How to make clang compile to llvm IR

Question

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.

If you have multiple files and you don't want to have to type each file, I would recommend that you follow these simple steps (I am using clang-3.8 but you can use any other version):

1. generate all .ll files

   clang-3.8 -S -emit-llvm *.c
   

2. link them into a single one

   llvm-link-3.8 -S -v -o single.ll *.ll
   

3. (Optional) Optimise your code (maybe some alias analysis)

   opt-3.8 -S -O3 -aa -basicaaa -tbaa -licm single.ll -o optimised.ll
   

4. Generate assembly (generates a optimised.s file)

   llc-3.8 optimised.ll
   

5. Create executable (named a.out)

   clang-3.8 optimised.s
   

Did you read clang documentation ? You're probably looking for -emit-llvm.