################################################################################################
# Exclude and prepend functionalities
function(exclude OUTPUT INPUT)
set(EXCLUDES ${ARGN})
foreach(EXCLUDE ${EXCLUDES})
        list(REMOVE_ITEM INPUT "${EXCLUDE}")
endforeach()
set(${OUTPUT} ${INPUT} PARENT_SCOPE)
endfunction(exclude)

function(prepend OUTPUT PREPEND)
set(OUT "")
foreach(ITEM ${ARGN})
        list(APPEND OUT "${PREPEND}${ITEM}")
endforeach()
set(${OUTPUT} ${OUT} PARENT_SCOPE)
endfunction(prepend)


################################################################################################
# Clears variables from list
# Usage:
#   caffe_clear_vars(<variables_list>)
macro(caffe_clear_vars)
  foreach(_var ${ARGN})
    unset(${_var})
  endforeach()
endmacro()

################################################################################################
# Prints list element per line
# Usage:
#   caffe_print_list(<list>)
function(caffe_print_list)
  foreach(e ${ARGN})
    message(STATUS ${e})
  endforeach()
endfunction()

################################################################################################
# Reads set of version defines from the header file
# Usage:
#   caffe_parse_header(<file> <define1> <define2> <define3> ..)
macro(caffe_parse_header FILENAME FILE_VAR)
  set(vars_regex "")
  set(__parnet_scope OFF)
  set(__add_cache OFF)
  foreach(name ${ARGN})
    if("${name}" STREQUAL "PARENT_SCOPE")
      set(__parnet_scope ON)
    elseif("${name}" STREQUAL "CACHE")
      set(__add_cache ON)
    elseif(vars_regex)
      set(vars_regex "${vars_regex}|${name}")
    else()
      set(vars_regex "${name}")
    endif()
  endforeach()
  if(EXISTS "${FILENAME}")
    file(STRINGS "${FILENAME}" ${FILE_VAR} REGEX "#define[ \t]+(${vars_regex})[ \t]+[0-9]+" )
  else()
    unset(${FILE_VAR})
  endif()
  foreach(name ${ARGN})
    if(NOT "${name}" STREQUAL "PARENT_SCOPE" AND NOT "${name}" STREQUAL "CACHE")
      if(${FILE_VAR})
        if(${FILE_VAR} MATCHES ".+[ \t]${name}[ \t]+([0-9]+).*")
          string(REGEX REPLACE ".+[ \t]${name}[ \t]+([0-9]+).*" "\\1" ${name} "${${FILE_VAR}}")
        else()
          set(${name} "")
        endif()
        if(__add_cache)
          set(${name} ${${name}} CACHE INTERNAL "${name} parsed from ${FILENAME}" FORCE)
        elseif(__parnet_scope)
          set(${name} "${${name}}" PARENT_SCOPE)
        endif()
      else()
        unset(${name} CACHE)
      endif()
    endif()
  endforeach()
endmacro()

################################################################################################
# Parses a version string that might have values beyond major, minor, and patch
# and set version variables for the library.
# Usage:
#   caffe2_parse_version_str(<library_name> <version_string>)
function(caffe2_parse_version_str LIBNAME VERSIONSTR)
  string(REGEX REPLACE "^([0-9]+).*$" "\\1" ${LIBNAME}_VERSION_MAJOR "${VERSIONSTR}")
  string(REGEX REPLACE "^[0-9]+\\.([0-9]+).*$" "\\1" ${LIBNAME}_VERSION_MINOR  "${VERSIONSTR}")
  string(REGEX REPLACE "[0-9]+\\.[0-9]+\\.([0-9]+).*$" "\\1" ${LIBNAME}_VERSION_PATCH "${VERSIONSTR}")
  set(${LIBNAME}_VERSION_MAJOR ${${LIBNAME}_VERSION_MAJOR} ${ARGN} PARENT_SCOPE)
  set(${LIBNAME}_VERSION_MINOR ${${LIBNAME}_VERSION_MINOR} ${ARGN} PARENT_SCOPE)
  set(${LIBNAME}_VERSION_PATCH ${${LIBNAME}_VERSION_PATCH} ${ARGN} PARENT_SCOPE)
  set(${LIBNAME}_VERSION "${${LIBNAME}_VERSION_MAJOR}.${${LIBNAME}_VERSION_MINOR}.${${LIBNAME}_VERSION_PATCH}" PARENT_SCOPE)
endfunction()

###
# Removes common indentation from a block of text to produce code suitable for
# setting to `python -c`, or using with pycmd. This allows multiline code to be
# nested nicely in the surrounding code structure.
#
# This function respsects Python_EXECUTABLE if it defined, otherwise it uses
# `python` and hopes for the best. An error will be thrown if it is not found.
#
# Args:
#     outvar : variable that will hold the stdout of the python command
#     text   : text to remove indentation from
#
function(dedent outvar text)
  # Use Python_EXECUTABLE if it is defined, otherwise default to python
  if("${Python_EXECUTABLE}" STREQUAL "")
    set(_python_exe "python3")
  else()
    set(_python_exe "${Python_EXECUTABLE}")
  endif()
  set(_fixup_cmd "import sys; from textwrap import dedent; print(dedent(sys.stdin.read()))")
  file(WRITE "${CMAKE_BINARY_DIR}/indented.txt" "${text}")
  execute_process(
    COMMAND "${_python_exe}" -c "${_fixup_cmd}"
    INPUT_FILE "${CMAKE_BINARY_DIR}/indented.txt"
    RESULT_VARIABLE _dedent_exitcode
    OUTPUT_VARIABLE _dedent_text)
  if(NOT _dedent_exitcode EQUAL 0)
    message(ERROR " Failed to remove indentation from: \n\"\"\"\n${text}\n\"\"\"
    Python dedent failed with error code: ${_dedent_exitcode}")
    message(FATAL_ERROR " Python dedent failed with error code: ${_dedent_exitcode}")
  endif()
  # Remove supurflous newlines (artifacts of print)
  string(STRIP "${_dedent_text}" _dedent_text)
  set(${outvar} "${_dedent_text}" PARENT_SCOPE)
endfunction()


function(pycmd_no_exit outvar exitcode cmd)
  # Use Python_EXECUTABLE if it is defined, otherwise default to python
  if("${Python_EXECUTABLE}" STREQUAL "")
    set(_python_exe "python")
  else()
    set(_python_exe "${Python_EXECUTABLE}")
  endif()
  # run the actual command
  execute_process(
    COMMAND "${_python_exe}" -c "${cmd}"
    RESULT_VARIABLE _exitcode
    OUTPUT_VARIABLE _output)
  # Remove supurflous newlines (artifacts of print)
  string(STRIP "${_output}" _output)
  set(${outvar} "${_output}" PARENT_SCOPE)
  set(${exitcode} "${_exitcode}" PARENT_SCOPE)
endfunction()


###
# Helper function to run `python -c "<cmd>"` and capture the results of stdout
#
# Runs a python command and populates an outvar with the result of stdout.
# Common indentation in the text of `cmd` is removed before the command is
# executed, so the caller does not need to worry about indentation issues.
#
# This function respsects Python_EXECUTABLE if it defined, otherwise it uses
# `python` and hopes for the best. An error will be thrown if it is not found.
#
# Args:
#     outvar : variable that will hold the stdout of the python command
#     cmd    : text representing a (possibly multiline) block of python code
#
function(pycmd outvar cmd)
  dedent(_dedent_cmd "${cmd}")
  pycmd_no_exit(_output _exitcode "${_dedent_cmd}")

  if(NOT _exitcode EQUAL 0)
    message(ERROR " Failed when running python code: \"\"\"\n${_dedent_cmd}\n\"\"\"")
    message(FATAL_ERROR " Python command failed with error code: ${_exitcode}")
  endif()
  # Remove supurflous newlines (artifacts of print)
  string(STRIP "${_output}" _output)
  set(${outvar} "${_output}" PARENT_SCOPE)
endfunction()


##############################################################################
# Macro to update cached options.
macro(caffe2_update_option variable value)
  if(CAFFE2_CMAKE_BUILDING_WITH_MAIN_REPO)
    get_property(__help_string CACHE ${variable} PROPERTY HELPSTRING)
    set(${variable} ${value} CACHE BOOL ${__help_string} FORCE)
  else()
    set(${variable} ${value})
  endif()
endmacro()


##############################################################################
# Add an interface library definition that is dependent on the source.
#
# It's probably easiest to explain why this macro exists, by describing
# what things would look like if we didn't have this macro.
#
# Let's suppose we want to statically link against torch.  We've defined
# a library in cmake called torch, and we might think that we just
# target_link_libraries(my-app PUBLIC torch).  This will result in a
# linker argument 'libtorch.a' getting passed to the linker.
#
# Unfortunately, this link command is wrong!  We have static
# initializers in libtorch.a that would get improperly pruned by
# the default link settings.  What we actually need is for you
# to do -Wl,--whole-archive,libtorch.a -Wl,--no-whole-archive to ensure
# that we keep all symbols, even if they are (seemingly) not used.
#
# What caffe2_interface_library does is create an interface library
# that indirectly depends on the real library, but sets up the link
# arguments so that you get all of the extra link settings you need.
# The result is not a "real" library, and so we have to manually
# copy over necessary properties from the original target.
#
# (The discussion above is about static libraries, but a similar
# situation occurs for dynamic libraries: if no symbols are used from
# a dynamic library, it will be pruned unless you are --no-as-needed)
macro(caffe2_interface_library SRC DST)
  add_library(${DST} INTERFACE)
  add_dependencies(${DST} ${SRC})
  # Depending on the nature of the source library as well as the compiler,
  # determine the needed compilation flags.
  get_target_property(__src_target_type ${SRC} TYPE)
  # Depending on the type of the source library, we will set up the
  # link command for the specific SRC library.
  if(${__src_target_type} STREQUAL "STATIC_LIBRARY")
    # In the case of static library, we will need to add whole-static flags.
    if(APPLE)
      target_link_libraries(
          ${DST} INTERFACE -Wl,-force_load,\"$<TARGET_FILE:${SRC}>\")
    elseif(MSVC)
      # In MSVC, we will add whole archive in default.
      target_link_libraries(
         ${DST} INTERFACE "$<TARGET_FILE:${SRC}>")
      target_link_options(
         ${DST} INTERFACE "-WHOLEARCHIVE:$<TARGET_FILE:${SRC}>")
    else()
      # Assume everything else is like gcc
      target_link_libraries(${DST} INTERFACE
          "-Wl,--whole-archive,\"$<TARGET_FILE:${SRC}>\" -Wl,--no-whole-archive")
    endif()
    # Link all interface link libraries of the src target as well.
    # For static library, we need to explicitly depend on all the libraries
    # that are the dependent library of the source library. Note that we cannot
    # use the populated INTERFACE_LINK_LIBRARIES property, because if one of the
    # dependent library is not a target, cmake creates a $<LINK_ONLY:src> wrapper
    # and then one is not able to find target "src". For more discussions, check
    #   https://gitlab.kitware.com/cmake/cmake/issues/15415
    #   https://cmake.org/pipermail/cmake-developers/2013-May/019019.html
    # Specifically the following quote
    #
    # """
    # For STATIC libraries we can define that the PUBLIC/PRIVATE/INTERFACE keys
    # are ignored for linking and that it always populates both LINK_LIBRARIES
    # LINK_INTERFACE_LIBRARIES.  Note that for STATIC libraries the
    # LINK_LIBRARIES property will not be used for anything except build-order
    # dependencies.
    # """
    target_link_libraries(${DST} INTERFACE
        $<TARGET_PROPERTY:${SRC},LINK_LIBRARIES>)
  elseif(${__src_target_type} STREQUAL "SHARED_LIBRARY")
    if("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU")
      target_link_libraries(${DST} INTERFACE
          "-Wl,--no-as-needed,\"$<TARGET_FILE:${SRC}>\" -Wl,--as-needed")
    else()
      target_link_libraries(${DST} INTERFACE ${SRC})
    endif()
    # Link all interface link libraries of the src target as well.
    # For shared libraries, we can simply depend on the INTERFACE_LINK_LIBRARIES
    # property of the target.
    target_link_libraries(${DST} INTERFACE
        $<TARGET_PROPERTY:${SRC},INTERFACE_LINK_LIBRARIES>)
  else()
    message(FATAL_ERROR
        "You made a CMake build file error: target " ${SRC}
        " must be of type either STATIC_LIBRARY or SHARED_LIBRARY. However, "
        "I got " ${__src_target_type} ".")
  endif()
  # For all other interface properties, manually inherit from the source target.
  set_target_properties(${DST} PROPERTIES
    INTERFACE_COMPILE_DEFINITIONS
    $<TARGET_PROPERTY:${SRC},INTERFACE_COMPILE_DEFINITIONS>
    INTERFACE_COMPILE_OPTIONS
    $<TARGET_PROPERTY:${SRC},INTERFACE_COMPILE_OPTIONS>
    INTERFACE_INCLUDE_DIRECTORIES
    $<TARGET_PROPERTY:${SRC},INTERFACE_INCLUDE_DIRECTORIES>
    INTERFACE_SYSTEM_INCLUDE_DIRECTORIES
    $<TARGET_PROPERTY:${SRC},INTERFACE_SYSTEM_INCLUDE_DIRECTORIES>)
endmacro()


##############################################################################
# Creating a Caffe2 binary target with sources specified with relative path.
# Usage:
#   caffe2_binary_target(target_name_or_src <src1> [<src2>] [<src3>] ...)
# If only target_name_or_src is specified, this target is build with one single
# source file and the target name is autogen from the filename. Otherwise, the
# target name is given by the first argument and the rest are the source files
# to build the target.
function(caffe2_binary_target target_name_or_src)
  # https://cmake.org/cmake/help/latest/command/function.html
  # Checking that ARGC is greater than # is the only way to ensure
  # that ARGV# was passed to the function as an extra argument.
  if(ARGC GREATER 1)
    set(__target ${target_name_or_src})
    prepend(__srcs "${CMAKE_CURRENT_SOURCE_DIR}/" "${ARGN}")
  else()
    get_filename_component(__target ${target_name_or_src} NAME_WE)
    prepend(__srcs "${CMAKE_CURRENT_SOURCE_DIR}/" "${target_name_or_src}")
  endif()
  add_executable(${__target} ${__srcs})
  target_link_libraries(${__target} torch_library)
  # If we have Caffe2_MODULES defined, we will also link with the modules.
  if(DEFINED Caffe2_MODULES)
    target_link_libraries(${__target} ${Caffe2_MODULES})
  endif()
  install(TARGETS ${__target} DESTINATION bin)
endfunction()

function(caffe2_hip_binary_target target_name_or_src)
  if(ARGC GREATER 1)
    set(__target ${target_name_or_src})
    prepend(__srcs "${CMAKE_CURRENT_SOURCE_DIR}/" "${ARGN}")
  else()
    get_filename_component(__target ${target_name_or_src} NAME_WE)
    prepend(__srcs "${CMAKE_CURRENT_SOURCE_DIR}/" "${target_name_or_src}")
  endif()

  caffe2_binary_target(${target_name_or_src})

  target_compile_options(${__target} PRIVATE ${HIP_CXX_FLAGS})
  target_include_directories(${__target} PRIVATE ${Caffe2_HIP_INCLUDE})
endfunction()


##############################################################################
# Multiplex between adding libraries for CUDA versus HIP (AMD Software Stack).
# Usage:
#   torch_cuda_based_add_library(cuda_target)
#
macro(torch_cuda_based_add_library cuda_target)
  if(USE_ROCM)
    hip_add_library(${cuda_target} ${ARGN})
  elseif(USE_CUDA)
    add_library(${cuda_target} ${ARGN})
  else()
  endif()
endmacro()

##############################################################################
# Get the HIP arch flags specified by PYTORCH_ROCM_ARCH.
# Usage:
#   torch_hip_get_arch_list(variable_to_store_flags)
#
macro(torch_hip_get_arch_list store_var)
  if(DEFINED ENV{PYTORCH_ROCM_ARCH})
    set(_TMP $ENV{PYTORCH_ROCM_ARCH})
  else()
    # Use arch of installed GPUs as default
    execute_process(COMMAND "rocm_agent_enumerator" COMMAND bash "-c" "grep -v gfx000 | sort -u | xargs | tr -d '\n'"
                    RESULT_VARIABLE ROCM_AGENT_ENUMERATOR_RESULT
                    OUTPUT_VARIABLE ROCM_ARCH_INSTALLED)
    if(NOT ROCM_AGENT_ENUMERATOR_RESULT EQUAL 0)
      message(FATAL_ERROR " Could not detect ROCm arch for GPUs on machine. Result: '${ROCM_AGENT_ENUMERATOR_RESULT}'")
    endif()
    set(_TMP ${ROCM_ARCH_INSTALLED})
  endif()
  string(REPLACE " " ";" ${store_var} "${_TMP}")
endmacro()

##############################################################################
# Get the NVCC arch flags specified by TORCH_CUDA_ARCH_LIST and CUDA_ARCH_NAME.
# Usage:
#   torch_cuda_get_nvcc_gencode_flag(variable_to_store_flags)
#
macro(torch_cuda_get_nvcc_gencode_flag store_var)
  # setting nvcc arch flags
  if((NOT DEFINED TORCH_CUDA_ARCH_LIST) AND (DEFINED ENV{TORCH_CUDA_ARCH_LIST}))
    message(WARNING
        "In the future we will require one to explicitly pass "
        "TORCH_CUDA_ARCH_LIST to cmake instead of implicitly setting it as an "
        "env variable. This will become a FATAL_ERROR in future version of "
        "pytorch.")
    set(TORCH_CUDA_ARCH_LIST $ENV{TORCH_CUDA_ARCH_LIST})
  endif()
  if(DEFINED CUDA_ARCH_NAME)
    message(WARNING
        "CUDA_ARCH_NAME is no longer used. Use TORCH_CUDA_ARCH_LIST instead. "
        "Right now, CUDA_ARCH_NAME is ${CUDA_ARCH_NAME} and "
        "TORCH_CUDA_ARCH_LIST is ${TORCH_CUDA_ARCH_LIST}.")
    set(TORCH_CUDA_ARCH_LIST TORCH_CUDA_ARCH_LIST ${CUDA_ARCH_NAME})
  endif()

  # Invoke cuda_select_nvcc_arch_flags from proper cmake FindCUDA.
  cuda_select_nvcc_arch_flags(${store_var} ${TORCH_CUDA_ARCH_LIST})
endmacro()


##############################################################################
# Add standard compile options.
# Usage:
#   torch_compile_options(lib_name)
function(torch_compile_options libname)
  set_property(TARGET ${libname} PROPERTY CXX_STANDARD 17)
  set(private_compile_options "")

  # ---[ Check if warnings should be errors.
  if(WERROR)
    list(APPEND private_compile_options -Werror)
  endif()

  # until they can be unified, keep these lists synced with setup.py
  if(MSVC)

    if(MSVC_Z7_OVERRIDE)
      set(MSVC_DEBINFO_OPTION "/Z7")
    else()
      set(MSVC_DEBINFO_OPTION "/Zi")
    endif()

    target_compile_options(${libname} PUBLIC
      $<$<COMPILE_LANGUAGE:CXX>:
        ${MSVC_RUNTIME_LIBRARY_OPTION}
        $<$<OR:$<CONFIG:Debug>,$<CONFIG:RelWithDebInfo>>:${MSVC_DEBINFO_OPTION}>
        /EHsc
        /bigobj>
      )
  else()
    list(APPEND private_compile_options
      -Wall
      -Wextra
      -Wdeprecated
      -Wno-unused-parameter
      -Wno-unused-function
      -Wno-missing-field-initializers
      -Wno-unknown-pragmas
      -Wno-type-limits
      -Wno-array-bounds
      -Wno-unknown-pragmas
      -Wno-strict-overflow
      -Wno-strict-aliasing
      )
    if(NOT "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
      list(APPEND private_compile_options
        # Considered to be flaky.  See the discussion at
        # https://github.com/pytorch/pytorch/pull/9608
        -Wno-maybe-uninitialized)
    endif()

  endif()

  if(MSVC)
  elseif(WERROR)
    list(APPEND private_compile_options -Wno-strict-overflow)
  endif()

  target_compile_options(${libname} PRIVATE
      $<$<COMPILE_LANGUAGE:CXX>:${private_compile_options}>)
  if(USE_CUDA)
    string(FIND "${private_compile_options}" " " space_position)
    if(NOT space_position EQUAL -1)
      message(FATAL_ERROR "Found spaces in private_compile_options='${private_compile_options}'")
    endif()
    # Convert CMake list to comma-separated list
    string(REPLACE ";" "," private_compile_options "${private_compile_options}")
    target_compile_options(${libname} PRIVATE
        $<$<COMPILE_LANGUAGE:CUDA>:-Xcompiler=${private_compile_options}>)
  endif()

  if(NOT WIN32 AND NOT USE_ASAN)
    # Enable hidden visibility by default to make it easier to debug issues with
    # TORCH_API annotations. Hidden visibility with selective default visibility
    # behaves close enough to Windows' dllimport/dllexport.
    #
    # Unfortunately, hidden visibility messes up some ubsan warnings because
    # templated classes crossing library boundary get duplicated (but identical)
    # definitions. It's easier to just disable it.
    target_compile_options(${libname} PRIVATE
        $<$<COMPILE_LANGUAGE:CXX>: -fvisibility=hidden>)
  endif()

  # Use -O2 for release builds (-O3 doesn't improve perf, and -Os results in perf regression)
  target_compile_options(${libname} PRIVATE
      $<$<AND:$<COMPILE_LANGUAGE:CXX>,$<OR:$<CONFIG:Release>,$<CONFIG:RelWithDebInfo>>>:-O2>)

endfunction()

##############################################################################
# Set old-style FindCuda.cmake compile flags from modern CMake cuda flags.
# Usage:
#   torch_update_find_cuda_flags()
function(torch_update_find_cuda_flags)
  # Convert -O2 -Xcompiler="-O2 -Wall" to "-O2;-Xcompiler=-O2,-Wall"
  if(USE_CUDA)
    separate_arguments(FLAGS UNIX_COMMAND "${CMAKE_CUDA_FLAGS}")
    string(REPLACE " " "," FLAGS "${FLAGS}")
    set(CUDA_NVCC_FLAGS ${FLAGS} PARENT_SCOPE)

    separate_arguments(FLAGS_DEBUG UNIX_COMMAND "${CMAKE_CUDA_FLAGS_DEBUG}")
    string(REPLACE " " "," FLAGS_DEBUG "${FLAGS_DEBUG}")
    set(CUDA_NVCC_FLAGS_DEBUG "${FLAGS_DEBUG}" PARENT_SCOPE)

    separate_arguments(FLAGS_RELEASE UNIX_COMMAND "${CMAKE_CUDA_FLAGS_RELEASE}")
    string(REPLACE " " "," FLAGS_RELEASE "${FLAGS_RELEASE}")
    set(CUDA_NVCC_FLAGS_RELEASE "${FLAGS_RELEASE}" PARENT_SCOPE)

    separate_arguments(FLAGS_MINSIZEREL UNIX_COMMAND "${CMAKE_CUDA_FLAGS_MINSIZEREL}")
    string(REPLACE " " "," FLAGS_MINSIZEREL "${FLAGS_MINSIZEREL}")
    set(CUDA_NVCC_FLAGS_MINSIZEREL "${FLAGS_MINSIZEREL}" PARENT_SCOPE)

    separate_arguments(FLAGS_RELWITHDEBINFO UNIX_COMMAND "${CMAKE_CUDA_FLAGS_RELWITHDEBINFO}")
    string(REPLACE " " "," FLAGS_RELWITHDEBINFO "${FLAGS_RELWITHDEBINFO}")
    set(CUDA_NVCC_FLAGS_RELWITHDEBINFO "${FLAGS_RELWITHDEBINFO}" PARENT_SCOPE)

    message(STATUS "Converting CMAKE_CUDA_FLAGS to CUDA_NVCC_FLAGS:\n"
                    "    CUDA_NVCC_FLAGS                = ${FLAGS}\n"
                    "    CUDA_NVCC_FLAGS_DEBUG          = ${FLAGS_DEBUG}\n"
                    "    CUDA_NVCC_FLAGS_RELEASE        = ${FLAGS_RELEASE}\n"
                    "    CUDA_NVCC_FLAGS_RELWITHDEBINFO = ${FLAGS_RELWITHDEBINFO}\n"
                    "    CUDA_NVCC_FLAGS_MINSIZEREL     = ${FLAGS_MINSIZEREL}")
  endif()
endfunction()

include(CheckCXXCompilerFlag)

##############################################################################
# CHeck if given flag is supported and append it to provided outputvar
# Also define HAS_UPPER_CASE_FLAG_NAME variable
# Usage:
#   append_cxx_flag_if_supported("-Werror" CMAKE_CXX_FLAGS)
function(append_cxx_flag_if_supported flag outputvar)
    string(TOUPPER "HAS${flag}" _FLAG_NAME)
    string(REGEX REPLACE "[=-]" "_" _FLAG_NAME "${_FLAG_NAME}")
    # GCC silents unknown -Wno-XXX flags, so we detect the corresponding -WXXX.
    if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
      string(REGEX REPLACE "Wno-" "W" new_flag "${flag}")
    else()
      set(new_flag ${flag})
    endif()
    check_cxx_compiler_flag("${new_flag}" ${_FLAG_NAME})
    if(${_FLAG_NAME})
        string(APPEND ${outputvar} " ${flag}")
        set(${outputvar} "${${outputvar}}" PARENT_SCOPE)
    endif()
endfunction()

function(target_compile_options_if_supported target flag)
  set(_compile_options "")
  append_cxx_flag_if_supported("${flag}" _compile_options)
  if(NOT "${_compile_options}" STREQUAL "")
    target_compile_options(${target} PRIVATE ${flag})
  endif()
endfunction()