I don’t think CMAKE_PROJECT_INCLUDE_BEFORE helps here.
It’s unclear to me how someone might come to that conclusion.
I’m advising the following:
# On the command line:
-DCMAKE_PROJECT_INCLUDE=my_llvm_configuration.cmake
# In my_llvm_configuration.cmake:
set(BOOTSTRAP_CMAKE_PROJECT_INCLUDE "${CMAKE_CURRENT_LIST_FILE}" CACHE PATH "")
if(CLANG_STAGE STREQUAL "stage1")
# stage1-specific settings here
if(CLANG_STAGE STREQUAL "stage2")
# stage2-specific settings here
if(CLANG_STAGE STREQUAL "stage3")
# stage3-specific settings here
Then they can do whatever is appropriate for each stage in those files (which may well go beyond setting variables. Maybe they want to define debug macros that print the output from feature test procedures to see why they’re getting different results in stage2 and stage3, for example.).
Fuchsia takes one of the approaches Singh described, of prepending a prescribed list of variables with BOOTSTRAP_:
# From clang/cmake/caches/Fuchsia.cmake:
foreach(variable ${_FUCHSIA_BOOTSTRAP_PASSTHROUGH})
get_property(is_value_set CACHE ${variable} PROPERTY VALUE SET)
if(${is_value_set})
get_property(value CACHE ${variable} PROPERTY VALUE)
get_property(type CACHE ${variable} PROPERTY TYPE)
set(BOOTSTRAP_${variable} "${value}" CACHE ${type} "")
endif()
endforeach()
Then later the BOOTSTRAP_ prefix gets stripped and each corresponding variable is added to PASSTHROUGH_VARIABLES:
# From clang/CMakeLists.txt:
# Find all variables that start with BOOTSTRAP_ and populate a variable with them.
get_cmake_property(variableNames VARIABLES)
foreach(variableName ${variableNames})
if(variableName MATCHES "^BOOTSTRAP_")
string(SUBSTRING ${variableName} 10 -1 varName)
string(REPLACE ";" "|" value "${${variableName}}")
list(APPEND PASSTHROUGH_VARIABLES -D${varName}=${value})
endif()
if(${variableName} AND variableName MATCHES "LLVM_EXTERNAL_.*_SOURCE_DIR")
list(APPEND PASSTHROUGH_VARIABLES -D${variableName}=${${variableName}})
endif()
endforeach()
Then the contents of PASSTHROUGH_VARIABLES are handed to ExternalProject_Add:
# From clang/CMakeLists.txt:
ExternalProject_Add(${NEXT_CLANG_STAGE}
DEPENDS clang-bootstrap-deps
PREFIX ${NEXT_CLANG_STAGE}
SOURCE_DIR ${CMAKE_SOURCE_DIR}
STAMP_DIR ${STAMP_DIR}
BINARY_DIR ${BINARY_DIR}
EXCLUDE_FROM_ALL 1
CMAKE_ARGS
# We shouldn't need to set this here, but INSTALL_DIR doesn't
# seem to work, so instead I'm passing this through
-DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
${PASSTHROUGH_VARIABLES}
${CLANG_BOOTSTRAP_CMAKE_ARGS}
-DCLANG_STAGE=${NEXT_CLANG_STAGE}
${COMPILER_OPTIONS}
${${CLANG_STAGE}_TABLEGEN}
${LTO_LIBRARY} ${verbose} ${PGO_OPT}
${${CLANG_STAGE}_LINKER}
${${CLANG_STAGE}_AR}
${${CLANG_STAGE}_RANLIB}
${${CLANG_STAGE}_OBJCOPY}
${${CLANG_STAGE}_STRIP}
BUILD_COMMAND ${CMAKE_COMMAND} --build ${BINARY_DIR}
--config ${build_configuration}
${build_tool_args}
INSTALL_COMMAND ""
STEP_TARGETS configure build
USES_TERMINAL_CONFIGURE 1
USES_TERMINAL_BUILD 1
USES_TERMINAL_INSTALL 1
LIST_SEPARATOR |
)
That certainly works (unless you exceed the maximum command buffer size, which I’ve encountered on Windows), though you can run into issues with escape sequences, semicolons, pipe symbols, and spaces during the translation from CMake to the command line, and then back into CMake (which I’ve encountered). It’s also doing a lot of string manipulation, and it’s possible (I haven’t measured it) that that’s burning enough cycles to be meaningfully contributing to build times.
A similar situation arises when trying to pass configuration settings to the “Builtins” or “Runtimes” external projects, where you need to prepend BUILTINS_<target>_ or RUNTIMES_<target>_ to each variable you want to pass through (see: Fuchsia-stage2.cmake, CrossWinToArmLinux.cmake, etc). Instead, I would suggest adding the following to the my_llvm_configuration.cmake example above:
# in my_llvm_configuration.cmake:
# Configure the builtins and runtimes projects to load this file as well.
set( BUILTINS_<target>_CMAKE_PROJECT_INCLUDE "${CMAKE_CURRENT_LIST_FILE}" CACHE PATH "" )
set( RUNTIMES_<target>_CMAKE_PROJECT_INCLUDE "${CMAKE_CURRENT_LIST_FILE}" CACHE PATH "" )
if(CMAKE_PROJECT_NAME STREQUAL "Runtimes")
# runtimes-specific settings here
if(CMAKE_PROJECT_NAME STREQUAL "CompilerRTBuiltins")
# builtins-specific settings here
# Note that CMAKE_PROJECT_NAME will not be set when this
# configuration file is invoked if you've used one of the
# *_BEFORE variants (such as CMAKE_PROJECT_INCLUDE_BEFORE).
This allows you to avoid rewriting options using these various prefixes multiple times. For example:
# in clang/cmake/caches/Fuchsia.cmake:
foreach(target aarch64-unknown-linux-gnu; ... )
set(RUNTIMES_${target}_LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
if(BOOTSTRAP_CMAKE_SYSTEM_NAME)
if(STAGE2_LINUX_${target}_SYSROOT)
set(RUNTIMES_${target}_LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
# in clang/cmake/caches/Fuchsia-stage2.cmake:
if(APPLE)
set(LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
foreach(target aarch64-unknown-linux-gnu; ... )
if(LINUX_${target}_SYSROOT)
set(RUNTIMES_${target}_LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
if(FUCHSIA_SDK)
foreach(target x86_64-unknown-fuchsia; ... )
set(RUNTIMES_${target}_LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
(And this isn’t to pick on Fuchsia; I’m using their project configurations in my examples here because they’re by far and away the most thoroughly specified project configurations currently in the codebase.)