Different files are generated with cmake and with qmake

posktomten

No, then the files will be much larger.
(I'm using MinGW GCC 11.2.0, haven't tested with MSVC)

posktomten

Now I have tested with MSVC. The files don't get bigger.

Chris Kawa

CMake and qmake don't compile or link your app. All they do is set up parameters for the compiler/linker, and those are the same in both cases, so you can set both build tools to produce exactly the same outputs.
Don't guess. Just pick one file and look at the compilation log from both build tools. Compare what options go directly into compiling/linking. Some default is probably different, that's all. Maybe one sets optimizer for size and the other for speed or something like that.

ChrisW67

On my Linux box with GCC the qmake generated Makefile builds with the -O2 compiler flag.

...
g++ -c -pipe -O2 -std=gnu++1z -Wall -Wextra -D_REENTRANT -fPIC -DQT_DISABLE_DEPRECATED_BEFORE=0x060000 -DABOUT_LIBRARY -DQT_NO_DEBUG -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_CORE_LIB -I. -I/usr/include/x86_64-linux-gnu/qt5 -I/usr/include/x86_64-linux-gnu/qt5/QtWidgets -I/usr/include/x86_64-linux-gnu/qt5/QtGui -I/usr/include/x86_64-linux-gnu/qt5/QtCore -I. -I/usr/lib/x86_64-linux-gnu/qt5/mkspecs/linux-g++ -o about.o about.cpp
...

chrisw@newton:/tmp/libabout/code$ ls -l ../code-lib/
total 44
lrwxrwxrwx 1 chrisw chrisw    17 Dec  4 11:05 libabout.so -> libabout.so.1.0.0
lrwxrwxrwx 1 chrisw chrisw    17 Dec  4 11:05 libabout.so.1 -> libabout.so.1.0.0
lrwxrwxrwx 1 chrisw chrisw    17 Dec  4 11:05 libabout.so.1.0 -> libabout.so.1.0.0
-rwxrwxr-x 1 chrisw chrisw 43616 Dec  4 11:05 libabout.so.1.0.0

The CMake generated Makefile does not seem to specify optimization options

...
[75%] Building CXX object CMakeFiles/about.dir/about.cpp.o
/usr/bin/c++ -DABOUT_LIBRARY -DQT_CORE_LIB -DQT_DISABLE_DEPRECATED_BEFORE=0x060000 -DQT_GUI_LIB -DQT_NO_DEBUG -DQT_WIDGETS_LIB -Dabout_EXPORTS -I/tmp/libabout/code -I/tmp/libabout/code/about_autogen/include -isystem /home/chrisw/Qt/6.4.0/gcc_64/include/QtWidgets -isystem /home/chrisw/Qt/6.4.0/gcc_64/include -isystem /home/chrisw/Qt/6.4.0/gcc_64/include/QtCore -isystem /home/chrisw/Qt/6.4.0/gcc_64/mkspecs/linux-g++ -isystem /home/chrisw/Qt/6.4.0/gcc_64/include/QtGui -fPIC -fPIC -std=gnu++17 -MD -MT CMakeFiles/about.dir/about.cpp.o -MF CMakeFiles/about.dir/about.cpp.o.d -o CMakeFiles/about.dir/about.cpp.o -c /tmp/libabout/code/about.cpp
...

chrisw@newton:/tmp/libabout/code$ ls -l ../code-lib/
total 56
-rwxrwxr-x 1 chrisw chrisw 56512 Dec  4 11:01 libabout.so

JoeCFD

@posktomten you can check if the default flags are the same in both builds.

posktomten

Fixed a typo
And a big thank you for the help and all the points of view!

Been doing this all day. Something I don't understand.
Same inputs to the compiler? But different results when using cmake and when using qmake.

// cmake
// Changes -O3 to -O2
macro(remove_cxx_flag flag)
   string(REPLACE "${flag}" "" CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE}")
endmacro()
remove_cxx_flag("-O3")
set(CMAKE_CXX_FLAGS_RELEASE "-O2${CMAKE_CXX_FLAGS_RELEASE}")

message(${CMAKE_CXX_FLAGS_RELEASE}) // -O2 -DNDEBUG
// size: 84 KB

// qmake
QMAKE_CXXFLAGS_RELEASE += -DNDEBUG
message($$QMAKE_CXXFLAGS_RELEASE)  // -O2 -DNDEBUG
// size: 35 KB


// Compiled using the command prompt

qmake -project ../code/about.pro
qmake ../code/about.pro
mingw32-make

// size 38 KB
  message($$QMAKE_CXXFLAGS_RELEASE)  // -O2 -DNDEBUGG


cmake -G "MinGW Makefiles"
cmake --build .
mingw32-make

// size 94 KB
  message(${CMAKE_CXX_FLAGS_RELEASE}) // -O2 -DNDEBUG

Chris Kawa

@posktomten I rather meant to put CMake into verbose mode, to see something like what @ChrisW67 posted, not just look at one variable.
QMAKE_CXXFLAGS_RELEASE might not contain everything that is passed. At least look at the linker flags too, QMAKE_LFLAGS_RELEASE, but better would be to look at the actual commands executed.

posktomten

Hey!
Thanks for all the suggestions.
@Chris-Kawa probably has the best solution. I haven't succeeded yet.
I can state:
The difference in the created file size (*.dll, *.so) depending on whether cmake or qmake is used:
Linux, GCC: insignificant
Windows, MSVC: insignificant
Windows, MinGW GCC: significant

ChrisW67

The qmake-generated Makefile is verbose enough. You can get the verbose form from the CMake generated Makefile:

mingw32-make VERBOSE=1

I see that CMake does not specify -D_REENTRANT which could change code inclusions. It also calls the C++ compiler via c++ which, even if symlinked to g++, might trigger g++ to use different defaults. Lots of possibilities.

posktomten

Thanks!
I'll have to test that