2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
5 % Code generation for tail calls.
9 cgTailCall, performTailCall,
10 performReturn, performPrimReturn,
11 returnUnboxedTuple, ccallReturnUnboxedTuple,
18 #include "HsVersions.h"
41 -----------------------------------------------------------------------------
44 cgTailCall :: Id -> [StgArg] -> Code
46 -- Here's the code we generate for a tail call. (NB there may be no
47 -- arguments, in which case this boils down to just entering a variable.)
49 -- * Put args in the top locations of the stack.
50 -- * Adjust the stack ptr
51 -- * Make R1 point to the function closure if necessary.
52 -- * Perform the call.
54 -- Things to be careful about:
56 -- * Don't overwrite stack locations before you have finished with
57 -- them (remember you need the function and the as-yet-unmoved
59 -- * Preferably, generate no code to replace x by x on the stack (a
60 -- common situation in tail-recursion).
61 -- * Adjust the stack high water mark appropriately.
63 -- Treat unboxed locals exactly like literals (above) except use the addr
64 -- mode for the local instead of (CLit lit) in the assignment.
67 = do { fun_info <- getCgIdInfo fun
69 ; if isUnLiftedType (idType fun)
70 then -- Primitive return
72 do { fun_amode <- idInfoToAmode fun_info
73 ; performPrimReturn (cgIdInfoArgRep fun_info) fun_amode }
75 else -- Normal case, fun is boxed
76 do { arg_amodes <- getArgAmodes args
77 ; performTailCall fun_info arg_amodes noStmts }
81 -- -----------------------------------------------------------------------------
82 -- The guts of a tail-call
85 :: CgIdInfo -- The function
86 -> [(CgRep,CmmExpr)] -- Args
87 -> CmmStmts -- Pending simultaneous assignments
88 -- *** GUARANTEED to contain only stack assignments.
91 performTailCall fun_info arg_amodes pending_assts
92 | Just join_sp <- maybeLetNoEscape fun_info
93 = -- A let-no-escape is slightly different, because we
94 -- arrange the stack arguments into pointers and non-pointers
95 -- to make the heap check easier. The tail-call sequence
96 -- is very similar to returning an unboxed tuple, so we
98 do { (final_sp, arg_assts) <- pushUnboxedTuple join_sp arg_amodes
99 ; emitSimultaneously (pending_assts `plusStmts` arg_assts)
100 ; let lbl = enterReturnPtLabel (idUnique (cgIdInfoId fun_info))
101 ; doFinalJump final_sp True {- Is LNE -} (jumpToLbl lbl) }
104 = do { fun_amode <- idInfoToAmode fun_info
105 ; let assignSt = CmmAssign nodeReg fun_amode
106 node_asst = oneStmt assignSt
107 opt_node_asst | nodeMustPointToIt lf_info = node_asst
108 | otherwise = noStmts
109 ; EndOfBlockInfo sp _ <- getEndOfBlockInfo
111 ; case (getCallMethod fun_name fun_has_cafs lf_info (length arg_amodes)) of
113 -- Node must always point to things we enter
115 { emitSimultaneously (node_asst `plusStmts` pending_assts)
116 ; let target = entryCode (closureInfoPtr (CmmReg nodeReg))
117 enterClosure = stmtC (CmmJump target [])
118 -- If this is a scrutinee
119 -- let's check if the closure is a constructor
120 -- so we can directly jump to the alternatives switch
122 jumpInstr = getEndOfBlockInfo >>=
123 maybeSwitchOnCons enterClosure
124 ; doFinalJump sp False jumpInstr }
126 -- A function, but we have zero arguments. It is already in WHNF,
127 -- so we can just return it.
128 -- As with any return, Node must point to it.
130 { emitSimultaneously (node_asst `plusStmts` pending_assts)
131 ; doFinalJump sp False emitReturnInstr }
133 -- A real constructor. Don't bother entering it,
134 -- just do the right sort of return instead.
135 -- As with any return, Node must point to it.
137 { emitSimultaneously (node_asst `plusStmts` pending_assts)
138 ; doFinalJump sp False emitReturnInstr }
141 { emitSimultaneously (opt_node_asst `plusStmts` pending_assts)
142 ; doFinalJump sp False (jumpToLbl lbl) }
144 -- A slow function call via the RTS apply routines
145 -- Node must definitely point to the thing
147 { when (not (null arg_amodes)) $ do
148 { if (isKnownFun lf_info)
149 then tickyKnownCallTooFewArgs
150 else tickyUnknownCall
151 ; tickySlowCallPat (map fst arg_amodes)
154 ; let (apply_lbl, args, extra_args)
155 = constructSlowCall arg_amodes
157 ; directCall sp apply_lbl args extra_args
158 (node_asst `plusStmts` pending_assts)
162 -- A direct function call (possibly with some left-over arguments)
163 DirectEntry lbl arity -> do
164 { if arity == length arg_amodes
165 then tickyKnownCallExact
166 else do tickyKnownCallExtraArgs
167 tickySlowCallPat (map fst (drop arity arg_amodes))
170 -- The args beyond the arity go straight on the stack
171 (arity_args, extra_args) = splitAt arity arg_amodes
173 ; directCall sp lbl arity_args extra_args
174 (opt_node_asst `plusStmts` pending_assts)
178 fun_id = cgIdInfoId fun_info
179 fun_name = idName fun_id
180 lf_info = cgIdInfoLF fun_info
181 fun_has_cafs = idCafInfo fun_id
182 untag_node = CmmAssign nodeReg (cmmUntag (CmmReg nodeReg))
183 -- Test if closure is a constructor
184 maybeSwitchOnCons enterClosure eob
185 | EndOfBlockInfo _ (CaseAlts lbl _ _) <- eob
186 = do { is_constr <- newLabelC
187 -- Is the pointer tagged?
188 -- Yes, jump to switch statement
189 ; stmtC (CmmCondBranch (cmmIsTagged (CmmReg nodeReg))
191 -- No, enter the closure.
194 ; stmtC (CmmJump (entryCode $ CmmLit (CmmLabel lbl)) [])
197 -- This is a scrutinee for a case expression
198 -- so let's see if we can directly inspect the closure
199 | EndOfBlockInfo _ (CaseAlts lbl _ _ _) <- eob
200 = do { no_cons <- newLabelC
201 -- Both the NCG and gcc optimize away the temp
202 ; z <- newTemp wordRep
203 ; stmtC (CmmAssign z tag_expr)
205 -- Is the closure a cons?
206 ; stmtC (CmmCondBranch (cond1 tag) no_cons)
207 ; stmtC (CmmCondBranch (cond2 tag) no_cons)
208 -- Yes, jump to switch statement
209 ; stmtC (CmmJump (CmmLit (CmmLabel lbl)) [])
211 -- No, enter the closure.
215 -- No case expression involved, enter the closure.
217 = do { stmtC untag_node
221 --cond1 tag = cmmULtWord tag lowCons
222 -- More efficient than the above?
224 tag_expr = cmmGetClosureType (CmmReg nodeReg)
225 cond1 tag = cmmEqWord tag (CmmLit (mkIntCLit 0))
226 cond2 tag = cmmUGtWord tag highCons
227 lowCons = CmmLit (mkIntCLit 1)
229 highCons = CmmLit (mkIntCLit 8)
230 -- CONSTR_NOCAF_STATIC (from ClosureType.h)
233 directCall :: VirtualSpOffset -> CLabel -> [(CgRep, CmmExpr)]
234 -> [(CgRep, CmmExpr)] -> CmmStmts
236 directCall sp lbl args extra_args assts = do
238 -- First chunk of args go in registers
239 (reg_arg_amodes, stk_args) = assignCallRegs args
241 -- Any "extra" arguments are placed in frames on the
242 -- stack after the other arguments.
243 slow_stk_args = slowArgs extra_args
245 reg_assts = assignToRegs reg_arg_amodes
247 (final_sp, stk_assts) <- mkStkAmodes sp (stk_args ++ slow_stk_args)
249 emitSimultaneously (reg_assts `plusStmts`
250 stk_assts `plusStmts`
253 doFinalJump final_sp False (jumpToLbl lbl)
255 -- -----------------------------------------------------------------------------
256 -- The final clean-up before we do a jump at the end of a basic block.
257 -- This code is shared by tail-calls and returns.
259 doFinalJump :: VirtualSpOffset -> Bool -> Code -> Code
260 doFinalJump final_sp is_let_no_escape jump_code
261 = do { -- Adjust the high-water mark if necessary
262 adjustStackHW final_sp
264 -- Push a return address if necessary (after the assignments
265 -- above, in case we clobber a live stack location)
267 -- DONT push the return address when we're about to jump to a
268 -- let-no-escape: the final tail call in the let-no-escape
270 ; eob <- getEndOfBlockInfo
271 ; whenC (not is_let_no_escape) (pushReturnAddress eob)
273 -- Final adjustment of Sp/Hp
274 ; adjustSpAndHp final_sp
279 -- ----------------------------------------------------------------------------
280 -- A general return (just a special case of doFinalJump, above)
282 performReturn :: Code -- The code to execute to actually do the return
285 performReturn finish_code
286 = do { EndOfBlockInfo args_sp _sequel <- getEndOfBlockInfo
287 ; doFinalJump args_sp False{-not a LNE-} finish_code }
289 -- ----------------------------------------------------------------------------
291 -- Just load the return value into the right register, and return.
293 performPrimReturn :: CgRep -> CmmExpr -- The thing to return
295 performPrimReturn rep amode
296 = do { whenC (not (isVoidArg rep))
297 (stmtC (CmmAssign ret_reg amode))
298 ; performReturn emitReturnInstr }
300 ret_reg = dataReturnConvPrim rep
302 -- ---------------------------------------------------------------------------
303 -- Unboxed tuple returns
305 -- These are a bit like a normal tail call, except that:
307 -- - The tail-call target is an info table on the stack
309 -- - We separate stack arguments into pointers and non-pointers,
310 -- to make it easier to leave things in a sane state for a heap check.
311 -- This is OK because we can never partially-apply an unboxed tuple,
312 -- unlike a function. The same technique is used when calling
313 -- let-no-escape functions, because they also can't be partially
316 returnUnboxedTuple :: [(CgRep, CmmExpr)] -> Code
317 returnUnboxedTuple amodes
318 = do { (EndOfBlockInfo args_sp _sequel) <- getEndOfBlockInfo
319 ; tickyUnboxedTupleReturn (length amodes)
320 ; (final_sp, assts) <- pushUnboxedTuple args_sp amodes
321 ; emitSimultaneously assts
322 ; doFinalJump final_sp False{-not a LNE-} emitReturnInstr }
324 pushUnboxedTuple :: VirtualSpOffset -- Sp at which to start pushing
325 -> [(CgRep, CmmExpr)] -- amodes of the components
326 -> FCode (VirtualSpOffset, -- final Sp
327 CmmStmts) -- assignments (regs+stack)
329 pushUnboxedTuple sp []
330 = return (sp, noStmts)
331 pushUnboxedTuple sp amodes
332 = do { let (reg_arg_amodes, stk_arg_amodes) = assignReturnRegs amodes
334 -- separate the rest of the args into pointers and non-pointers
335 (ptr_args, nptr_args) = separateByPtrFollowness stk_arg_amodes
336 reg_arg_assts = assignToRegs reg_arg_amodes
338 -- push ptrs, then nonptrs, on the stack
339 ; (ptr_sp, ptr_assts) <- mkStkAmodes sp ptr_args
340 ; (final_sp, nptr_assts) <- mkStkAmodes ptr_sp nptr_args
342 ; returnFC (final_sp,
343 reg_arg_assts `plusStmts`
344 ptr_assts `plusStmts` nptr_assts) }
347 -- -----------------------------------------------------------------------------
348 -- Returning unboxed tuples. This is mainly to support _ccall_GC_, where
349 -- we want to do things in a slightly different order to normal:
351 -- - push return address
352 -- - adjust stack pointer
353 -- - r = call(args...)
354 -- - assign regs for unboxed tuple (usually just R1 = r)
355 -- - return to continuation
357 -- The return address (i.e. stack frame) must be on the stack before
358 -- doing the call in case the call ends up in the garbage collector.
360 -- Sadly, the information about the continuation is lost after we push it
361 -- (in order to avoid pushing it again), so we end up doing a needless
362 -- indirect jump (ToDo).
364 ccallReturnUnboxedTuple :: [(CgRep, CmmExpr)] -> Code -> Code
365 ccallReturnUnboxedTuple amodes before_jump
366 = do { eob@(EndOfBlockInfo args_sp _) <- getEndOfBlockInfo
368 -- Push a return address if necessary
369 ; pushReturnAddress eob
370 ; setEndOfBlockInfo (EndOfBlockInfo args_sp OnStack)
371 (do { adjustSpAndHp args_sp
373 ; returnUnboxedTuple amodes })
376 -- -----------------------------------------------------------------------------
377 -- Calling an out-of-line primop
379 tailCallPrimOp :: PrimOp -> [StgArg] -> Code
380 tailCallPrimOp op args
381 = do { -- We're going to perform a normal-looking tail call,
382 -- except that *all* the arguments will be in registers.
383 -- Hence the ASSERT( null leftovers )
384 arg_amodes <- getArgAmodes args
385 ; let (arg_regs, leftovers) = assignPrimOpCallRegs arg_amodes
386 jump_to_primop = jumpToLbl (mkRtsPrimOpLabel op)
388 ; ASSERT(null leftovers) -- no stack-resident args
389 emitSimultaneously (assignToRegs arg_regs)
391 ; EndOfBlockInfo args_sp _ <- getEndOfBlockInfo
392 ; doFinalJump args_sp False{-not a LNE-} jump_to_primop }
394 -- -----------------------------------------------------------------------------
397 -- We always push the return address just before performing a tail call
398 -- or return. The reason we leave it until then is because the stack
399 -- slot that the return address is to go into might contain something
402 -- If the end of block info is 'CaseAlts', then we're in the scrutinee of a
403 -- case expression and the return address is still to be pushed.
405 -- There are cases where it doesn't look necessary to push the return
406 -- address: for example, just before doing a return to a known
407 -- continuation. However, the continuation will expect to find the
408 -- return address on the stack in case it needs to do a heap check.
410 pushReturnAddress :: EndOfBlockInfo -> Code
412 pushReturnAddress (EndOfBlockInfo args_sp (CaseAlts lbl _ _))
413 = do { sp_rel <- getSpRelOffset args_sp
414 ; stmtC (CmmStore sp_rel (mkLblExpr lbl)) }
416 pushReturnAddress _ = nopC
418 -- -----------------------------------------------------------------------------
421 jumpToLbl :: CLabel -> Code
422 -- Passes no argument to the destination procedure
423 jumpToLbl lbl = stmtC (CmmJump (CmmLit (CmmLabel lbl)) [{- No args -}])
425 assignToRegs :: [(CmmExpr, GlobalReg)] -> CmmStmts
426 assignToRegs reg_args
427 = mkStmts [ CmmAssign (CmmGlobal reg_id) expr
428 | (expr, reg_id) <- reg_args ]
432 %************************************************************************
434 \subsection[CgStackery-adjust]{Adjusting the stack pointers}
436 %************************************************************************
438 This function adjusts the stack and heap pointers just before a tail
439 call or return. The stack pointer is adjusted to its final position
440 (i.e. to point to the last argument for a tail call, or the activation
441 record for a return). The heap pointer may be moved backwards, in
442 cases where we overallocated at the beginning of the basic block (see
443 CgCase.lhs for discussion).
445 These functions {\em do not} deal with high-water-mark adjustment.
446 That's done by functions which allocate stack space.
449 adjustSpAndHp :: VirtualSpOffset -- New offset for Arg stack ptr
451 adjustSpAndHp newRealSp
452 = do { -- Adjust stack, if necessary.
453 -- NB: the conditional on the monad-carried realSp
454 -- is out of line (via codeOnly), to avoid a black hole
455 ; new_sp <- getSpRelOffset newRealSp
456 ; checkedAbsC (CmmAssign spReg new_sp) -- Will generate no code in the case
457 ; setRealSp newRealSp -- where realSp==newRealSp
459 -- Adjust heap. The virtual heap pointer may be less than the real Hp
460 -- because the latter was advanced to deal with the worst-case branch
461 -- of the code, and we may be in a better-case branch. In that case,
462 -- move the real Hp *back* and retract some ticky allocation count.
463 ; hp_usg <- getHpUsage
464 ; let rHp = realHp hp_usg
466 ; new_hp <- getHpRelOffset vHp
467 ; checkedAbsC (CmmAssign hpReg new_hp) -- Generates nothing when vHp==rHp
468 ; tickyAllocHeap (vHp - rHp) -- ...ditto