nativeGen/RegAlloc/Linear/JoinToTargets.hs

    -- | 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 RegAlloc.Linear.JoinToTargets (joinToTargets) where

    import RegAlloc.Linear.State
    import RegAlloc.Linear.Base
    import RegAlloc.Linear.FreeRegs
    import RegAlloc.Liveness
    import Instruction
    import Reg

    import BlockId
    import Hoopl
    import Digraph
    import DynFlags
    import Outputable
    import Unique
    import UniqFM
    import UniqSet

    import Data.Foldable (foldl')

    -- | 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 mapLookup 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   dflags <- getDynFlags
            let platform = targetPlatform dflags

            -- 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 (mapInsert 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 Loc
                        -> UniqFM 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 <- getUniqueR
                    let block = BasicBlock (mkBlockId fixup_block_id)
                                    $ fixUpInstrs ++ mkJumpInstr dest

    {-              pprTrace
                            ("joinToTargets: fixup code is:")
                            (vcat   [ ppr block
                                    , text ""])
                            (return ())
    -}
                    -- 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 mkBlockId fixup_block_id
                                                                    else bid) -- no change!

                               in   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 -> [(Unique, Loc, [Loc])]
    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
            -> [(a, Loc, [Loc])]

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

    expandNode vreg loc@(InMem src) (InBoth dst mem)
            | src == mem = [(vreg, loc, [InReg dst])]
            | otherwise  = [(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  = [(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 (Unique, Loc, [Loc])
            -> 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 (vreg, src, dsts))
            = mapM (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 ((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 dflags <- getDynFlags
          let platform = targetPlatform dflags

          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 dflags (RegReal d) delta s
              (InReg s, InMem d) ->
                  do recordSpill (SpillJoinRM vreg)
                     return $ mkSpillInstr dflags (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.")