[Relocated to haskell-cafe] Dirk Kleeblatt wrote:
apfelmus wrote:
Note that even currently, your operations cannot be strict in the address a label refers to because this may be determined later than the first use of the label. In other words, your example code
fac = do [...] (1) jmp loopTest [...] (2) loopTest @@ cmp ecx (0 :: Word32) [...] already shows that the function jmp that generates a jmp-instruction may not be strict in the position it jumps to as the address behind loopTest is only known later at line (2).
When generating code for (1), the label loopTest is used as an index into a map, to see whether there's a code position associated with it. If it is, the code position is used to compute the jump offset for the jmp instruction, if not (as in this example), a dummy value is placed in the code buffer, and Harpy remembers this position to be patched later on. At (2), the label is defined, and this leads to patching all instructions that have been emitted before this definition.
So, yes, the code position is only used after the definition of the label. But the "look up in a map"-part makes the jmp operation strict in the label parameter.
Ah, I slowly get what you mean. In short, the point is that you're reinventing lazy evaluation because you're writing directly to a memory buffer and writing a raw _|_ is impossible. So, you invent a scheme to delay the evaluation of the _|_ until they are defined. With raw writing, we indeed have the strictness writeByte buf _|_ = _|_ which implies jmp _|_ = _|_ I thought that you'd simply store the instructions in a list/monoid like the Builder-Monoid from Data.Binary with the effect that jmp _|_ /= _|_ as monadic action although it stores _|_ as instruction. Lazy evaluation then takes care of filling the _|_-instructions with content in times of need. Here's an illustration with a fictional robodog toy assembly language import Control.Monad.RWS type Address = Int data Dogcode = Bark | Bite | Jump Address data CodeGen a = RWS () [Dogcode] Address a emitDogcode :: Dogcode -> CodeGen Address emitDogcode c = x <- get tell c put (x+1) return x jump :: Label -> CodeGen Address jump = emitDogcode . Jump bark, bite :: CodeGen Address bark = emitDogcode Bark bite = emitDogcode Bite generateDogCode :: CodeGen a -> CodeBuffer generateDogCode c = let (_,instructionCount, code) = runRWST c () 0 in ... whatever to do with a list of instructions ... Now, you magically get recursive do-notation without lifting a finger because RWS is already an instance of MonadFix. For example, the following works out of the box pluto :: CodeGen Address pluto = mdo bark jump loop bite loop <- bark bark jump loop (The return value of type Address encapsulated in the monad is the line number of the last instruction of pluto.) If you really want to directly write to a memory buffer, I think that you can still use lazy evaluation to fill in the _|_ by emitting a "jump PATCHME" dummy instruction and build a stack (f.i. of type [Address]) of addresses. The stack takes the role of the map you currently use, I think that a stack is enough. The addresses therein may well be _|_. After having written all instructions into the buffer, all the _|_ are resolved and you can patch the "PATCHME"s in a second pass over the buffer. However, because of omnipresent lazy evaluation, it is unclear whether "directly" writing to a buffer instead of first building a monoid does give a speed/space advantage. So, your current approach may well be slower than a naïve "first (difference) list, then raw memory" approach.
We could omit the map, and just remember where to patch the code, but then we'd need to call explicitly some function after code generation that does the patching. We had implemented this, but found the current solution easier to use, since backpatching is completely automatic and hidden from the user.
Huh, I thought that runCodeGen would do an additional backpatch pass as transparently?
I also think that having liftIO in the CodeGen-monad is plain wrong. I mean, CodeGen is a monad that generates code without any execution taking place. The execution part is already handled by runCodeGen. Having liftIO means that arbitrary Haskell programs can be intertwined with assembly generation and I doubt that you want that.
Feel free to doubt, but this is exactly what we want. :-)
Also, note that runCodeGen runs the code _generation_, executing the generated code is done _within_ the CodeGen monad via the functions generated by callDecl (or the predefined functions in the Harpy.Call module). This is even more intertwined, but intentional.
Huh? That means that code gets executed during it's own generation? But why do you mix separate concerns? I don't see what use this is besides being an opportunity to mess up.
Of course, again a different design is possible, making runCodeGen return a binary code object, that can be called from the IO monad. But then, the user has to care about releasing code buffers, and not to have unevaluated closures having code pointers to already released run-time generated code.
Huh, why does the user have to care? Shouldn't wrapping the raw memory block into a Foreign.ForeignPtr do the job? Regards, apfelmus