make CMakeLists.txt work with user defined LLVM location

[mo-xiaoming]

If llvm is not installed in default location, or there are multiple installations on the system, then specify LLVM_DIR alone is not suffice

llvm-config path

cmake searches LLVM-Config.cmake under LLVM_DIR. However, current CMakeLists.txt relies on llvm-config to give proper compile/link flags, and llvm-config is still gonna be searched in default locations. The result it, -I and -L will refer to different LLVM build.

If LLVM_DIR and default LLVM installation on the system have different versions/build flags etc., it will throw all kinds of weird linking errors

To solve this problem, LLVM_TOOLS_BINARY_DIR not only has to be added to llvm-config search path, but also must have higher precedence.

-find_program(llvm-config llvm-config PATHS ${LLVM_TOOLS_BINARY_DIR})
+find_program(llvm-config llvm-config PATHS ${LLVM_TOOLS_BINARY_DIR} NO_DEFAULT_PATH)
+find_program(llvm-config llvm-config)

warnings from LLVM header

Another thing is -Werror fails compilation because of warnings emitted from LLVM header files. To solve this, LLVM header files should be considered as SYSTEM header

-include_directories(${LLVM_INCLUDE_DIRS})
+include_directories(SYSTEM ${LLVM_INCLUDE_DIRS})

no-rtti

Because hobbes replies on RTTI, so LLVM it self must be built with RTTI support, that is not default for LLVM

If no-rtti, linking llvm with hobbes will give errors like

/usr/bin/ld: libhobbes.a(jitcc.C.o):(.data.rel.ro._ZTIN6hobbes8LenWatchE[_ZTIN6hobbes8LenWatchE]+0x10): undefined reference to `typeinfo for llvm::JITEventListener'
/usr/bin/ld: libhobbes.a(jitcc.C.o):(.data.rel.ro._ZTIN6hobbes5jitmmE[_ZTIN6hobbes5jitmmE]+0x10): undefined reference to `typeinfo for llvm::SectionMemoryManager

to sum up

To work with custom LLVM location

LLVM must be built with an extra -DLLVM_ENABLE_RTTI=ON

cmake -DCMAKE_INSTALL_PREFIX=$HOME/llmv-project/install-10 -DLLVM_ENABLE_RTTI=ON ../llvm

hobbes must be built with an extra -DLLVM_TOOLS_BINARY_DIR

cmake -Bbuild -S. -DLLVM_DIR=$HOME/llmv-project/install-10/lib/cmake/llvm -DLLVM_TOOLS_BINARY_DIR=$HOME/llmv-project/install-10/bin

[smunix]

should we update this section of the doc as well?

[chenkai036]

Thanks for the change @mo-xiaoming, you must have done lots of research!