Sun, 18 Jul 2010

LLVM Backend : Milestone #1.

About 3 weeks ago I started work on the LLVM backend for DDC and I have now reached the first milestone.

Over the weekend I attended AusHac2010 and during Friday and Saturday I managed to get DDC modified so I could compile a Main module via the existing C backend and another module via the LLVM backend to produce an executable that ran, but gave an incorrect answer.

Today, I managed to get a very simple function actually working correctly. The function is trivial:

  identInt :: Int -> Int
  identInt a = a

and the generated LLVM code looks like this:

  define external ccc %struct.Obj* @Test_identInt(%struct.Obj* %_va)  
      ; _ENTER (1)
      %local.slotPtr = load %struct.Obj*** @_ddcSlotPtr
      %enter.1 = getelementptr inbounds %struct.Obj** %local.slotPtr, i64 1
      store %struct.Obj** %enter.1, %struct.Obj*** @_ddcSlotPtr
      %enter.2 = load %struct.Obj*** @_ddcSlotMax
      %enter.3 = icmp ult %struct.Obj** %enter.1, %enter.2
      br i1 %enter.3, label %enter.good, label %enter.panic
      call ccc void ()* @_panicOutOfSlots(  ) noreturn
      br label %enter.good
      ; ----- Slot initialization ----- = getelementptr  %struct.Obj** %local.slotPtr, i64 0
      store %struct.Obj* null, %struct.Obj**
      ; ---------------------------------------------------------------
      %u.2 = getelementptr inbounds %struct.Obj** %local.slotPtr, i64 0
      store %struct.Obj* %_va, %struct.Obj** %u.2
      br label %_Test_identInt_start
      ; alt default
      br label %_dEF1_a0
      br label %_dEF0_match_end
      %u.3 = getelementptr inbounds %struct.Obj** %local.slotPtr, i64 0
      %_vxSS0 = load %struct.Obj** %u.3
      ; ---------------------------------------------------------------
      ; _LEAVE
      store %struct.Obj** %local.slotPtr, %struct.Obj*** @_ddcSlotPtr
      ; ---------------------------------------------------------------
      ret %struct.Obj* %_vxSS0

That looks like a lot of code but there are a couple of points to remember:

I have found David Terei's LLVM AST code that I pulled from the GHC sources very easy to use. Choosing this code was definitely not a mistake and I have been corresponding with David, which has resulted in a few updates to this code, including a commit with my name on it.

LLVM is also conceptually very, very sound and easy to work with. For instance, variables in LLVM code are allowed to contain the dot character, so that its easy to avoid name clashes between C function/variable names and names generated during the generation of LLVM code, by making generated names contain a dot.

Finally, I love the fact that LLVM is a typed assembly language. There would have been dozens of times over the weekend that I generated LLVM code that the LLVM compiler rejected because it would't type check. Just like when programming with Haskell, once the code type checked, it actually worked correctly.

Anyway, this is a good first step. Lots more work to be done.

Posted at: 22:18 | Category: CodeHacking/DDC | Permalink