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%
% (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
%
% $Id: CgLetNoEscape.lhs,v 1.26 2004/09/30 10:35:47 simonpj Exp $
%
%********************************************************
%* *
\section[CgLetNoEscape]{Handling ``let-no-escapes''}
%* *
%********************************************************
\begin{code}
module CgLetNoEscape ( cgLetNoEscapeClosure ) where
#include "HsVersions.h"
import {-# SOURCE #-} CgExpr ( cgExpr )
import StgSyn
import CgMonad
import CgBindery ( CgIdInfo, letNoEscapeIdInfo, nukeDeadBindings )
import CgCase ( restoreCurrentCostCentre )
import CgCon ( bindUnboxedTupleComponents )
import CgHeapery ( unbxTupleHeapCheck )
import CgInfoTbls ( emitDirectReturnTarget )
import CgStackery ( allocStackTop, deAllocStackTop, getSpRelOffset )
import Cmm ( CmmStmt(..) )
import CmmUtils ( mkLblExpr, oneStmt )
import CLabel ( mkReturnInfoLabel )
import ClosureInfo ( mkLFLetNoEscape )
import CostCentre ( CostCentreStack )
import Id ( Id, idName )
import Var ( idUnique )
import SMRep ( retAddrSizeW )
import BasicTypes ( RecFlag(..) )
import Outputable
\end{code}
%************************************************************************
%* *
\subsection[what-is-non-escaping]{What {\em is} a ``non-escaping let''?}
%* *
%************************************************************************
[The {\em code} that detects these things is elsewhere.]
Consider:
\begin{verbatim}
let x = fvs \ args -> e
in
if ... then x else
if ... then x else ...
\end{verbatim}
@x@ is used twice (so we probably can't unfold it), but when it is
entered, the stack is deeper than it was when the definition of @x@
happened. Specifically, if instead of allocating a closure for @x@,
we saved all @x@'s fvs on the stack, and remembered the stack depth at
that moment, then whenever we enter @x@ we can simply set the stack
pointer(s) to these remembered (compile-time-fixed) values, and jump
to the code for @x@.
All of this is provided x is:
\begin{enumerate}
\item
non-updatable;
\item
guaranteed to be entered before the stack retreats -- ie x is not
buried in a heap-allocated closure, or passed as an argument to something;
\item
all the enters have exactly the right number of arguments,
no more no less;
\item
all the enters are tail calls; that is, they return to the
caller enclosing the definition of @x@.
\end{enumerate}
Under these circumstances we say that @x@ is {\em non-escaping}.
An example of when (4) does {\em not} hold:
\begin{verbatim}
let x = ...
in case x of ...alts...
\end{verbatim}
Here, @x@ is certainly entered only when the stack is deeper than when
@x@ is defined, but here it must return to \tr{...alts...} So we can't
just adjust the stack down to @x@'s recalled points, because that
would lost @alts@' context.
Things can get a little more complicated. Consider:
\begin{verbatim}
let y = ...
in let x = fvs \ args -> ...y...
in ...x...
\end{verbatim}
Now, if @x@ is used in a non-escaping way in \tr{...x...}, {\em and}
@y@ is used in a non-escaping way in \tr{...y...}, {\em then} @y@ is
non-escaping.
@x@ can even be recursive! Eg:
\begin{verbatim}
letrec x = [y] \ [v] -> if v then x True else ...
in
...(x b)...
\end{verbatim}
%************************************************************************
%* *
\subsection[codeGen-for-non-escaping]{Generating code for a ``non-escaping let''}
%* *
%************************************************************************
Generating code for this is fun. It is all very very similar to what
we do for a case expression. The duality is between
\begin{verbatim}
let-no-escape x = b
in e
\end{verbatim}
and
\begin{verbatim}
case e of ... -> b
\end{verbatim}
That is, the RHS of @x@ (ie @b@) will execute {\em later}, just like
the alternative of the case; it needs to be compiled in an environment
in which all volatile bindings are forgotten, and the free vars are
bound only to stable things like stack locations.. The @e@ part will
execute {\em next}, just like the scrutinee of a case.
First, we need to save all @x@'s free vars
on the stack, if they aren't there already.
\begin{code}
cgLetNoEscapeClosure
:: Id -- binder
-> CostCentreStack -- NB: *** NOT USED *** ToDo (WDP 94/06)
-> StgBinderInfo -- NB: ditto
-> SRT
-> StgLiveVars -- variables live in RHS, including the binders
-- themselves in the case of a recursive group
-> EndOfBlockInfo -- where are we going to?
-> Maybe VirtualSpOffset -- Slot for current cost centre
-> RecFlag -- is the binding recursive?
-> [Id] -- args (as in \ args -> body)
-> StgExpr -- body (as in above)
-> FCode (Id, CgIdInfo)
-- ToDo: deal with the cost-centre issues
cgLetNoEscapeClosure
bndr cc binder_info srt full_live_in_rhss
rhs_eob_info cc_slot rec args body
= let
arity = length args
lf_info = mkLFLetNoEscape arity
in
-- saveVolatileVarsAndRegs done earlier in cgExpr.
do { (vSp, _) <- forkEvalHelp rhs_eob_info
(do { allocStackTop retAddrSizeW
; nukeDeadBindings full_live_in_rhss })
(do { deAllocStackTop retAddrSizeW
; abs_c <- forkProc $ cgLetNoEscapeBody bndr cc
cc_slot args body
-- Ignore the label that comes back from
-- mkRetDirectTarget. It must be conjured up elswhere
; emitDirectReturnTarget (idName bndr) abs_c srt
; return () })
; returnFC (bndr, letNoEscapeIdInfo bndr vSp lf_info) }
\end{code}
\begin{code}
cgLetNoEscapeBody :: Id -- Name of the joint point
-> CostCentreStack
-> Maybe VirtualSpOffset
-> [Id] -- Args
-> StgExpr -- Body
-> Code
cgLetNoEscapeBody bndr cc cc_slot all_args body = do
{ (arg_regs, ptrs, nptrs, ret_slot) <- bindUnboxedTupleComponents all_args
-- restore the saved cost centre. BUT: we must not free the stack slot
-- containing the cost centre, because it might be needed for a
-- recursive call to this let-no-escape.
; restoreCurrentCostCentre cc_slot False{-don't free-}
-- Enter the closures cc, if required
; -- enterCostCentreCode closure_info cc IsFunction
-- The "return address" slot doesn't have a return address in it;
-- but the heap-check needs it filled in if the heap-check fails.
-- So we pass code to fill it in to the heap-check macro
; sp_rel <- getSpRelOffset ret_slot
; let lbl = mkReturnInfoLabel (idUnique bndr)
frame_hdr_asst = oneStmt (CmmStore sp_rel (mkLblExpr lbl))
-- Do heap check [ToDo: omit for non-recursive case by recording in
-- in envt and absorbing at call site]
; unbxTupleHeapCheck arg_regs ptrs nptrs frame_hdr_asst
(cgExpr body)
}
\end{code}
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