2 % (c) The GRASP/AQUA Project, Glasgow University, 1993-1994
4 %********************************************************
6 \section[CgLetNoEscape]{Handling ``let-no-escapes''}
8 %********************************************************
11 module CgLetNoEscape ( cgLetNoEscapeClosure ) where
13 #include "HsVersions.h"
15 import {-# SOURCE #-} CgExpr ( cgExpr )
21 import CgBindery ( letNoEscapeIdInfo, bindArgsToRegs,
22 bindNewToAStack, bindNewToBStack,
25 import CgHeapery ( heapCheck )
26 import CgRetConv ( assignRegs )
27 import CgStackery ( mkVirtStkOffsets )
28 import CgUsages ( setRealAndVirtualSps, getVirtSps )
29 import CLabel ( mkStdEntryLabel )
30 import ClosureInfo ( mkLFLetNoEscape )
31 import CostCentre ( CostCentre )
32 import HeapOffs ( VirtualSpBOffset )
33 import Id ( idPrimRep, Id )
36 %************************************************************************
38 \subsection[what-is-non-escaping]{What {\em is} a ``non-escaping let''?}
40 %************************************************************************
42 [The {\em code} that detects these things is elsewhere.]
46 let x = fvs \ args -> e
49 if ... then x else ...
51 @x@ is used twice (so we probably can't unfold it), but when it is
52 entered, the stack is deeper than it was then the definition of @x@
53 happened. Specifically, if instead of allocating a closure for @x@,
54 we saved all @x@'s fvs on the stack, and remembered the stack depth at
55 that moment, then whenever we enter @x@ we can simply set the stack
56 pointer(s) to these remembered (compile-time-fixed) values, and jump
59 All of this is provided x is:
64 guaranteed to be entered before the stack retreats -- ie x is not
65 buried in a heap-allocated closure, or passed as an argument to something;
67 all the enters have exactly the right number of arguments,
70 all the enters are tail calls; that is, they return to the
71 caller enclosing the definition of @x@.
74 Under these circumstances we say that @x@ is {\em non-escaping}.
76 An example of when (4) does {\em not} hold:
79 in case x of ...alts...
82 Here, @x@ is certainly entered only when the stack is deeper than when
83 @x@ is defined, but here it must return to \tr{...alts...} So we can't
84 just adjust the stack down to @x@'s recalled points, because that
85 would lost @alts@' context.
87 Things can get a little more complicated. Consider:
90 in let x = fvs \ args -> ...y...
94 Now, if @x@ is used in a non-escaping way in \tr{...x...}, {\em and}
95 @y@ is used in a non-escaping way in \tr{...y...}, {\em then} @y@ is
98 @x@ can even be recursive! Eg:
100 letrec x = [y] \ [v] -> if v then x True else ...
106 %************************************************************************
108 \subsection[codeGen-for-non-escaping]{Generating code for a ``non-escaping let''}
110 %************************************************************************
113 Generating code for this is fun. It is all very very similar to what
114 we do for a case expression. The duality is between
124 That is, the RHS of @x@ (ie @b@) will execute {\em later}, just like
125 the alternative of the case; it needs to be compiled in an environment
126 in which all volatile bindings are forgotten, and the free vars are
127 bound only to stable things like stack locations.. The @e@ part will
128 execute {\em next}, just like the scrutinee of a case.
130 First, we need to save all @x@'s free vars
131 on the stack, if they aren't there already.
136 -> CostCentre -- NB: *** NOT USED *** ToDo (WDP 94/06)
137 -> StgBinderInfo -- NB: ditto
138 -> StgLiveVars -- variables live in RHS, including the binders
139 -- themselves in the case of a recursive group
140 -> EndOfBlockInfo -- where are we going to?
141 -> Maybe VirtualSpBOffset -- Slot for current cost centre
142 -> [Id] -- args (as in \ args -> body)
143 -> StgExpr -- body (as in above)
144 -> FCode (Id, CgIdInfo)
146 -- ToDo: deal with the cost-centre issues
148 cgLetNoEscapeClosure binder cc bi full_live_in_rhss rhs_eob_info maybe_cc_slot args body
151 lf_info = mkLFLetNoEscape arity full_live_in_rhss{-used???-}
155 (nukeDeadBindings full_live_in_rhss)
156 (forkAbsC (cgLetNoEscapeBody args body))
157 `thenFC` \ (vA, vB, code) ->
159 label = mkStdEntryLabel binder -- arity
161 absC (CCodeBlock label code) `thenC`
162 returnFC (binder, letNoEscapeIdInfo binder vA vB lf_info)
166 cgLetNoEscapeBody :: [Id] -- Args
170 cgLetNoEscapeBody all_args rhs
171 = getVirtSps `thenFC` \ (vA, vB) ->
173 arg_kinds = map idPrimRep all_args
174 (arg_regs, _) = assignRegs [{-nothing live-}] arg_kinds
175 (reg_args, stk_args) = splitAt (length arg_regs) all_args
177 -- stk_args is the args which are passed on the stack at the fast-entry point
178 -- Using them, we define the stack layout
179 (spA_stk_args, spB_stk_args, stk_bxd_w_offsets, stk_ubxd_w_offsets)
181 vA vB -- Initial virtual SpA, SpB
186 -- Bind args to appropriate regs/stk locns
187 bindArgsToRegs reg_args arg_regs `thenC`
188 mapCs bindNewToAStack stk_bxd_w_offsets `thenC`
189 mapCs bindNewToBStack stk_ubxd_w_offsets `thenC`
190 setRealAndVirtualSps spA_stk_args spB_stk_args `thenC`
192 {- ToDo: NOT SURE ABOUT COST CENTRES!
193 -- Enter the closures cc, if required
194 lexEnterCCcode closure_info maybe_cc `thenC`
197 -- [No need for stack check; forkEvalHelp dealt with that]
199 -- Do heap check [ToDo: omit for non-recursive case by recording in
200 -- in envt and absorbing at call site]
201 heapCheck arg_regs False {- Node doesn't point to it -} (
202 -- heapCheck *encloses* the rest