CMake: Adjust compiler flag optimizations to qmake mkspec ones

There are inconsistencies in the default optimization flags added by
CMake across configurations like Release and RelWithDebInfo.
In particular Release uses -O3, whereas RelWithDebInfo uses -O2,
as well as usage of /INCREMENTAL in release configs with MSVC, etc.

To make sure that the Qt 6 binaries built with CMake are consistent
across configs, as well as consistent with the flags we used when
building Qt 5 with qmake, add a horrible search and replace mechanism
to replaces the CMake flags with what our mkspecs indicate to use.

Ideally this would be done by providing custom CMake toolchain files
for each platform we support, and we might revisit that later if the
need really arises.

To implement the replacing, we first need the flags that should be
added. Port the QMAKE_CFLAGS_OPTIMIZE variables to CMake, which is
done in QtCompilerOptimization.cmake.

Then a new function called
qt_internal_set_up_config_optimizations_like_in_qmake will look for
any kind of optimization flags set in the
CMAKE_<LANG>_FLAGS_<CONFIG> style variables, remove them, and add
the appropriate flags that qmake mkspecs provide.

On some platforms (like Windows MSVC) the function also alters the
linker CMAKE_${TYPE}_LINKER_FLAGS_<CONFIG> style variables.

The mechanism allows opting out of this replacing by
setting the QT_USE_DEFAULT_CMAKE_OPTIMIZATION_FLAGS value.

It also allows opting into removal of flags for custom configs by
providing QT_ADDITIONAL_OPTIMIZATION_FLAG_CONFIGS. It's only removal,
because we wouldn't know what kind of config it is, and thus what
flags to add.

The currently modified configs are: Release, RelWithDebInfo,
MinSizeRel, Debug aka the usual default CMake provided ones.

The mechanism is only applied to C-like languages.
ASM is not handled to be on the safe side due to not knowing what kind
of compiler flags the platform assembler might take.
It's also important to skip RC on MSVC platforms.

Task-number: QTBUG-85992
Change-Id: I3712d5cd5a34fceab54f56a6fa46b4e678952362
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

1 files changed, 2 insertions, 0 deletions

diff --git a/cmake/QtBuildInternals/QtBuildInternalsConfig.cmake b/cmake/QtBuildInternals/QtBuildInternalsConfig.cmake
index 9c5a2633d2..3c8692a903 100644
--- a/cmake/QtBuildInternals/QtBuildInternalsConfig.cmake
+++ b/cmake/QtBuildInternals/QtBuildInternalsConfig.cmake
@@ -179,6 +179,8 @@ macro(qt_enable_cmake_languages)
             enable_language(${__qt_lang})
         endif()
     endforeach()
+
+    qt_internal_set_up_config_optimizations_like_in_qmake()
 endmacro()
 
 # Minimum setup required to have any CMakeList.txt build as as a standalone