%
-% (c) The AQUA Project, Glasgow University, 1993-1995
+% (c) The AQUA Project, Glasgow University, 1993-1998
%
\begin{code}
-#include "HsVersions.h"
+module AbsCStixGen ( genCodeAbstractC ) where
-module AbsCStixGen (
- genCodeAbstractC,
+#include "HsVersions.h"
- -- and, of course, that's not enough...
- AbstractC, Target, StixTree, UniqSupply, UniqSM(..)
- ) where
+import Ratio ( Rational )
import AbsCSyn
-import PrelInfo ( PrimOp(..), primOpNeedsWrapper, isCompareOp
- IF_ATTACK_PRAGMAS(COMMA tagOf_PrimOp)
- IF_ATTACK_PRAGMAS(COMMA pprPrimOp)
+import Stix
+import MachMisc
+
+import AbsCUtils ( getAmodeRep, mixedTypeLocn,
+ nonemptyAbsC, mkAbsCStmts, mkAbsCStmtList
)
-import CgCompInfo ( mIN_UPD_SIZE )
-import ClosureInfo ( infoTableLabelFromCI, entryLabelFromCI, fastLabelFromCI,
- closureUpdReqd
+import PprAbsC ( dumpRealC )
+import SMRep ( fixedItblSize,
+ rET_SMALL, rET_BIG,
+ rET_VEC_SMALL, rET_VEC_BIG
)
-import MachDesc
-import Maybes ( Maybe(..), maybeToBool )
-import Outputable
-import PrimRep ( isFloatingRep )
-import SMRep ( SMRep(..), SMSpecRepKind(..), SMUpdateKind(..) )
-import Stix
-import StixInfo ( genCodeInfoTable )
-import UniqSupply
-import Util
+import Constants ( mIN_UPD_SIZE )
+import CLabel ( CLabel, mkReturnInfoLabel, mkReturnPtLabel,
+ mkClosureTblLabel, mkClosureLabel,
+ moduleRegdLabel )
+import ClosureInfo ( infoTableLabelFromCI, entryLabelFromCI,
+ fastLabelFromCI, closureUpdReqd,
+ staticClosureNeedsLink
+ )
+import Literal ( Literal(..), word2IntLit )
+import Maybes ( maybeToBool )
+import PrimOp ( primOpNeedsWrapper, PrimOp(..) )
+import PrimRep ( isFloatingRep, PrimRep(..) )
+import StixInfo ( genCodeInfoTable, genBitmapInfoTable )
+import StixMacro ( macroCode, checkCode )
+import StixPrim ( primCode, amodeToStix, amodeToStix' )
+import Outputable ( pprPanic, ppr )
+import UniqSupply ( returnUs, thenUs, mapUs, getUniqueUs, UniqSM )
+import Util ( naturalMergeSortLe )
+import Panic ( panic )
+import TyCon ( tyConDataCons )
+import DataCon ( dataConWrapId )
+import BitSet ( intBS )
+import Name ( NamedThing(..) )
+import Char ( ord )
\end{code}
-For each independent chunk of AbstractC code, we generate a list of @StixTree@s,
-where each tree corresponds to a single Stix instruction. We leave the chunks
-separated so that register allocation can be performed locally within the chunk.
+For each independent chunk of AbstractC code, we generate a list of
+@StixTree@s, where each tree corresponds to a single Stix instruction.
+We leave the chunks separated so that register allocation can be
+performed locally within the chunk.
\begin{code}
--- hacking with Uncle Will:
-#define target_STRICT target@(Target _ _ _ _ _ _ _ _)
+genCodeAbstractC :: AbstractC -> UniqSM [[StixTree]]
-genCodeAbstractC
- :: Target
- -> AbstractC
- -> UniqSM [[StixTree]]
-
-genCodeAbstractC target_STRICT absC =
- mapUs gentopcode (mkAbsCStmtList absC) `thenUs` \ trees ->
+genCodeAbstractC absC
+ = mapUs gentopcode (mkAbsCStmtList absC) `thenUs` \ trees ->
returnUs ([StComment SLIT("Native Code")] : trees)
where
- -- "target" munging things... ---
- a2stix = amodeToStix target
- a2stix' = amodeToStix' target
- volsaves = volatileSaves target
- volrestores = volatileRestores target
- p2stix = primToStix target
- macro_code = macroCode target
- hp_rel = hpRel target
+ a2stix = amodeToStix
+ a2stix' = amodeToStix'
+ volsaves = volatileSaves
+ volrestores = volatileRestores
+ p2stix = primCode
+ macro_code = macroCode
-- real code follows... ---------
\end{code}
\begin{code}
{-
genCodeTopAbsC
- :: Target
- -> AbstractC
+ :: AbstractC
-> UniqSM [StixTree]
-}
- gentopcode (CCodeBlock label absC) =
- gencode absC `thenUs` \ code ->
- returnUs (StSegment TextSegment : StFunBegin label : code [StFunEnd label])
+ gentopcode (CCodeBlock lbl absC)
+ = gencode absC `thenUs` \ code ->
+ returnUs (StSegment TextSegment : StFunBegin lbl : code [StFunEnd lbl])
- gentopcode stmt@(CStaticClosure label _ _ _) =
- genCodeStaticClosure stmt `thenUs` \ code ->
- returnUs (StSegment DataSegment : StLabel label : code [])
+ gentopcode stmt@(CStaticClosure lbl _ _ _)
+ = genCodeStaticClosure stmt `thenUs` \ code ->
+ returnUs (StSegment DataSegment : StLabel lbl : code [])
- gentopcode stmt@(CRetUnVector _ _) = returnUs []
+ gentopcode stmt@(CRetVector lbl _ _ _)
+ = genCodeVecTbl stmt `thenUs` \ code ->
+ returnUs (StSegment TextSegment : code [StLabel lbl])
- gentopcode stmt@(CFlatRetVector label _) =
- genCodeVecTbl stmt `thenUs` \ code ->
- returnUs (StSegment TextSegment : code [StLabel label])
+ gentopcode stmt@(CRetDirect uniq absC srt liveness)
+ = gencode absC `thenUs` \ code ->
+ genBitmapInfoTable liveness srt closure_type False `thenUs` \ itbl ->
+ returnUs (StSegment TextSegment :
+ itbl (StLabel lbl_info : StLabel lbl_ret : code []))
+ where
+ lbl_info = mkReturnInfoLabel uniq
+ lbl_ret = mkReturnPtLabel uniq
+ closure_type = case liveness of
+ LvSmall _ -> rET_SMALL
+ LvLarge _ -> rET_BIG
- gentopcode stmt@(CClosureInfoAndCode cl_info slow Nothing _ _ _)
+ gentopcode stmt@(CClosureInfoAndCode cl_info slow Nothing _)
| slow_is_empty
- = genCodeInfoTable hp_rel a2stix stmt `thenUs` \ itbl ->
+ = genCodeInfoTable stmt `thenUs` \ itbl ->
returnUs (StSegment TextSegment : itbl [])
| otherwise
- = genCodeInfoTable hp_rel a2stix stmt `thenUs` \ itbl ->
- gencode slow `thenUs` \ slow_code ->
+ = genCodeInfoTable stmt `thenUs` \ itbl ->
+ gencode slow `thenUs` \ slow_code ->
returnUs (StSegment TextSegment : itbl (StFunBegin slow_lbl :
slow_code [StFunEnd slow_lbl]))
where
slow_is_empty = not (maybeToBool (nonemptyAbsC slow))
slow_lbl = entryLabelFromCI cl_info
- gentopcode stmt@(CClosureInfoAndCode cl_info slow (Just fast) _ _ _) =
+ gentopcode stmt@(CClosureInfoAndCode cl_info slow (Just fast) _) =
-- ToDo: what if this is empty? ------------------------^^^^
- genCodeInfoTable hp_rel a2stix stmt `thenUs` \ itbl ->
- gencode slow `thenUs` \ slow_code ->
- gencode fast `thenUs` \ fast_code ->
+ genCodeInfoTable stmt `thenUs` \ itbl ->
+ gencode slow `thenUs` \ slow_code ->
+ gencode fast `thenUs` \ fast_code ->
returnUs (StSegment TextSegment : itbl (StFunBegin slow_lbl :
slow_code (StFunEnd slow_lbl : StFunBegin fast_lbl :
fast_code [StFunEnd fast_lbl])))
slow_lbl = entryLabelFromCI cl_info
fast_lbl = fastLabelFromCI cl_info
- gentopcode absC =
- gencode absC `thenUs` \ code ->
+ gentopcode stmt@(CSRT lbl closures)
+ = returnUs [ StSegment TextSegment
+ , StLabel lbl
+ , StData DataPtrRep (map StCLbl closures)
+ ]
+
+ gentopcode stmt@(CBitmap lbl mask)
+ = returnUs [ StSegment TextSegment
+ , StLabel lbl
+ , StData WordRep (StInt (toInteger (length mask)) :
+ map (StInt . toInteger . intBS) mask)
+ ]
+
+ gentopcode stmt@(CClosureTbl tycon)
+ = returnUs [ StSegment TextSegment
+ , StLabel (mkClosureTblLabel tycon)
+ , StData DataPtrRep (map (StCLbl . mkClosureLabel . getName . dataConWrapId)
+ (tyConDataCons tycon) )
+ ]
+
+ gentopcode stmt@(CModuleInitBlock lbl absC)
+ = gencode absC `thenUs` \ code ->
+ getUniqLabelNCG `thenUs` \ tmp_lbl ->
+ returnUs ( StSegment DataSegment
+ : StLabel moduleRegdLabel
+ : StData IntRep [StInt 0]
+ : StSegment TextSegment
+ : StLabel lbl
+ : StCondJump tmp_lbl (StPrim IntNeOp
+ [StInd IntRep (StCLbl moduleRegdLabel),
+ StInt 0])
+ : StAssign IntRep (StInd IntRep (StCLbl moduleRegdLabel)) (StInt 1)
+ : code
+ [ StLabel tmp_lbl
+ , StAssign PtrRep stgSp (StPrim IntSubOp [stgSp, StInt 4])
+ , StJump (StInd WordRep stgSp)
+ ])
+
+ gentopcode absC
+ = gencode absC `thenUs` \ code ->
returnUs (StSegment TextSegment : code [])
-
\end{code}
-Vector tables are trivial!
-
\begin{code}
{-
genCodeVecTbl
- :: Target
- -> AbstractC
+ :: AbstractC
-> UniqSM StixTreeList
-}
- genCodeVecTbl (CFlatRetVector label amodes) =
- returnUs (\xs -> vectbl : xs)
+ genCodeVecTbl (CRetVector lbl amodes srt liveness)
+ = genBitmapInfoTable liveness srt closure_type True `thenUs` \itbl ->
+ returnUs (\xs -> vectbl : itbl xs)
where
vectbl = StData PtrRep (reverse (map a2stix amodes))
+ closure_type = case liveness of
+ LvSmall _ -> rET_VEC_SMALL
+ LvLarge _ -> rET_VEC_BIG
\end{code}
-Static closures are not so hard either.
-
\begin{code}
{-
genCodeStaticClosure
- :: Target
- -> AbstractC
+ :: AbstractC
-> UniqSM StixTreeList
-}
- genCodeStaticClosure (CStaticClosure _ cl_info cost_centre amodes) =
- returnUs (\xs -> table : xs)
+ genCodeStaticClosure (CStaticClosure _ cl_info cost_centre amodes)
+ = returnUs (\xs -> table ++ xs)
where
- table = StData PtrRep (StCLbl info_lbl : body)
- info_lbl = infoTableLabelFromCI cl_info
+ table = StData PtrRep [StCLbl (infoTableLabelFromCI cl_info)] :
+ map (\amode -> StData (getAmodeRep amode) [a2stix amode]) amodes ++
+ [StData PtrRep (padding_wds ++ static_link)]
- body = if closureUpdReqd cl_info then
- take (max mIN_UPD_SIZE (length amodes')) (amodes' ++ zeros)
- else
- amodes'
+ -- always at least one padding word: this is the static link field
+ -- for the garbage collector.
+ padding_wds = if closureUpdReqd cl_info then
+ take (max 0 (mIN_UPD_SIZE - length amodes)) zeros
+ else
+ []
- zeros = StInt 0 : zeros
+ static_link | staticClosureNeedsLink cl_info = [StInt 0]
+ | otherwise = []
- amodes' = map amodeZeroVoid amodes
+ zeros = StInt 0 : zeros
+ {- needed??? --SDM
-- Watch out for VoidKinds...cf. PprAbsC
amodeZeroVoid item
| getAmodeRep item == VoidRep = StInt 0
| otherwise = a2stix item
+ -}
\end{code}
\begin{code}
{-
gencode
- :: Target
- -> AbstractC
+ :: AbstractC
-> UniqSM StixTreeList
-}
\end{code}
\begin{code}
- gencode (AbsCStmts c1 c2) =
- gencode c1 `thenUs` \ b1 ->
+ gencode (AbsCStmts c1 c2)
+ = gencode c1 `thenUs` \ b1 ->
gencode c2 `thenUs` \ b2 ->
returnUs (b1 . b2)
\begin{code}
- gencode (CInitHdr cl_info reg_rel _ _) =
- let
- lhs = a2stix (CVal reg_rel PtrRep)
+ gencode (CInitHdr cl_info reg_rel _)
+ = let
+ lhs = a2stix reg_rel
lbl = infoTableLabelFromCI cl_info
in
- returnUs (\xs -> StAssign PtrRep lhs (StCLbl lbl) : xs)
+ returnUs (\xs -> StAssign PtrRep (StInd PtrRep lhs) (StCLbl lbl) : xs)
+
+\end{code}
+
+Heap/Stack Checks.
+
+\begin{code}
+
+ gencode (CCheck macro args assts)
+ = gencode assts `thenUs` \assts_stix ->
+ checkCode macro args assts_stix
\end{code}
gencode (CAssign lhs rhs)
| getAmodeRep lhs == VoidRep = returnUs id
- | otherwise =
- let pk = getAmodeRep lhs
+ | otherwise
+ = let pk = getAmodeRep lhs
pk' = if mixedTypeLocn lhs && not (isFloatingRep pk) then IntRep else pk
lhs' = a2stix lhs
rhs' = a2stix' rhs
Note that the new entry convention requires that we load the InfoPtr (R2)
with the address of the info table before jumping to the entry code for Node.
+For a vectored return, we must subtract the size of the info table to
+get at the return vector. This depends on the size of the info table,
+which varies depending on whether we're profiling etc.
+
\begin{code}
- gencode (CJump dest) =
- returnUs (\xs -> StJump (a2stix dest) : xs)
+ gencode (CJump dest)
+ = returnUs (\xs -> StJump (a2stix dest) : xs)
- gencode (CFallThrough (CLbl lbl _)) =
- returnUs (\xs -> StFallThrough lbl : xs)
+ gencode (CFallThrough (CLbl lbl _))
+ = returnUs (\xs -> StFallThrough lbl : xs)
- gencode (CReturn dest DirectReturn) =
- returnUs (\xs -> StJump (a2stix dest) : xs)
+ gencode (CReturn dest DirectReturn)
+ = returnUs (\xs -> StJump (a2stix dest) : xs)
- gencode (CReturn table (StaticVectoredReturn n)) =
- returnUs (\xs -> StJump dest : xs)
+ gencode (CReturn table (StaticVectoredReturn n))
+ = returnUs (\xs -> StJump dest : xs)
where
dest = StInd PtrRep (StIndex PtrRep (a2stix table)
- (StInt (toInteger (-n-1))))
+ (StInt (toInteger (-n-fixedItblSize-1))))
- gencode (CReturn table (DynamicVectoredReturn am)) =
- returnUs (\xs -> StJump dest : xs)
+ gencode (CReturn table (DynamicVectoredReturn am))
+ = returnUs (\xs -> StJump dest : xs)
where
dest = StInd PtrRep (StIndex PtrRep (a2stix table) dyn_off)
- dyn_off = StPrim IntSubOp [StPrim IntNegOp [a2stix am], StInt 1]
+ dyn_off = StPrim IntSubOp [StPrim IntNegOp [a2stix am],
+ StInt (toInteger (fixedItblSize+1))]
\end{code}
\begin{code}
- gencode (COpStmt results op args liveness_mask vols)
+ gencode (COpStmt results op args vols)
-- ToDo (ADR?): use that liveness mask
- | primOpNeedsWrapper op =
- let
+ | primOpNeedsWrapper op
+ = let
saves = volsaves vols
restores = volrestores vols
in
Nothing -> gencode alt_code
Just dc -> mkIfThenElse discrim tag alt_code dc
- [(tag1@(MachInt i1 _), alt_code1),
- (tag2@(MachInt i2 _), alt_code2)]
+ [(tag1@(MachInt i1), alt_code1),
+ (tag2@(MachInt i2), alt_code2)]
| deflt_is_empty && i1 == 0 && i2 == 1
-> mkIfThenElse discrim tag1 alt_code1 alt_code2
| deflt_is_empty && i1 == 1 && i2 == 0
other | simple_discrim -> mkSimpleSwitches discrim alts deflt
-- Otherwise, we need to do a bit of work.
- other -> getUnique `thenUs` \ u ->
+ other -> getUniqueUs `thenUs` \ u ->
gencode (AbsCStmts
(CAssign (CTemp u pk) discrim)
(CSwitch (CTemp u pk) alts deflt))
gencode (CMacroStmt macro args) = macro_code macro args
- gencode (CCallProfCtrMacro macro _) =
- returnUs (\xs -> StComment macro : xs)
+ gencode (CCallProfCtrMacro macro _)
+ = returnUs (\xs -> StComment macro : xs)
- gencode (CCallProfCCMacro macro _) =
- returnUs (\xs -> StComment macro : xs)
+ gencode (CCallProfCCMacro macro _)
+ = returnUs (\xs -> StComment macro : xs)
+ gencode other
+ = pprPanic "AbsCStixGen.gencode" (dumpRealC other)
\end{code}
-Here, we generate a jump table if there are more than four (integer) alternatives and
-the jump table occupancy is greater than 50%. Otherwise, we generate a binary
-comparison tree. (Perhaps this could be tuned.)
+Here, we generate a jump table if there are more than four (integer)
+alternatives and the jump table occupancy is greater than 50%.
+Otherwise, we generate a binary comparison tree. (Perhaps this could
+be tuned.)
\begin{code}
intTag :: Literal -> Integer
- intTag (MachChar c) = toInteger (ord c)
- intTag (MachInt i _) = i
- intTag _ = panic "intTag"
+ intTag (MachChar c) = toInteger (ord c)
+ intTag (MachInt i) = i
+ intTag (MachWord w) = intTag (word2IntLit (MachWord w))
+ intTag _ = panic "intTag"
fltTag :: Literal -> Rational
- fltTag (MachFloat f) = f
+ fltTag (MachFloat f) = f
fltTag (MachDouble d) = d
- fltTag _ = panic "fltTag"
+ fltTag x = pprPanic "fltTag" (ppr x)
{-
mkSimpleSwitches
- :: Target
- -> CAddrMode -> [(Literal,AbstractC)] -> AbstractC
+ :: CAddrMode -> [(Literal,AbstractC)] -> AbstractC
-> UniqSM StixTreeList
-}
- mkSimpleSwitches am alts absC =
- getUniqLabelNCG `thenUs` \ udlbl ->
+ mkSimpleSwitches am alts absC
+ = getUniqLabelNCG `thenUs` \ udlbl ->
getUniqLabelNCG `thenUs` \ ujlbl ->
let am' = a2stix am
joinedAlts = map (\ (tag,code) -> (tag, mkJoin code ujlbl)) alts
-- lowest and highest possible values the discriminant could take
lowest = if floating then targetMinDouble else targetMinInt
highest = if floating then targetMaxDouble else targetMaxInt
-
- -- These should come from somewhere else, depending on the target arch
- -- (Note that the floating point values aren't terribly important.)
- -- ToDo: Fix!(JSM)
- targetMinDouble = MachDouble (-1.7976931348623157e+308)
- targetMaxDouble = MachDouble (1.7976931348623157e+308)
- targetMinInt = mkMachInt (-2147483647)
- targetMaxInt = mkMachInt 2147483647
in
(
if not floating && choices > 4 && highTag - lowTag < toInteger (2 * choices) then
choices = length alts
(x@(MachChar _),_) `leAlt` (y,_) = intTag x <= intTag y
- (x@(MachInt _ _),_) `leAlt` (y,_) = intTag x <= intTag y
+ (x@(MachInt _), _) `leAlt` (y,_) = intTag x <= intTag y
+ (x@(MachWord _), _) `leAlt` (y,_) = intTag x <= intTag y
(x,_) `leAlt` (y,_) = fltTag x <= fltTag y
\end{code}
-We use jump tables when doing an integer switch on a relatively dense list of
-alternatives. We expect to be given a list of alternatives, sorted by tag,
-and a range of values for which we are to generate a table. Of course, the tags of
-the alternatives should lie within the indicated range. The alternatives need
-not cover the range; a default target is provided for the missing alternatives.
+We use jump tables when doing an integer switch on a relatively dense
+list of alternatives. We expect to be given a list of alternatives,
+sorted by tag, and a range of values for which we are to generate a
+table. Of course, the tags of the alternatives should lie within the
+indicated range. The alternatives need not cover the range; a default
+target is provided for the missing alternatives.
-If a join is necessary after the switch, the alternatives should already finish
-with a jump to the join point.
+If a join is necessary after the switch, the alternatives should
+already finish with a jump to the join point.
\begin{code}
{-
mkJumpTable
- :: Target
- -> StixTree -- discriminant
+ :: StixTree -- discriminant
-> [(Literal, AbstractC)] -- alternatives
-> Integer -- low tag
-> Integer -- high tag
-> UniqSM StixTreeList
-}
- mkJumpTable am alts lowTag highTag dflt =
- getUniqLabelNCG `thenUs` \ utlbl ->
+ mkJumpTable am alts lowTag highTag dflt
+ = getUniqLabelNCG `thenUs` \ utlbl ->
mapUs genLabel alts `thenUs` \ branches ->
- let cjmpLo = StCondJump dflt (StPrim IntLtOp [am, StInt lowTag])
- cjmpHi = StCondJump dflt (StPrim IntGtOp [am, StInt highTag])
+ let cjmpLo = StCondJump dflt (StPrim IntLtOp [am, StInt (toInteger lowTag)])
+ cjmpHi = StCondJump dflt (StPrim IntGtOp [am, StInt (toInteger highTag)])
offset = StPrim IntSubOp [am, StInt lowTag]
- jump = StJump (StInd PtrRep (StIndex PtrRep (StCLbl utlbl) offset))
+ jump = StJump (StInd PtrRep (StIndex PtrRep (StCLbl utlbl) offset))
tlbl = StLabel utlbl
table = StData PtrRep (mkTable branches [lowTag..highTag] [])
in
\begin{code}
{-
mkBinaryTree
- :: Target
- -> StixTree -- discriminant
+ :: StixTree -- discriminant
-> Bool -- floating point?
-> [(Literal, AbstractC)] -- alternatives
-> Int -- number of choices
mkBinaryTree am floating [(tag,alt)] _ lowTag highTag udlbl
| rangeOfOne = gencode alt
- | otherwise =
- let tag' = a2stix (CLit tag)
+ | otherwise
+ = let tag' = a2stix (CLit tag)
cmpOp = if floating then DoubleNeOp else IntNeOp
test = StPrim cmpOp [am, tag']
cjmp = StCondJump udlbl test
rangeOfOne = not floating && intTag lowTag + 1 >= intTag highTag
-- When there is only one possible tag left in range, we skip the comparison
- mkBinaryTree am floating alts choices lowTag highTag udlbl =
- getUniqLabelNCG `thenUs` \ uhlbl ->
+ mkBinaryTree am floating alts choices lowTag highTag udlbl
+ = getUniqLabelNCG `thenUs` \ uhlbl ->
let tag' = a2stix (CLit splitTag)
cmpOp = if floating then DoubleGeOp else IntGeOp
test = StPrim cmpOp [am, tag']
\begin{code}
{-
mkIfThenElse
- :: Target
- -> CAddrMode -- discriminant
+ :: CAddrMode -- discriminant
-> Literal -- tag
-> AbstractC -- if-part
-> AbstractC -- else-part
-> UniqSM StixTreeList
-}
- mkIfThenElse discrim tag alt deflt =
- getUniqLabelNCG `thenUs` \ ujlbl ->
+ mkIfThenElse discrim tag alt deflt
+ = getUniqLabelNCG `thenUs` \ ujlbl ->
getUniqLabelNCG `thenUs` \ utlbl ->
let discrim' = a2stix discrim
tag' = a2stix (CLit tag)
ft _ if_empty = if_empty
{- Old algorithm, which called nonemptyAbsC for every subexpression! =========
-fallThroughAbsC (AbsCStmts c1 c2) =
- case nonemptyAbsC c2 of
+fallThroughAbsC (AbsCStmts c1 c2)
+ = case nonemptyAbsC c2 of
Nothing -> fallThroughAbsC c1
Just x -> fallThroughAbsC x
fallThroughAbsC (CJump _) = False