How to use LLVM library in c++ Xcode project? - c++

Question

I'm loosely following the tutorial to implement Kaleidoscope on the LLVM website, and I'm at the point where I need to actually use the LLVM library for code generation. I've installed LLVM 13 using homebrew (brew install llvm), but I can't figure out how to use it in a c++ project in Xcode. I just get the error 'llvm/whatever/whatever' file not found for every file I try to include:
#include "llvm/ADT/STLExtras.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Verifier.h"
I've tried adding the include folder (/opt/homebrew/Cellar/llvm/13.0.1_1/include for me) to the header search paths in Xcode, which seems to do something but I end up with the error Undefined symbol: llvm::DisableABIBreakingChecks, and hundreds of different warnings.
All I want is to be able to use LLVM code generation in a small c++ project. Is there any simple way to do this?

Answer 1

I'm also doing that tutorial. I'll tell you how I got it to work, but since I'm not an LLVM expert, I don't know if this is the best way.
I checked out and built LLVM from source, as described here. Installing with brew like you did should also work fine.
Next, I looked at the build command they give in the tutorial:
clang++ -g -O3 toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core`
-o toy
I can run that llvm-config command from my build directory and it gives me the flags that I need to somehow give to Xcode.
> bin/llvm-config --cxxflags --ldflags --system-libs --libs core
-I/Users/rob/Foo/llvm-project/llvm/include
-I/Users/rob/Foo/llvm-project/build/include
-std=c++14 -fno-exceptions -fno-rtti
-D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS
-L/Users/rob/Foo/llvm-project/build/lib
-Wl,-search_paths_first -Wl,-headerpad_max_install_names
-lLLVMCore -lLLVMRemarks -lLLVMBitstreamReader
-lLLVMBinaryFormat -lLLVMSupport -lLLVMDemangle
-lm -lz -lcurses -lxml2
I put those -I paths in the HEADER_SEARCH_PATHS build setting, and the -L path in LIBRARY_SEARCH_PATHS. In Xcode those values are a list of paths, so you add the paths without the -I or -L part. I then put the -l library arguments in OTHER_LDFLAGS, and the -D arguments in OTHER_CPLUSPLUSFLAGS.
After that, it compiled and ran from Xcode. I did not yet translate all of the output from llvm-config into Xcode build settings. For example, I did nothing about these:
-std=c++14 -fno-exceptions -fno-rtti
-Wl,-search_paths_first -Wl,-headerpad_max_install_names
Remains to be seen if that will cause problems later.