2 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1996
4 %********************************************************
6 \section[CgTailCall]{Tail calls: converting @StgApps@}
8 %********************************************************
11 #include "HsVersions.h"
16 mkStaticAlgReturnCode, mkDynamicAlgReturnCode,
27 import AbsCUtils ( mkAbstractCs, mkAbsCStmts, getAmodeRep )
28 import CgBindery ( getArgAmodes, getCAddrMode, getCAddrModeAndInfo )
29 import CgRetConv ( dataReturnConvPrim, dataReturnConvAlg,
30 ctrlReturnConvAlg, CtrlReturnConvention(..),
31 DataReturnConvention(..)
33 import CgStackery ( adjustRealSps, mkStkAmodes )
34 import CgUsages ( getSpARelOffset )
35 import CLabel ( mkStdUpdCodePtrVecLabel, mkConUpdCodePtrVecLabel )
36 import ClosureInfo ( nodeMustPointToIt,
37 getEntryConvention, EntryConvention(..),
40 import CmdLineOpts ( opt_DoSemiTagging )
41 import HeapOffs ( zeroOff, SYN_IE(VirtualSpAOffset) )
42 import Id ( idType, dataConTyCon, dataConTag,
45 import Literal ( mkMachInt )
46 import Maybes ( assocMaybe )
47 import PrimRep ( PrimRep(..) )
48 import StgSyn ( SYN_IE(StgArg), GenStgArg(..), SYN_IE(StgLiveVars) )
49 import Type ( isPrimType )
50 import Util ( zipWithEqual, panic, assertPanic )
53 %************************************************************************
55 \subsection[tailcall-doc]{Documentation}
57 %************************************************************************
60 cgTailCall :: StgArg -> [StgArg] -> StgLiveVars -> Code
63 Here's the code we generate for a tail call. (NB there may be no
64 arguments, in which case this boils down to just entering a variable.)
67 \item Adjust the stack ptr to \tr{tailSp + #args}.
68 \item Put args in the top locations of the resulting stack.
69 \item Make Node point to the function closure.
70 \item Enter the function closure.
73 Things to be careful about:
75 \item Don't overwrite stack locations before you have finished with
76 them (remember you need the function and the as-yet-unmoved
78 \item Preferably, generate no code to replace x by x on the stack (a
79 common situation in tail-recursion).
80 \item Adjust the stack high water mark appropriately.
83 Literals are similar to constructors; they return by putting
84 themselves in an appropriate register and returning to the address on
88 cgTailCall (StgLitArg lit) [] live_vars
89 = performPrimReturn (CLit lit) live_vars
92 Treat unboxed locals exactly like literals (above) except use the addr
93 mode for the local instead of (CLit lit) in the assignment.
95 Case for unboxed @Ids@ first:
97 cgTailCall atom@(StgVarArg fun) [] live_vars
98 | isPrimType (idType fun)
99 = getCAddrMode fun `thenFC` \ amode ->
100 performPrimReturn amode live_vars
103 The general case (@fun@ is boxed):
105 cgTailCall (StgVarArg fun) args live_vars = performTailCall fun args live_vars
108 %************************************************************************
110 \subsection[return-and-tail-call]{Return and tail call}
112 %************************************************************************
116 A quick bit of hacking to try to solve my void#-leaking blues...
118 I think I'm getting bitten by this stuff because code like
121 case ds.s12 :: IoWorld of {
122 -- lvs: [ds.s12]; rhs lvs: []; uniq: c0
123 IoWorld ds.s13# -> ds.s13#;
127 causes me to try to allocate a register to return the result in. The
128 hope is that the following will avoid such problems (and that Will
129 will do this in a cleaner way when he hits the same problem).
134 performPrimReturn :: CAddrMode -- The thing to return
138 performPrimReturn amode live_vars
140 kind = getAmodeRep amode
141 ret_reg = dataReturnConvPrim kind
143 assign_possibly = case kind of
145 kind -> (CAssign (CReg ret_reg) amode)
147 performReturn assign_possibly mkPrimReturnCode live_vars
149 mkPrimReturnCode :: Sequel -> Code
150 mkPrimReturnCode (UpdateCode _) = panic "mkPrimReturnCode: Upd"
151 mkPrimReturnCode sequel = sequelToAmode sequel `thenFC` \ dest_amode ->
152 absC (CReturn dest_amode DirectReturn)
153 -- Direct, no vectoring
155 -- All constructor arguments in registers; Node and InfoPtr are set.
156 -- All that remains is
157 -- (a) to set TagReg, if necessary
158 -- (b) to set InfoPtr to the info ptr, if necessary
159 -- (c) to do the right sort of jump.
161 mkStaticAlgReturnCode :: Id -- The constructor
162 -> Maybe CLabel -- The info ptr, if it isn't already set
163 -> Sequel -- where to return to
166 mkStaticAlgReturnCode con maybe_info_lbl sequel
167 = -- Generate profiling code if necessary
168 (case return_convention of
169 VectoredReturn sz -> profCtrC SLIT("VEC_RETURN") [mkIntCLit sz]
173 -- Set tag if necessary
174 -- This is done by a macro, because if we are short of registers
175 -- we don't set TagReg; instead the continuation gets the tag
176 -- by indexing off the info ptr
177 (case return_convention of
179 UnvectoredReturn no_of_constrs
181 -> absC (CMacroStmt SET_TAG [mkIntCLit zero_indexed_tag])
186 -- Generate the right jump or return
188 UpdateCode _ -> -- Ha! We know the constructor,
189 -- so we can go direct to the correct
190 -- update code for that constructor
192 -- Set the info pointer, and jump
194 absC (CJump (CLbl update_label CodePtrRep))
196 CaseAlts _ (Just (alts, _)) -> -- Ho! We know the constructor so
197 -- we can go right to the alternative
199 -- No need to set info ptr when returning to a
200 -- known join point. After all, the code at
201 -- the destination knows what constructor it
202 -- is going to handle.
204 case assocMaybe alts tag of
205 Just (alt_absC, join_lbl) -> absC (CJump (CLbl join_lbl CodePtrRep))
206 Nothing -> panic "mkStaticAlgReturnCode: default"
207 -- The Nothing case should never happen; it's the subject
208 -- of a wad of special-case code in cgReturnCon
210 other -> -- OnStack, or (CaseAlts) ret_amode Nothing)
211 -- Set the info pointer, and jump
213 sequelToAmode sequel `thenFC` \ ret_amode ->
214 absC (CReturn ret_amode return_info)
219 tycon = dataConTyCon con
220 return_convention = ctrlReturnConvAlg tycon
221 zero_indexed_tag = tag - fIRST_TAG -- Adjust tag to be zero-indexed
222 -- cf AbsCUtils.mkAlgAltsCSwitch
225 = case (dataReturnConvAlg con) of
226 ReturnInHeap -> mkStdUpdCodePtrVecLabel tycon tag
227 ReturnInRegs _ -> mkConUpdCodePtrVecLabel tycon tag
229 return_info = case return_convention of
230 UnvectoredReturn _ -> DirectReturn
231 VectoredReturn _ -> StaticVectoredReturn zero_indexed_tag
233 set_info_ptr = case maybe_info_lbl of
235 Just info_lbl -> absC (CAssign (CReg infoptr) (CLbl info_lbl DataPtrRep))
238 mkDynamicAlgReturnCode :: TyCon -> CAddrMode -> Sequel -> Code
240 mkDynamicAlgReturnCode tycon dyn_tag sequel
241 = case ctrlReturnConvAlg tycon of
244 profCtrC SLIT("VEC_RETURN") [mkIntCLit sz] `thenC`
245 sequelToAmode sequel `thenFC` \ ret_addr ->
246 absC (CReturn ret_addr (DynamicVectoredReturn dyn_tag))
248 UnvectoredReturn no_of_constrs ->
250 -- Set tag if necessary
251 -- This is done by a macro, because if we are short of registers
252 -- we don't set TagReg; instead the continuation gets the tag
253 -- by indexing off the info ptr
254 (if no_of_constrs > 1 then
255 absC (CMacroStmt SET_TAG [dyn_tag])
261 sequelToAmode sequel `thenFC` \ ret_addr ->
262 -- Generate the right jump or return
263 absC (CReturn ret_addr DirectReturn)
267 performReturn :: AbstractC -- Simultaneous assignments to perform
268 -> (Sequel -> Code) -- The code to execute to actually do
269 -- the return, given an addressing mode
270 -- for the return address
274 performReturn sim_assts finish_code live_vars
275 = getEndOfBlockInfo `thenFC` \ (EndOfBlockInfo args_spa args_spb sequel) ->
277 -- Do the simultaneous assignments,
278 doSimAssts args_spa live_vars sim_assts `thenC`
280 -- Adjust stack pointers
281 adjustRealSps args_spa args_spb `thenC`
284 finish_code sequel -- "sequel" is `robust' in that it doesn't
285 -- depend on stk-ptr values
289 performTailCall :: Id -- Function
294 performTailCall fun args live_vars
295 = -- Get all the info we have about the function and args and go on to
297 getCAddrModeAndInfo fun `thenFC` \ (fun_amode, lf_info) ->
298 getArgAmodes args `thenFC` \ arg_amodes ->
301 fun fun_amode lf_info arg_amodes
302 live_vars AbsCNop {- No pending assignments -}
305 tailCallBusiness :: Id -> CAddrMode -- Function and its amode
306 -> LambdaFormInfo -- Info about the function
307 -> [CAddrMode] -- Arguments
308 -> StgLiveVars -- Live in continuation
310 -> AbstractC -- Pending simultaneous assignments
311 -- *** GUARANTEED to contain only stack assignments.
312 -- In ptic, we don't need to look in here to
313 -- discover all live regs
317 tailCallBusiness fun fun_amode lf_info arg_amodes live_vars pending_assts
318 = nodeMustPointToIt lf_info `thenFC` \ node_points ->
319 getEntryConvention fun lf_info
320 (map getAmodeRep arg_amodes) `thenFC` \ entry_conv ->
322 getEndOfBlockInfo `thenFC` \ (EndOfBlockInfo args_spa args_spb sequel) ->
326 = if node_points then
327 CAssign (CReg node) fun_amode
331 (arg_regs, finish_code)
336 CCallProfCtrMacro SLIT("ENT_VIA_NODE") [],
337 CJump (CMacroExpr CodePtrKind ENTRY_CODE [(CMacroExpr DataPtrKind INFO_PTR [CReg node])])
339 StdEntry lbl Nothing -> ([], CJump (CLbl lbl CodePtrRep))
340 StdEntry lbl (Just itbl) -> ([], CAssign (CReg infoptr) (CLbl itbl DataPtrRep)
342 CJump (CLbl lbl CodePtrRep))
343 DirectEntry lbl arity regs ->
344 (regs, CJump (CLbl lbl CodePtrRep))
346 no_of_args = length arg_amodes
348 (reg_arg_amodes, stk_arg_amodes) = splitAt (length arg_regs) arg_amodes
349 -- We get some stk_arg_amodes if (a) no regs, or (b) args beyond arity
352 = mkAbstractCs (zipWithEqual "assign_to_reg2" assign_to_reg arg_regs reg_arg_amodes)
354 assign_to_reg reg_id amode = CAssign (CReg reg_id) amode
357 CJoinPoint join_spa join_spb -> -- Ha! A let-no-escape thingy
359 ASSERT(not (args_spa > join_spa) || (args_spb > join_spb))
360 -- If ASSERTion fails: Oops: the join point has *lower*
361 -- stack ptrs than the continuation Note that we take
362 -- the SpB point without the return address here. The
363 -- return address is put on by the let-no-escapey thing
366 mkStkAmodes join_spa join_spb stk_arg_amodes
367 `thenFC` \ (final_spa, final_spb, stk_arg_assts) ->
369 -- Do the simultaneous assignments,
370 doSimAssts join_spa live_vars
371 (mkAbstractCs [pending_assts, reg_arg_assts, stk_arg_assts])
375 adjustRealSps final_spa final_spb `thenC`
377 -- Jump to join point
380 _ -> -- else: not a let-no-escape (the common case)
382 -- Make instruction to save return address
383 loadRetAddrIntoRetReg sequel `thenFC` \ ret_asst ->
385 mkStkAmodes args_spa args_spb stk_arg_amodes
387 \ (final_spa, final_spb, stk_arg_assts) ->
389 -- The B-stack space for the pushed return addess, with any args pushed
390 -- on top, is recorded in final_spb.
392 -- Do the simultaneous assignments,
393 doSimAssts args_spa live_vars
394 (mkAbstractCs [pending_assts, node_asst, ret_asst,
395 reg_arg_assts, stk_arg_assts])
398 -- Final adjustment of stack pointers
399 adjustRealSps final_spa final_spb `thenC`
401 -- Now decide about semi-tagging
403 semi_tagging_on = opt_DoSemiTagging
405 case (semi_tagging_on, arg_amodes, node_points, sequel) of
408 -- *************** The semi-tagging case ***************
410 ( True, [], True, CaseAlts _ (Just (st_alts, maybe_deflt_join_details))) ->
412 -- Whoppee! Semi-tagging rules OK!
413 -- (a) semi-tagging is switched on
414 -- (b) there are no arguments,
415 -- (c) Node points to the closure
416 -- (d) we have a case-alternative sequel with
417 -- some visible alternatives
419 -- Why is test (c) necessary?
420 -- Usually Node will point to it at this point, because we're
421 -- scrutinsing something which is either a thunk or a
423 -- But not always! The example I came across is when we have
424 -- a top-level Double:
426 -- ... (case lit.3 of ...) ...
427 -- Here, lit.3 is built as a re-entrant thing, which you must enter.
428 -- (OK, the simplifier should have eliminated this, but it's
429 -- easy to deal with the case anyway.)
431 join_details_to_code (load_regs_and_profiling_code, join_lbl)
432 = load_regs_and_profiling_code `mkAbsCStmts`
433 CJump (CLbl join_lbl CodePtrRep)
435 semi_tagged_alts = [ (mkMachInt (toInteger (tag - fIRST_TAG)),
436 join_details_to_code join_details)
437 | (tag, join_details) <- st_alts
441 -- Enter Node (we know infoptr will have the info ptr in it)!
443 CCallProfCtrMacro SLIT("RET_SEMI_FAILED")
444 [CMacroExpr IntRep INFO_TAG [CReg infoptr]],
445 CJump (CMacroExpr CodePtrRep ENTRY_CODE [CReg infoptr]) ]
449 CAssign (CReg infoptr)
450 (CVal (NodeRel zeroOff) DataPtrRep),
452 case maybe_deflt_join_details of
454 CSwitch (CMacroExpr IntRep INFO_TAG [CReg infoptr])
458 CSwitch (CMacroExpr IntRep EVAL_TAG [CReg infoptr])
459 [(mkMachInt 0, enter_jump)]
461 (CMacroExpr IntRep INFO_TAG [CReg infoptr])
463 (join_details_to_code details))
467 -- *************** The non-semi-tagging case ***************
469 other -> absC finish_code
473 loadRetAddrIntoRetReg :: Sequel -> FCode AbstractC
475 loadRetAddrIntoRetReg InRetReg
476 = returnFC AbsCNop -- Return address already there
478 loadRetAddrIntoRetReg sequel
479 = sequelToAmode sequel `thenFC` \ amode ->
480 returnFC (CAssign (CReg RetReg) amode)
484 %************************************************************************
486 \subsection[doSimAssts]{@doSimAssts@}
488 %************************************************************************
490 @doSimAssts@ happens at the end of every block of code.
491 They are separate because we sometimes do some jiggery-pokery in between.
494 doSimAssts :: VirtualSpAOffset -- tail_spa: SpA as seen by continuation
495 -> StgLiveVars -- Live in continuation
499 doSimAssts tail_spa live_vars sim_assts
500 = -- Do the simultaneous assignments
501 absC (CSimultaneous sim_assts) `thenC`
503 -- Stub any unstubbed slots; the only live variables are indicated in
504 -- the end-of-block info in the monad
505 nukeDeadBindings live_vars `thenC`
506 getUnstubbedAStackSlots tail_spa `thenFC` \ a_slots ->
507 -- Passing in tail_spa here should actually be redundant, because
508 -- the stack should be trimmed (by nukeDeadBindings) to
509 -- exactly the tail_spa position anyhow.
511 -- Emit code to stub dead regs; this only generates actual
512 -- machine instructions in in the DEBUG version
513 -- *** NOT DONE YET ***
517 else profCtrC SLIT("A_STK_STUB") [mkIntCLit (length a_slots)] `thenC`
518 mapCs stub_A_slot a_slots
521 stub_A_slot :: VirtualSpAOffset -> Code
522 stub_A_slot offset = getSpARelOffset offset `thenFC` \ spa_rel ->
523 absC (CAssign (CVal spa_rel PtrRep)