1 {-# OPTIONS_GHC -XGADTs -XNoMonoLocalBinds #-}
2 -- Norman likes local bindings
3 -- If this module lives on I'd like to get rid of this flag in due course
6 {-# OPTIONS_GHC -fno-warn-warnings-deprecations #-}
9 ( SlotEnv, liveSlotAnal, liveSlotTransfers, removeLiveSlotDefs
10 , layout, manifestSP, igraph, areaBuilder
11 , stubSlotsOnDeath ) -- to help crash early during debugging
15 import Prelude hiding (succ, zip, unzip, last)
22 import MkGraph (stackStubExpr)
24 import OptimizationFuel
26 import SMRep (ByteOff)
31 import qualified Data.Map as Map
32 import qualified FiniteMap as Map
34 ------------------------------------------------------------------------
36 ------------------------------------------------------------------------
38 -- | Before we lay out the stack, we need to know something about the
39 -- liveness of the stack slots. In particular, to decide whether we can
40 -- reuse a stack location to hold multiple stack slots, we need to know
41 -- when each of the stack slots is used.
42 -- Although tempted to use something simpler, we really need a full interference
43 -- graph. Consider the following case:
45 -- 1 -> <spill x>; // y is dead out
46 -- 2 -> <spill y>; // x is dead out
47 -- 3 -> <spill x and y>
48 -- If we consider the arms in order and we use just the deadness information given by a
49 -- dataflow analysis, we might decide to allocate the stack slots for x and y
50 -- to the same stack location, which will lead to incorrect code in the third arm.
51 -- We won't make this mistake with an interference graph.
53 -- First, the liveness analysis.
54 -- We represent a slot with an area, an offset into the area, and a width.
55 -- Tracking the live slots is a bit tricky because there may be loads and stores
56 -- into only a part of a stack slot (e.g. loading the low word of a 2-word long),
57 -- e.g. Slot A 0 8 overlaps with Slot A 4 4.
59 -- The definition of a slot set is intended to reduce the number of overlap
60 -- checks we have to make. There's no reason to check for overlap between
61 -- slots in different areas, so we segregate the map by Area's.
62 -- We expect few slots in each Area, so we collect them in an unordered list.
63 -- To keep these lists short, any contiguous live slots are coalesced into
64 -- a single slot, on insertion.
66 slotLattice :: DataflowLattice SubAreaSet
67 slotLattice = DataflowLattice "live slots" Map.empty add
68 where add _ (OldFact old) (NewFact new) = case Map.foldRightWithKey addArea (False, old) new of
69 (change, x) -> (changeIf change, x)
70 addArea a newSlots z = foldr (addSlot a) z newSlots
71 addSlot a slot (changed, map) =
72 let (c, live) = liveGen slot $ Map.findWithDefault [] a map
73 in (c || changed, Map.insert a live map)
75 slotLatticeJoin :: [SubAreaSet] -> SubAreaSet
76 slotLatticeJoin facts = foldr extend (fact_bot slotLattice) facts
77 where extend fact res = snd $ fact_join slotLattice undefined (OldFact fact) (NewFact res)
79 type SlotEnv = BlockEnv SubAreaSet
80 -- The sub-areas live on entry to the block
82 liveSlotAnal :: CmmGraph -> FuelUniqSM SlotEnv
83 liveSlotAnal g = liftM snd $ dataflowPassBwd g [] $ analBwd slotLattice liveSlotTransfers
85 -- Add the subarea s to the subareas in the list-set (possibly coalescing it with
86 -- adjacent subareas), and also return whether s was a new addition.
87 liveGen :: SubArea -> [SubArea] -> (Bool, [SubArea])
88 liveGen s set = liveGen' s set []
89 where liveGen' s [] z = (True, s : z)
90 liveGen' s@(a, hi, w) (s'@(a', hi', w') : rst) z =
91 if a /= a' || hi < lo' || lo > hi' then -- no overlap
92 liveGen' s rst (s' : z)
93 else if s' `contains` s then -- old contains new
95 else -- overlap: coalesce the slots
96 let new_hi = max hi hi'
98 in liveGen' (a, new_hi, new_hi - new_lo) rst z
99 where lo = hi - w -- remember: areas grow down
101 contains (a, hi, w) (a', hi', w') =
102 a == a' && hi >= hi' && hi - w <= hi' - w'
104 liveKill :: SubArea -> [SubArea] -> [SubArea]
105 liveKill (a, hi, w) set = -- pprTrace "killing slots in area" (ppr a) $
107 where liveKill' [] z = z
108 liveKill' (s'@(a', hi', w') : rst) z =
109 if a /= a' || hi < lo' || lo > hi' then -- no overlap
110 liveKill' rst (s' : z)
111 else -- overlap: split the old slot
112 let z' = if hi' > hi then (a, hi', hi' - hi) : z else z
113 z'' = if lo > lo' then (a, lo, lo - lo') : z' else z'
115 where lo = hi - w -- remember: areas grow down
118 -- Note: the stack slots that hold variables returned on the stack are not
119 -- considered live in to the block -- we treat the first node as a definition site.
120 -- BEWARE?: Am I being a little careless here in failing to check for the
121 -- entry Id (which would use the CallArea Old).
122 liveSlotTransfers :: BwdTransfer CmmNode SubAreaSet
123 liveSlotTransfers = mkBTransfer3 frt mid lst
124 where frt :: CmmNode C O -> SubAreaSet -> SubAreaSet
125 frt (CmmEntry l) f = Map.delete (CallArea (Young l)) f
126 mid :: CmmNode O O -> SubAreaSet -> SubAreaSet
127 mid n f = foldSlotsUsed addSlot (removeLiveSlotDefs f n) n
128 lst :: CmmNode O C -> FactBase SubAreaSet -> SubAreaSet
129 lst n f = liveInSlots n $ case n of
130 CmmCall {cml_cont=Nothing, cml_args=args} -> add_area (CallArea Old) args out
131 CmmCall {cml_cont=Just k, cml_args=args} -> add_area (CallArea Old) args (add_area (CallArea (Young k)) args out)
132 CmmForeignCall {succ=k, updfr=oldend} -> add_area (CallArea Old) oldend (add_area (CallArea (Young k)) wORD_SIZE out)
134 where out = joinOutFacts slotLattice n f
135 add_area _ n live | n == 0 = live
136 add_area a n live = Map.insert a (snd $ liveGen (a, n, n) $ Map.findWithDefault [] a live) live
138 -- Slot sets: adding slots, removing slots, and checking for membership.
139 liftToArea :: Area -> ([SubArea] -> [SubArea]) -> SubAreaSet -> SubAreaSet
140 addSlot, removeSlot :: SubAreaSet -> SubArea -> SubAreaSet
141 elemSlot :: SubAreaSet -> SubArea -> Bool
142 liftToArea a f map = Map.insert a (f (Map.findWithDefault [] a map)) map
143 addSlot live (a, i, w) = liftToArea a (snd . liveGen (a, i, w)) live
144 removeSlot live (a, i, w) = liftToArea a (liveKill (a, i, w)) live
145 elemSlot live (a, i, w) =
146 not $ fst $ liveGen (a, i, w) (Map.findWithDefault [] a live)
148 removeLiveSlotDefs :: (DefinerOfSlots s, UserOfSlots s) => SubAreaSet -> s -> SubAreaSet
149 removeLiveSlotDefs = foldSlotsDefd removeSlot
151 liveInSlots :: (DefinerOfSlots s, UserOfSlots s) => s -> SubAreaSet -> SubAreaSet
152 liveInSlots x live = foldSlotsUsed addSlot (removeLiveSlotDefs live x) x
154 liveLastIn :: CmmNode O C -> (BlockId -> SubAreaSet) -> SubAreaSet
155 liveLastIn l env = liveInSlots l (liveLastOut env l)
157 -- Don't forget to keep the outgoing parameters in the CallArea live,
158 -- as well as the update frame.
159 -- Note: We have to keep the update frame live at a call because of the
160 -- case where the function doesn't return -- in that case, there won't
161 -- be a return to keep the update frame live. We'd still better keep the
162 -- info pointer in the update frame live at any call site;
163 -- otherwise we could screw up the garbage collector.
164 liveLastOut :: (BlockId -> SubAreaSet) -> CmmNode O C -> SubAreaSet
167 CmmCall _ Nothing n _ _ ->
168 add_area (CallArea Old) n out -- add outgoing args (includes upd frame)
169 CmmCall _ (Just k) n _ _ ->
170 add_area (CallArea Old) n (add_area (CallArea (Young k)) n out)
171 CmmForeignCall { succ = k, updfr = oldend } ->
172 add_area (CallArea Old) oldend (add_area (CallArea (Young k)) wORD_SIZE out)
174 where out = slotLatticeJoin $ map env $ successors l
175 add_area _ n live | n == 0 = live
177 Map.insert a (snd $ liveGen (a, n, n) $ Map.findWithDefault [] a live) live
179 -- The liveness analysis must be precise: otherwise, we won't know if a definition
180 -- should really kill a live-out stack slot.
181 -- But the interference graph does not have to be precise -- it might decide that
182 -- any live areas interfere. To maintain both a precise analysis and an imprecise
183 -- interference graph, we need to convert the live-out stack slots to graph nodes
184 -- at each and every instruction; rather than reconstruct a new list of nodes
185 -- every time, I provide a function to fold over the nodes, which should be a
186 -- reasonably efficient approach for the implementations we envision.
187 -- Of course, it will probably be much easier to program if we just return a list...
188 type Set x = Map x ()
189 data IGraphBuilder n =
190 Builder { foldNodes :: forall z. SubArea -> (n -> z -> z) -> z -> z
191 , _wordsOccupied :: AreaMap -> AreaMap -> n -> [Int]
194 areaBuilder :: IGraphBuilder Area
195 areaBuilder = Builder fold words
196 where fold (a, _, _) f z = f a z
197 words areaSize areaMap a =
198 case Map.lookup a areaMap of
199 Just addr -> [addr .. addr + (Map.lookup a areaSize `orElse`
200 pprPanic "wordsOccupied: unknown area" (ppr areaSize <+> ppr a))]
203 --slotBuilder :: IGraphBuilder (Area, Int)
204 --slotBuilder = undefined
206 -- Now, we can build the interference graph.
207 -- The usual story: a definition interferes with all live outs and all other
209 type IGraph x = Map x (Set x)
210 type IGPair x = (IGraph x, IGraphBuilder x)
211 igraph :: (Ord x) => IGraphBuilder x -> SlotEnv -> CmmGraph -> IGraph x
212 igraph builder env g = foldr interfere Map.empty (postorderDfs g)
213 where foldN = foldNodes builder
214 interfere block igraph = foldBlockNodesB3 (first, middle, last) block igraph
215 where first _ (igraph, _) = igraph
216 middle node (igraph, liveOut) =
217 (addEdges igraph node liveOut, liveInSlots node liveOut)
219 (addEdges igraph node $ liveLastOut env' node, liveLastIn node env')
221 -- add edges between a def and the other defs and liveouts
222 addEdges igraph i out = fst $ foldSlotsDefd addDef (igraph, out) i
223 addDef (igraph, out) def@(a, _, _) =
224 (foldN def (addDefN out) igraph,
225 Map.insert a (snd $ liveGen def (Map.findWithDefault [] a out)) out)
226 addDefN out n igraph =
227 let addEdgeNO o igraph = foldN o addEdgeNN igraph
228 addEdgeNN n' igraph = addEdgeNN' n n' $ addEdgeNN' n' n igraph
229 addEdgeNN' n n' igraph = Map.insert n (Map.insert n' () set) igraph
230 where set = Map.findWithDefault Map.empty n igraph
231 in Map.foldRightWithKey (\ _ os igraph -> foldr addEdgeNO igraph os) igraph out
232 env' bid = mapLookup bid env `orElse` panic "unknown blockId in igraph"
234 -- Before allocating stack slots, we need to collect one more piece of information:
235 -- what's the highest offset (in bytes) used in each Area?
236 -- We'll need to allocate that much space for each Area.
238 -- JD: WHY CAN'T THIS COME FROM THE slot-liveness info?
239 getAreaSize :: ByteOff -> CmmGraph -> AreaMap
240 -- The domain of the returned mapping consists only of Areas
241 -- used for (a) variable spill slots, and (b) parameter passing ares for calls
242 getAreaSize entry_off g =
243 foldGraphBlocks (foldBlockNodesF3 (first, add_regslots, last))
244 (Map.singleton (CallArea Old) entry_off) g
246 last :: CmmNode O C -> Map Area Int -> Map Area Int
247 last l@(CmmCall _ Nothing args res _) z = add_regslots l (add (add z area args) area res)
248 where area = CallArea Old
249 last l@(CmmCall _ (Just k) args res _) z = add_regslots l (add (add z area args) area res)
250 where area = CallArea (Young k)
251 last l@(CmmForeignCall {succ = k}) z = add_regslots l (add z area wORD_SIZE)
252 where area = CallArea (Young k)
253 last l z = add_regslots l z
254 add_regslots i z = foldSlotsUsed addSlot (foldSlotsDefd addSlot z i) i
255 addSlot z (a@(RegSlot (LocalReg _ ty)), _, _) =
256 add z a $ widthInBytes $ typeWidth ty
258 add z a off = Map.insert a (max off (Map.findWithDefault 0 a z)) z
259 -- The 'max' is important. Two calls, to f and g, might share a common
260 -- continuation (and hence a common CallArea), but their number of overflow
261 -- parameters might differ.
264 -- Find the Stack slots occupied by the subarea's conflicts
265 conflictSlots :: Ord x => IGPair x -> AreaMap -> AreaMap -> SubArea -> Set Int
266 conflictSlots (ig, Builder foldNodes wordsOccupied) areaSize areaMap subarea =
267 foldNodes subarea foldNode Map.empty
268 where foldNode n set = Map.foldRightWithKey conflict set $ Map.findWithDefault Map.empty n ig
269 conflict n' () set = liveInSlots areaMap n' set
270 -- Add stack slots occupied by igraph node n
271 liveInSlots areaMap n set = foldr setAdd set (wordsOccupied areaSize areaMap n)
272 setAdd w s = Map.insert w () s
274 -- Find any open space on the stack, starting from the offset.
275 -- If the area is a CallArea or a spill slot for a pointer, then it must
277 freeSlotFrom :: Ord x => IGPair x -> AreaMap -> Int -> AreaMap -> Area -> Int
278 freeSlotFrom ig areaSize offset areaMap area =
279 let size = Map.lookup area areaSize `orElse` 0
280 conflicts = conflictSlots ig areaSize areaMap (area, size, size)
281 -- CallAreas and Ptrs need to be word-aligned (round up!)
282 align = case area of CallArea _ -> align'
283 RegSlot r | isGcPtrType (localRegType r) -> align'
285 align' n = (n + (wORD_SIZE - 1)) `div` wORD_SIZE * wORD_SIZE
286 -- Find a space big enough to hold the area
287 findSpace curr 0 = curr
288 findSpace curr cnt = -- part of target slot, # of bytes left to check
289 if Map.member curr conflicts then
290 findSpace (align (curr + size)) size -- try the next (possibly) open space
291 else findSpace (curr - 1) (cnt - 1)
292 in findSpace (align (offset + size)) size
294 -- Find an open space on the stack, and assign it to the area.
295 allocSlotFrom :: Ord x => IGPair x -> AreaMap -> Int -> AreaMap -> Area -> AreaMap
296 allocSlotFrom ig areaSize from areaMap area =
297 if Map.member area areaMap then areaMap
298 else Map.insert area (freeSlotFrom ig areaSize from areaMap area) areaMap
300 -- | Greedy stack layout.
301 -- Compute liveness, build the interference graph, and allocate slots for the areas.
302 -- We visit each basic block in a (generally) forward order.
304 -- At each instruction that names a register subarea r, we immediately allocate
305 -- any available slot on the stack by the following procedure:
306 -- 1. Find the sub-areas S that conflict with r
307 -- 2. Find the stack slots used for S
308 -- 3. Choose a contiguous stack space s not in S (s must be large enough to hold r)
310 -- For a CallArea, we allocate the stack space only when we reach a function
311 -- call that returns to the CallArea's blockId.
312 -- Then, we allocate the Area subject to the following constraints:
313 -- a) It must be younger than all the sub-areas that are live on entry to the block
314 -- This constraint is only necessary for the successor of a call
315 -- b) It must not overlap with any already-allocated Area with which it conflicts
316 -- (ie at some point, not necessarily now, is live at the same time)
317 -- Part (b) is just the 1,2,3 part above
319 -- Note: The stack pointer only has to be younger than the youngest live stack slot
320 -- at proc points. Otherwise, the stack pointer can point anywhere.
322 layout :: ProcPointSet -> SlotEnv -> ByteOff -> CmmGraph -> AreaMap
323 -- The domain of the returned map includes an Area for EVERY block
324 -- including each block that is not the successor of a call (ie is not a proc-point)
325 -- That's how we return the info of what the SP should be at the entry of every block
327 layout procPoints env entry_off g =
328 let ig = (igraph areaBuilder env g, areaBuilder)
329 env' bid = mapLookup bid env `orElse` panic "unknown blockId in igraph"
330 areaSize = getAreaSize entry_off g
332 -- Find the youngest live stack slot that has already been allocated
333 youngest_live :: AreaMap -- Already allocated
334 -> SubAreaSet -- Sub-areas live here
335 -> ByteOff -- Offset of the youngest byte of any
336 -- already-allocated, live sub-area
337 youngest_live areaMap live = fold_subareas young_slot live 0
338 where young_slot (a, o, _) z = case Map.lookup a areaMap of
339 Just top -> max z $ top + o
341 fold_subareas f m z = Map.foldRightWithKey (\_ s z -> foldr f z s) z m
343 -- Allocate space for spill slots and call areas
344 allocVarSlot = allocSlotFrom ig areaSize 0
346 -- Update the successor's incoming SP.
347 setSuccSPs inSp bid areaMap =
348 case (Map.lookup area areaMap , mapLookup bid (toBlockMap g)) of
349 (Just _, _) -> areaMap -- succ already knows incoming SP
351 if setMember bid procPoints then
352 let young = youngest_live areaMap $ env' bid
353 -- start = case returnOff stackInfo of Just b -> max b young
355 start = young -- maybe wrong, but I don't understand
356 -- why the preceding is necessary...
357 in allocSlotFrom ig areaSize start areaMap area
358 else Map.insert area inSp areaMap
359 (_, Nothing) -> panic "Block not found in cfg"
360 where area = CallArea (Young bid)
362 layoutAreas areaMap block = foldBlockNodesF3 (flip const, allocMid, allocLast (entryLabel block)) block areaMap
363 allocMid m areaMap = foldSlotsDefd alloc' (foldSlotsUsed alloc' areaMap m) m
364 allocLast bid l areaMap =
365 foldr (setSuccSPs inSp) areaMap' (successors l)
366 where inSp = expectJust "sp in" $ Map.lookup (CallArea (Young bid)) areaMap
367 areaMap' = foldSlotsDefd alloc' (foldSlotsUsed alloc' areaMap l) l
368 alloc' areaMap (a@(RegSlot _), _, _) = allocVarSlot areaMap a
369 alloc' areaMap _ = areaMap
371 initMap = Map.insert (CallArea (Young (g_entry g))) 0 $
372 Map.insert (CallArea Old) 0 Map.empty
374 areaMap = foldl layoutAreas initMap (postorderDfs g)
375 in -- pprTrace "ProcPoints" (ppr procPoints) $
376 -- pprTrace "Area SizeMap" (ppr areaSize) $
377 -- pprTrace "Entry SP" (ppr entrySp) $
378 -- pprTrace "Area Map" (ppr areaMap) $
381 -- After determining the stack layout, we can:
382 -- 1. Replace references to stack Areas with addresses relative to the stack
384 -- 2. Insert adjustments to the stack pointer to ensure that it is at a
385 -- conventional location at each proc point.
386 -- Because we don't take interrupts on the execution stack, we only need the
387 -- stack pointer to be younger than the live values on the stack at proc points.
388 -- 3. Compute the maximum stack offset used in the procedure and replace
389 -- the stack high-water mark with that offset.
390 manifestSP :: AreaMap -> ByteOff -> CmmGraph -> FuelUniqSM CmmGraph
391 manifestSP areaMap entry_off g@(CmmGraph {g_entry=entry}) =
392 ofBlockMap entry `liftM` foldl replB (return mapEmpty) (postorderDfs g)
393 where slot a = -- pprTrace "slot" (ppr a) $
394 Map.lookup a areaMap `orElse` panic "unallocated Area"
395 slot' (Just id) = slot $ CallArea (Young id)
396 slot' Nothing = slot $ CallArea Old
397 sp_high = maxSlot slot g
398 proc_entry_sp = slot (CallArea Old) + entry_off
400 add_sp_off :: CmmBlock -> BlockEnv Int -> BlockEnv Int
403 CmmCall {cml_cont=Just succ, cml_ret_args=off} -> mapInsert succ off env
404 CmmForeignCall {succ=succ} -> mapInsert succ wORD_SIZE env
406 spEntryMap = foldGraphBlocks add_sp_off (mapInsert entry entry_off emptyBlockMap) g
407 spOffset id = mapLookup id spEntryMap `orElse` 0
409 sp_on_entry id | id == entry = proc_entry_sp
410 sp_on_entry id = slot' (Just id) + spOffset id
412 -- On entry to procpoints, the stack pointer is conventional;
413 -- otherwise, we check the SP set by predecessors.
414 replB :: FuelUniqSM (BlockEnv CmmBlock) -> CmmBlock -> FuelUniqSM (BlockEnv CmmBlock)
416 do let (head, middles, JustC tail :: MaybeC C (CmmNode O C)) = blockToNodeList block
417 middles' = map (middle spIn) middles
418 bs <- replLast head middles' tail
419 flip (foldr insertBlock) bs `liftM` blocks
420 where spIn = sp_on_entry (entryLabel block)
422 middle spOff m = mapExpDeep (replSlot spOff) m
423 last spOff l = mapExpDeep (replSlot spOff) l
424 replSlot spOff (CmmStackSlot a i) = CmmRegOff (CmmGlobal Sp) (spOff - (slot a + i))
425 replSlot _ (CmmLit CmmHighStackMark) = -- replacing the high water mark
426 CmmLit (CmmInt (toInteger (max 0 (sp_high - proc_entry_sp))) (typeWidth bWord))
429 replLast :: MaybeC C (CmmNode C O) -> [CmmNode O O] -> CmmNode O C -> FuelUniqSM [CmmBlock]
430 replLast h m l@(CmmCall _ k n _ _) = updSp (slot' k + n) h m l
431 -- JD: LastForeignCall probably ought to have an outgoing
432 -- arg size, just like LastCall
433 replLast h m l@(CmmForeignCall {succ=k}) = updSp (slot' (Just k) + wORD_SIZE) h m l
434 replLast h m l@(CmmBranch k) = updSp (sp_on_entry k) h m l
435 replLast h m l = uncurry (:) `liftM` foldr succ (return (b, [])) (successors l)
437 b = updSp' spIn h m l
439 let succSp = sp_on_entry succId in
440 if succSp /= spIn then
442 (b', bs') <- insertBetween b (adjustSp succSp) succId
443 return (b', bs' ++ bs)
446 updSp sp h m l = return [updSp' sp h m l]
447 updSp' sp h m l | sp == spIn = blockOfNodeList (h, m, JustC $ last sp l)
448 | otherwise = blockOfNodeList (h, m ++ adjustSp sp, JustC $ last sp l)
449 adjustSp sp = [CmmAssign (CmmGlobal Sp) e]
450 where e = CmmMachOp (MO_Add wordWidth) [CmmReg (CmmGlobal Sp), off]
451 off = CmmLit $ CmmInt (toInteger $ spIn - sp) wordWidth
454 -- To compute the stack high-water mark, we fold over the graph and
455 -- compute the highest slot offset.
456 maxSlot :: (Area -> Int) -> CmmGraph -> Int
457 maxSlot slotOff g = foldGraphBlocks (foldBlockNodesF3 (flip const, highSlot, highSlot)) 0 g
458 where highSlot i z = foldSlotsUsed add (foldSlotsDefd add z i) i
459 add z (a, i, _) = max z (slotOff a + i)
461 -----------------------------------------------------------------------------
462 -- | Sanity check: stub pointers immediately after they die
463 -----------------------------------------------------------------------------
464 -- This will miss stack slots that are last used in a Last node,
465 -- but it should do pretty well...
467 stubSlotsOnDeath :: CmmGraph -> FuelUniqSM CmmGraph
468 stubSlotsOnDeath g = liftM fst $ dataflowPassBwd g [] $ analRewBwd slotLattice
471 where rewrites = mkBRewrite3 frt mid lst
472 frt _ _ = return Nothing
473 mid m liveSlots = return $ foldSlotsUsed (stub liveSlots m) Nothing m
474 lst _ _ = return Nothing
475 stub liveSlots m rst subarea@(a, off, w) =
476 if elemSlot liveSlots subarea then rst
477 else let store = mkMiddle $ CmmStore (CmmStackSlot a off)
478 (stackStubExpr (widthFromBytes w))
479 in case rst of Nothing -> Just (mkMiddle m <*> store)
480 Just g -> Just (g <*> store)