Annotation of gforth/glocals.fs, revision 1.10

1.1       anton       1: \ Local variables are quite important for writing readable programs, but
                      2: \ IMO (anton) they are the worst part of the standard. There they are very
                      3: \ restricted and have an ugly interface.
                      4: 
                      5: \ So, we implement the locals wordset, but do not recommend using
                      6: \ locals-ext (which is a really bad user interface for locals).
                      7: 
                      8: \ We also have a nice and powerful user-interface for locals: locals are
                      9: \ defined with
                     10: 
                     11: \ { local1 local2 ... }
                     12: \ or
                     13: \ { local1 local2 ... -- ... }
                     14: \ (anything after the -- is just a comment)
                     15: 
                     16: \ Every local in this list consists of an optional type specification
                     17: \ and a name. If there is only the name, it stands for a cell-sized
                     18: \ value (i.e., you get the value of the local variable, not it's
                     19: \ address). The following type specifiers stand before the name:
                     20: 
                     21: \ Specifier    Type    Access
                     22: \ W:           Cell    value
                     23: \ W^           Cell    address
                     24: \ D:           Double  value
                     25: \ D^           Double  address
                     26: \ F:           Float   value
                     27: \ F^           Float   address
                     28: \ C:           Char    value
                     29: \ C^           Char    address
                     30: 
                     31: \ The local variables are initialized with values from the appropriate
                     32: \ stack. In contrast to the examples in the standard document our locals
                     33: \ take the arguments in the expected way: The last local gets the top of
                     34: \ stack, the second last gets the second stack item etc. An example:
                     35: 
                     36: \ : CX* { F: Ar  F: Ai  F: Br  F: Bi -- Cr Ci }
                     37: \ \ complex multiplication
                     38: \  Ar Br f* Ai Bi f* f-
                     39: \  Ar Bi f* Ai Br f* f+ ;
                     40: 
                     41: \ There will also be a way to add user types, but it is not yet decided,
                     42: \ how. Ideas are welcome.
                     43: 
                     44: \ Locals defined in this manner live until (!! see below). 
                     45: \ Their names can be used during this time to get
                     46: \ their value or address; The addresses produced in this way become
                     47: \ invalid at the end of the lifetime.
                     48: 
                     49: \ Values can be changed with TO, but this is not recomended (TO is a
                     50: \ kludge and words lose the single-assignment property, which makes them
                     51: \ harder to analyse).
                     52: 
                     53: \ As for the internals, we use a special locals stack. This eliminates
                     54: \ the problems and restrictions of reusing the return stack and allows
                     55: \ to store floats as locals: the return stack is not guaranteed to be
                     56: \ aligned correctly, but our locals stack must be float-aligned between
                     57: \ words.
                     58: 
                     59: \ Other things about the internals are pretty unclear now.
                     60: 
                     61: \ Currently locals may only be
                     62: \ defined at the outer level and TO is not supported.
                     63: 
1.7       pazsan     64: include search-order.fs
1.1       anton      65: include float.fs
                     66: 
1.10    ! anton      67: : compile-@local ( n -- ) \ new compile-fetch-local
1.3       anton      68:  case
1.7       pazsan     69:     0       of postpone @local0 endof
                     70:     1 cells of postpone @local1 endof
                     71:     2 cells of postpone @local2 endof
                     72:     3 cells of postpone @local3 endof
1.3       anton      73:    ( otherwise ) dup postpone @local# ,
                     74:  endcase ;
                     75: 
1.10    ! anton      76: : compile-f@local ( n -- ) \ new compile-f-fetch-local
1.3       anton      77:  case
1.7       pazsan     78:     0        of postpone f@local0 endof
                     79:     1 floats of postpone f@local1 endof
1.3       anton      80:    ( otherwise ) dup postpone f@local# ,
                     81:  endcase ;
                     82: 
1.1       anton      83: \ the locals stack grows downwards (see primitives)
                     84: \ of the local variables of a group (in braces) the leftmost is on top,
                     85: \ i.e. by going onto the locals stack the order is reversed.
                     86: \ there are alignment gaps if necessary.
                     87: \ lp must have the strictest alignment (usually float) across calls;
                     88: \ for simplicity we align it strictly for every group.
                     89: 
1.5       anton      90: slowvoc @
                     91: slowvoc on \ we want a linked list for the vocabulary locals
1.1       anton      92: vocabulary locals \ this contains the local variables
1.3       anton      93: ' locals >body ' locals-list >body !
1.5       anton      94: slowvoc !
1.1       anton      95: 
                     96: create locals-buffer 1000 allot \ !! limited and unsafe
                     97:     \ here the names of the local variables are stored
                     98:     \ we would have problems storing them at the normal dp
                     99: 
                    100: variable locals-dp \ so here's the special dp for locals.
                    101: 
                    102: : alignlp-w ( n1 -- n2 )
                    103:     \ cell-align size and generate the corresponding code for aligning lp
1.3       anton     104:     aligned dup adjust-locals-size ;
1.1       anton     105: 
                    106: : alignlp-f ( n1 -- n2 )
1.3       anton     107:     faligned dup adjust-locals-size ;
1.1       anton     108: 
                    109: \ a local declaration group (the braces stuff) is compiled by calling
                    110: \ the appropriate compile-pushlocal for the locals, starting with the
                    111: \ righmost local; the names are already created earlier, the
                    112: \ compile-pushlocal just inserts the offsets from the frame base.
                    113: 
                    114: : compile-pushlocal-w ( a-addr -- ) ( run-time: w -- )
                    115: \ compiles a push of a local variable, and adjusts locals-size
                    116: \ stores the offset of the local variable to a-addr
                    117:     locals-size @ alignlp-w cell+ dup locals-size !
                    118:     swap !
                    119:     postpone >l ;
                    120: 
                    121: : compile-pushlocal-f ( a-addr -- ) ( run-time: f -- )
                    122:     locals-size @ alignlp-f float+ dup locals-size !
                    123:     swap !
                    124:     postpone f>l ;
                    125: 
                    126: : compile-pushlocal-d ( a-addr -- ) ( run-time: w1 w2 -- )
                    127:     locals-size @ alignlp-w cell+ cell+ dup locals-size !
                    128:     swap !
                    129:     postpone swap postpone >l postpone >l ;
                    130: 
                    131: : compile-pushlocal-c ( a-addr -- ) ( run-time: w -- )
1.3       anton     132:     -1 chars compile-lp+!
1.1       anton     133:     locals-size @ swap !
                    134:     postpone lp@ postpone c! ;
                    135: 
                    136: : create-local ( " name" -- a-addr )
1.9       anton     137:     \ defines the local "name"; the offset of the local shall be
                    138:     \ stored in a-addr
1.1       anton     139:     create
                    140:        immediate
                    141:        here 0 , ( place for the offset ) ;
                    142: 
1.3       anton     143: : lp-offset ( n1 -- n2 )
                    144: \ converts the offset from the frame start to an offset from lp and
                    145: \ i.e., the address of the local is lp+locals_size-offset
                    146:   locals-size @ swap - ;
                    147: 
1.1       anton     148: : lp-offset, ( n -- )
                    149: \ converts the offset from the frame start to an offset from lp and
                    150: \ adds it as inline argument to a preceding locals primitive
1.3       anton     151:   lp-offset , ;
1.1       anton     152: 
                    153: vocabulary locals-types \ this contains all the type specifyers, -- and }
                    154: locals-types definitions
                    155: 
                    156: : W:
                    157:     create-local ( "name" -- a-addr xt )
                    158:        \ xt produces the appropriate locals pushing code when executed
                    159:        ['] compile-pushlocal-w
                    160:     does> ( Compilation: -- ) ( Run-time: -- w )
                    161:         \ compiles a local variable access
1.3       anton     162:        @ lp-offset compile-@local ;
1.1       anton     163: 
                    164: : W^
                    165:     create-local ( "name" -- a-addr xt )
                    166:        ['] compile-pushlocal-w
                    167:     does> ( Compilation: -- ) ( Run-time: -- w )
                    168:        postpone laddr# @ lp-offset, ;
                    169: 
                    170: : F:
                    171:     create-local ( "name" -- a-addr xt )
                    172:        ['] compile-pushlocal-f
                    173:     does> ( Compilation: -- ) ( Run-time: -- w )
1.3       anton     174:        @ lp-offset compile-f@local ;
1.1       anton     175: 
                    176: : F^
                    177:     create-local ( "name" -- a-addr xt )
                    178:        ['] compile-pushlocal-f
                    179:     does> ( Compilation: -- ) ( Run-time: -- w )
                    180:        postpone laddr# @ lp-offset, ;
                    181: 
                    182: : D:
                    183:     create-local ( "name" -- a-addr xt )
                    184:        ['] compile-pushlocal-d
                    185:     does> ( Compilation: -- ) ( Run-time: -- w )
                    186:        postpone laddr# @ lp-offset, postpone 2@ ;
                    187: 
                    188: : D^
                    189:     create-local ( "name" -- a-addr xt )
                    190:        ['] compile-pushlocal-d
                    191:     does> ( Compilation: -- ) ( Run-time: -- w )
                    192:        postpone laddr# @ lp-offset, ;
                    193: 
                    194: : C:
                    195:     create-local ( "name" -- a-addr xt )
                    196:        ['] compile-pushlocal-c
                    197:     does> ( Compilation: -- ) ( Run-time: -- w )
                    198:        postpone laddr# @ lp-offset, postpone c@ ;
                    199: 
                    200: : C^
                    201:     create-local ( "name" -- a-addr xt )
                    202:        ['] compile-pushlocal-c
                    203:     does> ( Compilation: -- ) ( Run-time: -- w )
                    204:        postpone laddr# @ lp-offset, ;
                    205: 
                    206: \ you may want to make comments in a locals definitions group:
                    207: ' \ alias \ immediate
                    208: ' ( alias ( immediate
                    209: 
                    210: forth definitions
                    211: 
                    212: \ the following gymnastics are for declaring locals without type specifier.
                    213: \ we exploit a feature of our dictionary: every wordlist
                    214: \ has it's own methods for finding words etc.
                    215: \ So we create a vocabulary new-locals, that creates a 'w:' local named x
                    216: \ when it is asked if it contains x.
                    217: 
                    218: also locals-types
                    219: 
                    220: : new-locals-find ( caddr u w -- nfa )
                    221: \ this is the find method of the new-locals vocabulary
                    222: \ make a new local with name caddr u; w is ignored
                    223: \ the returned nfa denotes a word that produces what W: produces
                    224: \ !! do the whole thing without nextname
1.3       anton     225:     drop nextname
                    226:     ['] W: >name ;
1.1       anton     227: 
                    228: previous
                    229: 
                    230: : new-locals-reveal ( -- )
                    231:   true abort" this should not happen: new-locals-reveal" ;
                    232: 
                    233: create new-locals-map ' new-locals-find A, ' new-locals-reveal A,
                    234: 
                    235: vocabulary new-locals
                    236: new-locals-map ' new-locals >body cell+ A! \ !! use special access words
                    237: 
                    238: variable old-dpp
                    239: 
                    240: \ and now, finally, the user interface words
                    241: : { ( -- addr wid 0 )
                    242:     dp old-dpp !
                    243:     locals-dp dpp !
                    244:     also new-locals
                    245:     also get-current locals definitions  locals-types
                    246:     0 TO locals-wordlist
                    247:     0 postpone [ ; immediate
                    248: 
                    249: locals-types definitions
                    250: 
                    251: : } ( addr wid 0 a-addr1 xt1 ... -- )
                    252:     \ ends locals definitions
                    253:     ] old-dpp @ dpp !
                    254:     begin
                    255:        dup
                    256:     while
                    257:        execute
                    258:     repeat
                    259:     drop
                    260:     locals-size @ alignlp-f locals-size ! \ the strictest alignment
                    261:     set-current
                    262:     previous previous
                    263:     locals-list TO locals-wordlist ;
                    264: 
                    265: : -- ( addr wid 0 ... -- )
                    266:     }
1.9       anton     267:     [char] } parse 2drop ;
1.1       anton     268: 
                    269: forth definitions
                    270: 
                    271: \ A few thoughts on automatic scopes for locals and how they can be
                    272: \ implemented:
                    273: 
                    274: \ We have to combine locals with the control structures. My basic idea
                    275: \ was to start the life of a local at the declaration point. The life
                    276: \ would end at any control flow join (THEN, BEGIN etc.) where the local
                    277: \ is lot live on both input flows (note that the local can still live in
                    278: \ other, later parts of the control flow). This would make a local live
                    279: \ as long as you expected and sometimes longer (e.g. a local declared in
                    280: \ a BEGIN..UNTIL loop would still live after the UNTIL).
                    281: 
                    282: \ The following example illustrates the problems of this approach:
                    283: 
                    284: \ { z }
                    285: \ if
                    286: \   { x }
                    287: \ begin
                    288: \   { y }
                    289: \ [ 1 cs-roll ] then
                    290: \   ...
                    291: \ until
                    292: 
                    293: \ x lives only until the BEGIN, but the compiler does not know this
                    294: \ until it compiles the UNTIL (it can deduce it at the THEN, because at
                    295: \ that point x lives in no thread, but that does not help much). This is
                    296: \ solved by optimistically assuming at the BEGIN that x lives, but
                    297: \ warning at the UNTIL that it does not. The user is then responsible
                    298: \ for checking that x is only used where it lives.
                    299: 
                    300: \ The produced code might look like this (leaving out alignment code):
                    301: 
                    302: \ >l ( z )
                    303: \ ?branch <then>
                    304: \ >l ( x )
                    305: \ <begin>:
                    306: \ >l ( y )
                    307: \ lp+!# 8 ( RIP: x,y )
                    308: \ <then>:
                    309: \ ...
                    310: \ lp+!# -4 ( adjust lp to <begin> state )
                    311: \ ?branch <begin>
                    312: \ lp+!# 4 ( undo adjust )
                    313: 
                    314: \ The BEGIN problem also has another incarnation:
                    315: 
                    316: \ AHEAD
                    317: \ BEGIN
                    318: \   x
                    319: \ [ 1 CS-ROLL ] THEN
                    320: \   { x }
                    321: \   ...
                    322: \ UNTIL
                    323: 
                    324: \ should be legal: The BEGIN is not a control flow join in this case,
                    325: \ since it cannot be entered from the top; therefore the definition of x
                    326: \ dominates the use. But the compiler processes the use first, and since
                    327: \ it does not look ahead to notice the definition, it will complain
                    328: \ about it. Here's another variation of this problem:
                    329: 
                    330: \ IF
                    331: \   { x }
                    332: \ ELSE
                    333: \   ...
                    334: \ AHEAD
                    335: \ BEGIN
                    336: \   x
                    337: \ [ 2 CS-ROLL ] THEN
                    338: \   ...
                    339: \ UNTIL
                    340: 
                    341: \ In this case x is defined before the use, and the definition dominates
                    342: \ the use, but the compiler does not know this until it processes the
                    343: \ UNTIL. So what should the compiler assume does live at the BEGIN, if
                    344: \ the BEGIN is not a control flow join? The safest assumption would be
                    345: \ the intersection of all locals lists on the control flow
                    346: \ stack. However, our compiler assumes that the same variables are live
                    347: \ as on the top of the control flow stack. This covers the following case:
                    348: 
                    349: \ { x }
                    350: \ AHEAD
                    351: \ BEGIN
                    352: \   x
                    353: \ [ 1 CS-ROLL ] THEN
                    354: \   ...
                    355: \ UNTIL
                    356: 
                    357: \ If this assumption is too optimistic, the compiler will warn the user.
                    358: 
1.3       anton     359: \ Implementation: migrated to kernal.fs
1.1       anton     360: 
                    361: \ THEN (another control flow from before joins the current one):
                    362: \ The new locals-list is the intersection of the current locals-list and
                    363: \ the orig-local-list. The new locals-size is the (alignment-adjusted)
                    364: \ size of the new locals-list. The following code is generated:
                    365: \ lp+!# (current-locals-size - orig-locals-size)
                    366: \ <then>:
                    367: \ lp+!# (orig-locals-size - new-locals-size)
                    368: 
                    369: \ Of course "lp+!# 0" is not generated. Still this is admittedly a bit
                    370: \ inefficient, e.g. if there is a locals declaration between IF and
                    371: \ ELSE. However, if ELSE generates an appropriate "lp+!#" before the
                    372: \ branch, there will be none after the target <then>.
                    373: 
1.3       anton     374: \ explicit scoping
1.1       anton     375: 
1.3       anton     376: : scope ( -- scope )
                    377:  cs-push-part scopestart ; immediate
                    378: 
                    379: : endscope ( scope -- )
                    380:  scope?
1.1       anton     381:  drop
1.3       anton     382:  locals-list @ common-list
                    383:  dup list-size adjust-locals-size
                    384:  locals-list ! ; immediate
1.1       anton     385: 
1.3       anton     386: \ adapt the hooks
1.1       anton     387: 
1.3       anton     388: : locals-:-hook ( sys -- sys addr xt n )
                    389:     \ addr is the nfa of the defined word, xt its xt
1.1       anton     390:     DEFERS :-hook
                    391:     last @ lastcfa @
                    392:     clear-leave-stack
                    393:     0 locals-size !
                    394:     locals-buffer locals-dp !
1.3       anton     395:     0 locals-list !
                    396:     dead-code off
                    397:     defstart ;
1.1       anton     398: 
1.3       anton     399: : locals-;-hook ( sys addr xt sys -- sys )
                    400:     def?
1.1       anton     401:     0 TO locals-wordlist
1.3       anton     402:     0 adjust-locals-size ( not every def ends with an exit )
1.1       anton     403:     lastcfa ! last !
                    404:     DEFERS ;-hook ;
                    405: 
                    406: ' locals-:-hook IS :-hook
                    407: ' locals-;-hook IS ;-hook
                    408: 
                    409: \ The words in the locals dictionary space are not deleted until the end
                    410: \ of the current word. This is a bit too conservative, but very simple.
                    411: 
                    412: \ There are a few cases to consider: (see above)
                    413: 
                    414: \ after AGAIN, AHEAD, EXIT (the current control flow is dead):
                    415: \ We have to special-case the above cases against that. In this case the
                    416: \ things above are not control flow joins. Everything should be taken
                    417: \ over from the live flow. No lp+!# is generated.
                    418: 
                    419: \ !! The lp gymnastics for UNTIL are also a real problem: locals cannot be
                    420: \ used in signal handlers (or anything else that may be called while
                    421: \ locals live beyond the lp) without changing the locals stack.
                    422: 
                    423: \ About warning against uses of dead locals. There are several options:
                    424: 
                    425: \ 1) Do not complain (After all, this is Forth;-)
                    426: 
                    427: \ 2) Additional restrictions can be imposed so that the situation cannot
                    428: \ arise; the programmer would have to introduce explicit scoping
                    429: \ declarations in cases like the above one. I.e., complain if there are
                    430: \ locals that are live before the BEGIN but not before the corresponding
                    431: \ AGAIN (replace DO etc. for BEGIN and UNTIL etc. for AGAIN).
                    432: 
                    433: \ 3) The real thing: i.e. complain, iff a local lives at a BEGIN, is
                    434: \ used on a path starting at the BEGIN, and does not live at the
                    435: \ corresponding AGAIN. This is somewhat hard to implement. a) How does
                    436: \ the compiler know when it is working on a path starting at a BEGIN
                    437: \ (consider "{ x } if begin [ 1 cs-roll ] else x endif again")? b) How
                    438: \ is the usage info stored?
                    439: 
                    440: \ For now I'll resort to alternative 2. When it produces warnings they
                    441: \ will often be spurious, but warnings should be rare. And better
                    442: \ spurious warnings now and then than days of bug-searching.
                    443: 
                    444: \ Explicit scoping of locals is implemented by cs-pushing the current
                    445: \ locals-list and -size (and an unused cell, to make the size equal to
                    446: \ the other entries) at the start of the scope, and restoring them at
                    447: \ the end of the scope to the intersection, like THEN does.
                    448: 
                    449: 
                    450: \ And here's finally the ANS standard stuff
                    451: 
                    452: : (local) ( addr u -- )
1.3       anton     453:     \ a little space-inefficient, but well deserved ;-)
                    454:     \ In exchange, there are no restrictions whatsoever on using (local)
1.4       anton     455:     \ as long as you use it in a definition
1.3       anton     456:     dup
                    457:     if
                    458:        nextname POSTPONE { [ also locals-types ] W: } [ previous ]
                    459:     else
                    460:        2drop
                    461:     endif ;
1.1       anton     462: 
1.4       anton     463: : >definer ( xt -- definer )
                    464:     \ this gives a unique identifier for the way the xt was defined
                    465:     \ words defined with different does>-codes have different definers
                    466:     \ the definer can be used for comparison and in definer!
                    467:     dup >code-address [ ' bits >code-address ] Literal =
                    468:     \ !! this definition will not work on some implementations for `bits'
                    469:     if  \ if >code-address delivers the same value for all does>-def'd words
                    470:        >does-code 1 or \ bit 0 marks special treatment for does codes
                    471:     else
                    472:        >code-address
                    473:     then ;
                    474: 
                    475: : definer! ( definer xt -- )
                    476:     \ gives the word represented by xt the behaviour associated with definer
                    477:     over 1 and if
                    478:        does-code!
                    479:     else
                    480:        code-address!
                    481:     then ;
                    482: 
                    483: \ !! untested
                    484: : TO ( c|w|d|r "name" -- )
                    485: \ !! state smart
                    486:  0 0 0. 0.0e0 { c: clocal w: wlocal d: dlocal f: flocal }
                    487:  ' dup >definer
                    488:  state @ 
                    489:  if
                    490:    case
                    491:      [ ' locals-wordlist >definer ] literal \ value
                    492:      OF >body POSTPONE Aliteral POSTPONE ! ENDOF
                    493:      [ ' clocal >definer ] literal
                    494:      OF POSTPONE laddr# >body @ lp-offset, POSTPONE c! ENDOF
                    495:      [ ' wlocal >definer ] literal
                    496:      OF POSTPONE laddr# >body @ lp-offset, POSTPONE ! ENDOF
                    497:      [ ' dlocal >definer ] literal
                    498:      OF POSTPONE laddr# >body @ lp-offset, POSTPONE d! ENDOF
                    499:      [ ' flocal >definer ] literal
                    500:      OF POSTPONE laddr# >body @ lp-offset, POSTPONE f! ENDOF
                    501:      abort" can only store TO value or local value"
                    502:    endcase
                    503:  else
                    504:    [ ' locals-wordlist >definer ] literal =
                    505:    if
                    506:      >body !
                    507:    else
                    508:      abort" can only store TO value"
                    509:    endif
                    510:  endif ; immediate
1.1       anton     511: 
1.6       pazsan    512: : locals|
1.8       anton     513:     BEGIN
                    514:        name 2dup s" |" compare 0<>
                    515:     WHILE
                    516:        (local)
                    517:     REPEAT
                    518:     drop 0 (local) ;  immediate restrict

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