path: root/compiler/GHC/CmmToAsm/Reg/Linear/JoinToTargets.hs

{-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}

-- | Handles joining of a jump instruction to its targets.

--      The first time we encounter a jump to a particular basic block, we
--      record the assignment of temporaries.  The next time we encounter a
--      jump to the same block, we compare our current assignment to the
--      stored one.  They might be different if spilling has occurred in one
--      branch; so some fixup code will be required to match up the assignments.
--
module GHC.CmmToAsm.Reg.Linear.JoinToTargets (joinToTargets) where

import GHC.Prelude

import GHC.CmmToAsm.Reg.Linear.State
import GHC.CmmToAsm.Reg.Linear.Base
import GHC.CmmToAsm.Reg.Linear.FreeRegs
import GHC.CmmToAsm.Reg.Liveness
import GHC.CmmToAsm.Instr
import GHC.CmmToAsm.Config
import GHC.CmmToAsm.Types

import GHC.Platform.Reg

import GHC.Cmm.BlockId
import GHC.Cmm.Dataflow.Collections
import GHC.Data.Graph.Directed
import GHC.Utils.Panic
import GHC.Utils.Monad (concatMapM)
import GHC.Types.Unique
import GHC.Types.Unique.FM
import GHC.Types.Unique.Set

-- | For a jump instruction at the end of a block, generate fixup code so its
--      vregs are in the correct regs for its destination.
--
joinToTargets
        :: (FR freeRegs, Instruction instr)
        => BlockMap RegSet              -- ^ maps the unique of the blockid to the set of vregs
                                        --      that are known to be live on the entry to each block.

        -> BlockId                      -- ^ id of the current block
        -> instr                        -- ^ branch instr on the end of the source block.

        -> RegM freeRegs ([NatBasicBlock instr] -- fresh blocks of fixup code.
                         , instr)               -- the original branch
                                                -- instruction, but maybe
                                                -- patched to jump
                                                -- to a fixup block first.

joinToTargets block_live id instr

        -- we only need to worry about jump instructions.
        | not $ isJumpishInstr instr
        = return ([], instr)

        | otherwise
        = joinToTargets' block_live [] id instr (jumpDestsOfInstr instr)

-----
joinToTargets'
        :: (FR freeRegs, Instruction instr)
        => BlockMap RegSet              -- ^ maps the unique of the blockid to the set of vregs
                                        --      that are known to be live on the entry to each block.

        -> [NatBasicBlock instr]        -- ^ acc blocks of fixup code.

        -> BlockId                      -- ^ id of the current block
        -> instr                        -- ^ branch instr on the end of the source block.

        -> [BlockId]                    -- ^ branch destinations still to consider.

        -> RegM freeRegs ([NatBasicBlock instr], instr)

-- no more targets to consider. all done.
joinToTargets' _          new_blocks _ instr []
        = return (new_blocks, instr)

-- handle a branch target.
joinToTargets' block_live new_blocks block_id instr (dest:dests)
 = do
        -- get the map of where the vregs are stored on entry to each basic block.
        block_assig     <- getBlockAssigR

        -- get the assignment on entry to the branch instruction.
        assig           <- getAssigR

        -- adjust the current assignment to remove any vregs that are not live
        -- on entry to the destination block.
        let Just live_set       = mapLookup dest block_live
        let still_live uniq _   = uniq `elemUniqSet_Directly` live_set
        let adjusted_assig      = filterUFM_Directly still_live assig

        -- and free up those registers which are now free.
        let to_free =
                [ r     | (reg, loc) <- nonDetUFMToList assig
                        -- This is non-deterministic but we do not
                        -- currently support deterministic code-generation.
                        -- See Note [Unique Determinism and code generation]
                        , not (elemUniqSet_Directly reg live_set)
                        , r          <- regsOfLoc loc ]

        case lookupBlockAssignment  dest block_assig of
         Nothing
          -> joinToTargets_first
                        block_live new_blocks block_id instr dest dests
                        block_assig adjusted_assig to_free

         Just (_, dest_assig)
          -> joinToTargets_again
                        block_live new_blocks block_id instr dest dests
                        adjusted_assig dest_assig


-- this is the first time we jumped to this block.
joinToTargets_first :: (FR freeRegs, Instruction instr)
                    => BlockMap RegSet
                    -> [NatBasicBlock instr]
                    -> BlockId
                    -> instr
                    -> BlockId
                    -> [BlockId]
                    -> BlockAssignment freeRegs
                    -> RegMap Loc
                    -> [RealReg]
                    -> RegM freeRegs ([NatBasicBlock instr], instr)
joinToTargets_first block_live new_blocks block_id instr dest dests
        block_assig src_assig
        to_free

 = do   config <- getConfig
        let platform = ncgPlatform config

        -- free up the regs that are not live on entry to this block.
        freeregs        <- getFreeRegsR
        let freeregs' = foldl' (flip $ frReleaseReg platform) freeregs to_free

        -- remember the current assignment on entry to this block.
        setBlockAssigR (updateBlockAssignment dest (freeregs', src_assig) block_assig)

        joinToTargets' block_live new_blocks block_id instr dests


-- we've jumped to this block before
joinToTargets_again :: (Instruction instr, FR freeRegs)
                    => BlockMap RegSet
                    -> [NatBasicBlock instr]
                    -> BlockId
                    -> instr
                    -> BlockId
                    -> [BlockId]
                    -> UniqFM Reg Loc
                    -> UniqFM Reg Loc
                    -> RegM freeRegs ([NatBasicBlock instr], instr)
joinToTargets_again
    block_live new_blocks block_id instr dest dests
    src_assig dest_assig

        -- the assignments already match, no problem.
        | nonDetUFMToList dest_assig == nonDetUFMToList src_assig
        -- This is non-deterministic but we do not
        -- currently support deterministic code-generation.
        -- See Note [Unique Determinism and code generation]
        = joinToTargets' block_live new_blocks block_id instr dests

        -- assignments don't match, need fixup code
        | otherwise
        = do

                -- make a graph of what things need to be moved where.
                let graph = makeRegMovementGraph src_assig dest_assig

                -- look for cycles in the graph. This can happen if regs need to be swapped.
                -- Note that we depend on the fact that this function does a
                --      bottom up traversal of the tree-like portions of the graph.
                --
                --  eg, if we have
                --      R1 -> R2 -> R3
                --
                --  ie move value in R1 to R2 and value in R2 to R3.
                --
                -- We need to do the R2 -> R3 move before R1 -> R2.
                --
                let sccs  = stronglyConnCompFromEdgedVerticesOrdR graph

              -- debugging
                {-
                pprTrace
                        ("joinToTargets: making fixup code")
                        (vcat   [ text "        in block: "     <> ppr block_id
                                , text " jmp instruction: "     <> ppr instr
                                , text "  src assignment: "     <> ppr src_assig
                                , text " dest assignment: "     <> ppr dest_assig
                                , text "  movement graph: "     <> ppr graph
                                , text "   sccs of graph: "     <> ppr sccs
                                , text ""])
                        (return ())
                -}
                delta           <- getDeltaR
                fixUpInstrs_    <- mapM (handleComponent delta instr) sccs
                let fixUpInstrs = concat fixUpInstrs_

                -- make a new basic block containing the fixup code.
                --      A the end of the current block we will jump to the fixup one,
                --      then that will jump to our original destination.
                fixup_block_id <- mkBlockId <$> getUniqueR
                let block = BasicBlock fixup_block_id
                                $ fixUpInstrs ++ mkJumpInstr dest

                -- if we didn't need any fixups, then don't include the block
                case fixUpInstrs of
                 []     -> joinToTargets' block_live new_blocks block_id instr dests

                 -- patch the original branch instruction so it goes to our
                 --     fixup block instead.
                 _      -> let  instr'  =  patchJumpInstr instr
                                            (\bid -> if bid == dest
                                                        then fixup_block_id
                                                        else bid) -- no change!

                           in do
                                {- --debugging
                                pprTrace "FixUpEdge info:"
                                    (
                                    text "inBlock:" <> ppr block_id $$
                                    text "instr:" <> ppr instr $$
                                    text "instr':" <> ppr instr' $$
                                    text "fixup_block_id':" <>
                                        ppr fixup_block_id $$
                                    text "dest:" <> ppr dest
                                    ) (return ())
                                -}
                                recordFixupBlock block_id fixup_block_id dest
                                joinToTargets' block_live (block : new_blocks)
                                               block_id instr' dests


-- | Construct a graph of register\/spill movements.
--
--      Cyclic components seem to occur only very rarely.
--
--      We cut some corners by not handling memory-to-memory moves.
--      This shouldn't happen because every temporary gets its own stack slot.
--
makeRegMovementGraph :: RegMap Loc -> RegMap Loc -> [Node Loc Unique]
makeRegMovementGraph adjusted_assig dest_assig
 = [ node       | (vreg, src) <- nonDetUFMToList adjusted_assig
                    -- This is non-deterministic but we do not
                    -- currently support deterministic code-generation.
                    -- See Note [Unique Determinism and code generation]
                    -- source reg might not be needed at the dest:
                , Just loc <- [lookupUFM_Directly dest_assig vreg]
                , node <- expandNode vreg src loc ]


-- | Expand out the destination, so InBoth destinations turn into
--      a combination of InReg and InMem.

--      The InBoth handling is a little tricky here.  If the destination is
--      InBoth, then we must ensure that the value ends up in both locations.
--      An InBoth  destination must conflict with an InReg or InMem source, so
--      we expand an InBoth destination as necessary.
--
--      An InBoth source is slightly different: we only care about the register
--      that the source value is in, so that we can move it to the destinations.
--
expandNode
        :: a
        -> Loc                  -- ^ source of move
        -> Loc                  -- ^ destination of move
        -> [Node Loc a ]

expandNode vreg loc@(InReg src) (InBoth dst mem)
        | src == dst = [DigraphNode vreg loc [InMem mem]]
        | otherwise  = [DigraphNode vreg loc [InReg dst, InMem mem]]

expandNode vreg loc@(InMem src) (InBoth dst mem)
        | src == mem = [DigraphNode vreg loc [InReg dst]]
        | otherwise  = [DigraphNode vreg loc [InReg dst, InMem mem]]

expandNode _        (InBoth _ src) (InMem dst)
        | src == dst = [] -- guaranteed to be true

expandNode _        (InBoth src _) (InReg dst)
        | src == dst = []

expandNode vreg     (InBoth src _) dst
        = expandNode vreg (InReg src) dst

expandNode vreg src dst
        | src == dst = []
        | otherwise  = [DigraphNode vreg src [dst]]


-- | Generate fixup code for a particular component in the move graph
--      This component tells us what values need to be moved to what
--      destinations. We have eliminated any possibility of single-node
--      cycles in expandNode above.
--
handleComponent
        :: Instruction instr
        => Int -> instr -> SCC (Node Loc Unique)
        -> RegM freeRegs [instr]

-- If the graph is acyclic then we won't get the swapping problem below.
--      In this case we can just do the moves directly, and avoid having to
--      go via a spill slot.
--
handleComponent delta _  (AcyclicSCC (DigraphNode vreg src dsts))
        = concatMapM (makeMove delta vreg src) dsts


-- Handle some cyclic moves.
--      This can happen if we have two regs that need to be swapped.
--      eg:
--           vreg   source loc   dest loc
--          (vreg1, InReg r1,    [InReg r2])
--          (vreg2, InReg r2,    [InReg r1])
--
--      To avoid needing temp register, we just spill all the source regs, then
--      reaload them into their destination regs.
--
--      Note that we can not have cycles that involve memory locations as
--      sources as single destination because memory locations (stack slots)
--      are allocated exclusively for a virtual register and therefore can not
--      require a fixup.
--
handleComponent delta instr
        (CyclicSCC ((DigraphNode vreg (InReg sreg) ((InReg dreg: _))) : rest))
        -- dest list may have more than one element, if the reg is also InMem.
 = do
        -- spill the source into its slot
        (instrSpill, slot)
                        <- spillR (RegReal sreg) vreg

        -- reload into destination reg
        instrLoad       <- loadR (RegReal dreg) slot

        remainingFixUps <- mapM (handleComponent delta instr)
                                (stronglyConnCompFromEdgedVerticesOrdR rest)

        -- make sure to do all the reloads after all the spills,
        --      so we don't end up clobbering the source values.
        return (instrSpill ++ concat remainingFixUps ++ instrLoad)

handleComponent _ _ (CyclicSCC _)
 = panic "Register Allocator: handleComponent cyclic"


-- | Move a vreg between these two locations.
--
makeMove
    :: Instruction instr
    => Int      -- ^ current C stack delta.
    -> Unique   -- ^ unique of the vreg that we're moving.
    -> Loc      -- ^ source location.
    -> Loc      -- ^ destination location.
    -> RegM freeRegs [instr]  -- ^ move instruction.

makeMove delta vreg src dst
 = do config <- getConfig
      let platform = ncgPlatform config

      case (src, dst) of
          (InReg s, InReg d) ->
              do recordSpill (SpillJoinRR vreg)
                 return $ [mkRegRegMoveInstr platform (RegReal s) (RegReal d)]
          (InMem s, InReg d) ->
              do recordSpill (SpillJoinRM vreg)
                 return $ mkLoadInstr config (RegReal d) delta s
          (InReg s, InMem d) ->
              do recordSpill (SpillJoinRM vreg)
                 return $ mkSpillInstr config (RegReal s) delta d
          _ ->
              -- we don't handle memory to memory moves.
              -- they shouldn't happen because we don't share
              -- stack slots between vregs.
              panic ("makeMove " ++ show vreg ++ " (" ++ show src ++ ") ("
                  ++ show dst ++ ")"
                  ++ " we don't handle mem->mem moves.")