iOS: Use LC_MAIN to wrap user main() instead of mangling object files

With iOS 6.0 and above the LC_MAIN load command is available, which allows
dyld to call the application entrypoint directly instead of going through
_start in crt.o. By passing -e to the linker we can change this entrypoint
to our wrapper that sets up the separate stack before entering the native
iOS runloop through UIApplicationMain. As before, we call the user's main()
from applicationDidFinishLaunching.

By using LC_MAIN instead of messing with the object files we open up the
possibility of generating Bitcode instead of object code, which can be
useful for link-time optimizations, either locally or by Apple.

Change-Id: If2153bc919581cd93dfa10fb6ff1c305b3e39a52
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@theqtcompany.com>

3 files changed, 7 insertions, 107 deletions

diff --git a/mkspecs/macx-ios-clang/features/qt.prf b/mkspecs/macx-ios-clang/features/qt.prf
index 9e6b486c6e..474a195e4b 100644
--- a/mkspecs/macx-ios-clang/features/qt.prf
+++ b/mkspecs/macx-ios-clang/features/qt.prf
@@ -19,34 +19,12 @@ equals(TEMPLATE, app):contains(qt_depends, gui(-private)?) {
     QTPLUGIN.platforms = -
 
     !no_main_wrapper {
-        # We use ld to rename the _main symbol to _qt_main, so that we don't get a symbol clash
-        # with the _main we provide that calls UIApplicationMain. We need to make a copy of the
-        # original object file, as ld will not copy over DWARF debug information to the output
-        # file. Instead, it will inject a reference back to the original object file, so when
-        # Xcode runs dsymutil to make the final dSYM file it will still find the debug info
-        # for the object file that provided the original _main. This back-reference has the
-        # interesting side-effect of the debug information still referring to the original
-        # symbol name, so stack-traces will show both our wrapper main and the original
-        # user main as 'main', and adding a symbolic breakpoint for 'main' will break on
-        # both functions. Although a bit weird, it's a good thing, as the user will still be
-        # able to add symbolic breakpoints for 'main', not caring that the symbol is actually
-        # called 'qt_main' now.
-
-        macx-xcode {
-            objects_dir = "\"${OBJECT_FILE_DIR}-${CURRENT_VARIANT}\""
-            archs = "\"${ARCHS}\""
-        } else {
-            isEmpty(OBJECTS_DIR): \
-                objects_dir = .
-            else: \
-                objects_dir = $$shell_quote($$OBJECTS_DIR)
-            archs = $$shell_quote($$QMAKE_IOS_DEVICE_ARCHS $$QMAKE_IOS_SIMULATOR_ARCHS)
-        }
-
-        !isEmpty(QMAKE_PRE_LINK): \
-            QMAKE_PRE_LINK += ";"
-
-        QMAKE_PRE_LINK += $$shell_quote($$QMAKESPEC/rename_main.sh) $$objects_dir $$archs
+        # The LC_MAIN load command available in iOS 6.0 and above allows dyld to
+        # directly call the entrypoint instead of going through _start in crt.o.
+        # Passing -e to the linker changes the entrypoint from _main to our custom
+        # wrapper that calls UIApplicationMain and dispatches back to main() once
+        # the application has started up and is ready to initialize QApplication.
+        QMAKE_LFLAGS += -Wl,-e,_qt_main_wrapper
     }
 }
 
diff --git a/mkspecs/macx-ios-clang/qmake.conf b/mkspecs/macx-ios-clang/qmake.conf
index 0dd761760c..d5e5840902 100644
--- a/mkspecs/macx-ios-clang/qmake.conf
+++ b/mkspecs/macx-ios-clang/qmake.conf
@@ -7,7 +7,7 @@ CONFIG                 += app_bundle reduce_exports incremental global_init_link
 QMAKE_INCREMENTAL_STYLE = sublib
 
 QMAKE_MACOSX_DEPLOYMENT_TARGET  =
-QMAKE_IOS_DEPLOYMENT_TARGET     = 5.1.1
+QMAKE_IOS_DEPLOYMENT_TARGET     = 6.0
 
 INCLUDEPATH            += $$PWD/ios
 DEFINES                += DARWIN_NO_CARBON QT_NO_PRINTER QT_NO_PRINTDIALOG
diff --git a/mkspecs/macx-ios-clang/rename_main.sh b/mkspecs/macx-ios-clang/rename_main.sh
deleted file mode 100755
index 1547f5f75c..0000000000
--- a/mkspecs/macx-ios-clang/rename_main.sh
+++ /dev/null
@@ -1,78 +0,0 @@
-#!/bin/bash
-
-#############################################################################
-##
-## Copyright (C) 2015 The Qt Company Ltd.
-## Contact: http://www.qt.io/licensing/
-##
-## This file is the build configuration utility of the Qt Toolkit.
-##
-## $QT_BEGIN_LICENSE:LGPL21$
-## Commercial License Usage
-## Licensees holding valid commercial Qt licenses may use this file in
-## accordance with the commercial license agreement provided with the
-## Software or, alternatively, in accordance with the terms contained in
-## a written agreement between you and The Qt Company. For licensing terms
-## and conditions see http://www.qt.io/terms-conditions. For further
-## information use the contact form at http://www.qt.io/contact-us.
-##
-## GNU Lesser General Public License Usage
-## Alternatively, this file may be used under the terms of the GNU Lesser
-## General Public License version 2.1 or version 3 as published by the Free
-## Software Foundation and appearing in the file LICENSE.LGPLv21 and
-## LICENSE.LGPLv3 included in the packaging of this file. Please review the
-## following information to ensure the GNU Lesser General Public License
-## requirements will be met: https://www.gnu.org/licenses/lgpl.html and
-## http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
-##
-## As a special exception, The Qt Company gives you certain additional
-## rights. These rights are described in The Qt Company LGPL Exception
-## version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
-##
-## $QT_END_LICENSE$
-##
-#############################################################################
-
-if [ $# -ne 2 ]; then
-    echo "$0: wrong number of arguments for internal tool used by iOS mkspec"
-else
-    arch_paths=""
-    for a in $2; do
-        arch_paths="$arch_paths
-$1/$a"
-    done
-    for f in $(IFS="
-"; find $arch_paths -name '*.o'); do
-        # Skip object files without the _main symbol
-        nm $f 2>/dev/null | grep -q 'T _main$' || continue
-
-        fname=${f#$1/}
-
-        file -b $f | grep -qi 'llvm bit-code' && \
-            (cat \
-<<EOF >&2
-$f:: error: The file '$fname' contains LLVM bitcode, not object code. Automatic main() redirection could not be applied.
-note: This is most likely due to the use of link-time optimization (-flto). Please disable LTO, or work around the \
-issue by manually renaming your main() function to qtmn():
-
-#ifdef Q_OS_IOS
-extern "C" int qtmn(int argc, char *argv[])
-#else
-int main(int argc, char *argv[])
-#endif
-EOF
-            ) && exit 1
-
-        echo "Found main() in $fname"
-
-        strings -t d - $f | grep '_main\(\.eh\)\?$' | while read match; do
-            offset=$(echo $match | cut -d ' ' -f 1)
-            symbol=$(echo $match | cut -d ' ' -f 2)
-
-            echo "  Renaming '$symbol' at offset $offset to '${symbol/main/qtmn}'"
-
-            # In-place rename the string (keeping the same length)
-            printf '_qtmn' | dd of=$f bs=1 seek=$offset conv=notrunc >/dev/null 2>&1
-        done
-    done
-fi
\ No newline at end of file