[c] How to make clang compile to llvm IR

I want clang to compile my C/C++ code to LLVM bytecode rather than binary executable. How can I achieve that? And if I get the LLVM bytecode, how can I take it to further compile it to binary executable.

Basically I want to add some of my own code to the LLVM bytecode before compiling to binary executable.

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

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