{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
module GHC.Cmm.Dataflow.Graph
( Body
, Graph
, Graph'(..)
, NonLocal(..)
, addBlock
, bodyList
, emptyBody
, labelsDefined
, mapGraph
, mapGraphBlocks
, revPostorderFrom
) where
import GHC.Prelude
import GHC.Utils.Misc
import GHC.Cmm.Dataflow.Label
import GHC.Cmm.Dataflow.Block
import GHC.Cmm.Dataflow.Collections
import Data.Kind
-- | A (possibly empty) collection of closed/closed blocks
type Body n = LabelMap (Block n C C)
-- | @Body@ abstracted over @block@
type Body' block (n :: Extensibility -> Extensibility -> Type) = LabelMap (block n C C)
-------------------------------
-- | Gives access to the anchor points for
-- nonlocal edges as well as the edges themselves
class NonLocal thing where
entryLabel :: thing C x -> Label -- ^ The label of a first node or block
successors :: thing e C -> [Label] -- ^ Gives control-flow successors
instance NonLocal n => NonLocal (Block n) where
entryLabel (BlockCO f _) = entryLabel f
entryLabel (BlockCC f _ _) = entryLabel f
successors (BlockOC _ n) = successors n
successors (BlockCC _ _ n) = successors n
emptyBody :: Body' block n
emptyBody = mapEmpty
bodyList :: Body' block n -> [(Label,block n C C)]
bodyList body = mapToList body
addBlock
:: (NonLocal block, HasDebugCallStack)
=> block C C -> LabelMap (block C C) -> LabelMap (block C C)
addBlock block body = mapAlter add lbl body
where
lbl = entryLabel block
add Nothing = Just block
add _ = error $ "duplicate label " ++ show lbl ++ " in graph"
-- ---------------------------------------------------------------------------
-- Graph
-- | A control-flow graph, which may take any of four shapes (O/O,
-- O/C, C/O, C/C). A graph open at the entry has a single,
-- distinguished, anonymous entry point; if a graph is closed at the
-- entry, its entry point(s) are supplied by a context.
type Graph = Graph' Block
-- | @Graph'@ is abstracted over the block type, so that we can build
-- graphs of annotated blocks for example (Compiler.Hoopl.Dataflow
-- needs this).
data Graph' block (n :: Extensibility -> Extensibility -> Type) e x where
GNil :: Graph' block n O O
GUnit :: block n O O -> Graph' block n O O
GMany :: MaybeO e (block n O C)
-> Body' block n
-> MaybeO x (block n C O)
-> Graph' block n e x
-- -----------------------------------------------------------------------------
-- Mapping over graphs
-- | Maps over all nodes in a graph.
mapGraph :: (forall e x. n e x -> n' e x) -> Graph n e x -> Graph n' e x
mapGraph f = mapGraphBlocks (mapBlock f)
-- | Function 'mapGraphBlocks' enables a change of representation of blocks,
-- nodes, or both. It lifts a polymorphic block transform into a polymorphic
-- graph transform. When the block representation stabilizes, a similar
-- function should be provided for blocks.
mapGraphBlocks :: forall block n block' n' e x .
(forall e x . block n e x -> block' n' e x)
-> (Graph' block n e x -> Graph' block' n' e x)
mapGraphBlocks f = map
where map :: Graph' block n e x -> Graph' block' n' e x
map GNil = GNil
map (GUnit b) = GUnit (f b)
map (GMany e b x) = GMany (fmap f e) (mapMap f b) (fmap f x)
-- -----------------------------------------------------------------------------
-- Extracting Labels from graphs
labelsDefined :: forall block n e x . NonLocal (block n) => Graph' block n e x
-> LabelSet
labelsDefined GNil = setEmpty
labelsDefined (GUnit{}) = setEmpty
labelsDefined (GMany _ body x) = mapFoldlWithKey addEntry (exitLabel x) body
where addEntry :: forall a. LabelSet -> ElemOf LabelSet -> a -> LabelSet
addEntry labels label _ = setInsert label labels
exitLabel :: MaybeO x (block n C O) -> LabelSet
exitLabel NothingO = setEmpty
exitLabel (JustO b) = setSingleton (entryLabel b)
----------------------------------------------------------------
-- | Returns a list of blocks reachable from the provided Labels in the reverse
-- postorder.
--
-- This is the most important traversal over this data structure. It drops
-- unreachable code and puts blocks in an order that is good for solving forward
-- dataflow problems quickly. The reverse order is good for solving backward
-- dataflow problems quickly. The forward order is also reasonably good for
-- emitting instructions, except that it will not usually exploit Forrest
-- Baskett's trick of eliminating the unconditional branch from a loop. For
-- that you would need a more serious analysis, probably based on dominators, to
-- identify loop headers.
--
-- For forward analyses we want reverse postorder visitation, consider:
-- @
-- A -> [B,C]
-- B -> D
-- C -> D
-- @
-- Postorder: [D, C, B, A] (or [D, B, C, A])
-- Reverse postorder: [A, B, C, D] (or [A, C, B, D])
-- This matters for, e.g., forward analysis, because we want to analyze *both*
-- B and C before we analyze D.
revPostorderFrom
:: forall block. (NonLocal block)
=> LabelMap (block C C) -> Label -> [block C C]
revPostorderFrom graph start = go start_worklist setEmpty []
where
start_worklist = lookup_for_descend start Nil
-- To compute the postorder we need to "visit" a block (mark as done)
-- *after* visiting all its successors. So we need to know whether we
-- already processed all successors of each block (and @NonLocal@ allows
-- arbitrary many successors). So we use an explicit stack with an extra bit
-- of information:
-- * @ConsTodo@ means to explore the block if it wasn't visited before
-- * @ConsMark@ means that all successors were already done and we can add
-- the block to the result.
--
-- NOTE: We add blocks to the result list in postorder, but we *prepend*
-- them (i.e., we use @(:)@), which means that the final list is in reverse
-- postorder.
go :: DfsStack (block C C) -> LabelSet -> [block C C] -> [block C C]
go Nil !_ !result = result
go (ConsMark block rest) !wip_or_done !result =
go rest wip_or_done (block : result)
go (ConsTodo block rest) !wip_or_done !result
| entryLabel block `setMember` wip_or_done = go rest wip_or_done result
| otherwise =
let new_worklist =
foldr lookup_for_descend
(ConsMark block rest)
(successors block)
in go new_worklist (setInsert (entryLabel block) wip_or_done) result
lookup_for_descend :: Label -> DfsStack (block C C) -> DfsStack (block C C)
lookup_for_descend label wl
| Just b <- mapLookup label graph = ConsTodo b wl
| otherwise =
error $ "Label that doesn't have a block?! " ++ show label
data DfsStack a = ConsTodo a (DfsStack a) | ConsMark a (DfsStack a) | Nil