How can I disable compiler optimizations for a release build?

Guy Gizmo

I'm trying to build an app in Windows with Qt 6.2.1, qmake, and the Visual Studio 2019 C++ compiler. I want to build a release build with compiler optimizations turned off completely, i.e. passing in the -Od command line argument.

However, I cannot for the life of me get qmake to do this! It seems to always add in '-O2' no matter what I do.

First of all, adding QMAKE_CXXFLAGS_RELEASE += -Od has no effect whatsoever, which already is odd. If I use QMAKE_CXXFLAGS instead it works. I am most definitely building a release build, not a debug build.

I eventually tried adding the following to my .pro file to no avail:

message($$QMAKE_CFLAGS)
message($$QMAKE_CFLAGS_RELEASE)
message($$QMAKE_CXXFLAGS)
message($$QMAKE_CXXFLAGS_RELEASE)
QMAKE_CFLAGS -= -O2
QMAKE_CFLAGS_RELEASE -= -O2
QMAKE_CXXFLAGS -= -O2
QMAKE_CXXFLAGS_RELEASE -= -O2
QMAKE_CFLAGS -= /O2
QMAKE_CFLAGS_RELEASE -= /O2
QMAKE_CXXFLAGS -= /O2
QMAKE_CXXFLAGS_RELEASE -= /O2
message($$QMAKE_CFLAGS)
message($$QMAKE_CFLAGS_RELEASE)
message($$QMAKE_CXXFLAGS)
message($$QMAKE_CXXFLAGS_RELEASE)

QMAKE_CXXFLAGS += -Od

This prints the following to general messages in Qt Creator, or the terminal when compiling from there:

Project MESSAGE: Before:
Project MESSAGE: -nologo -Zc:wchar_t -FS -Zc:strictStrings
Project MESSAGE: -O2 -MD
Project MESSAGE: -nologo -Zc:wchar_t -FS -Zc:rvalueCast -Zc:inline -Zc:strictStrings -Zc:throwingNew -permissive- -Zc:__cplusplus -Zc:externConstexpr
Project MESSAGE: -O2 -MD
Project MESSAGE: After:
Project MESSAGE: -nologo -Zc:wchar_t -FS -Zc:strictStrings
Project MESSAGE: -MD
Project MESSAGE: -nologo -Zc:wchar_t -FS -Zc:rvalueCast -Zc:inline -Zc:strictStrings -Zc:throwingNew -permissive- -Zc:__cplusplus -Zc:externConstexpr -Od
Project MESSAGE: -MD

So I can see that it's removed -O2 and added in -Od. But when I compile, it's now passing both -O2 and -Od to cl, and producing this error:

cl : Command line warning D9025 : overriding '/Od' with '/O2'

Is there anything I can do to disable optimizations?