File:  [gforth] / gforth / glocals.fs
Revision 1.1: download - view: text, annotated - select for diffs
Sat May 7 14:55:56 1994 UTC (25 years, 7 months ago) by anton
Branches: MAIN
CVS tags: HEAD
local variables
rewrote primitives2c.el in Forth (prims2x.el)
various small changes
Added Files:
 	from-cut-here gforth.el gforth.texi glocals.fs gray.fs
 	locals-test.fs prims2x.fs

\ Local variables are quite important for writing readable programs, but
\ IMO (anton) they are the worst part of the standard. There they are very
\ restricted and have an ugly interface.

\ So, we implement the locals wordset, but do not recommend using
\ locals-ext (which is a really bad user interface for locals).

\ We also have a nice and powerful user-interface for locals: locals are
\ defined with

\ { local1 local2 ... }
\ or
\ { local1 local2 ... -- ... }
\ (anything after the -- is just a comment)

\ Every local in this list consists of an optional type specification
\ and a name. If there is only the name, it stands for a cell-sized
\ value (i.e., you get the value of the local variable, not it's
\ address). The following type specifiers stand before the name:

\ Specifier	Type	Access
\ W:		Cell	value
\ W^		Cell	address
\ D:		Double	value
\ D^		Double	address
\ F:		Float	value
\ F^		Float	address
\ C:		Char	value
\ C^		Char	address

\ The local variables are initialized with values from the appropriate
\ stack. In contrast to the examples in the standard document our locals
\ take the arguments in the expected way: The last local gets the top of
\ stack, the second last gets the second stack item etc. An example:

\ : CX* { F: Ar  F: Ai  F: Br  F: Bi -- Cr Ci }
\ \ complex multiplication
\  Ar Br f* Ai Bi f* f-
\  Ar Bi f* Ai Br f* f+ ;

\ There will also be a way to add user types, but it is not yet decided,
\ how. Ideas are welcome.

\ Locals defined in this manner live until (!! see below). 
\ Their names can be used during this time to get
\ their value or address; The addresses produced in this way become
\ invalid at the end of the lifetime.

\ Values can be changed with TO, but this is not recomended (TO is a
\ kludge and words lose the single-assignment property, which makes them
\ harder to analyse).

\ As for the internals, we use a special locals stack. This eliminates
\ the problems and restrictions of reusing the return stack and allows
\ to store floats as locals: the return stack is not guaranteed to be
\ aligned correctly, but our locals stack must be float-aligned between
\ words.

\ Other things about the internals are pretty unclear now.

\ Currently locals may only be
\ defined at the outer level and TO is not supported.

include float.fs
include search-order.fs

\ the locals stack grows downwards (see primitives)
\ of the local variables of a group (in braces) the leftmost is on top,
\ i.e. by going onto the locals stack the order is reversed.
\ there are alignment gaps if necessary.
\ lp must have the strictest alignment (usually float) across calls;
\ for simplicity we align it strictly for every group.

vocabulary locals \ this contains the local variables
' locals >body Constant locals-list \ acts like a variable that contains
				    \ a linear list of locals names

create locals-buffer 1000 allot \ !! limited and unsafe
    \ here the names of the local variables are stored
    \ we would have problems storing them at the normal dp

variable locals-dp \ so here's the special dp for locals.

: alignlp-w ( n1 -- n2 )
    \ cell-align size and generate the corresponding code for aligning lp
    dup aligned tuck - compile-lp+!# ;

: alignlp-f ( n1 -- n2 )
    dup faligned tuck - compile-lp+!# ;

\ a local declaration group (the braces stuff) is compiled by calling
\ the appropriate compile-pushlocal for the locals, starting with the
\ righmost local; the names are already created earlier, the
\ compile-pushlocal just inserts the offsets from the frame base.

: compile-pushlocal-w ( a-addr -- ) ( run-time: w -- )
\ compiles a push of a local variable, and adjusts locals-size
\ stores the offset of the local variable to a-addr
    locals-size @ alignlp-w cell+ dup locals-size !
    swap !
    postpone >l ;

: compile-pushlocal-f ( a-addr -- ) ( run-time: f -- )
    locals-size @ alignlp-f float+ dup locals-size !
    swap !
    postpone f>l ;

: compile-pushlocal-d ( a-addr -- ) ( run-time: w1 w2 -- )
    locals-size @ alignlp-w cell+ cell+ dup locals-size !
    swap !
    postpone swap postpone >l postpone >l ;

: compile-pushlocal-c ( a-addr -- ) ( run-time: w -- )
    -1 chars compile-lp+!#
    locals-size @ swap !
    postpone lp@ postpone c! ;

: create-local ( " name" -- a-addr )
	\ defines the local "name"; the offset of the local shall be stored in a-addr
    create
	immediate
	here 0 , ( place for the offset ) ;

: lp-offset, ( n -- )
\ converts the offset from the frame start to an offset from lp and
\ adds it as inline argument to a preceding locals primitive
\ i.e., the address of the local is lp+locals_size-offset
  locals-size @ swap - , ;

vocabulary locals-types \ this contains all the type specifyers, -- and }
locals-types definitions

: W:
    create-local ( "name" -- a-addr xt )
	\ xt produces the appropriate locals pushing code when executed
	['] compile-pushlocal-w
    does> ( Compilation: -- ) ( Run-time: -- w )
        \ compiles a local variable access
	postpone @local# @ lp-offset, ;

: W^
    create-local ( "name" -- a-addr xt )
	['] compile-pushlocal-w
    does> ( Compilation: -- ) ( Run-time: -- w )
	postpone laddr# @ lp-offset, ;

: F:
    create-local ( "name" -- a-addr xt )
	['] compile-pushlocal-f
    does> ( Compilation: -- ) ( Run-time: -- w )
	postpone f@local# @ lp-offset, ;

: F^
    create-local ( "name" -- a-addr xt )
	['] compile-pushlocal-f
    does> ( Compilation: -- ) ( Run-time: -- w )
	postpone laddr# @ lp-offset, ;

: D:
    create-local ( "name" -- a-addr xt )
	['] compile-pushlocal-d
    does> ( Compilation: -- ) ( Run-time: -- w )
	postpone laddr# @ lp-offset, postpone 2@ ;

: D^
    create-local ( "name" -- a-addr xt )
	['] compile-pushlocal-d
    does> ( Compilation: -- ) ( Run-time: -- w )
	postpone laddr# @ lp-offset, ;

: C:
    create-local ( "name" -- a-addr xt )
	['] compile-pushlocal-c
    does> ( Compilation: -- ) ( Run-time: -- w )
	postpone laddr# @ lp-offset, postpone c@ ;

: C^
    create-local ( "name" -- a-addr xt )
	['] compile-pushlocal-c
    does> ( Compilation: -- ) ( Run-time: -- w )
	postpone laddr# @ lp-offset, ;

\ you may want to make comments in a locals definitions group:
' \ alias \ immediate
' ( alias ( immediate

forth definitions

\ the following gymnastics are for declaring locals without type specifier.
\ we exploit a feature of our dictionary: every wordlist
\ has it's own methods for finding words etc.
\ So we create a vocabulary new-locals, that creates a 'w:' local named x
\ when it is asked if it contains x.

0. 2constant last-local \ !! actually a 2value

also locals-types

: new-locals-find ( caddr u w -- nfa )
\ this is the find method of the new-locals vocabulary
\ make a new local with name caddr u; w is ignored
\ the returned nfa denotes a word that produces what W: produces
\ !! do the whole thing without nextname
    drop nextname W: \ we don't want the thing that W: produces,
    ['] last-local >body 2!   \ but the nfa of a word that produces that value: last-local
    [ ' last-local >name ] Aliteral ;

previous

: new-locals-reveal ( -- )
  true abort" this should not happen: new-locals-reveal" ;

create new-locals-map ' new-locals-find A, ' new-locals-reveal A,

vocabulary new-locals
new-locals-map ' new-locals >body cell+ A! \ !! use special access words

variable old-dpp

\ and now, finally, the user interface words
: { ( -- addr wid 0 )
    dp old-dpp !
    locals-dp dpp !
    also new-locals
    also get-current locals definitions  locals-types
    0 TO locals-wordlist
    0 postpone [ ; immediate

locals-types definitions

: } ( addr wid 0 a-addr1 xt1 ... -- )
    \ ends locals definitions
    ] old-dpp @ dpp !
    begin
	dup
    while
	execute
    repeat
    drop
    locals-size @ alignlp-f locals-size ! \ the strictest alignment
    set-current
    previous previous
    locals-list TO locals-wordlist ;

: -- ( addr wid 0 ... -- )
    }
    [char] } word drop ;

forth definitions

\ A few thoughts on automatic scopes for locals and how they can be
\ implemented:

\ We have to combine locals with the control structures. My basic idea
\ was to start the life of a local at the declaration point. The life
\ would end at any control flow join (THEN, BEGIN etc.) where the local
\ is lot live on both input flows (note that the local can still live in
\ other, later parts of the control flow). This would make a local live
\ as long as you expected and sometimes longer (e.g. a local declared in
\ a BEGIN..UNTIL loop would still live after the UNTIL).

\ The following example illustrates the problems of this approach:

\ { z }
\ if
\   { x }
\ begin
\   { y }
\ [ 1 cs-roll ] then
\   ...
\ until

\ x lives only until the BEGIN, but the compiler does not know this
\ until it compiles the UNTIL (it can deduce it at the THEN, because at
\ that point x lives in no thread, but that does not help much). This is
\ solved by optimistically assuming at the BEGIN that x lives, but
\ warning at the UNTIL that it does not. The user is then responsible
\ for checking that x is only used where it lives.

\ The produced code might look like this (leaving out alignment code):

\ >l ( z )
\ ?branch <then>
\ >l ( x )
\ <begin>:
\ >l ( y )
\ lp+!# 8 ( RIP: x,y )
\ <then>:
\ ...
\ lp+!# -4 ( adjust lp to <begin> state )
\ ?branch <begin>
\ lp+!# 4 ( undo adjust )

\ The BEGIN problem also has another incarnation:

\ AHEAD
\ BEGIN
\   x
\ [ 1 CS-ROLL ] THEN
\   { x }
\   ...
\ UNTIL

\ should be legal: The BEGIN is not a control flow join in this case,
\ since it cannot be entered from the top; therefore the definition of x
\ dominates the use. But the compiler processes the use first, and since
\ it does not look ahead to notice the definition, it will complain
\ about it. Here's another variation of this problem:

\ IF
\   { x }
\ ELSE
\   ...
\ AHEAD
\ BEGIN
\   x
\ [ 2 CS-ROLL ] THEN
\   ...
\ UNTIL

\ In this case x is defined before the use, and the definition dominates
\ the use, but the compiler does not know this until it processes the
\ UNTIL. So what should the compiler assume does live at the BEGIN, if
\ the BEGIN is not a control flow join? The safest assumption would be
\ the intersection of all locals lists on the control flow
\ stack. However, our compiler assumes that the same variables are live
\ as on the top of the control flow stack. This covers the following case:

\ { x }
\ AHEAD
\ BEGIN
\   x
\ [ 1 CS-ROLL ] THEN
\   ...
\ UNTIL

\ If this assumption is too optimistic, the compiler will warn the user.

\ Implementation:

\ orig, dest and do-sys have the following structure:
\ address (of the branch or the instruction to be branched to) (TOS)
\ locals-list (valid at address) (second)
\ locals-size (at address; this could be computed from locals-list, but so what) (third)

3 constant cs-item-size

: CS-PICK ( ... u -- ... destu )
 1+ cs-item-size * 1- >r
 r@ pick  r@ pick  r@ pick
 rdrop ;

: CS-ROLL ( destu/origu .. dest0/orig0 u -- .. dest0/orig0 destu/origu )
 1+ cs-item-size * 1- >r
 r@ roll r@ roll r@ roll
 rdrop ; 

: CS-PUSH ( -- dest/orig )
 locals-size @
 locals-list @
 here ;

: BUT       sys? 1 cs-roll ;                      immediate restrict
: YET       sys? 0 cs-pick ;                       immediate restrict

: common-list ( list1 list2 -- list3 )
\ list1 and list2 are lists, where the heads are at higher addresses than
\ the tail. list3 is the largest sublist of both lists.
 begin
   2dup u<>
 while
   2dup u>
   if
     swap
   endif
   @
 repeat
 drop ;

: sub-list? ( list1 list2 -- f )
\ true iff list1 is a sublist of list2
 begin
   2dup u<
 while
   @
 repeat
 = ;

: list-size ( list -- u )
\ size of the locals frame represented by list
 0 ( list n )
 begin
   over 0<>
 while
   over
   cell+ name> >body @ max
   swap @ swap ( get next )
 repeat
 faligned nip ;

: x>mark ( -- orig )
 cs-push 0 , ;

variable dead-code \ true if normal code at "here" would be dead

: unreachable ( -- )
\ declares the current point of execution as unreachable and
\ prepares the assumptions for a possible upcoming BEGIN
 dead-code on
 dup 0<> if
   2 pick 2 pick
 else
   0 0
 endif
 locals-list !
 locals-size ! ;

: check-begin ( list -- )
\ warn if list is not a sublist of locals-list
 locals-list @ sub-list? 0= if
   \ !! print current position
   ." compiler was overly optimistic about locals at a BEGIN" cr
   \ !! print assumption and reality
 endif ;

: xahead ( -- orig )
 POSTPONE branch x>mark unreachable ; immediate

: xif ( -- orig )
 POSTPONE ?branch x>mark ; immediate

\ THEN (another control flow from before joins the current one):
\ The new locals-list is the intersection of the current locals-list and
\ the orig-local-list. The new locals-size is the (alignment-adjusted)
\ size of the new locals-list. The following code is generated:
\ lp+!# (current-locals-size - orig-locals-size)
\ <then>:
\ lp+!# (orig-locals-size - new-locals-size)

\ Of course "lp+!# 0" is not generated. Still this is admittedly a bit
\ inefficient, e.g. if there is a locals declaration between IF and
\ ELSE. However, if ELSE generates an appropriate "lp+!#" before the
\ branch, there will be none after the target <then>.
: xthen ( orig -- )
 sys? dup @ ?struc
 dead-code @
 if
   >resolve
   locals-list !
   locals-size !
 else
   locals-size @ 3 roll - compile-lp+!#
   >resolve
   locals-list @ common-list locals-list !
   locals-size @  locals-list @ list-size - compile-lp+!#
 endif
 dead-code off ; immediate

: scope ( -- dest )
 cs-push ; immediate

: endscope ( dest -- )
 drop
 locals-list @ common-list locals-list !
 locals-size @  locals-list @ list-size - compile-lp+!#
 drop ; immediate

: xexit ( -- )
    locals-size @ compile-lp+!# POSTPONE exit unreachable ; immediate

: x?exit ( -- )
    POSTPONE xif POSTPONE xexit POSTPONE xthen ; immediate

: xelse ( orig1 -- orig2 )
 sys?
 POSTPONE xahead
 1 cs-roll
 POSTPONE xthen ; immediate

: xbegin ( -- dest )
 cs-push dead-code off ; immediate

: xwhile ( dest -- orig dest )
 sys?
 POSTPONE xif
 1 cs-roll ; immediate

\ AGAIN (the current control flow joins another, earlier one):
\ If the dest-locals-list is not a subset of the current locals-list,
\ issue a warning (see below). The following code is generated:
\ lp+!# (current-local-size - dest-locals-size)
\ branch <begin>
: xagain ( dest -- )
 sys?
 locals-size @ 3 roll - compile-lp+!#
 POSTPONE branch
 <resolve
 check-begin
 unreachable ; immediate

\ UNTIL (the current control flow may join an earlier one or continue):
\ Similar to AGAIN. The new locals-list and locals-size are the current
\ ones. The following code is generated:
\ lp+!# (current-local-size - dest-locals-size)
\ ?branch <begin>
\ lp+!# (dest-local-size - current-locals-size)
\ (Another inefficiency. Maybe we should introduce a ?branch-lp+!#
\ primitive. This would also solve the interrupt problem)
: until-like ( dest xt -- )
 >r
 sys?
 locals-size @ dup 4 roll - compile-lp+!# ( list dest-addr old-locals-size )
 r> compile,
 >r <resolve
 check-begin
 locals-size @ r> - compile-lp+!# ;

: xuntil ( dest -- )
 ['] ?branch until-like ; immediate

: xrepeat ( orig dest -- )
 3 pick 0= ?struc
 postpone xagain
 postpone xthen ; immediate

\ counted loops

\ leave poses a little problem here
\ we have to store more than just the address of the branch, so the
\ traditional linked list approach is no longer viable.
\ This is solved by storing the information about the leavings in a
\ special stack. The leavings of different DO-LOOPs are separated
\ by a 0 entry

\ !! remove the fixed size limit. 'Tis easy.
20 constant leave-stack-size
create leave-stack leave-stack-size cs-item-size * cells allot
variable leave-sp  leave-stack leave-sp !

: clear-leave-stack ( -- )
 leave-stack leave-sp ! ;

\ : leave-empty? ( -- f )
\  leave-sp @ leave-stack = ;

: >leave ( orig -- )
\ push on leave-stack
 leave-sp @
 dup [ leave-stack leave-stack-size cs-item-size * cells + ] Aliteral >=
 if
   abort" leave-stack full"
 endif
 tuck ! cell+
 tuck ! cell+
 tuck ! cell+
 leave-sp ! ;

: leave> ( -- orig )
\ pop from leave-stack
 leave-sp @
 dup leave-stack <= if
   abort" leave-stack empty"
 endif
 cell - dup @ swap
 cell - dup @ swap
 cell - dup @ swap
 leave-sp ! ;

: done ( -- )
\ !! the original done had ( addr -- )
 begin
   leave>
   dup
 while
   POSTPONE xthen
 repeat
 2drop drop ; immediate

: xleave ( -- )
 POSTPONE xahead
 >leave ; immediate

: x?leave ( -- )
 POSTPONE 0= POSTPONE xif
 >leave ; immediate

: xdo ( -- do-sys )
 POSTPONE (do)
 POSTPONE xbegin
 0 0 0 >leave ; immediate

: x?do ( -- do-sys )
 0 0 0 >leave
 POSTPONE (?do)
 x>mark >leave
 POSTPONE xbegin ; immediate

: xfor ( -- do-sys )
 POSTPONE (for)
 POSTPONE xbegin
 0 0 0 >leave ; immediate

\ LOOP etc. are just like UNTIL
\ the generated code for ?DO ... LOOP with locals is inefficient, this 
\ could be changed by introducing (loop)-lp+!# etc.

: loop-like ( do-sys xt -- )
 until-like  POSTPONE done  POSTPONE unloop ;

: xloop ( do-sys -- )
 ['] (loop) loop-like ; immediate

: x+loop ( do-sys -- )
 ['] (+loop) loop-like ; immediate

: xs+loop ( do-sys -- )
 ['] (s+loop) loop-like ; immediate

: locals-:-hook ( sys -- sys addr xt )
    DEFERS :-hook
    last @ lastcfa @
    clear-leave-stack
    0 locals-size !
    locals-buffer locals-dp !
    0 locals-list ! ; ( clear locals vocabulary )

: locals-;-hook ( sys addr xt -- sys )
    0 TO locals-wordlist
    locals-size @ compile-lp+!#
    lastcfa ! last !
    DEFERS ;-hook ;

' locals-:-hook IS :-hook
' locals-;-hook IS ;-hook

\ The words in the locals dictionary space are not deleted until the end
\ of the current word. This is a bit too conservative, but very simple.

\ There are a few cases to consider: (see above)

\ after AGAIN, AHEAD, EXIT (the current control flow is dead):
\ We have to special-case the above cases against that. In this case the
\ things above are not control flow joins. Everything should be taken
\ over from the live flow. No lp+!# is generated.

\ !! The lp gymnastics for UNTIL are also a real problem: locals cannot be
\ used in signal handlers (or anything else that may be called while
\ locals live beyond the lp) without changing the locals stack.

\ About warning against uses of dead locals. There are several options:

\ 1) Do not complain (After all, this is Forth;-)

\ 2) Additional restrictions can be imposed so that the situation cannot
\ arise; the programmer would have to introduce explicit scoping
\ declarations in cases like the above one. I.e., complain if there are
\ locals that are live before the BEGIN but not before the corresponding
\ AGAIN (replace DO etc. for BEGIN and UNTIL etc. for AGAIN).

\ 3) The real thing: i.e. complain, iff a local lives at a BEGIN, is
\ used on a path starting at the BEGIN, and does not live at the
\ corresponding AGAIN. This is somewhat hard to implement. a) How does
\ the compiler know when it is working on a path starting at a BEGIN
\ (consider "{ x } if begin [ 1 cs-roll ] else x endif again")? b) How
\ is the usage info stored?

\ For now I'll resort to alternative 2. When it produces warnings they
\ will often be spurious, but warnings should be rare. And better
\ spurious warnings now and then than days of bug-searching.

\ Explicit scoping of locals is implemented by cs-pushing the current
\ locals-list and -size (and an unused cell, to make the size equal to
\ the other entries) at the start of the scope, and restoring them at
\ the end of the scope to the intersection, like THEN does.


\ And here's finally the ANS standard stuff

: (local) ( addr u -- )
\ a little space-inefficient, but well deserved ;-)
\ In exchange, there are no restrictions whatsoever on using (local)
 dup
 if
   nextname POSTPONE { [ also locals-types ] W: } [ previous ]
 else
   2drop
 endif ;

\ \ !! untested
\ : TO ( c|w|d|r "name" -- )
\ \ !! state smart
\  0 0 0. 0.0e0 { c: clocal w: wlocal d: dlocal f: flocal }
\  ' dup >definer
\  state @ 
\  if
\    case
\      [ ' locals-wordlist >definer ] literal \ value
\      OF >body POSTPONE Aliteral POSTPONE ! ENDOF
\      [ ' clocal >definer ] literal
\      OF POSTPONE laddr# >body @ lp-offset, POSTPONE c! ENDOF
\      [ ' wlocal >definer ] literal
\      OF POSTPONE laddr# >body @ lp-offset, POSTPONE ! ENDOF
\      [ ' dlocal >definer ] literal
\      OF POSTPONE laddr# >body @ lp-offset, POSTPONE d! ENDOF
\      [ ' flocal >definer ] literal
\      OF POSTPONE laddr# >body @ lp-offset, POSTPONE f! ENDOF
\      abort" can only store TO value or local value"
\    endcase
\  else
\    [ ' locals-wordlist >definer ] literal =
\    if
\      >body !
\    else
\      abort" can only store TO value"
\    endif
\  endif ;   

\ : locals|
\ !! should lie around somewhere

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