How do I make CMake play nice with a proprietary C++ compiler?

Question

I'm building an application for which I'd like to support a certain proprietary platform. It uses a modified version of ARMCC which CMake doesn't seem to like - no matter what I do, it keeps trying to provide strange flags to armlink where it should not, ignoring attempts to override its behavior.

Is it possible to essentially provide an all-new definition of how CMake should deal with a particular compiler? i.e. specify the binary and flags used for compilation, linking, etc., the expected file extensions, etc. throughout the whole process such that CMake doesn't do anything nasty? CMake seems to make custom compiler definitions an incredibly obscure thing to accomplish.

Edit: I've made it provide most of the right flags, but now I'm realizing I can't change the CMake test program - programs for this platform will fail to build if a certain atypical set of symbols (alternative entry point, not main) don't exist.

Edit 2: I've skipped the test program, but now it's failing while trying to get compiler information (why bother? I've given it everything it needs to know for the situation...) with this error:

Error: C9912E: No --cpu selected

To make it abundantly clear, the command it presents on the line above this clearly has the --cpu flag provided to a valid value:

armcc.exe -xc++ --cpu=MPCore -fpch-preprocess -v -dD -E

Though I'm entirely unsure as to what it's trying to do with the rest of it - I can't seem to override this part.

Answer 1

First, you need a toolchain.cmake file. Here, you provide the paths to your armcc compiler, linker, etc.

SET(CMAKE_SYSTEM_NAME Generic)
SET(CMAKE_SYSTEM_VERSION 1)

# this one is important
SET( CMAKE_SYSTEM_PROCESSOR  arm )

SET(MCU_ARCH Cortex-M4) #MCU architecture

SET(TOOLCHAIN_BIN_DIR "C:/Keil_v5/ARM/ARMCC/bin/")
set(LINKER_SCATTER_SCRIPT ${PROJECT_SOURCE_DIR}/Scatter_file.sct)

SET(CMAKE_C_COMPILER            ${TOOLCHAIN_BIN_DIR}/armcc.exe      CACHE FILEPATH "C compiler")
SET(CMAKE_CXX_COMPILER          ${TOOLCHAIN_BIN_DIR}/armcc.exe      CACHE FILEPATH "C++ compiler")
SET(CMAKE_LINKER                ${TOOLCHAIN_BIN_DIR}/armlink.exe    CACHE FILEPATH "linker")
SET(CMAKE_AR                    ${TOOLCHAIN_BIN_DIR}/armar.exe      CACHE FILEPATH "Archiver")
SET(CMAKE_ASM_COMPILER          ${TOOLCHAIN_BIN_DIR}/armasm.exe     CACHE FILEPATH "Assembler")
SET(CMAKE_FROMELF               ${TOOLCHAIN_BIN_DIR}/fromelf.exe    CACHE FILEPATH "From ELF tool")

SET(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)

SET(CMAKE_EXE_LINKER_FLAGS_INIT "--cpu ${MCU_ARCH} --myFlag1 --myFlag2 ${LINKER_SCATTER_SCRIPT}")

Next, within the CMakeLists.txt file, you can pass the compiler and linker flags:

cmake_minimum_required(VERSION 3.10)
SET(CMAKE_VERBOSE_MAKEFILE ON)
project (MyProject C CXX ASM)


#Source Files -> Let CMake know your source files
SET(SRC_FILES       ${CMAKE_SOURCE_DIR}/Dir/main.c) 

add_executable(
        ${PROJECT_NAME}
        ${SRC_FILES})

#Defining compiler preprocessor directives
target_compile_definitions(${PROJECT_NAME} PRIVATE
        $<$<COMPILE_LANGUAGE:C>:  DEFINE_1;__DEBUG;>
        $<$<COMPILE_LANGUAGE:CXX>:DEFINE_2;__DEBUG;>)

#Defining compiler flags
target_compile_options(${PROJECT_NAME} PRIVATE
        $<$<COMPILE_LANGUAGE:C>:    --c99 -c -O0 --cpu ${MCU_ARCH};>
        $<$<COMPILE_LANGUAGE:CXX>:--cpp11 -c -O0 --cpu ${MCU_ARCH};>
        $<$<COMPILE_LANGUAGE:ASM>:--cpu ${MCU_ARCH} -g>)

SET(CMAKE_ASM_FLAGS         "--pd \"DEFINE_3"")
target_link_libraries(${PROJECT_NAME} ${CMAKE_SOURCE_DIR}/SomeLibFile.lib --flagForLibFile)
target_link_options(${PROJECT_NAME} PRIVATE --linkerFlags)

target_include_directories(${PROJECT_NAME} PUBLIC
        ${CMAKE_SOURCE_DIR})

For the necessary flags, have a look at what Keil is using.

Then you can build your application by passing the toolchain.cmake file as -DCMAKE_TOOLCHAIN_FILE=toolchain.cmake

EDIT: Updated based on the feedback from KamilCuk