[CMake] Alternate compiler specific options - how to specify?

[Miller Henry]

We are using cmake 2.8.11 for out project.

Our local compiler is gcc-4.4.3.  There is desire to use a newer compiler, but we are not yet ready to commit to anything yet.  In the mean time we have installed binaries gcc-4.7 and gcc-4.8.  We can specify the alternate compiler with -DCMAKE_C_COMPILER=gcc-4.7, but we want to go a step farther:  one (believed important) advantage of gcc-4.7 is the option -mtune=atom since that is are target system. We want to force this option when using the newer compiler, but the older version of gcc doesn't accept it.

Toolchain files are not an option, when you use a toolchain file cmake sets CMAKE_CROSSCOMPILING meaning parts of our system that depend on running on x86 will not run.  (it is up to a different team to make a build for other processors - they are nowhere close to done but that variable is used in a few places to disable things that my team needs).

Here is what we come up with, which both feels icky, and seems like a bad compromise:

IF("4.8.0" STREQUAL ${CMAKE_CXX_COMPILER_VERSION} OR "4.8.1" STREQUAL ${CMAKE_CXX_COMPILER_VERSION} OR "4.8" STREQUAL ${USE_GCC_VERSION})
    SET(CMAKE_CXX_COMPILER "g++-4.8")
    SET(CMAKE_C_COMPILER "gcc-4.8")
    SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -g -std=c++11")
    SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unused-local-typedefs" CACHE STRING "" FORCE)
 ELSEIF("4.7.1" STREQUAL ${CMAKE_CXX_COMPILER_VERSION} OR "4.7.2" STREQUAL ${CMAKE_CXX_COMPILER_VERSION} OR "4.7" STREQUAL ${USE_GCC_VERSION})
    SET(CMAKE_CXX_COMPILER "g++-4.7")
    SET(CMAKE_C_COMPILER "gcc-4.7")
ENDIF("4.8.0" STREQUAL ${CMAKE_CXX_COMPILER_VERSION} OR "4.8.1" STREQUAL ${CMAKE_CXX_COMPILER_VERSION} OR "4.8" STREQUAL ${USE_GCC_VERSION})

The advantage of this is you can't accidentally use gcc-4.4 and g++-4.8 - we force them in sync. You can also set USE_GCC_VERSION on the command line and it takes both gcc and g++, instead of having to set both CMAKE_C_COMPILER and CMAKE_CXX_COMPILER (and the line to remember is shorter).  If you do this will work (until we upgrade gcc, but I can solve that)

What is the "right" thing to do - and why is it right?


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