How to make clang compile to llvm IR
I want clang to compile my C/C++ code to LLVM bitcode rather than a binary executable. How can I achieve that?
And if I have the LLVM bitcode, how can I further compile it to a binary executable?
I want to add some of my own code to the LLVM bitcode before compiling to a binary executable.
Given some C/C++ file foo.c:
> clang -S -emit-llvm foo.c
Produces foo.ll which is an LLVM IR file.
The -emit-llvm option can also be passed to the compiler front-end directly, and not the driver by means of -cc1:
> clang -cc1 foo.c -emit-llvm
Produces foo.ll with the IR. -cc1 adds some cool options like -ast-print. Check out -cc1 --help for more details.
To compile LLVM IR further to assembly, use the llc tool:
> llc foo.ll
Produces foo.s with assembly (defaulting to the machine architecture you run it on). llc is one of the LLVM tools - here is its documentation.
Use
clang -emit-llvm -o foo.bc -c foo.c
clang -o foo foo.bc
If you have multiple source files, you probably actually want to use link-time-optimization to output one bitcode file for the entire program. The other answers given will cause you to end up with a bitcode file for every source file.
Instead, you want to compile with link-time-optimization
clang -flto -c program1.c -o program1.o
clang -flto -c program2.c -o program2.o
and for the final linking step, add the argument -Wl,-plugin-opt=also-emit-llvm
clang -flto -Wl,-plugin-opt=also-emit-llvm program1.o program2.o -o program
This gives you both a compiled program and the bitcode corresponding to it (program.bc). You can then modify program.bc in any way you like, and recompile the modified program at any time by doing
clang program.bc -o program
although be aware that you need to include any necessary linker flags (for external libraries, etc) at this step again.
Note that you need to be using the gold linker for this to work. If you want to force clang to use a specific linker, create a symlink to that linker named "ld" in a special directory called "fakebin" somewhere on your computer, and add the option
-B/home/jeremy/fakebin
to any linking steps above.