CmmBuildInfoTables.hs 23.2 KB
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{-# LANGUAGE GADTs, NoMonoLocalBinds #-}
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{-# OPTIONS -fno-warn-tabs #-}
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and
-- detab the module (please do the detabbing in a separate patch). See
--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
-- for details

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-- Norman likes local bindings
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-- If this module lives on I'd like to get rid of the NoMonoLocalBinds
-- extension in due course
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-- Todo: remove -fno-warn-warnings-deprecations
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{-# OPTIONS_GHC -fno-warn-warnings-deprecations #-}
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module CmmBuildInfoTables
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    ( CAFSet, CAFEnv, cafAnal, localCAFInfo, mkTopCAFInfo
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    , setInfoTableSRT, setInfoTableStackMap
    , TopSRT, emptySRT, srtToData
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    , bundleCAFs
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    , lowerSafeForeignCalls
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    , cafTransfers, liveSlotTransfers
    , mkLiveness )
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where

#include "HsVersions.h"

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-- These should not be imported here!
import StgCmmForeign
import StgCmmUtils

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import Constants
import Digraph
import qualified Prelude as P
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import Prelude hiding (succ)
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import Util
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import BlockId
import Bitmap
import CLabel
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import Cmm
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import CmmUtils
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import CmmStackLayout
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import Module
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import FastString
import ForeignCall
import IdInfo
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import Data.List
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import Maybes
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import MkGraph as M
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import Control.Monad
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import Name
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import OptimizationFuel
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import Outputable
import SMRep
import UniqSupply
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import Compiler.Hoopl
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import Data.Map (Map)
import qualified Data.Map as Map
import qualified FiniteMap as Map

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----------------------------------------------------------------
-- 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 -> StackLayout
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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
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  where 
        res = mkLiveness (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)
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                           (Map.foldRightWithKey (\_ -> 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
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            (expectJust "add_slot" (Map.lookup a areaMap), r, w) : rst
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          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" $ mapLookup bid slotEnv
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        youngByte = expectJust "live_ptrs bid_pos" $ Map.lookup (CallArea (Young bid)) areaMap
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-- Construct the stack maps for a procedure _if_ it needs an infotable.
-- When wouldn't a procedure need an infotable? If it is a procpoint that
-- is not the successor of a call.
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setInfoTableStackMap :: SlotEnv -> AreaMap -> CmmDecl -> CmmDecl
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setInfoTableStackMap slotEnv areaMap
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     t@(CmmProc (TopInfo {stack_info=StackInfo {updfr_space = Just updfr_off}}) _ 
                (CmmGraph {g_entry = eid}))
  = updInfo (const (live_ptrs updfr_off slotEnv areaMap eid)) id t
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setInfoTableStackMap _ _ t = 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

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

-- First, an analysis to find live CAFs.
cafLattice :: DataflowLattice CAFSet
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cafLattice = DataflowLattice "live cafs" Map.empty add
  where add _ (OldFact old) (NewFact new) = case old `Map.union` new of
                                              new' -> (changeIf $ Map.size new' > Map.size old, new')

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cafTransfers :: BwdTransfer CmmNode CAFSet
cafTransfers = mkBTransfer3 first middle last
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  where first  _ live = live
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        middle m live = foldExpDeep addCaf m live
        last   l live = foldExpDeep addCaf l (joinOutFacts cafLattice l live)
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        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
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        add l s = if hasCAF l then Map.insert (toClosureLbl l) () s
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                              else s
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cafAnal :: CmmGraph -> FuelUniqSM CAFEnv
cafAnal g = liftM snd $ dataflowPassBwd g [] $ analBwd cafLattice cafTransfers
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-----------------------------------------------------------------------
-- 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]
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                     , elt_map  :: Map CLabel Int }
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                        -- map: CLabel -> its last entry in the table
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instance Outputable TopSRT where
  ppr (TopSRT lbl next elts eltmap) =
    text "TopSRT:" <+> ppr lbl
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                   <+> ppr next
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                   <+> ppr elts
                   <+> ppr eltmap
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emptySRT :: MonadUnique m => m TopSRT
emptySRT =
  do top_lbl <- getUniqueM >>= \ u -> return $ mkSRTLabel (mkFCallName u "srt") NoCafRefs
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     return TopSRT { lbl = top_lbl, next_elt = 0, rev_elts = [], elt_map = Map.empty }
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cafMember :: TopSRT -> CLabel -> Bool
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cafMember srt lbl = Map.member lbl (elt_map srt)
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cafOffset :: TopSRT -> CLabel -> Maybe Int
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cafOffset srt lbl = Map.lookup lbl (elt_map srt)
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addCAF :: CLabel -> TopSRT -> TopSRT
addCAF caf srt =
  srt { next_elt = last + 1
      , rev_elts = caf : rev_elts srt
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      , elt_map  = Map.insert caf last (elt_map srt) }
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    where last  = next_elt srt

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srtToData :: TopSRT -> CmmGroup
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srtToData srt = [CmmData RelocatableReadOnlyData (Statics (lbl srt) tbl)]
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    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.
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buildSRTs :: TopSRT -> Map CLabel CAFSet -> CAFSet ->
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             FuelUniqSM (TopSRT, Maybe CmmDecl, C_SRT)
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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 Map.lookup lbl topCAFMap of Just cafs -> z `Map.union` cafs
                                            Nothing   -> Map.insert lbl () z
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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 =
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           let cafs = Map.keys (Map.foldRightWithKey liftCAF Map.empty localCafs)
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               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
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               farthestFst x y = case (Map.lookup x m, Map.lookup y m) of
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                                   (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.
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procpointSRT :: CLabel -> Map CLabel Int -> [CLabel] ->
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                FuelUniqSM (Maybe CmmDecl, 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
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    ints = map (expectJust "constructSRT" . flip Map.lookup top_table) entries
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    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.
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to_SRT :: CLabel -> Int -> Int -> Bitmap -> FuelUniqSM (Maybe CmmDecl, C_SRT)
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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 $
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                   Statics srt_desc_lbl $ map CmmStaticLit
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                     ( 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 -> CmmDecl -> Maybe (CLabel, CAFSet)
localCAFInfo _      (CmmData _ _) = Nothing
localCAFInfo cafEnv (CmmProc top_info top_l (CmmGraph {g_entry=entry})) =
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  case info_tbl top_info of
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    CmmInfoTable { cit_rep = rep } 
      | not (isStaticRep rep) 
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      -> Just (toClosureLbl top_l,
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               expectJust "maybeBindCAFs" $ mapLookup entry cafEnv)
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    _ -> 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.
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mkTopCAFInfo :: [(CLabel, CAFSet)] -> Map CLabel CAFSet
mkTopCAFInfo localCAFs = foldl addToTop Map.empty g
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  where addToTop env (AcyclicSCC (l, cafset)) =
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          Map.insert l (flatten env cafset) env
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        addToTop env (CyclicSCC nodes) =
          let (lbls, cafsets) = unzip nodes
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              cafset  = lbls `Map.deleteList` foldl Map.union Map.empty cafsets
          in foldl (\env l -> Map.insert l (flatten env cafset) env) env lbls
        flatten env cafset = Map.foldRightWithKey (lookup env) Map.empty cafset
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        lookup env caf () cafset' =
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          case Map.lookup caf env of Just cafs -> Map.foldRightWithKey add cafset' cafs
                                     Nothing -> add caf () cafset'
        add caf () cafset' = Map.insert caf () cafset'
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        g = stronglyConnCompFromEdgedVertices
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              (map (\n@(l, cafs) -> (n, l, Map.keys cafs)) localCAFs)
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-- Bundle the CAFs used at a procpoint.
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bundleCAFs :: CAFEnv -> CmmDecl -> (CAFSet, CmmDecl)
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bundleCAFs cafEnv t@(CmmProc _ _ (CmmGraph {g_entry=entry})) =
  (expectJust "bundleCAFs" (mapLookup entry cafEnv), t)
bundleCAFs _ t = (Map.empty, t)
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-- Construct the SRTs for the given procedure.
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setInfoTableSRT :: Map CLabel CAFSet -> TopSRT -> (CAFSet, CmmDecl) ->
                   FuelUniqSM (TopSRT, [CmmDecl])
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setInfoTableSRT topCAFMap topSRT (cafs, t) =
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  setSRT cafs topCAFMap topSRT t

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setSRT :: CAFSet -> Map CLabel CAFSet -> TopSRT ->
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          CmmDecl -> FuelUniqSM (TopSRT, [CmmDecl])
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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
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       Just tbl -> return (topSRT, [t', tbl])
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       Nothing  -> return (topSRT, [t'])

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type StackLayout = Liveness

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updInfo :: (StackLayout -> StackLayout) -> (C_SRT -> C_SRT) -> CmmDecl -> CmmDecl
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updInfo toVars toSrt (CmmProc top_info top_l g) =
  CmmProc (top_info {info_tbl=updInfoTbl toVars toSrt (info_tbl top_info)}) top_l g
updInfo _ _ t = t

updInfoTbl :: (StackLayout -> StackLayout) -> (C_SRT -> C_SRT) -> CmmInfoTable -> CmmInfoTable
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updInfoTbl toVars toSrt info_tbl@(CmmInfoTable {})
  = info_tbl { cit_srt = toSrt (cit_srt info_tbl)
             , cit_rep = case cit_rep info_tbl of
                           StackRep ls -> StackRep (toVars ls)
                           other       -> other }
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updInfoTbl _ _ t@CmmNonInfoTable = t
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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.

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-- JD: Why not do this while splitting procedures?
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lowerSafeForeignCalls :: AreaMap -> CmmDecl -> FuelUniqSM CmmDecl
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lowerSafeForeignCalls _ t@(CmmData _ _) = return t
lowerSafeForeignCalls areaMap (CmmProc info l g@(CmmGraph {g_entry=entry})) = do
  let block b mblocks = mblocks >>= lowerSafeCallBlock entry areaMap b
  blocks <- foldGraphBlocks block (return mapEmpty) g
  return $ CmmProc info l (ofBlockMap entry blocks)

-- If the block ends with a safe call in the block, lower it to an unsafe
-- call (with appropriate saves and restores before and after).
lowerSafeCallBlock :: BlockId -> AreaMap -> CmmBlock -> BlockEnv CmmBlock
                              -> FuelUniqSM (BlockEnv CmmBlock)
lowerSafeCallBlock entry areaMap b blocks =
  case blockToNodeList b of
    (JustC (CmmEntry id), m, JustC l@(CmmForeignCall {})) -> lowerSafeForeignCall entry areaMap blocks id m l
    _                                                    -> return $ insertBlock b blocks
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-- Late in the code generator, we want to insert the code necessary
-- to lower a safe foreign call to a sequence of unsafe calls.
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lowerSafeForeignCall :: BlockId -> AreaMap -> BlockEnv CmmBlock -> BlockId -> [CmmNode O O] -> CmmNode O C
                                -> FuelUniqSM (BlockEnv CmmBlock)
lowerSafeForeignCall entry areaMap blocks bid m
    (CmmForeignCall {tgt=tgt, res=rs, args=as, succ=succ, updfr = updfr_off, intrbl = intrbl}) =
 do let newTemp rep = getUniqueM >>= \u -> return (LocalReg u rep)
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    -- 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))
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    let (caller_save, caller_load) = callerSaveVolatileRegs
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    load_tso <- newTemp gcWord -- TODO FIXME NOW
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    load_stack <- newTemp gcWord -- TODO FIXME NOW
    let (<**>) = (M.<*>)
    let suspendThread = foreignLbl "suspendThread"
        resumeThread  = foreignLbl "resumeThread"
        foreignLbl name = CmmLit (CmmLabel (mkCmmCodeLabel rtsPackageId (fsLit name)))
        suspend = saveThreadState <**>
                  caller_save <**>
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                  mkUnsafeCall (ForeignTarget suspendThread
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                                (ForeignConvention CCallConv [AddrHint, NoHint] [AddrHint]))
                               [id] [CmmReg (CmmGlobal BaseReg), CmmLit (CmmInt (fromIntegral (fromEnum intrbl)) wordWidth)]
        midCall = mkUnsafeCall tgt rs as
        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 load_stack
        -- We have to save the return value on the stack because its next use
        -- may appear in a different procedure due to procpoint splitting...
        saveRetVals = foldl (<**>) emptyAGraph $ map (M.mkMiddle . spill) rs
        spill r = CmmStore (regSlot r) (CmmReg $ CmmLocal r)
        regSlot r@(LocalReg _ _) = CmmRegOff (CmmGlobal Sp) (sp_off - offset)
          where offset = w + expectJust "lowerForeign" (Map.lookup (RegSlot r) areaMap)
                sp_off = wORD_SIZE + expectJust "lowerForeign" (Map.lookup (CallArea area) areaMap)
                area = if succ == entry then Old else Young succ
                w = widthInBytes $ typeWidth $ localRegType r
        -- Note: The successor must be a procpoint, and we have already split,
        --       so we use a jump, not a branch.
        succLbl = CmmLit (CmmLabel (infoTblLbl succ))
        jump = CmmCall { cml_target  = succLbl, cml_cont = Nothing
                       , cml_args    = widthInBytes wordWidth ,cml_ret_args = 0
                       , cml_ret_off = updfr_off}
    graph' <- liftUniq $ labelAGraph bid $ catAGraphs (map M.mkMiddle m) <**>
                                           suspend <**> midCall <**>
                                           resume  <**> saveRetVals <**> M.mkLast jump
    return $ blocks `mapUnion` toBlockMap graph'
lowerSafeForeignCall _ _ _ _ _ _ = panic "lowerSafeForeignCall was passed something else"
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