+-- Nothing if either of the nodes weren't in the graph
+
+coalesceNodes
+ :: (Uniquable k, Ord k, Eq cls, Outputable k)
+ => Bool -- ^ If True, coalesce nodes even if this might make the graph
+ -- less colorable (aggressive coalescing)
+ -> Triv k cls color
+ -> Graph k cls color
+ -> (k, k) -- ^ keys of the nodes to be coalesced
+ -> (Graph k cls color, Maybe (k, k))
+
+coalesceNodes aggressive triv graph (k1, k2)
+ | (kMin, kMax) <- if k1 < k2
+ then (k1, k2)
+ else (k2, k1)
+
+ -- the nodes being coalesced must be in the graph
+ , Just nMin <- lookupNode graph kMin
+ , Just nMax <- lookupNode graph kMax
+
+ -- can't coalesce conflicting modes
+ , not $ elementOfUniqSet kMin (nodeConflicts nMax)
+ , not $ elementOfUniqSet kMax (nodeConflicts nMin)
+
+ = coalesceNodes_merge aggressive triv graph kMin kMax nMin nMax
+
+ -- don't do the coalescing after all
+ | otherwise
+ = (graph, Nothing)
+
+coalesceNodes_merge aggressive triv graph kMin kMax nMin nMax
+
+ -- sanity checks
+ | nodeClass nMin /= nodeClass nMax
+ = error "GraphOps.coalesceNodes: can't coalesce nodes of different classes."
+
+ | not (isNothing (nodeColor nMin) && isNothing (nodeColor nMax))
+ = error "GraphOps.coalesceNodes: can't coalesce colored nodes."
+
+ ---
+ | otherwise
+ = let
+ -- the new node gets all the edges from its two components
+ node =
+ Node { nodeId = kMin
+ , nodeClass = nodeClass nMin
+ , nodeColor = Nothing
+
+ -- nodes don't conflict with themselves..
+ , nodeConflicts
+ = (unionUniqSets (nodeConflicts nMin) (nodeConflicts nMax))
+ `delOneFromUniqSet` kMin
+ `delOneFromUniqSet` kMax
+
+ , nodeExclusions = unionUniqSets (nodeExclusions nMin) (nodeExclusions nMax)
+ , nodePreference = nodePreference nMin ++ nodePreference nMax
+
+ -- nodes don't coalesce with themselves..
+ , nodeCoalesce
+ = (unionUniqSets (nodeCoalesce nMin) (nodeCoalesce nMax))
+ `delOneFromUniqSet` kMin
+ `delOneFromUniqSet` kMax
+ }
+
+ in coalesceNodes_check aggressive triv graph kMin kMax node
+
+coalesceNodes_check aggressive triv graph kMin kMax node
+
+ -- Unless we're coalescing aggressively, if the result node is not trivially
+ -- colorable then don't do the coalescing.
+ | not aggressive
+ , not $ triv (nodeClass node) (nodeConflicts node) (nodeExclusions node)
+ = (graph, Nothing)
+
+ | otherwise
+ = let -- delete the old nodes from the graph and add the new one
+ graph' = addNode kMin node
+ $ delNode kMin
+ $ delNode kMax
+ $ graph
+
+ in (graph', Just (kMax, kMin))
+
+
+-- | Freeze a node
+-- This is for the iterative coalescer.
+-- By freezing a node we give up on ever coalescing it.
+-- Move all its coalesce edges into the frozen set - and update
+-- back edges from other nodes.
+--
+freezeNode
+ :: Uniquable k
+ => k -- ^ key of the node to freeze
+ -> Graph k cls color -- ^ the graph
+ -> Graph k cls color -- ^ graph with that node frozen
+
+freezeNode k
+ = graphMapModify
+ $ \fm ->
+ let
+ -- freeze all the edges in the node to be frozen
+ Just node = lookupUFM fm k
+ node' = node
+ { nodeCoalesce = emptyUniqSet }
+
+ fm1 = addToUFM fm k node'
+
+ -- update back edges pointing to this node
+ freezeEdge k node
+ = if elementOfUniqSet k (nodeCoalesce node)
+ then node
+ { nodeCoalesce = delOneFromUniqSet (nodeCoalesce node) k }
+ else panic "GraphOps.freezeNode: edge to freeze wasn't in the coalesce set"
+
+ fm2 = foldUniqSet (adjustUFM (freezeEdge k)) fm1
+ $ nodeCoalesce node
+
+ in fm2
+
+
+-- | Freeze one node in the graph
+-- This if for the iterative coalescer.
+-- Look for a move related node of low degree and freeze it.
+--
+-- We probably don't need to scan the whole graph looking for the node of absolute
+-- lowest degree. Just sample the first few and choose the one with the lowest
+-- degree out of those. Also, we don't make any distinction between conflicts of different
+-- classes.. this is just a heuristic, after all.
+--
+-- IDEA: freezing a node might free it up for Simplify.. would be good to check for triv
+-- right here, and add it to a worklist if known triv/non-move nodes.
+--
+freezeOneInGraph
+ :: (Uniquable k, Outputable k)
+ => Graph k cls color
+ -> ( Graph k cls color -- the new graph
+ , Bool ) -- whether we found a node to freeze
+
+freezeOneInGraph graph
+ = let compareNodeDegree n1 n2
+ = compare (sizeUniqSet $ nodeConflicts n1) (sizeUniqSet $ nodeConflicts n2)
+
+ candidates
+ = sortBy compareNodeDegree
+ $ take 5 -- 5 isn't special, it's just a small number.
+ $ scanGraph (\node -> not $ isEmptyUniqSet (nodeCoalesce node)) graph
+
+ in case candidates of
+
+ -- there wasn't anything available to freeze
+ [] -> (graph, False)
+
+ -- we found something to freeze
+ (n : _)
+ -> ( freezeNode (nodeId n) graph
+ , True)
+
+
+-- | Freeze all the nodes in the graph
+-- for debugging the iterative allocator.
+--
+freezeAllInGraph
+ :: (Uniquable k, Outputable k)