LayoutStack.hs 46.4 KB
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{-# LANGUAGE BangPatterns, RecordWildCards, GADTs #-}
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module GHC.Cmm.LayoutStack (
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       cmmLayoutStack, setInfoTableStackMap
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  ) where

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import GHC.Prelude hiding ((<*>))
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import GHC.StgToCmm.Utils      ( callerSaveVolatileRegs, newTemp  ) -- XXX layering violation
import GHC.StgToCmm.Foreign    ( saveThreadState, loadThreadState ) -- XXX layering violation
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import GHC.Types.Basic
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import GHC.Cmm
import GHC.Cmm.Info
import GHC.Cmm.BlockId
import GHC.Cmm.CLabel
import GHC.Cmm.Utils
import GHC.Cmm.Graph
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import GHC.Types.ForeignCall
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import GHC.Cmm.Liveness
import GHC.Cmm.ProcPoint
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import GHC.Runtime.Heap.Layout
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import GHC.Cmm.Dataflow.Block
import GHC.Cmm.Dataflow.Collections
import GHC.Cmm.Dataflow
import GHC.Cmm.Dataflow.Graph
import GHC.Cmm.Dataflow.Label
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import GHC.Types.Unique.Supply
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import GHC.Data.Maybe
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import GHC.Types.Unique.FM
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import GHC.Utils.Misc
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import GHC.Platform
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import GHC.Driver.Session
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import GHC.Data.FastString
import GHC.Utils.Outputable hiding ( isEmpty )
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import qualified Data.Set as Set
import Control.Monad.Fix
import Data.Array as Array
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import Data.Bits
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import Data.List (nub)
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{- Note [Stack Layout]
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The job of this pass is to

 - replace references to abstract stack Areas with fixed offsets from Sp.

 - replace the CmmHighStackMark constant used in the stack check with
   the maximum stack usage of the proc.

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 - save any variables that are live across a call, and reload them as
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   necessary.

Before stack allocation, local variables remain live across native
calls (CmmCall{ cmm_cont = Just _ }), and after stack allocation local
variables are clobbered by native calls.

We want to do stack allocation so that as far as possible
 - stack use is minimized, and
 - unnecessary stack saves and loads are avoided.

The algorithm we use is a variant of linear-scan register allocation,
where the stack is our register file.

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We proceed in two passes, see Note [Two pass approach] for why they are not easy
to merge into one.

Pass 1:

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 - First, we do a liveness analysis, which annotates every block with
   the variables live on entry to the block.

 - We traverse blocks in reverse postorder DFS; that is, we visit at
   least one predecessor of a block before the block itself.  The
   stack layout flowing from the predecessor of the block will
   determine the stack layout on entry to the block.

 - We maintain a data structure

     Map Label StackMap

   which describes the contents of the stack and the stack pointer on
   entry to each block that is a successor of a block that we have
   visited.

 - For each block we visit:

    - Look up the StackMap for this block.

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    - If this block is a proc point (or a call continuation, if we aren't
      splitting proc points), we need to reload all the live variables from the
      stack - but this is done in Pass 2, which calculates more precise liveness
      information (see description of Pass 2).
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    - Walk forwards through the instructions:
      - At an assignment  x = Sp[loc]
        - Record the fact that Sp[loc] contains x, so that we won't
          need to save x if it ever needs to be spilled.
      - At an assignment  x = E
        - If x was previously on the stack, it isn't any more
      - At the last node, if it is a call or a jump to a proc point
        - Lay out the stack frame for the call (see setupStackFrame)
        - emit instructions to save all the live variables
        - Remember the StackMaps for all the successors
        - emit an instruction to adjust Sp
      - If the last node is a branch, then the current StackMap is the
        StackMap for the successors.

    - Manifest Sp: replace references to stack areas in this block
      with real Sp offsets. We cannot do this until we have laid out
      the stack area for the successors above.

      In this phase we also eliminate redundant stores to the stack;
      see elimStackStores.

  - There is one important gotcha: sometimes we'll encounter a control
    transfer to a block that we've already processed (a join point),
    and in that case we might need to rearrange the stack to match
    what the block is expecting. (exactly the same as in linear-scan
    register allocation, except here we have the luxury of an infinite
    supply of temporary variables).

  - Finally, we update the magic CmmHighStackMark constant with the
    stack usage of the function, and eliminate the whole stack check
    if there was no stack use. (in fact this is done as part of the
    main traversal, by feeding the high-water-mark output back in as
    an input. I hate cyclic programming, but it's just too convenient
    sometimes.)

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  There are plenty of tricky details: update frames, proc points, return
  addresses, foreign calls, and some ad-hoc optimisations that are
  convenient to do here and effective in common cases.  Comments in the
  code below explain these.

Pass 2:

- Calculate live registers, but taking into account that nothing is live at the
  entry to a proc point.

- At each proc point and call continuation insert reloads of live registers from
  the stack (they were saved by Pass 1).


Note [Two pass approach]

The main reason for Pass 2 is being able to insert only the reloads that are
needed and the fact that the two passes need different liveness information.
Let's consider an example:

  .....
   \ /
    D   <- proc point
   / \
  E   F
   \ /
    G   <- proc point
    |
    X

Pass 1 needs liveness assuming that local variables are preserved across calls.
This is important because it needs to save any local registers to the stack
(e.g., if register a is used in block X, it must be saved before any native
call).
However, for Pass 2, where we want to reload registers from stack (in a proc
point), this is overly conservative and would lead us to generate reloads in D
for things used in X, even though we're going to generate reloads in G anyway
(since it's also a proc point).
So Pass 2 calculates liveness knowing that nothing is live at the entry to a
proc point. This means that in D we only need to reload things used in E or F.
This can be quite important, for an extreme example see testcase for #3294.

Merging the two passes is not trivial - Pass 2 is a backward rewrite and Pass 1
is a forward one. Furthermore, Pass 1 is creating code that uses local registers
(saving them before a call), which the liveness analysis for Pass 2 must see to
be correct.
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-}
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-- All stack locations are expressed as positive byte offsets from the
-- "base", which is defined to be the address above the return address
-- on the stack on entry to this CmmProc.
--
-- Lower addresses have higher StackLocs.
--
type StackLoc = ByteOff

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{-
 A StackMap describes the stack at any given point.  At a continuation
 it has a particular layout, like this:

         |             | <- base
         |-------------|
         |     ret0    | <- base + 8
         |-------------|
         .  upd frame  . <- base + sm_ret_off
         |-------------|
         |             |
         .    vars     .
         . (live/dead) .
         |             | <- base + sm_sp - sm_args
         |-------------|
         |    ret1     |
         .  ret vals   . <- base + sm_sp    (<--- Sp points here)
         |-------------|

Why do we include the final return address (ret0) in our stack map?  I
have absolutely no idea, but it seems to be done that way consistently
in the rest of the code generator, so I played along here. --SDM

Note that we will be constructing an info table for the continuation
(ret1), which needs to describe the stack down to, but not including,
the update frame (or ret0, if there is no update frame).
-}

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data StackMap = StackMap
 {  sm_sp   :: StackLoc
       -- ^ the offset of Sp relative to the base on entry
       -- to this block.
 ,  sm_args :: ByteOff
       -- ^ the number of bytes of arguments in the area for this block
       -- Defn: the offset of young(L) relative to the base is given by
       -- (sm_sp - sm_args) of the StackMap for block L.
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 ,  sm_ret_off :: ByteOff
       -- ^ Number of words of stack that we do not describe with an info
       -- table, because it contains an update frame.
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 ,  sm_regs :: UniqFM LocalReg (LocalReg,StackLoc)
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       -- ^ regs on the stack
 }

instance Outputable StackMap where
  ppr StackMap{..} =
     text "Sp = " <> int sm_sp $$
     text "sm_args = " <> int sm_args $$
     text "sm_ret_off = " <> int sm_ret_off $$
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     text "sm_regs = " <> pprUFM sm_regs ppr
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cmmLayoutStack :: DynFlags -> ProcPointSet -> ByteOff -> CmmGraph
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               -> UniqSM (CmmGraph, LabelMap StackMap)
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cmmLayoutStack dflags procpoints entry_args
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               graph@(CmmGraph { g_entry = entry })
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  = do
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    -- We need liveness info. Dead assignments are removed later
    -- by the sinking pass.
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    let liveness = cmmLocalLiveness dflags graph
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        blocks = revPostorder graph
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    (final_stackmaps, _final_high_sp, new_blocks) <-
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          mfix $ \ ~(rec_stackmaps, rec_high_sp, _new_blocks) ->
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            layout dflags procpoints liveness entry entry_args
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                   rec_stackmaps rec_high_sp blocks

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    blocks_with_reloads <-
        insertReloadsAsNeeded dflags procpoints final_stackmaps entry new_blocks
    new_blocks' <- mapM (lowerSafeForeignCall dflags) blocks_with_reloads
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    return (ofBlockList entry new_blocks', final_stackmaps)
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-- -----------------------------------------------------------------------------
-- Pass 1
-- -----------------------------------------------------------------------------
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layout :: DynFlags
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       -> LabelSet                      -- proc points
       -> LabelMap CmmLocalLive         -- liveness
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       -> BlockId                       -- entry
       -> ByteOff                       -- stack args on entry

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       -> LabelMap StackMap             -- [final] stack maps
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       -> ByteOff                       -- [final] Sp high water mark

       -> [CmmBlock]                    -- [in] blocks

       -> UniqSM
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          ( LabelMap StackMap           -- [out] stack maps
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          , ByteOff                     -- [out] Sp high water mark
          , [CmmBlock]                  -- [out] new blocks
          )

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layout dflags procpoints liveness entry entry_args final_stackmaps final_sp_high blocks
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  = go blocks init_stackmap entry_args []
  where
    (updfr, cont_info)  = collectContInfo blocks

    init_stackmap = mapSingleton entry StackMap{ sm_sp   = entry_args
                                               , sm_args = entry_args
                                               , sm_ret_off = updfr
                                               , sm_regs = emptyUFM
                                               }

    go [] acc_stackmaps acc_hwm acc_blocks
      = return (acc_stackmaps, acc_hwm, acc_blocks)

    go (b0 : bs) acc_stackmaps acc_hwm acc_blocks
      = do
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       let (entry0@(CmmEntry entry_lbl tscope), middle0, last0) = blockSplit b0
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       let stack0@StackMap { sm_sp = sp0 }
               = mapFindWithDefault
                     (pprPanic "no stack map for" (ppr entry_lbl))
                     entry_lbl acc_stackmaps
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       -- (a) Update the stack map to include the effects of
       --     assignments in this block
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       let stack1 = foldBlockNodesF (procMiddle acc_stackmaps) middle0 stack0
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       -- (b) Look at the last node and if we are making a call or
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       --     jumping to a proc point, we must save the live
       --     variables, adjust Sp, and construct the StackMaps for
       --     each of the successor blocks.  See handleLastNode for
       --     details.
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       (middle1, sp_off, last1, fixup_blocks, out)
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           <- handleLastNode dflags procpoints liveness cont_info
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                             acc_stackmaps stack1 tscope middle0 last0
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       -- (c) Manifest Sp: run over the nodes in the block and replace
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       --     CmmStackSlot with CmmLoad from Sp with a concrete offset.
       --
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       -- our block:
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       --    middle0          -- the original middle nodes
       --    middle1          -- live variable saves from handleLastNode
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       --    Sp = Sp + sp_off -- Sp adjustment goes here
       --    last1            -- the last node
       --
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       let middle_pre = blockToList $ foldl' blockSnoc middle0 middle1
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       let final_blocks =
               manifestSp dflags final_stackmaps stack0 sp0 final_sp_high
                          entry0 middle_pre sp_off last1 fixup_blocks
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       let acc_stackmaps' = mapUnion acc_stackmaps out
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           -- If this block jumps to the GC, then we do not take its
           -- stack usage into account for the high-water mark.
           -- Otherwise, if the only stack usage is in the stack-check
           -- failure block itself, we will do a redundant stack
           -- check.  The stack has a buffer designed to accommodate
           -- the largest amount of stack needed for calling the GC.
           --
           this_sp_hwm | isGcJump last0 = 0
                       | otherwise      = sp0 - sp_off

           hwm' = maximum (acc_hwm : this_sp_hwm : map sm_sp (mapElems out))
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       go bs acc_stackmaps' hwm' (final_blocks ++ acc_blocks)
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-- -----------------------------------------------------------------------------

-- Not foolproof, but GCFun is the culprit we most want to catch
isGcJump :: CmmNode O C -> Bool
isGcJump (CmmCall { cml_target = CmmReg (CmmGlobal l) })
  = l == GCFun || l == GCEnter1
isGcJump _something_else = False

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-- -----------------------------------------------------------------------------
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-- This doesn't seem right somehow.  We need to find out whether this
-- proc will push some update frame material at some point, so that we
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-- can avoid using that area of the stack for spilling. Ideally we would
-- capture this information in the CmmProc (e.g. in CmmStackInfo; see #18232
-- for details on one ill-fated attempt at this).
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--
-- So we'll just take the max of all the cml_ret_offs.  This could be
-- unnecessarily pessimistic, but probably not in the code we
-- generate.

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collectContInfo :: [CmmBlock] -> (ByteOff, LabelMap ByteOff)
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collectContInfo blocks
  = (maximum ret_offs, mapFromList (catMaybes mb_argss))
 where
  (mb_argss, ret_offs) = mapAndUnzip get_cont blocks

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  get_cont :: Block CmmNode x C -> (Maybe (Label, ByteOff), ByteOff)
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  get_cont b =
     case lastNode b of
        CmmCall { cml_cont = Just l, .. }
           -> (Just (l, cml_ret_args), cml_ret_off)
        CmmForeignCall { .. }
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           -> (Just (succ, ret_args), ret_off)
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        _other -> (Nothing, 0)


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-- -----------------------------------------------------------------------------
-- Updating the StackMap from middle nodes
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-- Look for loads from stack slots, and update the StackMap.  This is
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-- purely for optimisation reasons, so that we can avoid saving a
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-- variable back to a different stack slot if it is already on the
-- stack.
--
-- This happens a lot: for example when function arguments are passed
-- on the stack and need to be immediately saved across a call, we
-- want to just leave them where they are on the stack.
--
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procMiddle :: LabelMap StackMap -> CmmNode e x -> StackMap -> StackMap
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procMiddle stackmaps node sm
  = case node of
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     CmmAssign (CmmLocal r) (CmmLoad (CmmStackSlot area off) _)
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       -> sm { sm_regs = addToUFM (sm_regs sm) r (r,loc) }
        where loc = getStackLoc area off stackmaps
     CmmAssign (CmmLocal r) _other
       -> sm { sm_regs = delFromUFM (sm_regs sm) r }
     _other
       -> sm

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getStackLoc :: Area -> ByteOff -> LabelMap StackMap -> StackLoc
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getStackLoc Old       n _         = n
getStackLoc (Young l) n stackmaps =
  case mapLookup l stackmaps of
    Nothing -> pprPanic "getStackLoc" (ppr l)
    Just sm -> sm_sp sm - sm_args sm + n

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-- -----------------------------------------------------------------------------
-- Handling stack allocation for a last node

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-- We take a single last node and turn it into:
--
--    C1 (some statements)
--    Sp = Sp + N
--    C2 (some more statements)
--    call f()          -- the actual last node
--
-- plus possibly some more blocks (we may have to add some fixup code
-- between the last node and the continuation).
--
-- C1: is the code for saving the variables across this last node onto
-- the stack, if the continuation is a call or jumps to a proc point.
--
-- C2: if the last node is a safe foreign call, we have to inject some
-- extra code that goes *after* the Sp adjustment.

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handleLastNode
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   :: DynFlags -> ProcPointSet -> LabelMap CmmLocalLive -> LabelMap ByteOff
   -> LabelMap StackMap -> StackMap -> CmmTickScope
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   -> Block CmmNode O O
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   -> CmmNode O C
   -> UniqSM
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      ( [CmmNode O O]      -- nodes to go *before* the Sp adjustment
      , ByteOff            -- amount to adjust Sp
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      , CmmNode O C        -- new last node
      , [CmmBlock]         -- new blocks
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      , LabelMap StackMap  -- stackmaps for the continuations
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      )

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handleLastNode dflags procpoints liveness cont_info stackmaps
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               stack0@StackMap { sm_sp = sp0 } tscp middle last
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 = case last of
    --  At each return / tail call,
    --  adjust Sp to point to the last argument pushed, which
    --  is cml_args, after popping any other junk from the stack.
    CmmCall{ cml_cont = Nothing, .. } -> do
      let sp_off = sp0 - cml_args
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      return ([], sp_off, last, [], mapEmpty)
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    --  At each CmmCall with a continuation:
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    CmmCall{ cml_cont = Just cont_lbl, .. } ->
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       return $ lastCall cont_lbl cml_args cml_ret_args cml_ret_off
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    CmmForeignCall{ succ = cont_lbl, .. } -> do
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       return $ lastCall cont_lbl (platformWordSizeInBytes platform) ret_args ret_off
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            -- one word of args: the return address
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    CmmBranch {}     ->  handleBranches
    CmmCondBranch {} ->  handleBranches
    CmmSwitch {}     ->  handleBranches
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  where
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     platform = targetPlatform dflags
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     -- Calls and ForeignCalls are handled the same way:
     lastCall :: BlockId -> ByteOff -> ByteOff -> ByteOff
              -> ( [CmmNode O O]
                 , ByteOff
                 , CmmNode O C
                 , [CmmBlock]
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                 , LabelMap StackMap
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                 )
     lastCall lbl cml_args cml_ret_args cml_ret_off
      =  ( assignments
         , spOffsetForCall sp0 cont_stack cml_args
         , last
         , [] -- no new blocks
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         , mapSingleton lbl cont_stack )
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      where
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         (assignments, cont_stack) = prepareStack lbl cml_ret_args cml_ret_off


     prepareStack lbl cml_ret_args cml_ret_off
       | Just cont_stack <- mapLookup lbl stackmaps
             -- If we have already seen this continuation before, then
             -- we just have to make the stack look the same:
       = (fixupStack stack0 cont_stack, cont_stack)
             -- Otherwise, we have to allocate the stack frame
       | otherwise
       = (save_assignments, new_cont_stack)
       where
        (new_cont_stack, save_assignments)
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           = setupStackFrame platform lbl liveness cml_ret_off cml_ret_args stack0
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     -- For other last nodes (branches), if any of the targets is a
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     -- proc point, we have to set up the stack to match what the proc
     -- point is expecting.
     --
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     handleBranches :: UniqSM ( [CmmNode O O]
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                                , ByteOff
                                , CmmNode O C
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                                , [CmmBlock]
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                                , LabelMap StackMap )
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     handleBranches
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         -- Note [diamond proc point]
       | Just l <- futureContinuation middle
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       , (nub $ filter (`setMember` procpoints) $ successors last) == [l]
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       = do
         let cont_args = mapFindWithDefault 0 l cont_info
             (assigs, cont_stack) = prepareStack l cont_args (sm_ret_off stack0)
             out = mapFromList [ (l', cont_stack)
                               | l' <- successors last ]
         return ( assigs
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                , spOffsetForCall sp0 cont_stack (platformWordSizeInBytes platform)
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                , last
                , []
                , out)
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        | otherwise = do
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          pps <- mapM handleBranch (successors last)
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          let lbl_map :: LabelMap Label
              lbl_map = mapFromList [ (l,tmp) | (l,tmp,_,_) <- pps ]
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              fix_lbl l = mapFindWithDefault l l lbl_map
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          return ( []
                 , 0
                 , mapSuccessors fix_lbl last
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                 , concat [ blk | (_,_,_,blk) <- pps ]
                 , mapFromList [ (l, sm) | (l,_,sm,_) <- pps ] )
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     -- For each successor of this block
     handleBranch :: BlockId -> UniqSM (BlockId, BlockId, StackMap, [CmmBlock])
     handleBranch l
        --   (a) if the successor already has a stackmap, we need to
        --       shuffle the current stack to make it look the same.
        --       We have to insert a new block to make this happen.
        | Just stack2 <- mapLookup l stackmaps
        = do
             let assigs = fixupStack stack0 stack2
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             (tmp_lbl, block) <- makeFixupBlock dflags sp0 l stack2 tscp assigs
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             return (l, tmp_lbl, stack2, block)

        --   (b) if the successor is a proc point, save everything
        --       on the stack.
        | l `setMember` procpoints
        = do
             let cont_args = mapFindWithDefault 0 l cont_info
                 (stack2, assigs) =
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                      setupStackFrame platform l liveness (sm_ret_off stack0)
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                                                        cont_args stack0
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             (tmp_lbl, block) <- makeFixupBlock dflags sp0 l stack2 tscp assigs
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             return (l, tmp_lbl, stack2, block)

        --   (c) otherwise, the current StackMap is the StackMap for
        --       the continuation.  But we must remember to remove any
        --       variables from the StackMap that are *not* live at
        --       the destination, because this StackMap might be used
        --       by fixupStack if this is a join point.
        | otherwise = return (l, l, stack1, [])
        where live = mapFindWithDefault (panic "handleBranch") l liveness
              stack1 = stack0 { sm_regs = filterUFM is_live (sm_regs stack0) }
              is_live (r,_) = r `elemRegSet` live


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makeFixupBlock :: DynFlags -> ByteOff -> Label -> StackMap
               -> CmmTickScope -> [CmmNode O O]
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               -> UniqSM (Label, [CmmBlock])
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makeFixupBlock dflags sp0 l stack tscope assigs
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  | null assigs && sp0 == sm_sp stack = return (l, [])
  | otherwise = do
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    tmp_lbl <- newBlockId
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    let sp_off = sp0 - sm_sp stack
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        block = blockJoin (CmmEntry tmp_lbl tscope)
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                          ( maybeAddSpAdj dflags sp0 sp_off
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                           $ blockFromList assigs )
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                          (CmmBranch l)
    return (tmp_lbl, [block])
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-- Sp is currently pointing to current_sp,
-- we want it to point to
--    (sm_sp cont_stack - sm_args cont_stack + args)
-- so the difference is
--    sp0 - (sm_sp cont_stack - sm_args cont_stack + args)
spOffsetForCall :: ByteOff -> StackMap -> ByteOff -> ByteOff
spOffsetForCall current_sp cont_stack args
  = current_sp - (sm_sp cont_stack - sm_args cont_stack + args)
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-- | create a sequence of assignments to establish the new StackMap,
-- given the old StackMap.
fixupStack :: StackMap -> StackMap -> [CmmNode O O]
fixupStack old_stack new_stack = concatMap move new_locs
 where
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     old_map  = sm_regs old_stack
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     new_locs = stackSlotRegs new_stack

     move (r,n)
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       | Just (_,m) <- lookupUFM old_map r, n == m = []
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       | otherwise = [CmmStore (CmmStackSlot Old n)
                               (CmmReg (CmmLocal r))]

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setupStackFrame
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             :: Platform
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             -> BlockId                 -- label of continuation
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             -> LabelMap CmmLocalLive   -- liveness
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             -> ByteOff      -- updfr
             -> ByteOff      -- bytes of return values on stack
             -> StackMap     -- current StackMap
             -> (StackMap, [CmmNode O O])

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setupStackFrame platform lbl liveness updfr_off ret_args stack0
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  = (cont_stack, assignments)
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  where
      -- get the set of LocalRegs live in the continuation
      live = mapFindWithDefault Set.empty lbl liveness

      -- the stack from the base to updfr_off is off-limits.
      -- our new stack frame contains:
      --   * saved live variables
      --   * the return address [young(C) + 8]
      --   * the args for the call,
      --     which are replaced by the return values at the return
      --     point.

      -- everything up to updfr_off is off-limits
      -- stack1 contains updfr_off, plus everything we need to save
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      (stack1, assignments) = allocate platform updfr_off live stack0
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      -- And the Sp at the continuation is:
      --   sm_sp stack1 + ret_args
      cont_stack = stack1{ sm_sp = sm_sp stack1 + ret_args
                         , sm_args = ret_args
                         , sm_ret_off = updfr_off
                         }


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-- -----------------------------------------------------------------------------
-- Note [diamond proc point]
--
-- This special case looks for the pattern we get from a typical
-- tagged case expression:
--
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--    Sp[young(L1)] = L1
--    if (R1 & 7) != 0 goto L1 else goto L2
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--  L2:
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--    call [R1] returns to L1
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--  L1: live: {y}
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--    x = R1
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--
-- If we let the generic case handle this, we get
--
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--    Sp[-16] = L1
--    if (R1 & 7) != 0 goto L1a else goto L2
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--  L2:
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--    Sp[-8] = y
--    Sp = Sp - 16
--    call [R1] returns to L1
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--  L1a:
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--    Sp[-8] = y
--    Sp = Sp - 16
--    goto L1
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--  L1:
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--    x = R1
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--
-- The code for saving the live vars is duplicated in each branch, and
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-- furthermore there is an extra jump in the fast path (assuming L1 is
-- a proc point, which it probably is if there is a heap check).
--
-- So to fix this we want to set up the stack frame before the
-- conditional jump.  How do we know when to do this, and when it is
-- safe?  The basic idea is, when we see the assignment
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--
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--   Sp[young(L)] = L
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--
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-- we know that
--   * we are definitely heading for L
--   * there can be no more reads from another stack area, because young(L)
--     overlaps with it.
--
-- We don't necessarily know that everything live at L is live now
-- (some might be assigned between here and the jump to L).  So we
-- simplify and only do the optimisation when we see
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--
--   (1) a block containing an assignment of a return address L
--   (2) ending in a branch where one (and only) continuation goes to L,
--       and no other continuations go to proc points.
--
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-- then we allocate the stack frame for L at the end of the block,
-- before the branch.
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--
-- We could generalise (2), but that would make it a bit more
-- complicated to handle, and this currently catches the common case.

futureContinuation :: Block CmmNode O O -> Maybe BlockId
futureContinuation middle = foldBlockNodesB f middle Nothing
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   where f :: CmmNode a b -> Maybe BlockId -> Maybe BlockId
         f (CmmStore (CmmStackSlot (Young l) _) (CmmLit (CmmBlock _))) _
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               = Just l
         f _ r = r

-- -----------------------------------------------------------------------------
-- Saving live registers

-- | Given a set of live registers and a StackMap, save all the registers
-- on the stack and return the new StackMap and the assignments to do
-- the saving.
--
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allocate :: Platform -> ByteOff -> LocalRegSet -> StackMap
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         -> (StackMap, [CmmNode O O])
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allocate platform ret_off live stackmap@StackMap{ sm_sp = sp0
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                                              , sm_regs = regs0 }
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 =
   -- we only have to save regs that are not already in a slot
   let to_save = filter (not . (`elemUFM` regs0)) (Set.elems live)
       regs1   = filterUFM (\(r,_) -> elemRegSet r live) regs0
   in

   -- make a map of the stack
   let stack = reverse $ Array.elems $
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               accumArray (\_ x -> x) Empty (1, toWords platform (max sp0 ret_off)) $
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                 ret_words ++ live_words
            where ret_words =
                   [ (x, Occupied)
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                   | x <- [ 1 .. toWords platform ret_off] ]
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                  live_words =
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                   [ (toWords platform x, Occupied)
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                   | (r,off) <- nonDetEltsUFM regs1,
                   -- See Note [Unique Determinism and code generation]
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                     let w = localRegBytes platform r,
                     x <- [ off, off - platformWordSizeInBytes platform .. off - w + 1] ]
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   in

   -- Pass over the stack: find slots to save all the new live variables,
   -- choosing the oldest slots first (hence a foldr).
   let
       save slot ([], stack, n, assigs, regs) -- no more regs to save
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          = ([], slot:stack, plusW platform n 1, assigs, regs)
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       save slot (to_save, stack, n, assigs, regs)
          = case slot of
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               Occupied ->  (to_save, Occupied:stack, plusW platform n 1, assigs, regs)
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               Empty
                 | Just (stack', r, to_save') <-
                       select_save to_save (slot:stack)
                 -> let assig = CmmStore (CmmStackSlot Old n')
                                         (CmmReg (CmmLocal r))
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                        n' = plusW platform n 1
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                   in
                        (to_save', stack', n', assig : assigs, (r,(r,n')):regs)

                 | otherwise
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                 -> (to_save, slot:stack, plusW platform n 1, assigs, regs)
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       -- we should do better here: right now we'll fit the smallest first,
       -- but it would make more sense to fit the biggest first.
       select_save :: [LocalReg] -> [StackSlot]
                   -> Maybe ([StackSlot], LocalReg, [LocalReg])
       select_save regs stack = go regs []
         where go []     _no_fit = Nothing
               go (r:rs) no_fit
                 | Just rest <- dropEmpty words stack
                 = Just (replicate words Occupied ++ rest, r, rs++no_fit)
                 | otherwise
                 = go rs (r:no_fit)
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                 where words = localRegWords platform r
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       -- fill in empty slots as much as possible
       (still_to_save, save_stack, n, save_assigs, save_regs)
          = foldr save (to_save, [], 0, [], []) stack

       -- push any remaining live vars on the stack
       (push_sp, push_assigs, push_regs)
          = foldr push (n, [], []) still_to_save
          where
              push r (n, assigs, regs)
                = (n', assig : assigs, (r,(r,n')) : regs)
                where
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                  n' = n + localRegBytes platform r
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                  assig = CmmStore (CmmStackSlot Old n')
                                   (CmmReg (CmmLocal r))

       trim_sp
          | not (null push_regs) = push_sp
          | otherwise
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          = plusW platform n (- length (takeWhile isEmpty save_stack))
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       final_regs = regs1 `addListToUFM` push_regs
                          `addListToUFM` save_regs

   in
  -- XXX should be an assert
   if ( n /= max sp0 ret_off ) then pprPanic "allocate" (ppr n <+> ppr sp0 <+> ppr ret_off) else

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   if (trim_sp .&. (platformWordSizeInBytes platform - 1)) /= 0  then pprPanic "allocate2" (ppr trim_sp <+> ppr final_regs <+> ppr push_sp) else
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   ( stackmap { sm_regs = final_regs , sm_sp = trim_sp }
   , push_assigs ++ save_assigs )


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-- -----------------------------------------------------------------------------
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-- Manifesting Sp
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-- | Manifest Sp: turn all the CmmStackSlots into CmmLoads from Sp.  The
-- block looks like this:
--
--    middle_pre       -- the middle nodes
--    Sp = Sp + sp_off -- Sp adjustment goes here
--    last             -- the last node
--
-- And we have some extra blocks too (that don't contain Sp adjustments)
--
-- The adjustment for middle_pre will be different from that for
-- middle_post, because the Sp adjustment intervenes.
--
manifestSp
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   :: DynFlags
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   -> LabelMap StackMap  -- StackMaps for other blocks
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   -> StackMap           -- StackMap for this block
   -> ByteOff            -- Sp on entry to the block
   -> ByteOff            -- SpHigh
   -> CmmNode C O        -- first node
   -> [CmmNode O O]      -- middle
   -> ByteOff            -- sp_off
   -> CmmNode O C        -- last node
   -> [CmmBlock]         -- new blocks
   -> [CmmBlock]         -- final blocks with Sp manifest

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manifestSp dflags stackmaps stack0 sp0 sp_high
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           first middle_pre sp_off last fixup_blocks
  = final_block : fixup_blocks'
  where
    area_off = getAreaOff stackmaps
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    platform = targetPlatform dflags
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    adj_pre_sp, adj_post_sp :: CmmNode e x -> CmmNode e x
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    adj_pre_sp  = mapExpDeep (areaToSp platform sp0            sp_high area_off)
    adj_post_sp = mapExpDeep (areaToSp platform (sp0 - sp_off) sp_high area_off)
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    final_middle = maybeAddSpAdj dflags sp0 sp_off
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                 . blockFromList
                 . map adj_pre_sp
                 . elimStackStores stack0 stackmaps area_off
                 $ middle_pre
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    final_last    = optStackCheck (adj_post_sp last)

    final_block   = blockJoin first final_middle final_last

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    fixup_blocks' = map (mapBlock3' (id, adj_post_sp, id)) fixup_blocks
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getAreaOff :: LabelMap StackMap -> (Area -> StackLoc)
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getAreaOff _ Old = 0
getAreaOff stackmaps (Young l) =
  case mapLookup l stackmaps of
    Just sm -> sm_sp sm - sm_args sm
    Nothing -> pprPanic "getAreaOff" (ppr l)


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maybeAddSpAdj
  :: DynFlags -> ByteOff -> ByteOff -> Block CmmNode O O -> Block CmmNode O O
maybeAddSpAdj dflags sp0 sp_off block =
  add_initial_unwind $ add_adj_unwind $ adj block
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  where
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    platform = targetPlatform dflags
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    adj block
      | sp_off /= 0
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      = block `blockSnoc` CmmAssign spReg (cmmOffset platform spExpr sp_off)
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      | otherwise = block
    -- Add unwind pseudo-instruction at the beginning of each block to
    -- document Sp level for debugging
    add_initial_unwind block
      | debugLevel dflags > 0
      = CmmUnwind [(Sp, Just sp_unwind)] `blockCons` block
      | otherwise
      = block
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      where sp_unwind = CmmRegOff spReg (sp0 - platformWordSizeInBytes platform)
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    -- Add unwind pseudo-instruction right after the Sp adjustment
    -- if there is one.
    add_adj_unwind block
      | debugLevel dflags > 0
      , sp_off /= 0
      = block `blockSnoc` CmmUnwind [(Sp, Just sp_unwind)]
      | otherwise
      = block
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      where sp_unwind = CmmRegOff spReg (sp0 - platformWordSizeInBytes platform - sp_off)
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{- Note [SP old/young offsets]

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Sp(L) is the Sp offset on entry to block L relative to the base of the
OLD area.

SpArgs(L) is the size of the young area for L, i.e. the number of
arguments.

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 - in block L, each reference to [old + N] turns into
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   [Sp + Sp(L) - N]

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 - in block L, each reference to [young(L') + N] turns into
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   [Sp + Sp(L) - Sp(L') + SpArgs(L') - N]

 - be careful with the last node of each block: Sp has already been adjusted
   to be Sp + Sp(L) - Sp(L')
-}

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areaToSp :: Platform -> ByteOff -> ByteOff -> (Area -> StackLoc) -> CmmExpr -> CmmExpr
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areaToSp platform sp_old _sp_hwm area_off (CmmStackSlot area n)
  = cmmOffset platform spExpr (sp_old - area_off area - n)
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    -- Replace (CmmStackSlot area n) with an offset from Sp

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areaToSp platform _ sp_hwm _ (CmmLit CmmHighStackMark)
  = mkIntExpr platform sp_hwm
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    -- Replace CmmHighStackMark with the number of bytes of stack used,
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    -- the sp_hwm.   See Note [Stack usage] in GHC.StgToCmm.Heap
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areaToSp platform _ _ _ (CmmMachOp (MO_U_Lt _) args)
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  | falseStackCheck args
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  = zeroExpr platform
areaToSp platform _ _ _ (CmmMachOp (MO_U_Ge _) args)
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  | falseStackCheck args
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  = mkIntExpr platform 1
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    -- Replace a stack-overflow test that cannot fail with a no-op
    -- See Note [Always false stack check]

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areaToSp _ _ _ _ other = other
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-- | Determine whether a stack check cannot fail.
falseStackCheck :: [CmmExpr] -> Bool
falseStackCheck [ CmmMachOp (MO_Sub _)
                      [ CmmRegOff (CmmGlobal Sp) x_off
                      , CmmLit (CmmInt y_lit _)]
                , CmmReg (CmmGlobal SpLim)]
  = fromIntegral x_off >= y_lit
falseStackCheck _ = False

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-- Note [Always false stack check]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- We can optimise stack checks of the form
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--
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--   if ((Sp + x) - y < SpLim) then .. else ..
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--
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-- where are non-negative integer byte offsets.  Since we know that
-- SpLim <= Sp (remember the stack grows downwards), this test must
-- yield False if (x >= y), so we can rewrite the comparison to False.
-- A subsequent sinking pass will later drop the dead code.
-- Optimising this away depends on knowing that SpLim <= Sp, so it is
-- really the job of the stack layout algorithm, hence we do it now.
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--
-- The control flow optimiser may negate a conditional to increase
-- the likelihood of a fallthrough if the branch is not taken.  But
-- not every conditional is inverted as the control flow optimiser
-- places some requirements on the predecessors of both branch targets.
-- So we better look for the inverted comparison too.
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optStackCheck :: CmmNode O C -> CmmNode O C
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optStackCheck n = -- Note [Always false stack check]
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 case n of
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   CmmCondBranch (CmmLit (CmmInt 0 _)) _true false _ -> CmmBranch false
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   CmmCondBranch (CmmLit (CmmInt _ _)) true _false _ -> CmmBranch true
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   other -> other

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-- -----------------------------------------------------------------------------

-- | Eliminate stores of the form
--
--    Sp[area+n] = r
--
-- when we know that r is already in the same slot as Sp[area+n].  We
-- could do this in a later optimisation pass, but that would involve
-- a separate analysis and we already have the information to hand
-- here.  It helps clean up some extra stack stores in common cases.
--
-- Note that we may have to modify the StackMap as we walk through the
-- code using procMiddle, since an assignment to a variable in the
-- StackMap will invalidate its mapping there.
--
elimStackStores :: StackMap
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                -> LabelMap StackMap
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                -> (Area -> ByteOff)
                -> [CmmNode O O]
                -> [CmmNode O O]
elimStackStores stackmap stackmaps area_off nodes
  = go stackmap nodes
  where
    go _stackmap [] = []
    go stackmap (n:ns)
     = case n of
         CmmStore (CmmStackSlot area m) (CmmReg (CmmLocal r))
            | Just (_,off) <- lookupUFM (sm_regs stackmap) r
            , area_off area + m == off
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            -> go stackmap ns
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         _otherwise
            -> n : go (procMiddle stackmaps n stackmap) ns


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-- -----------------------------------------------------------------------------
-- Update info tables to include stack liveness


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setInfoTableStackMap :: Platform -> LabelMap StackMap -> CmmDecl -> CmmDecl
setInfoTableStackMap platform stackmaps (CmmProc top_info@TopInfo{..} l v g)
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  = CmmProc top_info{ info_tbls = mapMapWithKey fix_info info_tbls } l v g
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  where
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    fix_info lbl info_tbl@CmmInfoTable{ cit_rep = StackRep _ } =
       info_tbl { cit_rep = StackRep (get_liveness lbl) }
    fix_info _ other = other
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    get_liveness :: BlockId -> Liveness
    get_liveness lbl
      = case mapLookup lbl stackmaps of
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          Nothing -> pprPanic "setInfoTableStackMap" (ppr lbl <+> ppr info_tbls)
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          Just sm -> stackMapToLiveness platform sm
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setInfoTableStackMap _ _ d = d
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stackMapToLiveness :: Platform -> StackMap -> Liveness
stackMapToLiveness platform StackMap{..} =
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   reverse $ Array.elems $