = -ffunction-sections
+QMAKE_CFLAGS_SPLIT_SECTIONS = -ffunction-sections -fdata-sections
 QMAKE_CFLAGS_LTCG       = -ipo -fno-fat-lto-objects
 QMAKE_CFLAGS_LTCG_FATOBJECTS = -ffat-lto-objects
 QMAKE_CFLAGS_DISABLE_LTCG = -no-ipo

mkspecs/macx-icc/qmake.conf

@@ -25,7 +25,7 @@ QMAKE_CFLAGS_SHLIB      = -fPIC
 QMAKE_CFLAGS_STATIC_LIB = $$QMAKE_CFLAGS_SHLIB
 QMAKE_CFLAGS_YACC       = -Wno-unused -Wno-parentheses
 QMAKE_CFLAGS_THREAD     =
-QMAKE_CFLAGS_SPLIT_SECTIONS = -ffunction-sections
+QMAKE_CFLAGS_SPLIT_SECTIONS = -ffunction-sections -fdata-sections
 QMAKE_CFLAGS_LTCG       = -ipo
 QMAKE_CFLAGS_DISABLE_LTCG = -no-ipo
 

src/corelib/corelib.pro

@@ -4,6 +4,7 @@ CONFIG    += exceptions
 
 MODULE = core     # not corelib, as per project file
 MODULE_CONFIG = moc resources
+qtConfig(gc_binaries): MODULE_CONFIG += gc_binaries
 !isEmpty(QT_NAMESPACE): MODULE_DEFINES = QT_NAMESPACE=$$QT_NAMESPACE
 
 TRACEPOINT_PROVIDER = $$PWD/qtcore.tracepoints

src/tools/bootstrap/bootstrap.pro

@@ -2,7 +2,7 @@ option(host_build)
 
 TARGET = QtBootstrap
 QT =
-CONFIG += minimal_syncqt internal_module force_bootstrap
+CONFIG += minimal_syncqt internal_module force_bootstrap gc_binaries
 
 MODULE_INCNAME = QtCore QtXml
 MODULE_DEFINES = \
@@ -12,6 +12,7 @@ MODULE_DEFINES = \
         QT_VERSION_PATCH=$$QT_PATCH_VERSION \
         QT_BOOTSTRAPPED \
         QT_NO_CAST_TO_ASCII
+MODULE_CONFIG = gc_binaries
 
 DEFINES += \
     $$MODULE_DEFINES \