Detect 32-bit x86 processor in CMakeList.txt?

We are catching errors in our CMake makefiles due to lack of -fPIC. Her's one from a ci20 MIPS dev-board:

...
[ 92%] Built target cryptopp-object
Scanning dependencies of target cryptopp-shared
Scanning dependencies of target cryptopp-static
Linking CXX static library libcryptopp.a
Linking CXX shared library libcryptopp.so
/usr/bin/ld: CMakeFiles/cryptopp-object.dir/cryptlib.cpp.o: relocation R_MIPS_HI16 against
`a local symbol' can not be used when making a shared object; recompile with -fPIC
CMakeFiles/cryptopp-object.dir/cryptlib.cpp.o: could not read symbols: Bad value
collect2: ld returned 1 exit status

The project's policy is to us PIC everywhere except 32-bit x86 due to register pressures. That means x86_64, ARM-32, Aarch32, Aarch64, MIPS, MIPS64, UltraSparc, etc get PIC.

I believe the target processor is provided in CMAKE_SYSTEM_PROCESSOR. The problem I am having is the docs don't tell me the values, so I can't figure out how to craft a "not 32-bit x86" test.

How do I detect 32-bit x86 processor in CMakeList.txt?

Even better, I would like to see a comprehensive list of processors that CMake sets CMAKE_SYSTEM_PROCESSOR to. If anyone has the list, then it would be great to provide it.

Solution

I probably would build something around the compiler.

A close approximation using existing variables/modules would be:

include(TestBigEndian)

if (NOT WIN32)
    TEST_BIG_ENDIAN(_bigendian)
    if((CMAKE_SIZEOF_VOID_P GREATER 4) OR (_bigendian))
        message(
            STATUS "Setting ${CMAKE_CXX_COMPILE_OPTIONS_PIC} "
                   "for machine ${CMAKE_HOST_SYSTEM_PROCESSOR}"
        )
        set(CMAKE_POSITION_INDEPENDENT_CODE 1)
    endif()
endif()

In short what I have done:

WIN32 is also valid for 64 Bit Windows compilers/environments
CMAKE_SIZEOF_VOID_P GREATER 4 checks for "greater then 32 Bit"
The last is the biggest assumption: take all little endian processors as Intel/AMD based
Used more generic CMAKE_POSITION_INDEPENDENT_CODE to set -fPIC

I admit a more accurate method would be to build something around a pre-defined macros test.

Edit: Added "Predefined Macros Check" Alternative

Here is the more precise check for predefined macros:

include(CheckCXXSourceCompiles)

if (CMAKE_CXX_COMPILE_OPTIONS_PIC)
    set(
        _preDefMacrosX86 
            __i386 __i386__ __i486__ __i586__ __i686__      
            _M_I86 _M_IX86 __X86__ _X86_ __THW_INTEL__
            __I86__ __INTEL__ __386
    )
    set(_code "void main() {}")
    foreach(_macro IN LISTS _preDefMacrosX86)
        set(
            _code
            "${_code}\n\#ifdef ${_macro}\n\#error ${_macro} is defined\n\#endif"
        )
    endforeach()
    CHECK_CXX_SOURCE_COMPILES("${_code}" _canCompileX86DoesFailCheck)

    if((CMAKE_SIZEOF_VOID_P GREATER 4) OR (_canCompileX86DoesFailCheck))
        message(STATUS "Setting ${CMAKE_CXX_COMPILE_OPTIONS_PIC}")
        set(CMAKE_POSITION_INDEPENDENT_CODE 1)
    endif()
endif()

References