2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
7 module CgCase ( cgCase, saveVolatileVarsAndRegs,
8 restoreCurrentCostCentre
11 #include "HsVersions.h"
13 import {-# SOURCE #-} CgExpr ( cgExpr )
48 = GCMayHappen -- The scrutinee may involve GC, so everything must be
49 -- tidy before the code for the scrutinee.
51 | NoGC -- The scrutinee is a primitive value, or a call to a
52 -- primitive op which does no GC. Hence the case can
53 -- be done inline, without tidying up first.
56 It is quite interesting to decide whether to put a heap-check
57 at the start of each alternative. Of course we certainly have
58 to do so if the case forces an evaluation, or if there is a primitive
59 op which can trigger GC.
61 A more interesting situation is this:
68 default -> !C!; ...C...
71 where \tr{!x!} indicates a possible heap-check point. The heap checks
72 in the alternatives {\em can} be omitted, in which case the topmost
73 heapcheck will take their worst case into account.
75 In favour of omitting \tr{!B!}, \tr{!C!}:
77 - {\em May} save a heap overflow test,
78 if ...A... allocates anything. The other advantage
79 of this is that we can use relative addressing
80 from a single Hp to get at all the closures so allocated.
82 - No need to save volatile vars etc across the case
86 - May do more allocation than reqd. This sometimes bites us
87 badly. For example, nfib (ha!) allocates about 30\% more space if the
88 worst-casing is done, because many many calls to nfib are leaf calls
89 which don't need to allocate anything.
91 This never hurts us if there is only one alternative.
104 Special case #1: case of literal.
107 cgCase (StgLit lit) live_in_whole_case live_in_alts bndr srt
108 alt_type@(PrimAlt tycon) alts
109 = do { tmp_reg <- bindNewToTemp bndr
110 ; cm_lit <- cgLit lit
111 ; stmtC (CmmAssign (CmmLocal tmp_reg) (CmmLit cm_lit))
112 ; cgPrimAlts NoGC alt_type (CmmLocal tmp_reg) alts }
115 Special case #2: scrutinising a primitive-typed variable. No
116 evaluation required. We don't save volatile variables, nor do we do a
117 heap-check in the alternatives. Instead, the heap usage of the
118 alternatives is worst-cased and passed upstream. This can result in
119 allocating more heap than strictly necessary, but it will sometimes
120 eliminate a heap check altogether.
123 cgCase (StgApp v []) live_in_whole_case live_in_alts bndr srt
124 alt_type@(PrimAlt tycon) alts
125 = do { -- Careful! we can't just bind the default binder to the same thing
126 -- as the scrutinee, since it might be a stack location, and having
127 -- two bindings pointing at the same stack locn doesn't work (it
128 -- confuses nukeDeadBindings). Hence, use a new temp.
129 v_info <- getCgIdInfo v
130 ; amode <- idInfoToAmode v_info
131 ; tmp_reg <- bindNewToTemp bndr
132 ; stmtC (CmmAssign (CmmLocal tmp_reg) amode)
133 ; cgPrimAlts NoGC alt_type (CmmLocal tmp_reg) alts }
136 Special case #3: inline PrimOps and foreign calls.
139 cgCase (StgOpApp op@(StgPrimOp primop) args _)
140 live_in_whole_case live_in_alts bndr srt alt_type alts
141 | not (primOpOutOfLine primop)
142 = cgInlinePrimOp primop args bndr alt_type live_in_alts alts
145 TODO: Case-of-case of primop can probably be done inline too (but
146 maybe better to translate it out beforehand). See
147 ghc/lib/misc/PackedString.lhs for examples where this crops up (with
150 Special case #4: inline foreign calls: an unsafe foreign call can be done
151 right here, just like an inline primop.
154 cgCase (StgOpApp op@(StgFCallOp fcall _) args _)
155 live_in_whole_case live_in_alts bndr srt alt_type alts
156 | unsafe_foreign_call
157 = ASSERT( isSingleton alts )
158 do -- *must* be an unboxed tuple alt.
159 -- exactly like the cgInlinePrimOp case for unboxed tuple alts..
160 { res_tmps <- mapFCs bindNewToTemp non_void_res_ids
161 ; let res_hints = map (typeHint.idType) non_void_res_ids
162 ; cgForeignCall (zip res_tmps res_hints) fcall args live_in_alts
165 (_, res_ids, _, rhs) = head alts
166 non_void_res_ids = filter (nonVoidArg . idCgRep) res_ids
170 CCall (CCallSpec _ _ s) -> not (playSafe s)
174 Special case: scrutinising a non-primitive variable.
175 This can be done a little better than the general case, because
176 we can reuse/trim the stack slot holding the variable (if it is in one).
179 cgCase (StgApp fun args)
180 live_in_whole_case live_in_alts bndr srt alt_type alts
181 = do { fun_info <- getCgIdInfo fun
182 ; arg_amodes <- getArgAmodes args
184 -- Nuking dead bindings *before* calculating the saves is the
185 -- value-add here. We might end up freeing up some slots currently
186 -- occupied by variables only required for the call.
187 -- NOTE: we need to look up the variables used in the call before
188 -- doing this, because some of them may not be in the environment
190 ; nukeDeadBindings live_in_alts
191 ; (save_assts, alts_eob_info, maybe_cc_slot)
192 <- saveVolatileVarsAndRegs live_in_alts
195 <- forkEval alts_eob_info
196 (allocStackTop retAddrSizeW >> nopC)
197 (do { deAllocStackTop retAddrSizeW
198 ; cgEvalAlts maybe_cc_slot bndr srt alt_type alts })
200 ; setEndOfBlockInfo scrut_eob_info
201 (performTailCall fun_info arg_amodes save_assts) }
204 Note about return addresses: we *always* push a return address, even
205 if because of an optimisation we end up jumping direct to the return
206 code (not through the address itself). The alternatives always assume
207 that the return address is on the stack. The return address is
208 required in case the alternative performs a heap check, since it
209 encodes the liveness of the slots in the activation record.
211 On entry to the case alternative, we can re-use the slot containing
212 the return address immediately after the heap check. That's what the
213 deAllocStackTop call is doing above.
215 Finally, here is the general case.
218 cgCase expr live_in_whole_case live_in_alts bndr srt alt_type alts
219 = do { -- Figure out what volatile variables to save
220 nukeDeadBindings live_in_whole_case
222 ; (save_assts, alts_eob_info, maybe_cc_slot)
223 <- saveVolatileVarsAndRegs live_in_alts
225 -- Save those variables right now!
226 ; emitStmts save_assts
228 -- generate code for the alts
230 <- forkEval alts_eob_info
231 (do { nukeDeadBindings live_in_alts
232 ; allocStackTop retAddrSizeW -- space for retn address
234 (do { deAllocStackTop retAddrSizeW
235 ; cgEvalAlts maybe_cc_slot bndr srt alt_type alts })
237 ; setEndOfBlockInfo scrut_eob_info (cgExpr expr)
241 There's a lot of machinery going on behind the scenes to manage the
242 stack pointer here. forkEval takes the virtual Sp and free list from
243 the first argument, and turns that into the *real* Sp for the second
244 argument. It also uses this virtual Sp as the args-Sp in the EOB info
245 returned, so that the scrutinee will trim the real Sp back to the
246 right place before doing whatever it does.
247 --SDM (who just spent an hour figuring this out, and didn't want to
250 Why don't we push the return address just before evaluating the
251 scrutinee? Because the slot reserved for the return address might
252 contain something useful, so we wait until performing a tail call or
253 return before pushing the return address (see
254 CgTailCall.pushReturnAddress).
256 This also means that the environment doesn't need to know about the
257 free stack slot for the return address (for generating bitmaps),
258 because we don't reserve it until just before the eval.
260 TODO!! Problem: however, we have to save the current cost centre
261 stack somewhere, because at the eval point the current CCS might be
262 different. So we pick a free stack slot and save CCCS in it. One
263 consequence of this is that activation records on the stack don't
264 follow the layout of closures when we're profiling. The CCS could be
265 anywhere within the record).
267 %************************************************************************
271 %************************************************************************
274 cgInlinePrimOp primop args bndr (PrimAlt tycon) live_in_alts alts
275 | isVoidArg (idCgRep bndr)
276 = ASSERT( con == DEFAULT && isSingleton alts && null bs )
277 do { -- VOID RESULT; just sequencing,
278 -- so get in there and do it
279 cgPrimOp [] primop args live_in_alts
282 (con,bs,_,rhs) = head alts
284 cgInlinePrimOp primop args bndr (PrimAlt tycon) live_in_alts alts
285 = do { -- PRIMITIVE ALTS, with non-void result
286 tmp_reg <- bindNewToTemp bndr
287 ; cgPrimOp [tmp_reg] primop args live_in_alts
288 ; cgPrimAlts NoGC (PrimAlt tycon) (CmmLocal tmp_reg) alts }
290 cgInlinePrimOp primop args bndr (UbxTupAlt tycon) live_in_alts alts
291 = ASSERT( isSingleton alts )
292 do { -- UNBOXED TUPLE ALTS
293 -- No heap check, no yield, just get in there and do it.
294 -- NB: the case binder isn't bound to anything;
295 -- it has a unboxed tuple type
297 res_tmps <- mapFCs bindNewToTemp non_void_res_ids
298 ; cgPrimOp res_tmps primop args live_in_alts
301 (_, res_ids, _, rhs) = head alts
302 non_void_res_ids = filter (nonVoidArg . idCgRep) res_ids
304 cgInlinePrimOp primop args bndr (AlgAlt tycon) live_in_alts alts
305 = do { -- ENUMERATION TYPE RETURN
306 -- Typical: case a ># b of { True -> ..; False -> .. }
307 -- The primop itself returns an index into the table of
308 -- closures for the enumeration type.
309 tag_amode <- ASSERT( isEnumerationTyCon tycon )
310 do_enum_primop primop
312 -- Bind the default binder if necessary
313 -- (avoiding it avoids the assignment)
314 -- The deadness info is set by StgVarInfo
315 ; this_pkg <- getThisPackage
316 ; whenC (not (isDeadBinder bndr))
317 (do { tmp_reg <- bindNewToTemp bndr
320 (tagToClosure this_pkg tycon tag_amode)) })
323 ; (branches, mb_deflt) <- cgAlgAlts NoGC Nothing{-cc_slot-}
327 ; emitSwitch tag_amode branches mb_deflt 0 (tyConFamilySize tycon - 1)
331 do_enum_primop :: PrimOp -> FCode CmmExpr -- Returns amode for result
332 do_enum_primop TagToEnumOp -- No code!
334 (_,e) <- getArgAmode arg
336 do_enum_primop primop
337 = do tmp <- newNonPtrTemp wordRep
338 cgPrimOp [tmp] primop args live_in_alts
339 returnFC (CmmReg (CmmLocal tmp))
341 cgInlinePrimOp primop arg_amodes bndr PolyAlt live_in_alts alts
342 = pprPanic "cgCase: case of primop has polymorphic type" (ppr bndr)
345 %************************************************************************
347 \subsection[CgCase-alts]{Alternatives}
349 %************************************************************************
351 @cgEvalAlts@ returns an addressing mode for a continuation for the
352 alternatives of a @case@, used in a context when there
353 is some evaluation to be done.
356 cgEvalAlts :: Maybe VirtualSpOffset -- Offset of cost-centre to be restored, if any
358 -> SRT -- SRT for the continuation
361 -> FCode Sequel -- Any addr modes inside are guaranteed
362 -- to be a label so that we can duplicate it
363 -- without risk of duplicating code
365 cgEvalAlts cc_slot bndr srt alt_type@(PrimAlt tycon) alts
366 = do { let rep = tyConCgRep tycon
367 reg = dataReturnConvPrim rep -- Bottom for voidRep
369 ; abs_c <- forkProc $ do
370 { -- Bind the case binder, except if it's void
371 -- (reg is bottom in that case)
372 whenC (nonVoidArg rep) $
373 bindNewToReg bndr reg (mkLFArgument bndr)
374 ; restoreCurrentCostCentre cc_slot True
375 ; cgPrimAlts GCMayHappen alt_type reg alts }
377 ; lbl <- emitReturnTarget (idName bndr) abs_c srt
378 ; returnFC (CaseAlts lbl Nothing bndr) }
380 cgEvalAlts cc_slot bndr srt (UbxTupAlt _) [(con,args,_,rhs)]
381 = -- Unboxed tuple case
382 -- By now, the simplifier should have have turned it
383 -- into case e of (# a,b #) -> e
384 -- There shouldn't be a
385 -- case e of DEFAULT -> e
386 ASSERT2( case con of { DataAlt _ -> True; other -> False },
387 text "cgEvalAlts: dodgy case of unboxed tuple type" )
388 do { -- forkAbsC for the RHS, so that the envt is
389 -- not changed for the emitReturn call
390 abs_c <- forkProc $ do
391 { (live_regs, ptrs, nptrs, _) <- bindUnboxedTupleComponents args
392 -- Restore the CC *after* binding the tuple components,
393 -- so that we get the stack offset of the saved CC right.
394 ; restoreCurrentCostCentre cc_slot True
395 -- Generate a heap check if necessary
396 -- and finally the code for the alternative
397 ; unbxTupleHeapCheck live_regs ptrs nptrs noStmts
399 ; lbl <- emitReturnTarget (idName bndr) abs_c srt
400 ; returnFC (CaseAlts lbl Nothing bndr) }
402 cgEvalAlts cc_slot bndr srt alt_type alts
403 = -- Algebraic and polymorphic case
404 do { -- Bind the default binder
405 bindNewToReg bndr nodeReg (mkLFArgument bndr)
407 -- Generate sequel info for use downstream
408 -- At the moment, we only do it if the type is vector-returnable.
409 -- Reason: if not, then it costs extra to label the
410 -- alternatives, because we'd get return code like:
412 -- switch TagReg { 0 : JMP(alt_1); 1 : JMP(alt_2) ..etc }
414 -- which is worse than having the alt code in the switch statement
416 ; (alts, mb_deflt) <- cgAlgAlts GCMayHappen cc_slot alt_type alts
418 ; (lbl, branches) <- emitAlgReturnTarget (idName bndr)
419 alts mb_deflt srt fam_sz
421 ; returnFC (CaseAlts lbl branches bndr) }
423 fam_sz = case alt_type of
424 AlgAlt tc -> tyConFamilySize tc
429 HWL comment on {\em GrAnSim\/} (adding GRAN_YIELDs for context switch): If
430 we do an inlining of the case no separate functions for returning are
431 created, so we don't have to generate a GRAN_YIELD in that case. This info
432 must be propagated to cgAlgAltRhs (where the GRAN_YIELD macro might be
433 emitted). Hence, the new Bool arg to cgAlgAltRhs.
435 %************************************************************************
437 \subsection[CgCase-alg-alts]{Algebraic alternatives}
439 %************************************************************************
441 In @cgAlgAlts@, none of the binders in the alternatives are
442 assumed to be yet bound.
444 HWL comment on {\em GrAnSim\/} (adding GRAN_YIELDs for context switch): The
445 last arg of cgAlgAlts indicates if we want a context switch at the
446 beginning of each alternative. Normally we want that. The only exception
447 are inlined alternatives.
451 -> Maybe VirtualSpOffset
452 -> AltType -- ** AlgAlt or PolyAlt only **
453 -> [StgAlt] -- The alternatives
454 -> FCode ( [(ConTagZ, CgStmts)], -- The branches
455 Maybe CgStmts ) -- The default case
457 cgAlgAlts gc_flag cc_slot alt_type alts
458 = do alts <- forkAlts [ cgAlgAlt gc_flag cc_slot alt_type alt | alt <- alts]
460 mb_deflt = case alts of -- DEFAULT is always first, if present
461 ((DEFAULT,blks) : _) -> Just blks
464 branches = [(dataConTagZ con, blks)
465 | (DataAlt con, blks) <- alts]
467 return (branches, mb_deflt)
471 -> Maybe VirtualSpOffset -- Turgid state
472 -> AltType -- ** AlgAlt or PolyAlt only **
474 -> FCode (AltCon, CgStmts)
476 cgAlgAlt gc_flag cc_slot alt_type (con, args, use_mask, rhs)
477 = do { abs_c <- getCgStmts $ do
478 { bind_con_args con args
479 ; restoreCurrentCostCentre cc_slot True
480 ; maybeAltHeapCheck gc_flag alt_type (cgExpr rhs) }
481 ; return (con, abs_c) }
483 bind_con_args DEFAULT args = nopC
484 bind_con_args (DataAlt dc) args = bindConArgs dc args
488 %************************************************************************
490 \subsection[CgCase-prim-alts]{Primitive alternatives}
492 %************************************************************************
494 @cgPrimAlts@ generates suitable a @CSwitch@
495 for dealing with the alternatives of a primitive @case@, given an
496 addressing mode for the thing to scrutinise. It also keeps track of
497 the maximum stack depth encountered down any branch.
499 As usual, no binders in the alternatives are yet bound.
503 -> AltType -- Always PrimAlt, but passed to maybeAltHeapCheck
504 -> CmmReg -- Scrutinee
505 -> [StgAlt] -- Alternatives
507 -- NB: cgPrimAlts emits code that does the case analysis.
508 -- It's often used in inline situations, rather than to genearte
509 -- a labelled return point. That's why its interface is a little
510 -- different to cgAlgAlts
512 -- INVARIANT: the default binder is already bound
513 cgPrimAlts gc_flag alt_type scrutinee alts
514 = do { tagged_absCs <- forkAlts (map (cgPrimAlt gc_flag alt_type) alts)
515 ; let ((DEFAULT, deflt_absC) : others) = tagged_absCs -- There is always a default
516 alt_absCs = [(lit,rhs) | (LitAlt lit, rhs) <- others]
517 ; emitLitSwitch (CmmReg scrutinee) alt_absCs deflt_absC }
521 -> StgAlt -- The alternative
522 -> FCode (AltCon, CgStmts) -- Its compiled form
524 cgPrimAlt gc_flag alt_type (con, [], [], rhs)
525 = ASSERT( case con of { DEFAULT -> True; LitAlt _ -> True; other -> False } )
526 do { abs_c <- getCgStmts (maybeAltHeapCheck gc_flag alt_type (cgExpr rhs))
527 ; returnFC (con, abs_c) }
531 %************************************************************************
533 \subsection[CgCase-tidy]{Code for tidying up prior to an eval}
535 %************************************************************************
540 -> AltType -- PolyAlt, PrimAlt, AlgAlt, but *not* UbxTupAlt
541 -> Code -- Continuation
543 maybeAltHeapCheck NoGC _ code = code
544 maybeAltHeapCheck GCMayHappen alt_type code = altHeapCheck alt_type code
546 saveVolatileVarsAndRegs
547 :: StgLiveVars -- Vars which should be made safe
548 -> FCode (CmmStmts, -- Assignments to do the saves
549 EndOfBlockInfo, -- sequel for the alts
550 Maybe VirtualSpOffset) -- Slot for current cost centre
552 saveVolatileVarsAndRegs vars
553 = do { var_saves <- saveVolatileVars vars
554 ; (maybe_cc_slot, cc_save) <- saveCurrentCostCentre
555 ; eob_info <- getEndOfBlockInfo
556 ; returnFC (var_saves `plusStmts` cc_save,
561 saveVolatileVars :: StgLiveVars -- Vars which should be made safe
562 -> FCode CmmStmts -- Assignments to to the saves
564 saveVolatileVars vars
565 = do { stmts_s <- mapFCs save_it (varSetElems vars)
566 ; return (foldr plusStmts noStmts stmts_s) }
569 = do { v <- getCAddrModeIfVolatile var
571 Nothing -> return noStmts -- Non-volatile
572 Just vol_amode -> save_var var vol_amode -- Aha! It's volatile
575 save_var var vol_amode
576 = do { slot <- allocPrimStack (idCgRep var)
577 ; rebindToStack var slot
578 ; sp_rel <- getSpRelOffset slot
579 ; returnFC (oneStmt (CmmStore sp_rel vol_amode)) }
582 ---------------------------------------------------------------------------
584 When we save the current cost centre (which is done for lexical
585 scoping), we allocate a free stack location, and return (a)~the
586 virtual offset of the location, to pass on to the alternatives, and
587 (b)~the assignment to do the save (just as for @saveVolatileVars@).
590 saveCurrentCostCentre ::
591 FCode (Maybe VirtualSpOffset, -- Where we decide to store it
592 CmmStmts) -- Assignment to save it
594 saveCurrentCostCentre
595 | not opt_SccProfilingOn
596 = returnFC (Nothing, noStmts)
598 = do { slot <- allocPrimStack PtrArg
599 ; sp_rel <- getSpRelOffset slot
600 ; returnFC (Just slot,
601 oneStmt (CmmStore sp_rel curCCS)) }
603 -- Sometimes we don't free the slot containing the cost centre after restoring it
604 -- (see CgLetNoEscape.cgLetNoEscapeBody).
605 restoreCurrentCostCentre :: Maybe VirtualSpOffset -> Bool -> Code
606 restoreCurrentCostCentre Nothing _freeit = nopC
607 restoreCurrentCostCentre (Just slot) freeit
608 = do { sp_rel <- getSpRelOffset slot
609 ; whenC freeit (freeStackSlots [slot])
610 ; stmtC (CmmStore curCCSAddr (CmmLoad sp_rel wordRep)) }