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I have very little experience with cmake, this is really the first time I used it for a project. In the past I used some autotools and recently mostly bazel. I would appreciate some suggestions how to better structure the code. For example during compilation I noticed that the same targets get compiled multiple times, which ideally I would like to avoid.

cmake_minimum_required(VERSION 3.10)
project(schwifty)

set(CMAKE_CXX_STANDARD 14)

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DELPP_FEATURE_CRASH_LOG")

set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/out)
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/out)

find_package(PythonInterp 3.6 REQUIRED)

file(MAKE_DIRECTORY downloads external)

################################################################################
# Easylogging++
################################################################################
if(EXISTS "external/easyloggingpp")
else()
file(MAKE_DIRECTORY external/easyloggingpp)
file(DOWNLOAD
        https://github.com/muflihun/easyloggingpp/archive/v9.96.4.zip
        downloads/easyloggingpp.zip)
execute_process(COMMAND unzip downloads/easyloggingpp.zip -d downloads)
file(GLOB easyloggingpp_files downloads/easyloggingpp-9.96.4/src/easylogging++.*)
file(COPY ${easyloggingpp_files} DESTINATION external/easyloggingpp)
endif()

set(ast ast.h ast.cc)
set(codegen codegen.h codegen.cc)
set(functions functions.h functions.cc)
set(parser parser.h parser.cc)

include_directories(external/easyloggingpp)
set(easyloggingpp external/easyloggingpp/easylogging++.cc)

set(SOURCE_FILES
        ast_compare_visitor.cc
        ast_compare_visitor.h
        classes.cc
        classes.h
        compilation_context.cc
        compilation_context.h
        common.h
        errors.h
        errors.cc
        expression_type_visitor.cc
        expression_type_visitor.h
        functions.cc
        functions.h
        jit.cc
        jit.h
        lexer.cc
        lexer.h
        lexer_common.cc
        lexer_common.h
        runtime.h
        runtime.cc
        utils.h
        utils.cc
        type.cc
        type.h
        type_inference_visitor.cc
        type_inference_visitor.h
        enum.cc
        enum.h
        type_inference.cc
        type_inference.h
        operators.cc
        operators.h
        symbol_visitor.cc
        symbol_visitor.h)

add_library(sources ${SOURCE_FILES})

find_package(LLVM REQUIRED CONFIG)

message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
message(STATUS "Using LLVMConfig.cmake in: ${LLVM_DIR}")

include_directories(${LLVM_INCLUDE_DIRS})
add_definitions(${LLVM_DEFINITIONS})

llvm_map_components_to_libnames(llvm_libs all)

find_package(FMT REQUIRED CONFIG)

add_executable(schwifty
        schwifty.cc
        ${ast}
        ${codegen}
        ${easyloggingpp}
        ${parser})

target_link_libraries(schwifty ${llvm_libs})
target_link_libraries(schwifty fmt::fmt)
target_link_libraries(schwifty sources)

################################################################################
# Testing
################################################################################

enable_testing()
find_package(gtest REQUIRED)

include_directories(${GTEST_INCLUDE_DIRS})

add_executable(codegen_test codegen_test.cc ${ast} ${codegen} ${easyloggingpp}
        ${functions} ${parser})
target_link_libraries(codegen_test ${GTEST_BOTH_LIBRARIES})
target_link_libraries(codegen_test ${llvm_libs})
target_link_libraries(codegen_test fmt::fmt)
target_link_libraries(codegen_test sources)
add_test(codegen_test COMMAND out/codegen_test)

add_executable(lexer_test lexer_test.cc ${ast} ${codegen} ${easyloggingpp}
        ${functions} ${parser})
target_link_libraries(lexer_test ${GTEST_BOTH_LIBRARIES})
target_link_libraries(lexer_test ${llvm_libs})
target_link_libraries(lexer_test fmt::fmt)
target_link_libraries(lexer_test sources)
add_test(lexer_test COMMAND out/lexer_test)

add_executable(parser_test parser_test.cc ${ast} ${codegen} ${easyloggingpp}
        ${functions} ${parser})
target_link_libraries(parser_test ${GTEST_BOTH_LIBRARIES})
target_link_libraries(parser_test ${llvm_libs})
target_link_libraries(parser_test fmt::fmt)
target_link_libraries(parser_test sources)
add_test(parser_test COMMAND out/parser_test)

add_executable(type_test type_test.cc ${ast} ${codegen} ${easyloggingpp}
        ${functions} ${parser})
target_link_libraries(type_test ${GTEST_BOTH_LIBRARIES})
target_link_libraries(type_test ${llvm_libs})
target_link_libraries(type_test fmt::fmt)
target_link_libraries(type_test sources)
add_test(type_test COMMAND out/type_test)

add_executable(type_inference_test type_inference_test.cc ${ast} ${codegen}
        ${easyloggingpp} ${functions} ${parser})
target_link_libraries(type_inference_test ${GTEST_BOTH_LIBRARIES})
target_link_libraries(type_inference_test ${llvm_libs})
target_link_libraries(type_inference_test fmt::fmt)
target_link_libraries(type_inference_test sources)
add_test(type_inference_test COMMAND ./out/type_inference_test)

add_test(NAME end_to_end_tests WORKING_DIRECTORY ${CTEST_SOURCE_DIRECTORY}
        COMMAND ${PYTHON_EXECUTABLE} ${CTEST_SOURCE_DIRECTORY}/end_to_end_tests.py)
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2 Answers 2

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I'm by no means a professional CMake user, I try to follow best practices though and have seen many talks and articles about modern CMake best practices, so lets go through your CMakeLists.

set(CMAKE_CXX_STANDARD 14)

This is generally frowned upon for two different reasons. First and most important, you set this configuration globally for every target you create or import through add_subdirectory. In modern CMake best practices you should always prefer target_ functions whenever possible to configure exactly the target which needs the configuration instead of setting it globally. Second, you should not need to set the C++ standard directly, instead you should select features which you need to compile your project and let CMake decide the standard. See target_compile_features.

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DELPP_FEATURE_CRASH_LOG")

Same problem as previously, use target_compile_definitions(mytarget PUBLIC ELPP_FEATURE_CRASH_LOG)

#
# Easylogging++
#
if(EXISTS "external/easyloggingpp")
else()
    file(MAKE_DIRECTORY external/easyloggingpp)
    file(DOWNLOAD https://github.com/muflihun/easyloggingpp/archive/v9.96.4.zip
                  downloads/easyloggingpp.zip)
    execute_process(COMMAND unzip downloads/easyloggingpp.zip -d downloads)
    file(GLOB easyloggingpp_files
              downloads/easyloggingpp-9.96.4/src/easylogging++.*)
    file(COPY ${easyloggingpp_files} DESTINATION external/easyloggingpp)
endif()

This looks like a messy hack to pull in a dependency. If this dependency is required to build your project you should probably add it as a subrepository to your own git source repository and use it using add_subdirectory (assuming it's a CMake project). Alternatively, there is also the ExternalProject module which exists for this sole reason, to pull in and compile external dependencies. 

include_directories(external/easyloggingpp)
add_library(easyloggingpp external/easyloggingpp/easylogging++.cc)

You're configuring globally again with include_directories, use target_include_directories instead. Also, I'd really consider splitting up your CMakeLists file, there is too much going on. Subdivide your project repository to subdirectories, one per library, and then use add_subdirectory to pull in all the libraries you need.

find_package(gtest REQUIRED)
include_directories(${GTEST_INCLUDE_DIRS})

You're setting include directories globally again. Actually, you don't even need to set the directories at all. target_link_libraries does a lot more than linking. It probably should've called differently. Since gtest exports a target with it's INTERFACE_INCLUDE_DIRECTORIES set up, solely linking (target_link_libraries) to gtest sets up include directories automatically for the target.

If you follow best practices and set all configurations with target_ functions then all you need to pull in a library should be a sole target_link_library since all other configurations (compiler features, include directories, ...) are automatically pulled in, given they're either set PUBLIC or INTERFACE on this library. As I said, target_link_library does a lot more than just linking, it's name is very misleading. A perfect example is the fmt package you're using. All you're doing is find_package(FMT REQUIRED CONFIG) and target_link_libraries(mytarget fmt::fmt) and everything else to use this package is set up by the target_link_libraries command since the fmt package exports all its own requirements and include paths through its target.

I'm pretty sure I missed a few things but I hope these pointers help you to get started.

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Instead of using set for source files, which made them get recompiled multiple times, I switched add_library and now it builds everything much faster. No more unnecessarily compiling the same file again.

cmake_minimum_required(VERSION 3.10)
project(schwifty)

set(CMAKE_CXX_STANDARD 14)

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DELPP_FEATURE_CRASH_LOG")

set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/out)
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/out)

find_package(PythonInterp 3.6 REQUIRED)

file(MAKE_DIRECTORY downloads external)

#
# Easylogging++
#
if(EXISTS "external/easyloggingpp")
else()
    file(MAKE_DIRECTORY external/easyloggingpp)
    file(DOWNLOAD https://github.com/muflihun/easyloggingpp/archive/v9.96.4.zip
                  downloads/easyloggingpp.zip)
    execute_process(COMMAND unzip downloads/easyloggingpp.zip -d downloads)
    file(GLOB easyloggingpp_files
              downloads/easyloggingpp-9.96.4/src/easylogging++.*)
    file(COPY ${easyloggingpp_files} DESTINATION external/easyloggingpp)
endif()

include_directories(external/easyloggingpp)
add_library(easyloggingpp external/easyloggingpp/easylogging++.cc)

#
# Local lib targets
#

add_library(ast ast.h ast.cc)

add_library(ast_compare_visitor ast_compare_visitor.h ast_compare_visitor.cc)

add_library(classes classes.h classes.cc)

add_library(codegen
            codegen.h
            codegen.cc
            codegen_common.h
            codegen_common.cc
            expression_type_visitor.cc
            expression_type_visitor.h)

add_library(common common.h utils.h utils.cc)

add_library(compilation_context
            compilation_context.h
            compilation_context.cc
            enum.h
            enum.cc
            errors.h
            errors.cc
            operators.h
            operators.cc
            type.h
            type.cc)

add_library(functions functions.h functions.cc)

add_library(jit jit.cc jit.h)

add_library(lexer lexer.cc lexer.h lexer_common.cc lexer_common.h)

add_library(parser parser.h parser.cc)

add_library(runtime runtime.cc runtime.h)

add_library(type_inference
            type_inference.h
            type_inference.cc
            symbol_visitor.cc
            symbol_visitor.h
            type_inference_visitor.cc
            type_inference_visitor.h)

#
# External lib targets
#

find_package(LLVM REQUIRED CONFIG)

message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
message(STATUS "Using LLVMConfig.cmake in: ${LLVM_DIR}")

include_directories(${LLVM_INCLUDE_DIRS})
add_definitions(${LLVM_DEFINITIONS})

llvm_map_components_to_libnames(llvm_libs all)

find_package(FMT REQUIRED CONFIG)

#
# Schwifty main executable
#

add_executable(schwifty schwifty.cc)
target_link_libraries(schwifty
                      ${llvm_libs}
                      ast
                      classes
                      codegen
                      common
                      compilation_context
                      easyloggingpp
                      fmt::fmt
                      functions
                      lexer
                      parser
                      runtime
                      type_inference)

#
# Testing
#

enable_testing()
find_package(gtest REQUIRED)

include_directories(${GTEST_INCLUDE_DIRS})

add_executable(codegen_test codegen_test.cc)
target_link_libraries(codegen_test
                      ${GTEST_BOTH_LIBRARIES}
                      ${llvm_libs}
                      easyloggingpp
                      ast
                      classes
                      codegen
                      common
                      compilation_context
                      fmt::fmt
                      functions
                      jit
                      lexer
                      parser
                      runtime
                      type_inference)
add_test(codegen_test COMMAND out/codegen_test)

add_executable(lexer_test lexer_test.cc)
target_link_libraries(lexer_test
                      ${GTEST_BOTH_LIBRARIES}
                      ast
                      common
                      compilation_context
                      easyloggingpp
                      functions
                      lexer
                      parser
                      fmt::fmt)
add_test(lexer_test COMMAND out/lexer_test)

add_executable(parser_test parser_test.cc)
target_link_libraries(parser_test
                      ${GTEST_BOTH_LIBRARIES}
                      ast
                      ast_compare_visitor
                      compilation_context
                      common
                      easyloggingpp
                      functions
                      lexer
                      parser
                      fmt::fmt)
add_test(parser_test COMMAND out/parser_test)

add_executable(type_test type_test.cc)
target_link_libraries(type_test
                      ${GTEST_BOTH_LIBRARIES}
                      ast
                      common
                      compilation_context
                      easyloggingpp
                      functions
                      lexer
                      parser)
add_test(type_test COMMAND out/type_test)

add_executable(type_inference_test type_inference_test.cc)
target_link_libraries(type_inference_test
                      ${GTEST_BOTH_LIBRARIES}
                      easyloggingpp
                      ast
                      classes
                      common
                      compilation_context
                      functions
                      fmt::fmt
                      lexer
                      parser
                      runtime
                      type_inference)
add_test(type_inference_test COMMAND ./out/type_inference_test)

add_test(NAME end_to_end_tests
         WORKING_DIRECTORY ${CTEST_SOURCE_DIRECTORY}
         COMMAND ${PYTHON_EXECUTABLE} end_to_end_tests.py)
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