Question

I am writing a compiler for a small imperative language. The target language is Java bytecode, and the compiler is implemented in Haskell.

I’ve written a frontend for the language – i.e I have a lexer, parser and typechecker. I’m having trouble figuring out how to do code generation.

I keep a data structure representing the stack of local variables. I can query this structure with the name of a local variable and get its position in the stack. This data structure is passed around as I walk the syntax tree, and variables are popped and pushed as I enter and exit new scopes.

What I having trouble figuring out is how to emit the bytecode. Emitting strings at terminals and concatenating them at higher levels seems like a poor solution, both clarity- and performance-wise.

tl;dr How do I emit bytecode while waling the syntax tree?

Answer 1

My first project in Haskell a few months back was to write a c compiler, and what resulted was a fairly naive approach to code generation, which I’ll walk through here. Please do not take this as an example of good design for a code generator, but rather view it as a quick and dirty (and ultimately naive) way to get something that works fairly quickly with decent performance.

I began by defining an intermediate representation LIR (Lower Intermediate Representation) which closely corresponded to my instruction set (x86_64 in my case):

data LIRInst = LIRRegAssignInst LIRReg LIRExpr
             | LIRRegOffAssignInst LIRReg LIRReg LIRSize LIROperand
             | LIRStoreInst LIRMemAddr LIROperand
             | LIRLoadInst LIRReg LIRMemAddr
             | LIREnterInst LIRInt
             | LIRJumpLabelInst LIRLabel
             | LIRIfInst LIRRelExpr LIRLabel LIRLabel -- false, then true
             | LIRCallInst LIRLabel LIRLabel -- method label, return label
             | LIRCalloutInst String
             | LIRRetInst [LIRLabel] String -- list of successors, and the name of the method returning from
             | LIRLabelInst LIRLabel
             deriving (Show, Eq, Typeable)

Next up came a monad that would handle interleaving state throughout the translation (I was blissfully unaware of our friend-the State Monad-at the time):

newtype LIRTranslator a = LIRTranslator
    { runLIR :: Namespace -> (a, Namespace) }

instance Monad LIRTranslator where
    return a = LIRTranslator (\s -> (a, s))
    m >>= f = LIRTranslator (\s ->
        let (a, s') = runLIR m s
        in runLIR (f a) s')

along with the state that would be ‘threaded’ through the various translation phases:

data Namespace = Namespace
    { temp         :: Int                       -- id's for new temporaries
    , labels       :: Int                       -- id's for new labels
    , scope        :: [(LIRLabel, LIRLabel)]    -- current program scope
    , encMethod    :: String                    -- current enclosing method
    , blockindex   :: [Int]                     -- index into the SymbolTree
    , successorMap :: Map.Map String [LIRLabel]
    , ivarStack    :: [(LIRReg, [CFGInst])]     -- stack of ivars (see motioned code)
    }

For convenience, I also specified a series of translator monadic functions, for example:

-- |Increment our translator's label counter
incLabel :: LIRTranslator Int
incLabel = LIRTranslator (\ns@(Namespace{ labels = l }) -> (l, ns{ labels = (l+1) }))

I then proceeded to recursively pattern-match my AST, fragment-by-fragment, resulting in many functions of the form:

translateBlock :: SymbolTree -> ASTBlock -> LIRTranslator [LIRInst]
translateBlock st (DecafBlock _ [] _) = withBlock (return [])
translateBlock st block =
    withBlock (do b <- getBlock
                  let st' = select b st
                  declarations <- mapM (translateVarDeclaration st') (blockVars block)
                  statements <- mapM (translateStm st') (blockStms block)
                  return (concat declarations ++ concat statements))

(for translating a block of the target language’s code) or

-- | Given a SymbolTree, Translate a single DecafMethodStm into [LIRInst]
translateStm st (DecafMethodStm mc _) =
    do (instructions, operand) <- translateMethodCall st mc
       final <- motionCode instructions
       return final

(for translating a method call) or

translateMethodPrologue :: SymbolTree -> DecafMethod -> LIRTranslator [LIRInst]
translateMethodPrologue st (DecafMethod _ ident args _ _) =
    do let numRegVars = min (length args) 6
           regvars = map genRegVar (zip [LRDI, LRSI, LRDX, LRCX, LR8, LR9] args)
       stackvars <- mapM genStackVar (zip [1..] (drop numRegVars args))
       return (regvars ++ stackvars)
  where
    genRegVar (reg, arg) =
        LIRRegAssignInst (symVar arg st) (LIROperExpr $ LIRRegOperand reg)
    genStackVar (index, arg) =
        do let mem = LIRMemAddr LRBP Nothing ((index + 1) * 8) qword -- ^ [rbp] = old rbp; [rbp + 8] = ret address; [rbp + 16] = first stack param
                                  return $ LIRLoadInst (symVar arg st) mem

for an example of actually generating some LIR code. Hopefully these three examples will give you a good starting point; ultimately, you’ll want to go slowly, focusing on one fragment (or intermediate type) within your AST at a time.