Writing llvm byte code

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

One very important feature (and design goal) of the LLVM IR language is its 3-way representation:

• The textual representation you can see here
• The bytecode representation (or binary form)
• The in-memory representation

All 3 are indeed completely interchangeable. Nothing that can be expressed in one cannot be expressed in the 2 others as well.

Therefore, as long as you conform to the syntax, you can indeed write the IR yourself. It is rather pointless though, unless used as an exercise to accustom yourself with the format, whether to be better at reading (and diagnosing) the IR or to produce your own compiler :)

Answer 2

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.