1 {-# OPTIONS_GHC -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 ( SlotEnv, liveSlotAnal, liveSlotTransfers, removeLiveSlotDefs
7 , layout, manifestSP, igraph, areaBuilder
8 , stubSlotsOnDeath ) -- to help crash early during debugging
12 import Prelude hiding (zip, unzip, last)
22 import MkZipCfgCmm hiding (CmmBlock, CmmGraph)
25 import SMRep (ByteOff)
31 ------------------------------------------------------------------------
33 ------------------------------------------------------------------------
35 -- | Before we lay out the stack, we need to know something about the
36 -- liveness of the stack slots. In particular, to decide whether we can
37 -- reuse a stack location to hold multiple stack slots, we need to know
38 -- when each of the stack slots is used.
39 -- Although tempted to use something simpler, we really need a full interference
40 -- graph. Consider the following case:
42 -- 1 -> <spill x>; // y is dead out
43 -- 2 -> <spill y>; // x is dead out
44 -- 3 -> <spill x and y>
45 -- If we consider the arms in order and we use just the deadness information given by a
46 -- dataflow analysis, we might decide to allocate the stack slots for x and y
47 -- to the same stack location, which will lead to incorrect code in the third arm.
48 -- We won't make this mistake with an interference graph.
50 -- First, the liveness analysis.
51 -- We represent a slot with an area, an offset into the area, and a width.
52 -- Tracking the live slots is a bit tricky because there may be loads and stores
53 -- into only a part of a stack slot (e.g. loading the low word of a 2-word long),
54 -- e.g. Slot A 0 8 overlaps with Slot A 4 4.
56 -- The definition of a slot set is intended to reduce the number of overlap
57 -- checks we have to make. There's no reason to check for overlap between
58 -- slots in different areas, so we segregate the map by Area's.
59 -- We expect few slots in each Area, so we collect them in an unordered list.
60 -- To keep these lists short, any contiguous live slots are coalesced into
61 -- a single slot, on insertion.
63 slotLattice :: DataflowLattice SubAreaSet
64 slotLattice = DataflowLattice "live slots" emptyFM add False
65 where add new old = case foldFM addArea (False, old) new of
68 addArea a newSlots z = foldr (addSlot a) z newSlots
69 addSlot a slot (changed, map) =
70 let (c, live) = liveGen slot $ lookupWithDefaultFM map [] a
71 in (c || changed, addToFM map a live)
73 type SlotEnv = BlockEnv SubAreaSet
74 -- The sub-areas live on entry to the block
76 type SlotFix a = FuelMonad (BackwardFixedPoint Middle Last SubAreaSet a)
78 liveSlotAnal :: LGraph Middle Last -> FuelMonad SlotEnv
79 liveSlotAnal g = liftM zdfFpFacts (res :: SlotFix ())
80 where res = zdfSolveFromL emptyBlockEnv "live slot analysis" slotLattice
81 liveSlotTransfers (fact_bot slotLattice) g
83 -- Add the subarea s to the subareas in the list-set (possibly coalescing it with
84 -- adjacent subareas), and also return whether s was a new addition.
85 liveGen :: SubArea -> [SubArea] -> (Bool, [SubArea])
86 liveGen s set = liveGen' s set []
87 where liveGen' s [] z = (True, s : z)
88 liveGen' s@(a, hi, w) (s'@(a', hi', w') : rst) z =
89 if a /= a' || hi < lo' || lo > hi' then -- no overlap
90 liveGen' s rst (s' : z)
91 else if s' `contains` s then -- old contains new
93 else -- overlap: coalesce the slots
94 let new_hi = max hi hi'
96 in liveGen' (a, new_hi, new_hi - new_lo) rst z
97 where lo = hi - w -- remember: areas grow down
99 contains (a, hi, w) (a', hi', w') =
100 a == a' && hi >= hi' && hi - w <= hi' - w'
102 liveKill :: SubArea -> [SubArea] -> [SubArea]
103 liveKill (a, hi, w) set = -- pprTrace "killing slots in area" (ppr a) $
105 where liveKill' [] z = z
106 liveKill' (s'@(a', hi', w') : rst) z =
107 if a /= a' || hi < lo' || lo > hi' then -- no overlap
108 liveKill' rst (s' : z)
109 else -- overlap: split the old slot
110 let z' = if hi' > hi then (a, hi', hi' - hi) : z else z
111 z'' = if lo > lo' then (a, lo, lo - lo') : z' else z'
113 where lo = hi - w -- remember: areas grow down
116 -- Note: the stack slots that hold variables returned on the stack are not
117 -- considered live in to the block -- we treat the first node as a definition site.
118 -- BEWARE?: Am I being a little careless here in failing to check for the
119 -- entry Id (which would use the CallArea Old).
120 liveSlotTransfers :: BackwardTransfers Middle Last SubAreaSet
122 BackwardTransfers first liveInSlots liveLastIn
123 where first id live = delFromFM live (CallArea (Young id))
125 -- Slot sets: adding slots, removing slots, and checking for membership.
126 liftToArea :: Area -> ([SubArea] -> [SubArea]) -> SubAreaSet -> SubAreaSet
127 addSlot, removeSlot :: SubAreaSet -> SubArea -> SubAreaSet
128 elemSlot :: SubAreaSet -> SubArea -> Bool
129 liftToArea a f map = addToFM map a $ f (lookupWithDefaultFM map [] a)
130 addSlot live (a, i, w) = liftToArea a (snd . liveGen (a, i, w)) live
131 removeSlot live (a, i, w) = liftToArea a (liveKill (a, i, w)) live
132 elemSlot live (a, i, w) =
133 not $ fst $ liveGen (a, i, w) (lookupWithDefaultFM live [] a)
135 removeLiveSlotDefs :: (DefinerOfSlots s, UserOfSlots s) => SubAreaSet -> s -> SubAreaSet
136 removeLiveSlotDefs = foldSlotsDefd removeSlot
138 liveInSlots :: (DefinerOfSlots s, UserOfSlots s) => s -> SubAreaSet -> SubAreaSet
139 liveInSlots x live = foldSlotsUsed addSlot (removeLiveSlotDefs live x) x
141 liveLastIn :: Last -> (BlockId -> SubAreaSet) -> SubAreaSet
142 liveLastIn l env = liveInSlots l (liveLastOut env l)
144 -- Don't forget to keep the outgoing parameters in the CallArea live,
145 -- as well as the update frame.
146 -- Note: We have to keep the update frame live at a call because of the
147 -- case where the function doesn't return -- in that case, there won't
148 -- be a return to keep the update frame live. We'd still better keep the
149 -- info pointer in the update frame live at any call site;
150 -- otherwise we could screw up the garbage collector.
151 liveLastOut :: (BlockId -> SubAreaSet) -> Last -> SubAreaSet
154 LastCall _ Nothing n _ _ ->
155 add_area (CallArea Old) n out -- add outgoing args (includes upd frame)
156 LastCall _ (Just k) n _ (Just _) ->
157 add_area (CallArea Old) n (add_area (CallArea (Young k)) n out)
158 LastCall _ (Just k) n _ Nothing ->
159 add_area (CallArea (Young k)) n out
161 where out = joinOuts slotLattice env l
162 add_area _ n live | n == 0 = live
164 addToFM live a $ snd $ liveGen (a, n, n) $ lookupWithDefaultFM live [] a
166 -- The liveness analysis must be precise: otherwise, we won't know if a definition
167 -- should really kill a live-out stack slot.
168 -- But the interference graph does not have to be precise -- it might decide that
169 -- any live areas interfere. To maintain both a precise analysis and an imprecise
170 -- interference graph, we need to convert the live-out stack slots to graph nodes
171 -- at each and every instruction; rather than reconstruct a new list of nodes
172 -- every time, I provide a function to fold over the nodes, which should be a
173 -- reasonably efficient approach for the implementations we envision.
174 -- Of course, it will probably be much easier to program if we just return a list...
175 type Set x = FiniteMap x ()
176 data IGraphBuilder n =
177 Builder { foldNodes :: forall z. SubArea -> (n -> z -> z) -> z -> z
178 , _wordsOccupied :: AreaMap -> AreaMap -> n -> [Int]
181 areaBuilder :: IGraphBuilder Area
182 areaBuilder = Builder fold words
183 where fold (a, _, _) f z = f a z
184 words areaSize areaMap a =
185 case lookupFM areaMap a of
186 Just addr -> [addr .. addr + (lookupFM areaSize a `orElse`
187 pprPanic "wordsOccupied: unknown area" (ppr a))]
190 --slotBuilder :: IGraphBuilder (Area, Int)
191 --slotBuilder = undefined
193 -- Now, we can build the interference graph.
194 -- The usual story: a definition interferes with all live outs and all other
196 type IGraph x = FiniteMap x (Set x)
197 type IGPair x = (IGraph x, IGraphBuilder x)
198 igraph :: (Ord x) => IGraphBuilder x -> SlotEnv -> LGraph Middle Last -> IGraph x
199 igraph builder env g = foldr interfere emptyFM (postorder_dfs g)
200 where foldN = foldNodes builder
201 interfere block igraph =
202 let (h, l) = goto_end (unzip block)
203 --heads :: ZHead Middle -> (IGraph x, SubAreaSet) -> IGraph x
204 heads (ZFirst _) (igraph, _) = igraph
205 heads (ZHead h m) (igraph, liveOut) =
206 heads h (addEdges igraph m liveOut, liveInSlots m liveOut)
207 -- add edges between a def and the other defs and liveouts
208 addEdges igraph i out = fst $ foldSlotsDefd addDef (igraph, out) i
209 addDef (igraph, out) def@(a, _, _) =
210 (foldN def (addDefN out) igraph,
211 addToFM out a (snd $ liveGen def (lookupWithDefaultFM out [] a)))
212 addDefN out n igraph =
213 let addEdgeNO o igraph = foldN o addEdgeNN igraph
214 addEdgeNN n' igraph = addEdgeNN' n n' $ addEdgeNN' n' n igraph
215 addEdgeNN' n n' igraph = addToFM igraph n (addToFM set n' ())
216 where set = lookupWithDefaultFM igraph emptyFM n
217 in foldFM (\ _ os igraph -> foldr addEdgeNO igraph os) igraph out
218 env' bid = lookupBlockEnv env bid `orElse` panic "unknown blockId in igraph"
219 in heads h $ case l of LastExit -> (igraph, emptyFM)
220 LastOther l -> (addEdges igraph l $ liveLastOut env' l,
223 -- Before allocating stack slots, we need to collect one more piece of information:
224 -- what's the highest offset (in bytes) used in each Area?
225 -- We'll need to allocate that much space for each Area.
226 getAreaSize :: ByteOff -> LGraph Middle Last -> AreaMap
227 -- The domain of the returned mapping consists only of Areas
228 -- used for (a) variable spill slots, and (b) parameter passing ares for calls
229 getAreaSize entry_off g@(LGraph _ _) =
230 fold_blocks (fold_fwd_block first add_regslots last)
231 (unitFM (CallArea Old) entry_off) g
233 last l@(LastOther (LastCall _ Nothing args res _)) z =
234 add_regslots l (add (add z area args) area res)
235 where area = CallArea Old
236 last l@(LastOther (LastCall _ (Just k) args res _)) z =
237 add_regslots l (add (add z area args) area res)
238 where area = CallArea (Young k)
239 last l z = add_regslots l z
240 add_regslots i z = foldSlotsUsed addSlot (foldSlotsDefd addSlot z i) i
241 addSlot z (a@(RegSlot (LocalReg _ ty)), _, _) =
242 add z a $ widthInBytes $ typeWidth ty
244 add z a off = addToFM z a (max off (lookupWithDefaultFM z 0 a))
245 -- The 'max' is important. Two calls, to f and g, might share a common
246 -- continuation (and hence a common CallArea), but their number of overflow
247 -- parameters might differ.
250 -- Find the Stack slots occupied by the subarea's conflicts
251 conflictSlots :: Ord x => IGPair x -> AreaMap -> AreaMap -> SubArea -> Set Int
252 conflictSlots (ig, Builder foldNodes wordsOccupied) areaSize areaMap subarea =
253 foldNodes subarea foldNode emptyFM
254 where foldNode n set = foldFM conflict set $ lookupWithDefaultFM ig emptyFM n
255 conflict n' () set = liveInSlots areaMap n' set
256 -- Add stack slots occupied by igraph node n
257 liveInSlots areaMap n set = foldr setAdd set (wordsOccupied areaSize areaMap n)
258 setAdd w s = addToFM s w ()
260 -- Find any open space on the stack, starting from the offset.
261 -- If the area is a CallArea or a spill slot for a pointer, then it must
263 freeSlotFrom :: Ord x => IGPair x -> AreaMap -> Int -> AreaMap -> Area -> Int
264 freeSlotFrom ig areaSize offset areaMap area =
265 let size = lookupFM areaSize area `orElse` 0
266 conflicts = conflictSlots ig areaSize areaMap (area, size, size)
267 -- CallAreas and Ptrs need to be word-aligned (round up!)
268 align = case area of CallArea _ -> align'
269 RegSlot r | isGcPtrType (localRegType r) -> align'
271 align' n = (n + (wORD_SIZE - 1)) `div` wORD_SIZE * wORD_SIZE
272 -- Find a space big enough to hold the area
273 findSpace curr 0 = curr
274 findSpace curr cnt = -- part of target slot, # of bytes left to check
275 if elemFM curr conflicts then
276 findSpace (align (curr + size)) size -- try the next (possibly) open space
277 else findSpace (curr - 1) (cnt - 1)
278 in findSpace (align (offset + size)) size
280 -- Find an open space on the stack, and assign it to the area.
281 allocSlotFrom :: Ord x => IGPair x -> AreaMap -> Int -> AreaMap -> Area -> AreaMap
282 allocSlotFrom ig areaSize from areaMap area =
283 if elemFM area areaMap then areaMap
284 else addToFM areaMap area $ freeSlotFrom ig areaSize from areaMap area
286 -- | Greedy stack layout.
287 -- Compute liveness, build the interference graph, and allocate slots for the areas.
288 -- We visit each basic block in a (generally) forward order.
290 -- At each instruction that names a register subarea r, we immediately allocate
291 -- any available slot on the stack by the following procedure:
292 -- 1. Find the sub-areas S that conflict with r
293 -- 2. Find the stack slots used for S
294 -- 3. Choose a contiguous stack space s not in S (s must be large enough to hold r)
296 -- For a CallArea, we allocate the stack space only when we reach a function
297 -- call that returns to the CallArea's blockId.
298 -- Then, we allocate the Area subject to the following constraints:
299 -- a) It must be younger than all the sub-areas that are live on entry to the block
300 -- This constraint is only necessary for the successor of a call
301 -- b) It must not overlap with any already-allocated Area with which it conflicts
302 -- (ie at some point, not necessarily now, is live at the same time)
303 -- Part (b) is just the 1,2,3 part above
305 -- Note: The stack pointer only has to be younger than the youngest live stack slot
306 -- at proc points. Otherwise, the stack pointer can point anywhere.
308 layout :: ProcPointSet -> SlotEnv -> ByteOff -> LGraph Middle Last -> AreaMap
309 -- The domain of the returned map includes an Area for EVERY block
310 -- including each block that is not the successor of a call (ie is not a proc-point)
311 -- That's how we return the info of what the SP should be at the entry of every block
313 layout procPoints env entry_off g =
314 let ig = (igraph areaBuilder env g, areaBuilder)
315 env' bid = lookupBlockEnv env bid `orElse` panic "unknown blockId in igraph"
316 areaSize = getAreaSize entry_off g
317 -- Find the slots that are live-in to a block tail
318 live_in (ZTail m l) = liveInSlots m (live_in l)
319 live_in (ZLast (LastOther l)) = liveLastIn l env'
320 live_in (ZLast LastExit) = emptyFM
322 -- Find the youngest live stack slot that has already been allocated
323 youngest_live :: AreaMap -- Already allocated
324 -> SubAreaSet -- Sub-areas live here
325 -> ByteOff -- Offset of the youngest byte of any
326 -- already-allocated, live sub-area
327 youngest_live areaMap live = fold_subareas young_slot live 0
328 where young_slot (a, o, _) z = case lookupFM areaMap a of
329 Just top -> max z $ top + o
331 fold_subareas f m z = foldFM (\_ s z -> foldr f z s) z m
333 -- Allocate space for spill slots and call areas
334 allocVarSlot = allocSlotFrom ig areaSize 0
336 -- Update the successor's incoming SP.
337 setSuccSPs inSp bid areaMap =
338 case (lookupFM areaMap area, lookupBlockEnv (lg_blocks g) bid) of
339 (Just _, _) -> areaMap -- succ already knows incoming SP
340 (Nothing, Just (Block _ _)) ->
341 if elemBlockSet bid procPoints then
342 let young = youngest_live areaMap $ env' bid
343 -- start = case returnOff stackInfo of Just b -> max b young
345 start = young -- maybe wrong, but I don't understand
346 -- why the preceding is necessary...
347 in allocSlotFrom ig areaSize start areaMap area
348 else addToFM areaMap area inSp
349 (_, Nothing) -> panic "Block not found in cfg"
350 where area = CallArea (Young bid)
352 allocLast (Block id _) areaMap l =
353 fold_succs (setSuccSPs inSp) l areaMap
354 where inSp = expectJust "sp in" $ lookupFM areaMap (CallArea (Young id))
356 allocMidCall m@(MidForeignCall (Safe bid _) _ _ _) t areaMap =
357 let young = youngest_live areaMap $ removeLiveSlotDefs (live_in t) m
358 area = CallArea (Young bid)
359 areaSize' = addToFM areaSize area (widthInBytes (typeWidth gcWord))
360 in allocSlotFrom ig areaSize' young areaMap area
361 allocMidCall _ _ areaMap = areaMap
364 foldSlotsDefd alloc' (foldSlotsUsed alloc' (allocMidCall m t areaMap) m) m
365 where alloc' areaMap (a@(RegSlot _), _, _) = allocVarSlot areaMap a
366 alloc' areaMap _ = areaMap
368 layoutAreas areaMap b@(Block _ t) = layout areaMap t
369 where layout areaMap (ZTail m t) = layout (alloc m t areaMap) t
370 layout areaMap (ZLast l) = allocLast b areaMap l
371 initMap = addToFM (addToFM emptyFM (CallArea Old) 0)
372 (CallArea (Young (lg_entry g))) 0
373 areaMap = foldl layoutAreas initMap (postorder_dfs g)
374 in -- pprTrace "ProcPoints" (ppr procPoints) $
375 -- pprTrace "Area SizeMap" (ppr areaSize) $
376 -- pprTrace "Entry SP" (ppr entrySp) $
377 -- pprTrace "Area Map" (ppr areaMap) $
380 -- After determining the stack layout, we can:
381 -- 1. Replace references to stack Areas with addresses relative to the stack
383 -- 2. Insert adjustments to the stack pointer to ensure that it is at a
384 -- conventional location at each proc point.
385 -- Because we don't take interrupts on the execution stack, we only need the
386 -- stack pointer to be younger than the live values on the stack at proc points.
387 -- 3. Compute the maximum stack offset used in the procedure and replace
388 -- the stack high-water mark with that offset.
389 manifestSP :: AreaMap -> ByteOff -> LGraph Middle Last -> FuelMonad (LGraph Middle Last)
390 manifestSP areaMap entry_off g@(LGraph entry _blocks) =
391 liftM (LGraph entry) $ foldl replB (return emptyBlockEnv) (postorder_dfs g)
392 where slot a = -- pprTrace "slot" (ppr a) $
393 lookupFM areaMap a `orElse` panic "unallocated Area"
394 slot' (Just id) = slot $ CallArea (Young id)
395 slot' Nothing = slot $ CallArea Old
396 sp_high = maxSlot slot g
397 proc_entry_sp = slot (CallArea Old) + entry_off
400 case Z.last (unzip b) of
401 LastOther (LastCall {cml_cont = Just succ, cml_ret_args = off}) ->
402 extendBlockEnv env succ off
404 spEntryMap = fold_blocks add_sp_off (mkBlockEnv [(entry, entry_off)]) g
405 spOffset id = lookupBlockEnv spEntryMap id `orElse` 0
407 sp_on_entry id | id == entry = proc_entry_sp
408 sp_on_entry id = slot' (Just id) + spOffset id
410 -- On entry to procpoints, the stack pointer is conventional;
411 -- otherwise, we check the SP set by predecessors.
412 replB :: FuelMonad (BlockEnv CmmBlock) -> CmmBlock -> FuelMonad (BlockEnv CmmBlock)
413 replB blocks (Block id t) =
414 do bs <- replTail (Block id) spIn t
415 -- pprTrace "spIn" (ppr id <+> ppr spIn) $ do
416 liftM (flip (foldr insertBlock) bs) blocks
417 where spIn = sp_on_entry id
418 replTail :: (ZTail Middle Last -> CmmBlock) -> Int -> (ZTail Middle Last) ->
419 FuelMonad ([CmmBlock])
420 replTail h spOff (ZTail m@(MidForeignCall (Safe bid _) _ _ _) t) =
421 replTail (\t' -> h (setSp spOff spOff' (ZTail (middle spOff m) t'))) spOff' t
422 where spOff' = slot' (Just bid) + widthInBytes (typeWidth gcWord)
423 replTail h spOff (ZTail m t) = replTail (h . ZTail (middle spOff m)) spOff t
424 replTail h spOff (ZLast (LastOther l)) = fixSp h spOff l
425 replTail h _ l@(ZLast LastExit) = return [h l]
426 middle spOff m = mapExpDeepMiddle (replSlot spOff) m
427 last spOff l = mapExpDeepLast (replSlot spOff) l
428 replSlot spOff (CmmStackSlot a i) = CmmRegOff (CmmGlobal Sp) (spOff - (slot a + i))
429 replSlot _ (CmmLit CmmHighStackMark) = -- replacing the high water mark
430 CmmLit (CmmInt (toInteger (max 0 (sp_high - proc_entry_sp))) (typeWidth bWord))
432 -- The block must establish the SP expected at each successsor.
433 fixSp :: (ZTail Middle Last -> CmmBlock) -> Int -> Last -> FuelMonad ([CmmBlock])
434 fixSp h spOff l@(LastCall _ k n _ _) = updSp h spOff (slot' k + n) l
435 fixSp h spOff l@(LastBranch k) =
436 let succSp = sp_on_entry k in
437 if succSp /= spOff then
438 -- pprTrace "updSp" (ppr k <> ppr spOff <> ppr (sp_on_entry k)) $
439 updSp h spOff succSp l
440 else return $ [h (ZLast (LastOther (last spOff l)))]
441 fixSp h spOff l = liftM (uncurry (:)) $ fold_succs succ l $ return (b, [])
442 where b = h (ZLast (LastOther (last spOff l)))
444 let succSp = sp_on_entry succId in
445 if succSp /= spOff then
447 (b', bs') <- insertBetween b [setSpMid spOff succSp] succId
448 return (b', bs ++ bs')
450 updSp h old new l = return [h $ setSp old new $ ZLast $ LastOther (last new l)]
451 setSpMid sp sp' = MidAssign (CmmGlobal Sp) e
452 where e = CmmMachOp (MO_Add wordWidth) [CmmReg (CmmGlobal Sp), off]
453 off = CmmLit $ CmmInt (toInteger $ sp - sp') wordWidth
454 setSp sp sp' t = if sp == sp' then t else ZTail (setSpMid sp sp') t
457 -- To compute the stack high-water mark, we fold over the graph and
458 -- compute the highest slot offset.
459 maxSlot :: (Area -> Int) -> CmmGraph -> Int
460 maxSlot slotOff g = fold_blocks (fold_fwd_block (\ _ x -> x) highSlot highSlot) 0 g
461 where highSlot i z = foldSlotsUsed add (foldSlotsDefd add z i) i
462 add z (a, i, _) = max z (slotOff a + i)
464 -----------------------------------------------------------------------------
465 -- | Sanity check: stub pointers immediately after they die
466 -----------------------------------------------------------------------------
467 -- This will miss stack slots that are last used in a Last node,
468 -- but it should do pretty well...
470 type StubPtrFix = FuelMonad (BackwardFixedPoint Middle Last SubAreaSet CmmGraph)
472 stubSlotsOnDeath :: (LGraph Middle Last) -> FuelMonad (LGraph Middle Last)
473 stubSlotsOnDeath g = liftM zdfFpContents $ (res :: StubPtrFix)
474 where res = zdfBRewriteFromL RewriteShallow emptyBlockEnv "stub ptrs" slotLattice
475 liveSlotTransfers rewrites (fact_bot slotLattice) g
476 rewrites = BackwardRewrites first middle last Nothing
479 middle m liveSlots = foldSlotsUsed (stub liveSlots m) Nothing m
480 stub liveSlots m rst subarea@(a, off, w) =
481 if elemSlot liveSlots subarea then rst
482 else let store = mkStore (CmmStackSlot a off)
483 (stackStubExpr (widthFromBytes w))
484 in case rst of Nothing -> Just (mkMiddle m <*> store)
485 Just g -> Just (g <*> store)