README.md

Linking LLVM bitcode

In this section, we will link generated .bc files to get one single bitcode containing all the references.

We have 2 files of code, where one makes a references to the other.

First, we convert the C code to .bc file by lowering it twice :

$ clang -emit-llvm -S test1.c -o test1.ll
$ clang -emit-llvm -S test2.c -o test2.ll

$ llvm-as test1.ll -o test1.bc
$ llvm-as test2.ll -o test2.bc

Now we have our two .bc files ready. With test2.bc making a referenre to the func syntax from the test1.bc file.

Finally, we need to invoke the llvm-link command in order to link our two LLVM bitcode files :

$ llvm-link test1.bc test2.bc -o output.bc

If we upper our code back to LLVM IR using the llvm-dis tool, we get the output.ll file, which contains the definition of the two functions : main and func.

$ llvm-dis output.bc -o output.ll

Gives us one file containing the two function definitions :

...
; Function Attrs: noinline nounwind optnone uwtable
define dso_local i32 @func(i32 noundef %0) #0 {
  %2 = alloca i32, align 4
  store i32 %0, ptr %2, align 4
  %3 = load i32, ptr %2, align 4
  %4 = mul nsw i32 %3, 2
  store i32 %4, ptr %2, align 4
  %5 = load i32, ptr %2, align 4
  ret i32 %5
}

; Function Attrs: noinline nounwind optnone uwtable
define dso_local i32 @main() #0 {
  %1 = alloca i32, align 4
  %2 = alloca i32, align 4
  store i32 0, ptr %1, align 4
  store i32 5, ptr %2, align 4
  %3 = load i32, ptr %2, align 4
  %4 = call i32 @func(i32 noundef %3)
  store i32 %4, ptr %2, align 4
  %5 = load i32, ptr %2, align 4
  %6 = call i32 (ptr, ...) @printf(ptr noundef @.str, i32 noundef %5)
  %7 = load i32, ptr %2, align 4
  ret i32 %7
}
...