[feature] Design an internal macro to detect arch setting, _conan_detect_arch().

[hwhsu1231]

Feature Request

In the conan_cmake_autodetect() macro (so far 1.18.1), there are some internal macros to detect the required settings:

• _conan_detect_build_type()
• _conan_check_system_name()
• _conan_check_language()
• _conan_detect_compiler()
• _collect_settings()

As we can see, there isn't a macro to detect arch setting. Only when we use MSVC compilers, _conan_detect_compiler() will detect it specifically. Since arch is one of the required settings, I suggest that cmake-conan design an internal macro called _conan_detect_arch() to do so.

[hwhsu1231]

This issue is related to: #448

[hwhsu1231]

In fact, CMake already defined a variable called CMAKE_LANG_COMPILER_ARCHITECTURE_ID, which should be able to detect the architecture of compilers currently used. However, this variable is NOT computed everywhere for now. Here are the links of related issues in CMake repository:

• [bug] CMAKE_lang_COMPILER_ARCHITECTURE_ID is not working with C/C++ compilers installed in MSYS2.
• Compute CMAKE_C_COMPILER_ARCHITECTURE_ID everywhere

According to my experiments on Windows, the following compilers is computed:

• MSVC:

  %VCToolsInstallDir%\bin\Hostx64\x64\cl.exe
  %VCToolsInstallDir%\bin\Hostx64\x86\cl.exe
  %VCToolsInstallDir%\bin\Hostx86\x64\cl.exe
  %VCToolsInstallDir%\bin\Hostx86\x86\cl.exe
  

• MSVC-based Clang:

  C:\Program Files\LLVM\bin\clang.exe
  C:\Program Files\LLVM\bin\clang-cl.exe
  C:\Program Files (x86)\LLVM\bin\clang.exe
  C:\Program Files (x86)\LLVM\bin\clang-cl.exe
  %VCINSTALLDIR%\Tools\Llvm\x64\bin\clang.exe
  %VCINSTALLDIR%\Tools\Llvm\x64\bin\clang-cl.exe
  %VCINSTALLDIR%\Tools\Llvm\bin\clang.exe
  %VCINSTALLDIR%\Tools\Llvm\bin\clang-cl.exe
  

Therefore, the implementation of _conan_detect_arch() can be like:

macro(_conan_detect_arch)
    conan_parse_arguments(${ARGV})
    
    if (ARGUMENTS_ARCH)
        set(_CONAN_SETTING_ARCH ${ARGUMENTS_ARCH})
    else ()
        if ("${CMAKE_${LANGUAGE}_COMPILER_ID}" STREQUAL "MSVC" 
            OR ("${CMAKE_${LANGUAGE}_COMPILER_ID}" STREQUAL "Clang" 
                AND "${CMAKE_${LANGUAGE}_SIMULATE_ID}" STREQUAL "MSVC"))
            # using MSVC 
            # using MSVC-based Clang
            if (CMAKE_${LANGUAGE}_COMPILER_ARCHITECTURE_ID MATCHES "64")
                set(_CONAN_SETTING_ARCH x86_64)
            elseif (CMAKE_${LANGUAGE}_COMPILER_ARCHITECTURE_ID MATCHES "^ARM")
                set(_CONAN_SETTING_ARCH armv6)
            elseif (CMAKE_${LANGUAGE}_COMPILER_ARCHITECTURE_ID MATCHES "86")
                set(_CONAN_SETTING_ARCH x86)
            else ()
                # Other architectures...
            endif()
        else ()
            # Other C/C++ compilers...
            # Use other mechanism to detect its architecture 
        endif ()
    endif ()
endmacro()

Theoratically, once CMAKE_LANG_COMPILER_ARCHITECTURE_ID can be computed everywhere, the implementation should be able to reduce to:

macro(_conan_detect_arch)
    conan_parse_arguments(${ARGV})
    
    if (ARGUMENTS_ARCH)
        set(_CONAN_SETTING_ARCH ${ARGUMENTS_ARCH})
    else ()
        if (CMAKE_${LANGUAGE}_COMPILER_ARCHITECTURE_ID MATCHES "64")
            set(_CONAN_SETTING_ARCH x86_64)
        elseif (CMAKE_${LANGUAGE}_COMPILER_ARCHITECTURE_ID MATCHES "^ARM")
            set(_CONAN_SETTING_ARCH armv6)
        elseif (CMAKE_${LANGUAGE}_COMPILER_ARCHITECTURE_ID MATCHES "86")
            set(_CONAN_SETTING_ARCH x86)
        else ()
            # Other architectures...
        endif()
    endif ()
endmacro()