path: root/compiler/GHC/ByteCode/Asm.hs

{-# LANGUAGE CPP             #-}
{-# LANGUAGE DeriveFunctor   #-}
{-# LANGUAGE RecordWildCards #-}
{-# OPTIONS_GHC -optc-DNON_POSIX_SOURCE #-}
--
--  (c) The University of Glasgow 2002-2006
--

-- | Bytecode assembler and linker
module GHC.ByteCode.Asm (
        assembleBCOs, assembleOneBCO,
        bcoFreeNames,
        SizedSeq, sizeSS, ssElts,
        iNTERP_STACK_CHECK_THRESH,
        mkNativeCallInfoLit
  ) where

import GHC.Prelude

import GHC.ByteCode.Instr
import GHC.ByteCode.InfoTable
import GHC.ByteCode.Types
import GHCi.RemoteTypes
import GHC.Runtime.Interpreter
import GHC.Runtime.Heap.Layout hiding ( WordOff )

import GHC.Types.Name
import GHC.Types.Name.Set
import GHC.Types.Literal
import GHC.Types.Unique
import GHC.Types.Unique.DSet

import GHC.Utils.Outputable
import GHC.Utils.Panic

import GHC.Core.TyCon
import GHC.Data.FastString
import GHC.Data.SizedSeq

import GHC.StgToCmm.Layout     ( ArgRep(..) )
import GHC.Cmm.Expr
import GHC.Cmm.CallConv        ( allArgRegsCover )
import GHC.Platform
import GHC.Platform.Profile

import Control.Monad
import Control.Monad.ST ( runST )
import Control.Monad.Trans.Class
import Control.Monad.Trans.State.Strict

import Data.Array.MArray

import qualified Data.Array.Unboxed as Array
import Data.Array.Base  ( UArray(..) )

import Data.Array.Unsafe( castSTUArray )

import Foreign hiding (shiftL, shiftR)
import Data.Char        ( ord )
import Data.List        ( genericLength )
import Data.Map.Strict (Map)
import Data.Maybe (fromMaybe)
import qualified Data.Map.Strict as Map

-- -----------------------------------------------------------------------------
-- Unlinked BCOs

-- CompiledByteCode represents the result of byte-code
-- compiling a bunch of functions and data types

-- | Finds external references.  Remember to remove the names
-- defined by this group of BCOs themselves
bcoFreeNames :: UnlinkedBCO -> UniqDSet Name
bcoFreeNames bco
  = bco_refs bco `uniqDSetMinusUniqSet` mkNameSet [unlinkedBCOName bco]
  where
    bco_refs (UnlinkedBCO _ _ _ _ nonptrs ptrs)
        = unionManyUniqDSets (
             mkUniqDSet [ n | BCOPtrName n <- ssElts ptrs ] :
             mkUniqDSet [ n | BCONPtrItbl n <- ssElts nonptrs ] :
             map bco_refs [ bco | BCOPtrBCO bco <- ssElts ptrs ]
          )

-- -----------------------------------------------------------------------------
-- The bytecode assembler

-- The object format for bytecodes is: 16 bits for the opcode, and 16
-- for each field -- so the code can be considered a sequence of
-- 16-bit ints.  Each field denotes either a stack offset or number of
-- items on the stack (eg SLIDE), and index into the pointer table (eg
-- PUSH_G), an index into the literal table (eg PUSH_I/D/L), or a
-- bytecode address in this BCO.

-- Top level assembler fn.
assembleBCOs
  :: Interp
  -> Profile
  -> [ProtoBCO Name]
  -> [TyCon]
  -> AddrEnv
  -> Maybe ModBreaks
  -> IO CompiledByteCode
assembleBCOs interp profile proto_bcos tycons top_strs modbreaks = do
  -- TODO: the profile should be bundled with the interpreter: the rts ways are
  -- fixed for an interpreter
  itblenv <- mkITbls interp profile tycons
  bcos    <- mapM (assembleBCO (profilePlatform profile)) proto_bcos
  bcos'   <- mallocStrings interp bcos
  return CompiledByteCode
    { bc_bcos = bcos'
    , bc_itbls =  itblenv
    , bc_ffis = concatMap protoBCOFFIs proto_bcos
    , bc_strs = top_strs
    , bc_breaks = modbreaks
    }

-- Note [Allocating string literals]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- Our strategy for handling top-level string literal bindings is described in
-- Note [Generating code for top-level string literal bindings] in GHC.StgToByteCode,
-- but not all Addr# literals in a program are guaranteed to be lifted to the
-- top level. Our strategy for handling local Addr# literals is somewhat simpler:
-- after assembling, we find all the BCONPtrStr arguments in the program, malloc
-- memory for them, and bake the resulting addresses into the instruction stream
-- in the form of BCONPtrWord arguments.
--
-- Since we do this when assembling, we only allocate the memory when we compile
-- the module, not each time we relink it. However, we do want to take care to
-- malloc the memory all in one go, since that is more efficient with
-- -fexternal-interpreter, especially when compiling in parallel.
--
-- Note that, as with top-level string literal bindings, this memory is never
-- freed, so it just leaks if the BCO is unloaded. See Note [Generating code for
-- top-level string literal bindings] in GHC.StgToByteCode for some discussion
-- about why.
--
mallocStrings :: Interp -> [UnlinkedBCO] -> IO [UnlinkedBCO]
mallocStrings interp ulbcos = do
  let bytestrings = reverse (execState (mapM_ collect ulbcos) [])
  ptrs <- interpCmd interp (MallocStrings bytestrings)
  return (evalState (mapM splice ulbcos) ptrs)
 where
  splice bco@UnlinkedBCO{..} = do
    lits <- mapM spliceLit unlinkedBCOLits
    ptrs <- mapM splicePtr unlinkedBCOPtrs
    return bco { unlinkedBCOLits = lits, unlinkedBCOPtrs = ptrs }

  spliceLit (BCONPtrStr _) = do
    rptrs <- get
    case rptrs of
      (RemotePtr p : rest) -> do
        put rest
        return (BCONPtrWord (fromIntegral p))
      _ -> panic "mallocStrings:spliceLit"
  spliceLit other = return other

  splicePtr (BCOPtrBCO bco) = BCOPtrBCO <$> splice bco
  splicePtr other = return other

  collect UnlinkedBCO{..} = do
    mapM_ collectLit unlinkedBCOLits
    mapM_ collectPtr unlinkedBCOPtrs

  collectLit (BCONPtrStr bs) = do
    strs <- get
    put (bs:strs)
  collectLit _ = return ()

  collectPtr (BCOPtrBCO bco) = collect bco
  collectPtr _ = return ()


assembleOneBCO :: Interp -> Profile -> ProtoBCO Name -> IO UnlinkedBCO
assembleOneBCO interp profile pbco = do
  -- TODO: the profile should be bundled with the interpreter: the rts ways are
  -- fixed for an interpreter
  ubco <- assembleBCO (profilePlatform profile) pbco
  [ubco'] <- mallocStrings interp [ubco]
  return ubco'

assembleBCO :: Platform -> ProtoBCO Name -> IO UnlinkedBCO
assembleBCO platform (ProtoBCO { protoBCOName       = nm
                             , protoBCOInstrs     = instrs
                             , protoBCOBitmap     = bitmap
                             , protoBCOBitmapSize = bsize
                             , protoBCOArity      = arity }) = do
  -- pass 1: collect up the offsets of the local labels.
  let asm = mapM_ (assembleI platform) instrs

      initial_offset = 0

      -- Jump instructions are variable-sized, there are long and short variants
      -- depending on the magnitude of the offset.  However, we can't tell what
      -- size instructions we will need until we have calculated the offsets of
      -- the labels, which depends on the size of the instructions...  So we
      -- first create the label environment assuming that all jumps are short,
      -- and if the final size is indeed small enough for short jumps, we are
      -- done.  Otherwise, we repeat the calculation, and we force all jumps in
      -- this BCO to be long.
      (n_insns0, lbl_map0) = inspectAsm platform False initial_offset asm
      ((n_insns, lbl_map), long_jumps)
        | isLarge (fromIntegral $ Map.size lbl_map0)
          || isLarge n_insns0
                    = (inspectAsm platform True initial_offset asm, True)
        | otherwise = ((n_insns0, lbl_map0), False)

      env :: LocalLabel -> Word
      env lbl = fromMaybe
        (pprPanic "assembleBCO.findLabel" (ppr lbl))
        (Map.lookup lbl lbl_map)

  -- pass 2: run assembler and generate instructions, literals and pointers
  let initial_state = (emptySS, emptySS, emptySS)
  (final_insns, final_lits, final_ptrs) <- flip execStateT initial_state $ runAsm platform long_jumps env asm

  -- precomputed size should be equal to final size
  massertPpr (n_insns == sizeSS final_insns)
             (text "bytecode instruction count mismatch")

  let asm_insns = ssElts final_insns
      insns_arr = Array.listArray (0, fromIntegral n_insns - 1) asm_insns
      bitmap_arr = mkBitmapArray bsize bitmap
      ul_bco = UnlinkedBCO nm arity insns_arr bitmap_arr final_lits final_ptrs

  -- 8 Aug 01: Finalisers aren't safe when attached to non-primitive
  -- objects, since they might get run too early.  Disable this until
  -- we figure out what to do.
  -- when (notNull malloced) (addFinalizer ul_bco (mapM_ zonk malloced))

  return ul_bco

mkBitmapArray :: Word16 -> [StgWord] -> UArray Int Word64
-- Here the return type must be an array of Words, not StgWords,
-- because the underlying ByteArray# will end up as a component
-- of a BCO object.
mkBitmapArray bsize bitmap
  = Array.listArray (0, length bitmap) $
      fromIntegral bsize : map (fromInteger . fromStgWord) bitmap

-- instrs nonptrs ptrs
type AsmState = (SizedSeq Word16,
                 SizedSeq BCONPtr,
                 SizedSeq BCOPtr)

data Operand
  = Op Word
  | SmallOp Word16
  | LabelOp LocalLabel
-- (unused)  | LargeOp Word

data Assembler a
  = AllocPtr (IO BCOPtr) (Word -> Assembler a)
  | AllocLit [BCONPtr] (Word -> Assembler a)
  | AllocLabel LocalLabel (Assembler a)
  | Emit Word16 [Operand] (Assembler a)
  | NullAsm a
  deriving (Functor)

instance Applicative Assembler where
    pure = NullAsm
    (<*>) = ap

instance Monad Assembler where
  NullAsm x >>= f = f x
  AllocPtr p k >>= f = AllocPtr p (k >=> f)
  AllocLit l k >>= f = AllocLit l (k >=> f)
  AllocLabel lbl k >>= f = AllocLabel lbl (k >>= f)
  Emit w ops k >>= f = Emit w ops (k >>= f)

ioptr :: IO BCOPtr -> Assembler Word
ioptr p = AllocPtr p return

ptr :: BCOPtr -> Assembler Word
ptr = ioptr . return

lit :: [BCONPtr] -> Assembler Word
lit l = AllocLit l return

label :: LocalLabel -> Assembler ()
label w = AllocLabel w (return ())

emit :: Word16 -> [Operand] -> Assembler ()
emit w ops = Emit w ops (return ())

type LabelEnv = LocalLabel -> Word

largeOp :: Bool -> Operand -> Bool
largeOp long_jumps op = case op of
   SmallOp _ -> False
   Op w      -> isLarge w
   LabelOp _ -> long_jumps
-- LargeOp _ -> True

runAsm :: Platform -> Bool -> LabelEnv -> Assembler a -> StateT AsmState IO a
runAsm platform long_jumps e = go
  where
    go (NullAsm x) = return x
    go (AllocPtr p_io k) = do
      p <- lift p_io
      w <- state $ \(st_i0,st_l0,st_p0) ->
        let st_p1 = addToSS st_p0 p
        in (sizeSS st_p0, (st_i0,st_l0,st_p1))
      go $ k w
    go (AllocLit lits k) = do
      w <- state $ \(st_i0,st_l0,st_p0) ->
        let st_l1 = addListToSS st_l0 lits
        in (sizeSS st_l0, (st_i0,st_l1,st_p0))
      go $ k w
    go (AllocLabel _ k) = go k
    go (Emit w ops k) = do
      let largeOps = any (largeOp long_jumps) ops
          opcode
            | largeOps = largeArgInstr w
            | otherwise = w
          words = concatMap expand ops
          expand (SmallOp w) = [w]
          expand (LabelOp w) = expand (Op (e w))
          expand (Op w) = if largeOps then largeArg platform (fromIntegral w) else [fromIntegral w]
--        expand (LargeOp w) = largeArg platform w
      state $ \(st_i0,st_l0,st_p0) ->
        let st_i1 = addListToSS st_i0 (opcode : words)
        in ((), (st_i1,st_l0,st_p0))
      go k

type LabelEnvMap = Map LocalLabel Word

data InspectState = InspectState
  { instrCount :: !Word
  , ptrCount :: !Word
  , litCount :: !Word
  , lblEnv :: LabelEnvMap
  }

inspectAsm :: Platform -> Bool -> Word -> Assembler a -> (Word, LabelEnvMap)
inspectAsm platform long_jumps initial_offset
  = go (InspectState initial_offset 0 0 Map.empty)
  where
    go s (NullAsm _) = (instrCount s, lblEnv s)
    go s (AllocPtr _ k) = go (s { ptrCount = n + 1 }) (k n)
      where n = ptrCount s
    go s (AllocLit ls k) = go (s { litCount = n + genericLength ls }) (k n)
      where n = litCount s
    go s (AllocLabel lbl k) = go s' k
      where s' = s { lblEnv = Map.insert lbl (instrCount s) (lblEnv s) }
    go s (Emit _ ops k) = go s' k
      where
        s' = s { instrCount = instrCount s + size }
        size = sum (map count ops) + 1
        largeOps = any (largeOp long_jumps) ops
        count (SmallOp _) = 1
        count (LabelOp _) = count (Op 0)
        count (Op _) = if largeOps then largeArg16s platform else 1
--      count (LargeOp _) = largeArg16s platform

-- Bring in all the bci_ bytecode constants.
#include "Bytecodes.h"

largeArgInstr :: Word16 -> Word16
largeArgInstr bci = bci_FLAG_LARGE_ARGS .|. bci

largeArg :: Platform -> Word64 -> [Word16]
largeArg platform w = case platformWordSize platform of
   PW8 -> [fromIntegral (w `shiftR` 48),
           fromIntegral (w `shiftR` 32),
           fromIntegral (w `shiftR` 16),
           fromIntegral w]
   PW4 -> assertPpr (w < fromIntegral (maxBound :: Word32))
                    (text "largeArg too big:" <+> ppr w) $
          [fromIntegral (w `shiftR` 16),
           fromIntegral w]

largeArg16s :: Platform -> Word
largeArg16s platform = case platformWordSize platform of
   PW8 -> 4
   PW4 -> 2

assembleI :: Platform
          -> BCInstr
          -> Assembler ()
assembleI platform i = case i of
  STKCHECK n               -> emit bci_STKCHECK [Op n]
  PUSH_L o1                -> emit bci_PUSH_L [SmallOp o1]
  PUSH_LL o1 o2            -> emit bci_PUSH_LL [SmallOp o1, SmallOp o2]
  PUSH_LLL o1 o2 o3        -> emit bci_PUSH_LLL [SmallOp o1, SmallOp o2, SmallOp o3]
  PUSH8 o1                 -> emit bci_PUSH8 [SmallOp o1]
  PUSH16 o1                -> emit bci_PUSH16 [SmallOp o1]
  PUSH32 o1                -> emit bci_PUSH32 [SmallOp o1]
  PUSH8_W o1               -> emit bci_PUSH8_W [SmallOp o1]
  PUSH16_W o1              -> emit bci_PUSH16_W [SmallOp o1]
  PUSH32_W o1              -> emit bci_PUSH32_W [SmallOp o1]
  PUSH_G nm                -> do p <- ptr (BCOPtrName nm)
                                 emit bci_PUSH_G [Op p]
  PUSH_PRIMOP op           -> do p <- ptr (BCOPtrPrimOp op)
                                 emit bci_PUSH_G [Op p]
  PUSH_BCO proto           -> do let ul_bco = assembleBCO platform proto
                                 p <- ioptr (liftM BCOPtrBCO ul_bco)
                                 emit bci_PUSH_G [Op p]
  PUSH_ALTS proto          -> do let ul_bco = assembleBCO platform proto
                                 p <- ioptr (liftM BCOPtrBCO ul_bco)
                                 emit bci_PUSH_ALTS [Op p]
  PUSH_ALTS_UNLIFTED proto pk
                           -> do let ul_bco = assembleBCO platform proto
                                 p <- ioptr (liftM BCOPtrBCO ul_bco)
                                 emit (push_alts pk) [Op p]
  PUSH_ALTS_TUPLE proto call_info tuple_proto
                           -> do let ul_bco = assembleBCO platform proto
                                     ul_tuple_bco = assembleBCO platform
                                                                tuple_proto
                                 p <- ioptr (liftM BCOPtrBCO ul_bco)
                                 p_tup <- ioptr (liftM BCOPtrBCO ul_tuple_bco)
                                 info <- int (fromIntegral $
                                              mkNativeCallInfoSig platform call_info)
                                 emit bci_PUSH_ALTS_T
                                      [Op p, Op info, Op p_tup]
  PUSH_PAD8                -> emit bci_PUSH_PAD8 []
  PUSH_PAD16               -> emit bci_PUSH_PAD16 []
  PUSH_PAD32               -> emit bci_PUSH_PAD32 []
  PUSH_UBX8 lit            -> do np <- literal lit
                                 emit bci_PUSH_UBX8 [Op np]
  PUSH_UBX16 lit           -> do np <- literal lit
                                 emit bci_PUSH_UBX16 [Op np]
  PUSH_UBX32 lit           -> do np <- literal lit
                                 emit bci_PUSH_UBX32 [Op np]
  PUSH_UBX lit nws         -> do np <- literal lit
                                 emit bci_PUSH_UBX [Op np, SmallOp nws]

  -- see Note [Generating code for top-level string literal bindings] in GHC.StgToByteCode
  PUSH_ADDR nm             -> do np <- lit [BCONPtrAddr nm]
                                 emit bci_PUSH_UBX [Op np, SmallOp 1]

  PUSH_APPLY_N             -> emit bci_PUSH_APPLY_N []
  PUSH_APPLY_V             -> emit bci_PUSH_APPLY_V []
  PUSH_APPLY_F             -> emit bci_PUSH_APPLY_F []
  PUSH_APPLY_D             -> emit bci_PUSH_APPLY_D []
  PUSH_APPLY_L             -> emit bci_PUSH_APPLY_L []
  PUSH_APPLY_P             -> emit bci_PUSH_APPLY_P []
  PUSH_APPLY_PP            -> emit bci_PUSH_APPLY_PP []
  PUSH_APPLY_PPP           -> emit bci_PUSH_APPLY_PPP []
  PUSH_APPLY_PPPP          -> emit bci_PUSH_APPLY_PPPP []
  PUSH_APPLY_PPPPP         -> emit bci_PUSH_APPLY_PPPPP []
  PUSH_APPLY_PPPPPP        -> emit bci_PUSH_APPLY_PPPPPP []

  SLIDE     n by           -> emit bci_SLIDE [SmallOp n, SmallOp by]
  ALLOC_AP  n              -> emit bci_ALLOC_AP [SmallOp n]
  ALLOC_AP_NOUPD n         -> emit bci_ALLOC_AP_NOUPD [SmallOp n]
  ALLOC_PAP arity n        -> emit bci_ALLOC_PAP [SmallOp arity, SmallOp n]
  MKAP      off sz         -> emit bci_MKAP [SmallOp off, SmallOp sz]
  MKPAP     off sz         -> emit bci_MKPAP [SmallOp off, SmallOp sz]
  UNPACK    n              -> emit bci_UNPACK [SmallOp n]
  PACK      dcon sz        -> do itbl_no <- lit [BCONPtrItbl (getName dcon)]
                                 emit bci_PACK [Op itbl_no, SmallOp sz]
  LABEL     lbl            -> label lbl
  TESTLT_I  i l            -> do np <- int i
                                 emit bci_TESTLT_I [Op np, LabelOp l]
  TESTEQ_I  i l            -> do np <- int i
                                 emit bci_TESTEQ_I [Op np, LabelOp l]
  TESTLT_W  w l            -> do np <- word w
                                 emit bci_TESTLT_W [Op np, LabelOp l]
  TESTEQ_W  w l            -> do np <- word w
                                 emit bci_TESTEQ_W [Op np, LabelOp l]
  TESTLT_I64  i l          -> do np <- int64 i
                                 emit bci_TESTLT_I64 [Op np, LabelOp l]
  TESTEQ_I64  i l          -> do np <- int64 i
                                 emit bci_TESTEQ_I64 [Op np, LabelOp l]
  TESTLT_I32  i l          -> do np <- int (fromIntegral i)
                                 emit bci_TESTLT_I32 [Op np, LabelOp l]
  TESTEQ_I32 i l           -> do np <- int (fromIntegral i)
                                 emit bci_TESTEQ_I32 [Op np, LabelOp l]
  TESTLT_I16  i l          -> do np <- int (fromIntegral i)
                                 emit bci_TESTLT_I16 [Op np, LabelOp l]
  TESTEQ_I16 i l           -> do np <- int (fromIntegral i)
                                 emit bci_TESTEQ_I16 [Op np, LabelOp l]
  TESTLT_I8  i l           -> do np <- int (fromIntegral i)
                                 emit bci_TESTLT_I8 [Op np, LabelOp l]
  TESTEQ_I8 i l            -> do np <- int (fromIntegral i)
                                 emit bci_TESTEQ_I8 [Op np, LabelOp l]
  TESTLT_W64  w l          -> do np <- word64 w
                                 emit bci_TESTLT_W64 [Op np, LabelOp l]
  TESTEQ_W64  w l          -> do np <- word64 w
                                 emit bci_TESTEQ_W64 [Op np, LabelOp l]
  TESTLT_W32  w l          -> do np <- word (fromIntegral w)
                                 emit bci_TESTLT_W32 [Op np, LabelOp l]
  TESTEQ_W32  w l          -> do np <- word (fromIntegral w)
                                 emit bci_TESTEQ_W32 [Op np, LabelOp l]
  TESTLT_W16  w l          -> do np <- word (fromIntegral w)
                                 emit bci_TESTLT_W16 [Op np, LabelOp l]
  TESTEQ_W16  w l          -> do np <- word (fromIntegral w)
                                 emit bci_TESTEQ_W16 [Op np, LabelOp l]
  TESTLT_W8  w l           -> do np <- word (fromIntegral w)
                                 emit bci_TESTLT_W8 [Op np, LabelOp l]
  TESTEQ_W8  w l           -> do np <- word (fromIntegral w)
                                 emit bci_TESTEQ_W8 [Op np, LabelOp l]
  TESTLT_F  f l            -> do np <- float f
                                 emit bci_TESTLT_F [Op np, LabelOp l]
  TESTEQ_F  f l            -> do np <- float f
                                 emit bci_TESTEQ_F [Op np, LabelOp l]
  TESTLT_D  d l            -> do np <- double d
                                 emit bci_TESTLT_D [Op np, LabelOp l]
  TESTEQ_D  d l            -> do np <- double d
                                 emit bci_TESTEQ_D [Op np, LabelOp l]
  TESTLT_P  i l            -> emit bci_TESTLT_P [SmallOp i, LabelOp l]
  TESTEQ_P  i l            -> emit bci_TESTEQ_P [SmallOp i, LabelOp l]
  CASEFAIL                 -> emit bci_CASEFAIL []
  SWIZZLE   stkoff n       -> emit bci_SWIZZLE [SmallOp stkoff, SmallOp n]
  JMP       l              -> emit bci_JMP [LabelOp l]
  ENTER                    -> emit bci_ENTER []
  RETURN                   -> emit bci_RETURN []
  RETURN_UNLIFTED rep      -> emit (return_unlifted rep) []
  RETURN_TUPLE             -> emit bci_RETURN_T []
  CCALL off m_addr i       -> do np <- addr m_addr
                                 emit bci_CCALL [SmallOp off, Op np, SmallOp i]
  PRIMCALL                 -> emit bci_PRIMCALL []
  BRK_FUN index uniq cc    -> do p1 <- ptr BCOPtrBreakArray
                                 q <- int (getKey uniq)
                                 np <- addr cc
                                 emit bci_BRK_FUN [Op p1, SmallOp index,
                                                   Op q, Op np]

  where
    literal (LitLabel fs (Just sz) _)
     | platformOS platform == OSMinGW32
         = litlabel (appendFS fs (mkFastString ('@':show sz)))
     -- On Windows, stdcall labels have a suffix indicating the no. of
     -- arg words, e.g. foo@8.  testcase: ffi012(ghci)
    literal (LitLabel fs _ _) = litlabel fs
    literal LitNullAddr       = int 0
    literal (LitFloat r)      = float (fromRational r)
    literal (LitDouble r)     = double (fromRational r)
    literal (LitChar c)       = int (ord c)
    literal (LitString bs)    = lit [BCONPtrStr bs]
       -- LitString requires a zero-terminator when emitted
    literal (LitNumber nt i) = case nt of
      LitNumInt     -> int (fromIntegral i)
      LitNumWord    -> int (fromIntegral i)
      LitNumInt8    -> int8 (fromIntegral i)
      LitNumWord8   -> int8 (fromIntegral i)
      LitNumInt16   -> int16 (fromIntegral i)
      LitNumWord16  -> int16 (fromIntegral i)
      LitNumInt32   -> int32 (fromIntegral i)
      LitNumWord32  -> int32 (fromIntegral i)
      LitNumInt64   -> int64 (fromIntegral i)
      LitNumWord64  -> int64 (fromIntegral i)
      LitNumBigNat  -> panic "GHC.ByteCode.Asm.literal: LitNumBigNat"

    -- We can lower 'LitRubbish' to an arbitrary constant, but @NULL@ is most
    -- likely to elicit a crash (rather than corrupt memory) in case absence
    -- analysis messed up.
    literal (LitRubbish {}) = int 0

    litlabel fs = lit [BCONPtrLbl fs]
    addr (RemotePtr a) = words [fromIntegral a]
    float = words . mkLitF platform
    double = words . mkLitD platform
    int = words . mkLitI
    int8 = words . mkLitI64 platform
    int16 = words . mkLitI64 platform
    int32 = words . mkLitI64 platform
    int64 = words . mkLitI64 platform
    word64 = words . mkLitW64 platform
    words ws = lit (map BCONPtrWord ws)
    word w = words [w]

isLarge :: Word -> Bool
isLarge n = n > 65535

push_alts :: ArgRep -> Word16
push_alts V   = bci_PUSH_ALTS_V
push_alts P   = bci_PUSH_ALTS_P
push_alts N   = bci_PUSH_ALTS_N
push_alts L   = bci_PUSH_ALTS_L
push_alts F   = bci_PUSH_ALTS_F
push_alts D   = bci_PUSH_ALTS_D
push_alts V16 = error "push_alts: vector"
push_alts V32 = error "push_alts: vector"
push_alts V64 = error "push_alts: vector"

return_unlifted :: ArgRep -> Word16
return_unlifted V   = bci_RETURN_V
return_unlifted P   = bci_RETURN_P
return_unlifted N   = bci_RETURN_N
return_unlifted L   = bci_RETURN_L
return_unlifted F   = bci_RETURN_F
return_unlifted D   = bci_RETURN_D
return_unlifted V16 = error "return_unlifted: vector"
return_unlifted V32 = error "return_unlifted: vector"
return_unlifted V64 = error "return_unlifted: vector"

{-
  we can only handle up to a fixed number of words on the stack,
  because we need a stg_ctoi_tN stack frame for each size N. See
  Note [unboxed tuple bytecodes and tuple_BCO].

  If needed, you can support larger tuples by adding more in
  StgMiscClosures.cmm, Interpreter.c and MiscClosures.h and
  raising this limit.

  Note that the limit is the number of words passed on the stack.
  If the calling convention passes part of the tuple in registers, the
  maximum number of tuple elements may be larger. Elements can also
  take multiple words on the stack (for example Double# on a 32 bit
  platform).
 -}
maxTupleReturnNativeStackSize :: WordOff
maxTupleReturnNativeStackSize = 62

{-
  Construct the call_info word that stg_ctoi_t, stg_ret_t and stg_primcall
  use to convert arguments between the native calling convention and the
  interpreter.

  See Note [GHCi and native call registers] for more information.
 -}
mkNativeCallInfoSig :: Platform -> NativeCallInfo -> Word32
mkNativeCallInfoSig platform NativeCallInfo{..}
  | nativeCallType == NativeTupleReturn && nativeCallStackSpillSize > maxTupleReturnNativeStackSize
  = pprPanic "mkNativeCallInfoSig: tuple too big for the bytecode compiler"
             (ppr nativeCallStackSpillSize <+> text "stack words." <+>
              text "Use -fobject-code to get around this limit"
             )
  | otherwise
  = assertPpr (length regs <= 24) (text "too many registers for bitmap:" <+> ppr (length regs)) {- 24 bits for register bitmap -}
    assertPpr (cont_offset < 255) (text "continuation offset too large:" <+> ppr cont_offset) {- 8 bits for continuation offset (only for NativeTupleReturn) -}
    assertPpr (all (`elem` regs) (regSetToList nativeCallRegs)) (text "not all registers accounted for") {- all regs accounted for -}
    foldl' reg_bit 0 (zip regs [0..]) .|. (cont_offset `shiftL` 24)
  where
    cont_offset :: Word32
    cont_offset
      | nativeCallType == NativeTupleReturn = fromIntegral nativeCallStackSpillSize
      | otherwise                           = 0 -- there is no continuation for primcalls

    reg_bit :: Word32 -> (GlobalReg, Int) -> Word32
    reg_bit x (r, n)
      | r `elemRegSet` nativeCallRegs = x .|. 1 `shiftL` n
      | otherwise                     = x
    regs = allArgRegsCover platform

mkNativeCallInfoLit :: Platform -> NativeCallInfo -> Literal
mkNativeCallInfoLit platform call_info =
  mkLitWord platform . fromIntegral $ mkNativeCallInfoSig platform call_info

-- Make lists of host-sized words for literals, so that when the
-- words are placed in memory at increasing addresses, the
-- bit pattern is correct for the host's word size and endianness.
mkLitI   ::             Int    -> [Word]
mkLitF   :: Platform -> Float  -> [Word]
mkLitD   :: Platform -> Double -> [Word]
mkLitI64 :: Platform -> Int64  -> [Word]
mkLitW64 :: Platform -> Word64 -> [Word]

mkLitF platform f = case platformWordSize platform of
  PW4 -> runST $ do
        arr <- newArray_ ((0::Int),0)
        writeArray arr 0 f
        f_arr <- castSTUArray arr
        w0 <- readArray f_arr 0
        return [w0 :: Word]

  PW8 -> runST $ do
        arr <- newArray_ ((0::Int),1)
        writeArray arr 0 f
        -- on 64-bit architectures we read two (32-bit) Float cells when we read
        -- a (64-bit) Word: so we write a dummy value in the second cell to
        -- avoid an out-of-bound read.
        writeArray arr 1 0.0
        f_arr <- castSTUArray arr
        w0 <- readArray f_arr 0
        return [w0 :: Word]

mkLitD platform d = case platformWordSize platform of
   PW4 -> runST (do
        arr <- newArray_ ((0::Int),1)
        writeArray arr 0 d
        d_arr <- castSTUArray arr
        w0 <- readArray d_arr 0
        w1 <- readArray d_arr 1
        return [w0 :: Word, w1]
     )
   PW8 -> runST (do
        arr <- newArray_ ((0::Int),0)
        writeArray arr 0 d
        d_arr <- castSTUArray arr
        w0 <- readArray d_arr 0
        return [w0 :: Word]
     )

mkLitI64 platform ii = case platformWordSize platform of
   PW4 -> runST (do
        arr <- newArray_ ((0::Int),1)
        writeArray arr 0 ii
        d_arr <- castSTUArray arr
        w0 <- readArray d_arr 0
        w1 <- readArray d_arr 1
        return [w0 :: Word,w1]
     )
   PW8 -> [fromIntegral ii :: Word]

mkLitW64 platform ww = case platformWordSize platform of
   PW4 -> runST (do
        arr <- newArray_ ((0::Word),1)
        writeArray arr 0 ww
        d_arr <- castSTUArray arr
        w0 <- readArray d_arr 0
        w1 <- readArray d_arr 1
        return [w0 :: Word,w1]
     )
   PW8 -> [fromIntegral ww :: Word]

mkLitI i = [fromIntegral i :: Word]

iNTERP_STACK_CHECK_THRESH :: Int
iNTERP_STACK_CHECK_THRESH = INTERP_STACK_CHECK_THRESH