Question

I have just discovered LLVM and don’t know much about it yet. I have been trying it out using llvm in browser. I can see that any C code I write is converted to LLVM byte code which is then converted to native code. The page shows a textual representation of the byte code. For example for the following C code:

int array[] = { 1, 2, 3};

int foo(int X) {
  return array[X];
}

It shows the following byte code:

target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
target triple = "x86_64-linux-gnu"

@array = global [3 x i32] [i32 1, i32 2, i32 3]   ; <[3 x i32]*> [#uses=1]

define i32 @foo(i32 %X) nounwind readonly {
entry:
  %0 = sext i32 %X to i64                         ; <i64> [#uses=1]
  %1 = getelementptr inbounds [3 x i32]* @array, i64 0, i64 %0 ; <i32*> [#uses=1]
  %2 = load i32* %1, align 4                      ; <i32> [#uses=1]
   ret i32 %2
}

My question is: Can I write the byte code and give it to the llvm assembler to convert to native code skipping the first step of writing C code altogether? If yes, how do I do it? Does any one have any pointers for me?

Answer 1

Yes, surely you can. First, you can write LLVM IR by hand. All tools like llc (which will generate a native code for you) and opt (LLVM IR => LLVM IR optimizer) accept textual representation of LLVM IR as input.