CmmBuildInfoTables.hs 27.6 KB
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module CmmBuildInfoTables
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    ( CAFSet, CAFEnv, CmmTopForInfoTables(..), cafAnal, localCAFInfo, mkTopCAFInfo
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    , setInfoTableSRT, setInfoTableStackMap
    , TopSRT, emptySRT, srtToData
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    , bundleCAFs
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    , finishInfoTables, lowerSafeForeignCalls
    , cafTransfers, liveSlotTransfers
    , extendEnvWithSafeForeignCalls, extendEnvsForSafeForeignCalls )
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where

#include "HsVersions.h"

import Constants
import Digraph
import qualified Prelude as P
import Prelude
import Util (sortLe)

import BlockId
import Bitmap
import CLabel
import Cmm hiding (blockId)
import CmmInfo
import CmmProcPointZ
import CmmStackLayout
import CmmTx
import DFMonad
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import Module
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import FastString
import FiniteMap
import ForeignCall
import IdInfo
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import Data.List
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import Maybes
import MkZipCfg
import MkZipCfgCmm hiding (CmmAGraph, CmmBlock, CmmTopZ, CmmZ, CmmGraph)
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import Control.Monad
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import Name
import Outputable
import SMRep
import StgCmmClosure
import StgCmmForeign
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-- import StgCmmMonad
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import StgCmmUtils
import UniqSupply
import ZipCfg hiding (zip, unzip, last)
import qualified ZipCfg as G
import ZipCfgCmmRep
import ZipDataflow

----------------------------------------------------------------
-- Building InfoTables


-----------------------------------------------------------------------
-- Stack Maps

-- Given a block ID, we return a representation of the layout of the stack,
-- as suspended before entering that block.
-- (For a return site to a function call, the layout does not include the
--  parameter passing area (or the "return address" on the stack)).
-- If the element is `Nothing`, then it represents a word of the stack that
-- does not contain a live pointer.
-- If the element is `Just` a register, then it represents a live spill slot
-- for a pointer; we assume that a pointer is the size of a word.
-- The head of the list represents the young end of the stack where the infotable
-- pointer for the block `Bid` is stored.
-- The infotable pointer itself is not included in the list.
-- Call areas are also excluded from the list: besides the stuff in the update
-- frame (and the return infotable), call areas should never be live across
-- function calls.

-- RTS Invariant: All pointers must be word-aligned because each bit in the bitmap
-- represents a word. Consequently, we have to be careful when we see a live slot
-- on the stack: if we have packed multiple sub-word values into a word,
-- we have to make sure that we only mark the entire word as a non-pointer.

-- Also, don't forget to stop at the old end of the stack (oldByte),
-- which may differ depending on whether there is an update frame.
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type RegSlotInfo
   = ( Int	  -- Offset from oldest byte of Old area
     , LocalReg   -- The register
     , Int)       -- Width of the register

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live_ptrs :: ByteOff -> BlockEnv SubAreaSet -> AreaMap -> BlockId -> [Maybe LocalReg]
live_ptrs oldByte slotEnv areaMap bid =
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  -- pprTrace "live_ptrs for" (ppr bid <+> text (show oldByte ++ "-" ++ show youngByte) <+>
  --                           ppr liveSlots) $
  -- pprTrace ("stack layout for " ++ show bid ++ ": ") (ppr res) $ res
  res
  where res = reverse $ slotsToList youngByte liveSlots []
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        slotsToList :: Int -> [RegSlotInfo] -> [Maybe LocalReg] -> [Maybe LocalReg]
        -- n starts at youngByte and is decremented down to oldByte
	-- Returns a list, one element per word, with 
	--    (Just r) meaning 'pointer register r is saved here', 
	--    Nothing  meaning 'non-pointer or empty'

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        slotsToList n [] results | n == oldByte = results -- at old end of stack frame
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        slotsToList n (s : _) _  | n == oldByte =
          pprPanic "slot left off live_ptrs" (ppr s <+> ppr oldByte <+>
               ppr n <+> ppr liveSlots <+> ppr youngByte)
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        slotsToList n _ _ | n < oldByte =
          panic "stack slots not allocated on word boundaries?"
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        slotsToList n l@((n', r, w) : rst) results =
          if n == (n' + w) then -- slot's young byte is at n
            ASSERT (not (isPtr r) ||
                    (n `mod` wORD_SIZE == 0 && w == wORD_SIZE)) -- ptrs must be aligned
            slotsToList next (dropWhile (non_ptr_younger_than next) rst)
                        (stack_rep : results)
          else slotsToList next (dropWhile (non_ptr_younger_than next) l)
                           (Nothing : results)
          where next = n - wORD_SIZE
                stack_rep = if isPtr r then Just r else Nothing
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        slotsToList n [] results = slotsToList (n - wORD_SIZE) [] (Nothing : results)
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        non_ptr_younger_than next (n', r, w) =
          n' + w > next &&
            ASSERT (not (isPtr r))
            True
        isPtr = isGcPtrType . localRegType
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        liveSlots :: [RegSlotInfo]
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        liveSlots = sortBy (\ (off,_,_) (off',_,_) -> compare off' off)
                           (foldFM (\_ -> flip $ foldl add_slot) [] slots)
                    
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        add_slot :: [RegSlotInfo] -> SubArea -> [RegSlotInfo]
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        add_slot rst (a@(RegSlot r@(LocalReg _ ty)), off, w) = 
          if off == w && widthInBytes (typeWidth ty) == w then
            (expectJust "add_slot" (lookupFM areaMap a), r, w) : rst
          else panic "live_ptrs: only part of a variable live at a proc point"
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        add_slot rst (CallArea Old, _, _) =
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          rst -- the update frame (or return infotable) should be live
              -- would be nice to check that only that part of the callarea is live...
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        add_slot rst ((CallArea _), _, _) =
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          rst
          -- JD: THIS ISN'T CURRENTLY A CORRECTNESS PROBLEM, BUT WE SHOULD REALLY
          -- MAKE LIVENESS INFO AROUND CALLS MORE PRECISE -- FOR NOW, A 32-BIT
          -- FLOAT PADS OUT TO 64 BITS, BUT WE ASSUME THE WHOLE PARAMETER-PASSING
          -- AREA IS LIVE (WHICH IT ISN'T...).  WE SHOULD JUST PUT THE LIVE AREAS
          -- IN THE CALL NODES, WHICH SHOULD EVENTUALLY HAVE LIVE REGISTER AS WELL,
          -- SO IT'S ALL GOING IN THE SAME DIRECTION.
          -- pprPanic "CallAreas must not be live across function calls" (ppr bid <+> ppr c)
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        slots :: SubAreaSet	 -- The SubAreaSet for 'bid'
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        slots = expectJust "live_ptrs slots" $ lookupBlockEnv slotEnv bid
        youngByte = expectJust "live_ptrs bid_pos" $ lookupFM areaMap (CallArea (Young bid))

-- Construct the stack maps for the given procedure.
setInfoTableStackMap :: SlotEnv -> AreaMap -> CmmTopForInfoTables -> CmmTopForInfoTables 
setInfoTableStackMap _ _ t@(NoInfoTable _) = t
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setInfoTableStackMap slotEnv areaMap t@(FloatingInfoTable _ bid updfr_off) =
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  updInfo (const (live_ptrs updfr_off slotEnv areaMap bid)) id t
setInfoTableStackMap slotEnv areaMap
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     t@(ProcInfoTable (CmmProc (CmmInfo _ _ _) _ _ ((_, Just updfr_off), _)) procpoints) =
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  case blockSetToList procpoints of
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    [bid] -> updInfo (const (live_ptrs updfr_off slotEnv areaMap bid)) id t
    _ -> panic "setInfoTableStackMap: unexpected number of procpoints"
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           -- until we stop splitting the graphs at procpoints in the native path
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setInfoTableStackMap _ _ t = pprPanic "unexpected case for setInfoTableStackMap" (ppr t)
                 
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-----------------------------------------------------------------------
-- SRTs

-- WE NEED AN EXAMPLE HERE.
-- IN PARTICULAR, WE NEED TO POINT OUT THE DISTINCTION BETWEEN
-- FUNCTIONS WITH STATIC CLOSURES AND THOSE THAT MUST BE CONSTRUCTED
-- DYNAMICALLY (AND HENCE CAN'T BE REFERENCED IN AN SRT).
-- IN THE LATTER CASE, WE HAVE TO TAKE ALL THE CAFs REFERENCED BY
-- THE CLOSURE AND INLINE THEM INTO ANY SRT THAT MAY MENTION THE CLOSURE.
-- (I.E. TAKE THE TRANSITIVE CLOSURE, but only for non-static closures).


-----------------------------------------------------------------------
-- Finding the CAFs used by a procedure

type CAFSet = FiniteMap CLabel ()
type CAFEnv = BlockEnv CAFSet

-- First, an analysis to find live CAFs.
cafLattice :: DataflowLattice CAFSet
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cafLattice = DataflowLattice "live cafs" emptyFM add False
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  where add new old = if sizeFM new' > sizeFM old then aTx new' else noTx new'
          where new' = new `plusFM` old

cafTransfers :: BackwardTransfers Middle Last CAFSet
cafTransfers = BackwardTransfers first middle last
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  where first  _ live = live
        middle m live = foldExpDeepMiddle addCaf m live
        last   l env  = foldExpDeepLast   addCaf l (joinOuts cafLattice env l)
        addCaf e set = case e of
               CmmLit (CmmLabel c)              -> add c set
               CmmLit (CmmLabelOff c _)         -> add c set
               CmmLit (CmmLabelDiffOff c1 c2 _) -> add c1 $ add c2 set
               _ -> set
        add l s = if hasCAF l then addToFM s (cvtToClosureLbl l) () else s
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type CafFix a = FuelMonad (BackwardFixedPoint Middle Last CAFSet a)
cafAnal :: LGraph Middle Last -> FuelMonad CAFEnv
cafAnal g = liftM zdfFpFacts (res :: CafFix ())
  where res = zdfSolveFromL emptyBlockEnv "live CAF analysis" cafLattice
                            cafTransfers (fact_bot cafLattice) g

-----------------------------------------------------------------------
-- Building the SRTs

-- Description of the SRT for a given module.
-- Note that this SRT may grow as we greedily add new CAFs to it.
data TopSRT = TopSRT { lbl      :: CLabel
                     , next_elt :: Int -- the next entry in the table
                     , rev_elts :: [CLabel]
                     , elt_map  :: FiniteMap CLabel Int }
                        -- map: CLabel -> its last entry in the table
instance Outputable TopSRT where
  ppr (TopSRT lbl next elts eltmap) =
    text "TopSRT:" <+> ppr lbl <+> ppr next <+> ppr elts <+> ppr eltmap

emptySRT :: MonadUnique m => m TopSRT
emptySRT =
  do top_lbl <- getUniqueM >>= \ u -> return $ mkSRTLabel (mkFCallName u "srt") NoCafRefs
     return TopSRT { lbl = top_lbl, next_elt = 0, rev_elts = [], elt_map = emptyFM }

cafMember :: TopSRT -> CLabel -> Bool
cafMember srt lbl = elemFM lbl (elt_map srt)

cafOffset :: TopSRT -> CLabel -> Maybe Int
cafOffset srt lbl = lookupFM (elt_map srt) lbl

addCAF :: CLabel -> TopSRT -> TopSRT
addCAF caf srt =
  srt { next_elt = last + 1
      , rev_elts = caf : rev_elts srt
      , elt_map  = addToFM (elt_map srt) caf last }
    where last  = next_elt srt

srtToData :: TopSRT -> CmmZ
srtToData srt = Cmm [CmmData RelocatableReadOnlyData (CmmDataLabel (lbl srt) : tbl)]
    where tbl = map (CmmStaticLit . CmmLabel) (reverse (rev_elts srt))

-- Once we have found the CAFs, we need to do two things:
-- 1. Build a table of all the CAFs used in the procedure.
-- 2. Compute the C_SRT describing the subset of CAFs live at each procpoint.
--
-- When building the local view of the SRT, we first make sure that all the CAFs are 
-- in the SRT. Then, if the number of CAFs is small enough to fit in a bitmap,
-- we make sure they're all close enough to the bottom of the table that the
-- bitmap will be able to cover all of them.
buildSRTs :: TopSRT -> FiniteMap CLabel CAFSet -> CAFSet ->
             FuelMonad (TopSRT, Maybe CmmTopZ, C_SRT)
buildSRTs topSRT topCAFMap cafs =
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  do let liftCAF lbl () z = -- get CAFs for functions without static closures
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           case lookupFM topCAFMap lbl of Just cafs -> z `plusFM` cafs
                                          Nothing   -> addToFM z lbl ()
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         -- For each label referring to a function f without a static closure,
         -- replace it with the CAFs that are reachable from f.
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         sub_srt topSRT localCafs =
           let cafs = keysFM (foldFM liftCAF emptyFM localCafs)
               mkSRT topSRT =
                 do localSRTs <- procpointSRT (lbl topSRT) (elt_map topSRT) cafs
                    return (topSRT, localSRTs)
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           in if length cafs > maxBmpSize then
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                mkSRT (foldl add_if_missing topSRT cafs)
              else -- make sure all the cafs are near the bottom of the srt
                mkSRT (add_if_too_far topSRT cafs)
         add_if_missing srt caf =
           if cafMember srt caf then srt else addCAF caf srt
         -- If a CAF is more than maxBmpSize entries from the young end of the
         -- SRT, then we add it to the SRT again.
         -- (Note: Not in the SRT => infinitely far.)
         add_if_too_far srt@(TopSRT {elt_map = m}) cafs =
           add srt (sortBy farthestFst cafs)
             where
               farthestFst x y = case (lookupFM m x, lookupFM m y) of
                                   (Nothing, Nothing) -> EQ
                                   (Nothing, Just _)  -> LT
                                   (Just _,  Nothing) -> GT
                                   (Just d, Just d')  -> compare d' d
               add srt [] = srt
               add srt@(TopSRT {next_elt = next}) (caf : rst) =
                 case cafOffset srt caf of
                   Just ix -> if next - ix > maxBmpSize then
                                add (addCAF caf srt) rst
                              else srt
                   Nothing -> add (addCAF caf srt) rst
     (topSRT, subSRTs) <- sub_srt topSRT cafs
     let (sub_tbls, blockSRTs) = subSRTs
     return (topSRT, sub_tbls, blockSRTs)

-- Construct an SRT bitmap.
-- Adapted from simpleStg/SRT.lhs, which expects Id's.
procpointSRT :: CLabel -> FiniteMap CLabel Int -> [CLabel] ->
                FuelMonad (Maybe CmmTopZ, C_SRT)
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procpointSRT _ _ [] =
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 return (Nothing, NoC_SRT)
procpointSRT top_srt top_table entries =
 do (top, srt) <- bitmap `seq` to_SRT top_srt offset len bitmap
    return (top, srt)
  where
    ints = map (expectJust "constructSRT" . lookupFM top_table) entries
    sorted_ints = sortLe (<=) ints
    offset = head sorted_ints
    bitmap_entries = map (subtract offset) sorted_ints
    len = P.last bitmap_entries + 1
    bitmap = intsToBitmap len bitmap_entries

maxBmpSize :: Int
maxBmpSize = widthInBits wordWidth `div` 2

-- Adapted from codeGen/StgCmmUtils, which converts from SRT to C_SRT.
to_SRT :: CLabel -> Int -> Int -> Bitmap -> FuelMonad (Maybe CmmTopZ, C_SRT)
to_SRT top_srt off len bmp
  | len > maxBmpSize || bmp == [fromIntegral srt_escape]
  = do id <- getUniqueM
       let srt_desc_lbl = mkLargeSRTLabel id
           tbl = CmmData RelocatableReadOnlyData $
                   CmmDataLabel srt_desc_lbl : map CmmStaticLit
                     ( cmmLabelOffW top_srt off
                     : mkWordCLit (fromIntegral len)
                     : map mkWordCLit bmp)
       return (Just tbl, C_SRT srt_desc_lbl 0 srt_escape)
  | otherwise
  = return (Nothing, C_SRT top_srt off (fromIntegral (head bmp)))
	-- The fromIntegral converts to StgHalfWord

-- Gather CAF info for a procedure, but only if the procedure
-- doesn't have a static closure.
-- (If it has a static closure, it will already have an SRT to
--  keep its CAFs live.)
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-- Any procedure referring to a non-static CAF c must keep live
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-- any CAF that is reachable from c.
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localCAFInfo :: CAFEnv -> CmmTopZ -> Maybe (CLabel, CAFSet)
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localCAFInfo _      (CmmData _ _) = Nothing
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localCAFInfo cafEnv (CmmProc (CmmInfo _ _ infoTbl) top_l _ (_, LGraph entry _)) =
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  case infoTbl of
    CmmInfoTable False _ _ _ ->
      Just (cvtToClosureLbl top_l,
            expectJust "maybeBindCAFs" $ lookupBlockEnv cafEnv entry)
    _ -> Nothing

-- Once we have the local CAF sets for some (possibly) mutually
-- recursive functions, we can create an environment mapping
-- each function to its set of CAFs. Note that a CAF may
-- be a reference to a function. If that function f does not have
-- a static closure, then we need to refer specifically
-- to the set of CAFs used by f. Of course, the set of CAFs
-- used by f must be included in the local CAF sets that are input to
-- this function. To minimize lookup time later, we return
-- the environment with every reference to f replaced by its set of CAFs.
-- To do this replacement efficiently, we gather strongly connected
-- components, then we sort the components in topological order.
mkTopCAFInfo :: [(CLabel, CAFSet)] -> FiniteMap CLabel CAFSet
mkTopCAFInfo localCAFs = foldl addToTop emptyFM g
  where addToTop env (AcyclicSCC (l, cafset)) =
          addToFM env l (flatten env cafset)
        addToTop env (CyclicSCC nodes) =
          let (lbls, cafsets) = unzip nodes
              cafset  = foldl plusFM  emptyFM cafsets `delListFromFM` lbls
          in foldl (\env l -> addToFM env l (flatten env cafset)) env lbls
        flatten env cafset = foldFM (lookup env) emptyFM cafset
        lookup env caf () cafset' =
          case lookupFM env caf of Just cafs -> foldFM add cafset' cafs
                                   Nothing -> add caf () cafset'
        add caf () cafset' = addToFM cafset' caf ()
        g = stronglyConnCompFromEdgedVertices
              (map (\n@(l, cafs) -> (n, l, keysFM cafs)) localCAFs)

type StackLayout = [Maybe LocalReg]

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-- Bundle the CAFs used at a procpoint.
bundleCAFs :: CAFEnv -> CmmTopForInfoTables -> (CAFSet, CmmTopForInfoTables)
bundleCAFs cafEnv t@(ProcInfoTable _ procpoints) =
  case blockSetToList procpoints of
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    [bid] -> (expectJust "bundleCAFs" (lookupBlockEnv cafEnv bid), t)
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    _     -> panic "setInfoTableStackMap: unexpect number of procpoints"
             -- until we stop splitting the graphs at procpoints in the native path
bundleCAFs cafEnv t@(FloatingInfoTable _ bid _) =
  (expectJust "bundleCAFs " (lookupBlockEnv cafEnv bid), t)
bundleCAFs _ t@(NoInfoTable _) = (emptyFM, t)

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-- Construct the SRTs for the given procedure.
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setInfoTableSRT :: FiniteMap CLabel CAFSet -> TopSRT -> (CAFSet, CmmTopForInfoTables) ->
                   FuelMonad (TopSRT, [CmmTopForInfoTables])
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setInfoTableSRT topCAFMap topSRT (cafs, t@(ProcInfoTable _ procpoints)) =
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  case blockSetToList procpoints of
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    [_] -> setSRT cafs topCAFMap topSRT t
    _   -> panic "setInfoTableStackMap: unexpect number of procpoints"
           -- until we stop splitting the graphs at procpoints in the native path
setInfoTableSRT topCAFMap topSRT (cafs, t@(FloatingInfoTable _ _ _)) =
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  setSRT cafs topCAFMap topSRT t
setInfoTableSRT _ topSRT (_, t@(NoInfoTable _)) = return (topSRT, [t])

setSRT :: CAFSet -> FiniteMap CLabel CAFSet -> TopSRT ->
          CmmTopForInfoTables -> FuelMonad (TopSRT, [CmmTopForInfoTables])
setSRT cafs topCAFMap topSRT t =
  do (topSRT, cafTable, srt) <- buildSRTs topSRT topCAFMap cafs
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     let t' = updInfo id (const srt) t
     case cafTable of
       Just tbl -> return (topSRT, [t', NoInfoTable tbl])
       Nothing  -> return (topSRT, [t'])

updInfo :: (StackLayout -> StackLayout) -> (C_SRT -> C_SRT) ->
           CmmTopForInfoTables -> CmmTopForInfoTables 
updInfo toVars toSrt (ProcInfoTable (CmmProc info top_l top_args g) procpoints) =
  ProcInfoTable (CmmProc (updInfoTbl toVars toSrt info) top_l top_args g) procpoints
updInfo toVars toSrt (FloatingInfoTable info bid updfr_off) =
  FloatingInfoTable (updInfoTbl toVars toSrt info) bid updfr_off
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updInfo _ _ (NoInfoTable _) = panic "can't update NoInfoTable"
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updInfo _ _ _ = panic "unexpected arg to updInfo"

updInfoTbl :: (StackLayout -> StackLayout) -> (C_SRT -> C_SRT) -> CmmInfo -> CmmInfo 
updInfoTbl toVars toSrt (CmmInfo gc upd_fr (CmmInfoTable s p t typeinfo))
  = CmmInfo gc upd_fr (CmmInfoTable s p t typeinfo')
    where typeinfo' = case typeinfo of
            t@(ConstrInfo _ _ _)    -> t
            (FunInfo    c s a d e)  -> FunInfo c (toSrt s) a d e
            (ThunkInfo  c s)        -> ThunkInfo c (toSrt s)
            (ThunkSelectorInfo x s) -> ThunkSelectorInfo x (toSrt s)
            (ContInfo v s)          -> ContInfo (toVars v) (toSrt s)
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updInfoTbl _ _ t@(CmmInfo _ _ CmmNonInfoTable) = t
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-- Lower the CmmTopForInfoTables type down to good old CmmTopZ
-- by emitting info tables as data where necessary.
finishInfoTables :: CmmTopForInfoTables -> IO [CmmTopZ]
finishInfoTables (NoInfoTable t) = return [t]
finishInfoTables (ProcInfoTable p _) = return [p]
finishInfoTables (FloatingInfoTable (CmmInfo _ _ infotbl) bid _) =
  do uniq_supply <- mkSplitUniqSupply 'i'
     return $ mkBareInfoTable (retPtLbl bid) (uniqFromSupply uniq_supply) infotbl

----------------------------------------------------------------
-- Safe foreign calls:
-- Our analyses capture the dataflow facts at block boundaries, but we need
-- to extend the CAF and live-slot analyses to safe foreign calls as well,
-- which show up as middle nodes.
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extendEnvWithSafeForeignCalls ::
  BackwardTransfers Middle Last a -> BlockEnv a -> CmmGraph -> BlockEnv a
extendEnvWithSafeForeignCalls transfers env g = fold_blocks block env g
  where block b z =
          tail (bt_last_in transfers l (lookup env)) z head
           where (head, last) = goto_end (G.unzip b)
                 l = case last of LastOther l -> l
                                  LastExit -> panic "extendEnvs lastExit"
        tail _ z (ZFirst _) = z
        tail fact env (ZHead h m@(MidForeignCall (Safe bid _) _ _ _)) =
          tail (mid m fact) (extendBlockEnv env bid fact) h
        tail fact env (ZHead h m) = tail (mid m fact) env h
        lookup map k = expectJust "extendEnvWithSafeFCalls" $ lookupBlockEnv map k
        mid = bt_middle_in transfers


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extendEnvsForSafeForeignCalls :: CAFEnv -> SlotEnv -> CmmGraph -> (CAFEnv, SlotEnv)
extendEnvsForSafeForeignCalls cafEnv slotEnv g =
  fold_blocks block (cafEnv, slotEnv) g
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    where block b z =
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            tail ( bt_last_in cafTransfers      l (lookupFn cafEnv)
                 , bt_last_in liveSlotTransfers l (lookupFn slotEnv))
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                 z head
             where (head, last) = goto_end (G.unzip b)
                   l = case last of LastOther l -> l
                                    LastExit -> panic "extendEnvs lastExit"
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          tail _ z (ZFirst _) = z
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          tail lives@(cafs, slots) (cafEnv, slotEnv)
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               (ZHead h m@(MidForeignCall (Safe bid _) _ _ _)) =
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            let slots'   = removeLiveSlotDefs slots m
                slotEnv' = extendBlockEnv slotEnv bid slots'
                cafEnv'  = extendBlockEnv cafEnv  bid cafs
            in  tail (upd lives m) (cafEnv', slotEnv') h
          tail lives z (ZHead h m) = tail (upd lives m) z h
          lookupFn map k = expectJust "extendEnvsForSafeFCalls" $ lookupBlockEnv map k
          upd (cafs, slots) m =
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            (bt_middle_in cafTransfers m cafs, bt_middle_in liveSlotTransfers m slots)
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-- Safe foreign calls: We need to insert the code that suspends and resumes
-- the thread before and after a safe foreign call.
-- Why do we do this so late in the pipeline?
-- Because we need this code to appear without interrruption: you can't rely on the
-- value of the stack pointer between the call and resetting the thread state;
-- you need to have an infotable on the young end of the stack both when
-- suspending the thread and making the foreign call.
-- All of this is much easier if we insert the suspend and resume calls here.

-- At the same time, we prepare for the stages of the compiler that
-- build the proc points. We have to do this at the same time because
-- the safe foreign calls need special treatment with respect to infotables.
-- A safe foreign call needs an infotable even though it isn't
-- a procpoint. The following datatype captures the information
-- needed to generate the infotables along with the Cmm data and procedures.

data CmmTopForInfoTables
  = NoInfoTable       CmmTopZ  -- must be CmmData
  | ProcInfoTable     CmmTopZ BlockSet -- CmmProc; argument is its set of procpoints
  | FloatingInfoTable CmmInfo BlockId UpdFrameOffset
instance Outputable CmmTopForInfoTables where
  ppr (NoInfoTable t) = text "NoInfoTable: " <+> ppr t
  ppr (ProcInfoTable t bids) = text "ProcInfoTable: " <+> ppr t <+> ppr bids
  ppr (FloatingInfoTable info bid upd) =
    text "FloatingInfoTable: " <+> ppr info <+> ppr bid <+> ppr upd

-- The `safeState' record collects the info we update while lowering the
-- safe foreign calls in the graph.
data SafeState = State { s_blocks    :: BlockEnv CmmBlock
                       , s_pps       :: ProcPointSet
                       , s_safeCalls :: [CmmTopForInfoTables]}

lowerSafeForeignCalls
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  :: [[CmmTopForInfoTables]] -> CmmTopZ -> FuelMonad [[CmmTopForInfoTables]]
lowerSafeForeignCalls rst t@(CmmData _ _) = return $ [NoInfoTable t] : rst
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lowerSafeForeignCalls rst (CmmProc info l args (off, g@(LGraph entry _))) = do
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  let init = return $ State emptyBlockEnv emptyBlockSet []
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  let block b@(Block bid _) z = do
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        state@(State {s_pps = ppset, s_blocks = blocks}) <- z
        let ppset' = if bid == entry then extendBlockSet ppset bid else ppset
            state' = state { s_pps = ppset' }
        if hasSafeForeignCall b
         then lowerSafeCallBlock state' b
         else return (state' { s_blocks = insertBlock b blocks })
  State blocks' g_procpoints safeCalls <- fold_blocks block init g
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  let proc = (CmmProc info l args (off, LGraph entry blocks'))
      procTable = case off of
                    (_, Just _) -> [ProcInfoTable proc g_procpoints]
                    _ -> [NoInfoTable proc] -- not a successor of a call
  return $ safeCalls : procTable : rst
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-- Check for foreign calls -- if none, then we can avoid copying the block.
hasSafeForeignCall :: CmmBlock -> Bool
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hasSafeForeignCall (Block _ t) = tail t
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  where tail (ZTail (MidForeignCall (Safe _ _) _ _ _) _) = True
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        tail (ZTail _ t) = tail t
        tail (ZLast _)   = False

-- Lower each safe call in the block, update the CAF and slot environments
-- to include each of those calls, and insert the new block in the blockEnv.
lowerSafeCallBlock :: SafeState-> CmmBlock -> FuelMonad SafeState
lowerSafeCallBlock state b = tail (return state) (ZBlock head (ZLast last))
  where (head, last) = goto_end (G.unzip b)
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        tail s b@(ZBlock (ZFirst _) _) =
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          do state <- s
             return $ state { s_blocks = insertBlock (G.zip b) (s_blocks state) }
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        tail s (ZBlock (ZHead h m@(MidForeignCall (Safe bid updfr_off) _ _ _)) t) =
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          do state <- s
             let state' = state
                   { s_safeCalls = FloatingInfoTable emptyContInfoTable bid updfr_off :
                                     s_safeCalls state }
             (state'', t') <- lowerSafeForeignCall state' m t
             tail (return state'') (ZBlock h t')
        tail s (ZBlock (ZHead h m) t) = tail s (ZBlock h (ZTail m t))
           

-- Late in the code generator, we want to insert the code necessary
-- to lower a safe foreign call to a sequence of unsafe calls.
lowerSafeForeignCall ::
  SafeState -> Middle -> ZTail Middle Last -> FuelMonad (SafeState, ZTail Middle Last)
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lowerSafeForeignCall state m@(MidForeignCall (Safe infotable _) _ _ _) tail = do
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    let newTemp rep = getUniqueM >>= \u -> return (LocalReg u rep)
    -- Both 'id' and 'new_base' are KindNonPtr because they're
    -- RTS-only objects and are not subject to garbage collection
    id <- newTemp bWord
    new_base <- newTemp (cmmRegType (CmmGlobal BaseReg))
    let (caller_save, caller_load) = callerSaveVolatileRegs 
    load_tso <- newTemp gcWord -- TODO FIXME NOW
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    let suspendThread = CmmLit (CmmLabel (mkCmmCodeLabel rtsPackageId (fsLit "suspendThread")))
        resumeThread  = CmmLit (CmmLabel (mkCmmCodeLabel rtsPackageId (fsLit "resumeThread")))
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        suspend = mkStore (CmmReg spReg) (CmmLit (CmmBlock infotable)) <*>
                  saveThreadState <*>
                  caller_save <*>
                  mkUnsafeCall (ForeignTarget suspendThread
                                  (ForeignConvention CCallConv [AddrHint] [AddrHint]))
                    [id] [CmmReg (CmmGlobal BaseReg)]
        resume = mkUnsafeCall (ForeignTarget resumeThread
                                  (ForeignConvention CCallConv [AddrHint] [AddrHint]))
                    [new_base] [CmmReg (CmmLocal id)] <*>
                 -- Assign the result to BaseReg: we
                 -- might now have a different Capability!
                 mkAssign (CmmGlobal BaseReg) (CmmReg (CmmLocal new_base)) <*>
                 caller_load <*>
                 loadThreadState load_tso
    Graph tail' blocks' <-
      liftUniq (graphOfAGraph (suspend <*> mkMiddle m <*> resume <*> mkZTail tail))
    return (state {s_blocks = s_blocks state `plusBlockEnv` blocks'}, tail')
lowerSafeForeignCall _ _ _ = panic "lowerSafeForeignCall was passed something else"