Tune coalescing in non-iterative register allocator

If iterative coalescing isn't turned on, then do a single aggressive
coalescing pass for the first build/color cycle and then back off to 
conservative coalescing for subseqent passes.

Aggressive coalescing is a cheap way to eliminate lots of the reg-reg
moves, but it can make the graph less colorable - if we turn it on 
for every pass then allocation for code with a large amount of register
pressure (ie SHA1) doesn't converge in a sensible number of cycles.
parent 6a347ffc
-- | Graph coloring register allocator.
--
-- TODO:
-- The function that choosing the potential spills could be a bit cleverer.
-- Colors in graphviz graphs could be nicer.
-- TODO: The colors in graphviz graphs for x86_64 and ppc could be nicer.
--
{-# OPTIONS -fno-warn-missing-signatures #-}
......@@ -81,20 +79,8 @@ regAlloc_spin dflags (spinCount :: Int) triv regsFree slotsFree debug_codeGraphs
$ uniqSetToList $ unionManyUniqSets $ eltsUFM regsFree)
$$ text "slotsFree = " <> ppr (sizeUniqSet slotsFree))
-- Brig's algorithm does reckless coalescing for all but the first allocation stage
-- Doing this seems to reduce the number of reg-reg moves, but at the cost-
-- of creating more spills. Probably better just to stick with conservative
-- coalescing in Color.colorGraph for now.
--
{- code_coalesced1 <- if (spinCount > 0)
then regCoalesce code
else return code -}
let code_coalesced1 = code
-- build a conflict graph from the code.
graph <- {-# SCC "BuildGraph" #-} buildGraph code_coalesced1
graph <- {-# SCC "BuildGraph" #-} buildGraph code
-- VERY IMPORTANT:
-- We really do want the graph to be fully evaluated _before_ we start coloring.
......@@ -107,7 +93,7 @@ regAlloc_spin dflags (spinCount :: Int) triv regsFree slotsFree debug_codeGraphs
-- build a map of the cost of spilling each instruction
-- this will only actually be computed if we have to spill something.
let spillCosts = foldl' plusSpillCostInfo zeroSpillCostInfo
$ map slurpSpillCostInfo code_coalesced1
$ map slurpSpillCostInfo code
-- the function to choose regs to leave uncolored
let spill = chooseSpill spillCosts
......@@ -126,14 +112,15 @@ regAlloc_spin dflags (spinCount :: Int) triv regsFree slotsFree debug_codeGraphs
= {-# SCC "ColorGraph" #-}
Color.colorGraph
(dopt Opt_RegsIterative dflags)
spinCount
regsFree triv spill graph
-- rewrite regs in the code that have been coalesced
let patchF reg = case lookupUFM rmCoalesce reg of
Just reg' -> patchF reg'
Nothing -> reg
let code_coalesced2
= map (patchEraseLive patchF) code_coalesced1
let code_coalesced
= map (patchEraseLive patchF) code
-- see if we've found a coloring
......@@ -148,7 +135,7 @@ regAlloc_spin dflags (spinCount :: Int) triv regsFree slotsFree debug_codeGraphs
else graph_colored
-- patch the registers using the info in the graph
let code_patched = map (patchRegsFromGraph graph_colored_lint) code_coalesced2
let code_patched = map (patchRegsFromGraph graph_colored_lint) code_coalesced
-- clean out unneeded SPILL/RELOADs
let code_spillclean = map cleanSpills code_patched
......@@ -195,7 +182,7 @@ regAlloc_spin dflags (spinCount :: Int) triv regsFree slotsFree debug_codeGraphs
-- spill the uncolored regs
(code_spilled, slotsFree', spillStats)
<- regSpill code_coalesced2 slotsFree rsSpill
<- regSpill code_coalesced slotsFree rsSpill
-- recalculate liveness
let code_nat = map stripLive code_spilled
......
......@@ -39,6 +39,7 @@ colorGraph
, Eq color, Eq cls, Ord k
, Outputable k, Outputable cls, Outputable color)
=> Bool -- ^ whether to do iterative coalescing
-> Int -- ^ how many times we've tried to color this graph so far.
-> UniqFM (UniqSet color) -- ^ map of (node class -> set of colors available for this class).
-> Triv k cls color -- ^ fn to decide whether a node is trivially colorable.
-> (Graph k cls color -> k) -- ^ fn to choose a node to potentially leave uncolored if nothing is trivially colorable.
......@@ -49,14 +50,22 @@ colorGraph
, UniqFM k ) -- map of regs (r1 -> r2) that were coaleced
-- r1 should be replaced by r2 in the source
colorGraph iterative colors triv spill graph0
colorGraph iterative spinCount colors triv spill graph0
= let
-- If we're not doing iterative coalescing then just do a conservative
-- coalescing stage at the front.
-- If we're not doing iterative coalescing then do an aggressive coalescing first time
-- around and then conservative coalescing for subsequent passes.
--
-- Aggressive coalescing is a quick way to get rid of many reg-reg moves. However, if
-- there is a lot of register pressure and we do it on every round then it can make the
-- graph less colorable and prevent the algorithm from converging in a sensible number
-- of cycles.
--
(graph_coalesced, kksCoalesce1)
= if not iterative
then coalesceGraph True triv graph0
else (graph0, [])
= if iterative
then (graph0, [])
else if spinCount == 0
then coalesceGraph True triv graph0
else coalesceGraph False triv graph0
-- run the scanner to slurp out all the trivially colorable nodes
-- (and do coalescing if iterative coalescing is enabled)
......
-- | Basic operations on graphs.
--
-- TODO: refine coalescing crieteria
{-# OPTIONS -fno-warn-missing-signatures #-}
module GraphOps (
......
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