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|
{-# LANGUAGE GADTs, NoMonoLocalBinds, FlexibleContexts #-}
-- Norman likes local bindings
-- If this module lives on I'd like to get rid of this flag in due course
{-# OPTIONS_GHC -fno-warn-warnings-deprecations #-}
-- TODO: Get rid of this flag:
{-# OPTIONS_GHC -fno-warn-incomplete-patterns #-}
module CmmSpillReload
( dualLivenessWithInsertion
)
where
import BlockId
import Cmm
import CmmUtils
import CmmLive
import OptimizationFuel
import Control.Monad
import Outputable hiding (empty)
import qualified Outputable as PP
import UniqSet
import Hoopl
import Data.Maybe
import Prelude hiding (succ, zip)
{- Note [Overview of spill/reload]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The point of this module is to insert spills and reloads to establish
the invariant that at a call or any proc point with an established
protocol all live variables not expected in registers are sitting on the
stack. We use a backward dual liveness analysis (both traditional
register liveness as well as register slot liveness on the stack) to
insert spills and reloads. It should be followed by a forward
transformation to sink reloads as deeply as possible, so as to reduce
register pressure: this transformation is performed by
CmmRewriteAssignments.
A variable can be expected to be live in a register, live on the
stack, or both. This analysis ensures that spills and reloads are
inserted as needed to make sure that every live variable needed
after a call is available on the stack. Spills are placed immediately
after their reaching definitions, but reloads are placed immediately
after a return from a call (the entry point.)
Note that we offer no guarantees about the consistency of the value
in memory and the value in the register, except that they are
equal across calls/procpoints. If the variable is changed, this
mapping breaks: but as the original value of the register may still
be useful in a different context, the memory location is not updated.
-}
data DualLive = DualLive { on_stack :: RegSet, in_regs :: RegSet }
changeStack, changeRegs :: (RegSet -> RegSet) -> DualLive -> DualLive
changeStack f live = live { on_stack = f (on_stack live) }
changeRegs f live = live { in_regs = f (in_regs live) }
dualLiveLattice :: DataflowLattice DualLive
dualLiveLattice = DataflowLattice "variables live in registers and on stack" empty add
where empty = DualLive emptyRegSet emptyRegSet
add _ (OldFact old) (NewFact new) = (changeIf $ change1 || change2, DualLive stack regs)
where (change1, stack) = add1 (on_stack old) (on_stack new)
(change2, regs) = add1 (in_regs old) (in_regs new)
add1 old new = if sizeUniqSet join > sizeUniqSet old then (True, join) else (False, old)
where join = unionUniqSets old new
dualLivenessWithInsertion :: BlockSet -> CmmGraph -> FuelUniqSM CmmGraph
dualLivenessWithInsertion procPoints g =
liftM fst $ dataflowPassBwd g [] $ analRewBwd dualLiveLattice
(dualLiveTransfers (g_entry g) procPoints)
(insertSpillsAndReloads g procPoints)
-- Note [Live registers on entry to procpoints]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- Remember that the transfer function is only ever run on the rewritten
-- version of a graph, and the rewrite function for spills and reloads
-- enforces the invariant that no local registers are live on entry to
-- a procpoint. Accordingly, we check for this invariant here. An old
-- version of this code incorrectly claimed that any live registers were
-- live on the stack before entering the function: this is wrong, but
-- didn't cause bugs because it never actually was invoked.
dualLiveTransfers :: BlockId -> BlockSet -> (BwdTransfer CmmNode DualLive)
dualLiveTransfers entry procPoints = mkBTransfer3 first middle last
where first :: CmmNode C O -> DualLive -> DualLive
first (CmmEntry id) live -- See Note [Live registers on entry to procpoints]
| id == entry || setMember id procPoints = noLiveOnEntry id (in_regs live) live
| otherwise = live
middle :: CmmNode O O -> DualLive -> DualLive
middle m = changeStack updSlots
. changeRegs updRegs
where -- Reuse middle of liveness analysis from CmmLive
updRegs = case getBTransfer3 xferLive of (_, middle, _) -> middle m
updSlots live = foldSlotsUsed reload (foldSlotsDefd spill live m) m
spill live s@(RegSlot r, _, _) = check s $ deleteFromRegSet live r
spill live _ = live
reload live s@(RegSlot r, _, _) = check s $ extendRegSet live r
reload live _ = live
-- Ensure the assignment refers to the entirety of the
-- register slot (and not just a slice).
check (RegSlot (LocalReg _ ty), o, w) x
| o == w && w == widthInBytes (typeWidth ty) = x
check _ _ = panic "dualLiveTransfers: slices unsupported"
-- Register analysis is identical to liveness analysis from CmmLive.
last :: CmmNode O C -> FactBase DualLive -> DualLive
last l fb = changeRegs (gen_kill l) $ case l of
CmmCall {cml_cont=Nothing} -> empty
CmmCall {cml_cont=Just k} -> keep_stack_only k
CmmForeignCall {succ=k} -> keep_stack_only k
_ -> joinOutFacts dualLiveLattice l fb
where empty = fact_bot dualLiveLattice
lkp k = fromMaybe empty (lookupFact k fb)
keep_stack_only k = DualLive (on_stack (lkp k)) emptyRegSet
insertSpillsAndReloads :: CmmGraph -> BlockSet -> CmmBwdRewrite DualLive
insertSpillsAndReloads graph procPoints = deepBwdRw3 first middle nothing
-- Beware: deepBwdRw with one polymorphic function seems more reasonable here,
-- but GHC miscompiles it, see bug #4044.
where first :: CmmNode C O -> Fact O DualLive -> CmmReplGraph C O
first e@(CmmEntry id) live = return $
if id /= (g_entry graph) && setMember id procPoints then
case map reload (uniqSetToList (in_regs live)) of
[] -> Nothing
is -> Just $ mkFirst e <*> mkMiddles is
else Nothing
-- EZY: There was some dead code for handling the case where
-- we were not splitting procedures. Check Git history if
-- you're interested (circa e26ea0f41).
middle :: CmmNode O O -> Fact O DualLive -> CmmReplGraph O O
-- Don't add spills next to reloads.
middle (CmmAssign (CmmLocal reg) (CmmLoad (CmmStackSlot (RegSlot reg') _) _)) _ | reg == reg' = return Nothing
-- Spill if register is live on stack.
middle m@(CmmAssign (CmmLocal reg) _) live
| reg `elemRegSet` on_stack live = return (Just (mkMiddles [m, spill reg]))
middle _ _ = return Nothing
nothing _ _ = return Nothing
spill, reload :: LocalReg -> CmmNode O O
spill r = CmmStore (regSlot r) (CmmReg $ CmmLocal r)
reload r = CmmAssign (CmmLocal r) (CmmLoad (regSlot r) $ localRegType r)
---------------------
-- prettyprinting
ppr_regs :: String -> RegSet -> SDoc
ppr_regs s regs = text s <+> commafy (map ppr $ uniqSetToList regs)
where commafy xs = hsep $ punctuate comma xs
instance Outputable DualLive where
ppr (DualLive {in_regs = regs, on_stack = stack}) =
if isEmptyUniqSet regs && isEmptyUniqSet stack then
text "<nothing-live>"
else
nest 2 $ fsep [if isEmptyUniqSet regs then PP.empty
else (ppr_regs "live in regs =" regs),
if isEmptyUniqSet stack then PP.empty
else (ppr_regs "live on stack =" stack)]
|
