Diff for /gforth/Attic/gforth.ds between versions 1.1 and 1.2

version 1.1, 1994/10/24 19:15:57 version 1.2, 1994/11/14 19:01:16
Line 689  There are several variations on the coun Line 689  There are several variations on the coun
 index by @var{n} instead of by 1. The loop is terminated when the border  index by @var{n} instead of by 1. The loop is terminated when the border
 between @var{limit-1} and @var{limit} is crossed. E.g.:  between @var{limit-1} and @var{limit} is crossed. E.g.:
 4 0 ?DO  i .  2 +LOOP   prints 0 2  @code{4 0 ?DO  i .  2 +LOOP}   prints @code{0 2}
 4 1 ?DO  i .  2 +LOOP   prints 1 3  @code{4 1 ?DO  i .  2 +LOOP}   prints @code{1 3}
 The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:  The behaviour of @code{@var{n} +LOOP} is peculiar when @var{n} is negative:
 -1 0 ?DO  i .  -1 +LOOP  prints 0 -1  @code{-1 0 ?DO  i .  -1 +LOOP}  prints @code{0 -1}
  0 0 ?DO  i .  -1 +LOOP  prints nothing  @code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing
 Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative  Therefore we recommend avoiding using @code{@var{n} +LOOP} with negative
 @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative  @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative
 case behaves symmetrical to the positive case:  case behaves symmetrical to the positive case:
 -2 0 ?DO  i .  -1 +LOOP  prints 0 -1  @code{-2 0 ?DO  i .  -1 +LOOP}  prints @code{0 -1}
 -1 0 ?DO  i .  -1 +LOOP  prints 0  @code{-1 0 ?DO  i .  -1 +LOOP}  prints @code{0}
  0 0 ?DO  i .  -1 +LOOP  prints nothing  @code{ 0 0 ?DO  i .  -1 +LOOP}  prints nothing
 The loop is terminated when the border between @var{limit-sgn(n)} and  The loop is terminated when the border between @var{limit@minus{}sgn(n)} and
 @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS  @var{limit} is crossed. However, @code{S+LOOP} is not part of the ANS
 Forth standard.  Forth standard.
Line 734  iterates @var{n+1} times; @code{i} produ Line 734  iterates @var{n+1} times; @code{i} produ
 and ending with 0. Other Forth systems may behave differently, even if  and ending with 0. Other Forth systems may behave differently, even if
 they support @code{FOR} loops.  they support @code{FOR} loops.
   @subsection Arbitrary control structures
   ANS Forth permits and supports using control structures in a non-nested
   way. Information about incomplete control structures is stored on the
   control-flow stack. This stack may be implemented on the Forth data
   stack, and this is what we have done in gforth.
   An @i{orig} entry represents an unresolved forward branch, a @i{dest}
   entry represents a backward branch target. A few words are the basis for
   building any control structure possible (except control structures that
   need storage, like calls, coroutines, and backtracking).
   On many systems control-flow stack items take one word, in gforth they
   currently take three (this may change in the future). Therefore it is a
   really good idea to manipulate the control flow stack with
   @code{cs-pick} and @code{cs-roll}, not with data stack manipulation
   Some standard control structure words are built from these words:
   Counted loop words constitute a separate group of words:
   The standard does not allow using @code{cs-pick} and @code{cs-roll} on
   @i{do-sys}. Our system allows it, but it's your job to ensure that for
   every @code{?DO} etc. there is exactly one @code{UNLOOP} on any path
   through the program (@code{LOOP} etc. compile an @code{UNLOOP}). Also,
   you have to ensure that all @code{LEAVE}s are resolved (by using one of
   the loop-ending words or @code{UNDO}).
   Another group of control structure words are
   @i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
 @node Locals  @node Locals
 @section Locals  @section Locals
   Local variables can make Forth programming more enjoyable and Forth
   programs easier to read. Unfortunately, the locals of ANS Forth are
   laden with restrictions. Therefore, we provide not only the ANS Forth
   locals wordset, but also our own, more powerful locals wordset (we
   implemented the ANS Forth locals wordset through our locals wordset).
   @end menu
   @subsection gforth locals
   Locals can be defined with
   @{ local1 local2 ... -- comment @}
   @end example
   @{ local1 local2 ... @}
   @end example
   : max @{ n1 n2 -- n3 @}
    n1 n2 > if
    endif ;
   @end example
   The similarity of locals definitions with stack comments is intended. A
   locals definition often replaces the stack comment of a word. The order
   of the locals corresponds to the order in a stack comment and everything
   after the @code{--} is really a comment.
   This similarity has one disadvantage: It is too easy to confuse locals
   declarations with stack comments, causing bugs and making them hard to
   find. However, this problem can be avoided by appropriate coding
   conventions: Do not use both notations in the same program. If you do,
   they should be distinguished using additional means, e.g. by position.
   The name of the local may be preceded by a type specifier, e.g.,
   @code{F:} for a floating point value:
   : 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+ ;
   @end example
   GNU Forth currently supports cells (@code{W:}, @code{W^}), doubles
   (@code{D:}, @code{D^}), floats (@code{F:}, @code{F^}) and characters
   (@code{C:}, @code{C^}) in two flavours: a value-flavoured local (defined
   with @code{W:}, @code{D:} etc.) produces its value and can be changed
   with @code{TO}. A variable-flavoured local (defined with @code{W^} etc.)
   produces its address (which becomes invalid when the variable's scope is
   left). E.g., the standard word @code{emit} can be defined in therms of
   @code{type} like this:
   : emit @{ C^ char* -- @}
       char* 1 type ;
   @end example
   A local without type specifier is a @code{W:} local. Both flavours of
   locals are initialized with values from the data or FP stack.
   Currently there is no way to define locals with user-defined data
   structures, but we are working on it.
   GNU Forth allows defining locals everywhere in a colon definition. This poses the following questions:
   @subsubsection Where are locals visible by name?
   Basically, the answer is that locals are visible where you would expect
   it in block-structured languages, and sometimes a little longer. If you
   want to restrict the scope of a local, enclose its definition in
   These words behave like control structure words, so you can use them
   with @code{CS-PICK} and @code{CS-ROLL} to restrict the scope in
   arbitrary ways.
   If you want a more exact answer to the visibility question, here's the
   basic principle: A local is visible in all places that can only be
   reached through the definition of the local@footnote{In compiler
   construction terminology, all places dominated by the definition of the
   local.}. In other words, it is not visible in places that can be reached
   without going through the definition of the local. E.g., locals defined
   in @code{IF}...@code{ENDIF} are visible until the @code{ENDIF}, locals
   defined in @code{BEGIN}...@code{UNTIL} are visible after the
   @code{UNTIL} (until, e.g., a subsequent @code{ENDSCOPE}).
   The reasoning behind this solution is: We want to have the locals
   visible as long as it is meaningful. The user can always make the
   visibility shorter by using explicit scoping. In a place that can
   only be reached through the definition of a local, the meaning of a
   local name is clear. In other places it is not: How is the local
   initialized at the control flow path that does not contain the
   definition? Which local is meant, if the same name is defined twice in
   two independent control flow paths?
   This should be enough detail for nearly all users, so you can skip the
   rest of this section. If you relly must know all the gory details and
   options, read on.
   In order to implement this rule, the compiler has to know which places
   are unreachable. It knows this automatically after @code{AHEAD},
   @code{AGAIN}, @code{EXIT} and @code{LEAVE}; in other cases (e.g., after
   most @code{THROW}s), you can use the word @code{UNREACHABLE} to tell the
   compiler that the control flow never reaches that place. If
   @code{UNREACHABLE} is not used where it could, the only consequence is
   that the visibility of some locals is more limited than the rule above
   says. If @code{UNREACHABLE} is used where it should not (i.e., if you
   lie to the compiler), buggy code will be produced.
   Another problem with this rule is that at @code{BEGIN}, the compiler
   does not know which locals will be visible on the incoming back-edge
   . All problems discussed in the following are due to this ignorance of
   the compiler (we discuss the problems using @code{BEGIN} loops as
   examples; the discussion also applies to @code{?DO} and other
   loops). Perhaps the most insidious example is:
   [ 1 CS-ROLL ] THEN
     { x }
   @end example
   This should be legal according to the visibility rule. The use of
   @code{x} can only be reached through the definition; but that appears
   textually below the use.
   From this example it is clear that the visibility rules cannot be fully
   implemented without major headaches. Our implementation treats common
   cases as advertised and the exceptions are treated in a safe way: The
   compiler makes a reasonable guess about the locals visible after a
   @code{BEGIN}; if it is too pessimistic, the
   user will get a spurious error about the local not being defined; if the
   compiler is too optimistic, it will notice this later and issue a
   warning. In the case above the compiler would complain about @code{x}
   being undefined at its use. You can see from the obscure examples in
   this section that it takes quite unusual control structures to get the
   compiler into trouble, and even then it will often do fine.
   If the @code{BEGIN} is reachable from above, the most optimistic guess
   is that all locals visible before the @code{BEGIN} will also be
   visible after the @code{BEGIN}. This guess is valid for all loops that
   are entered only through the @code{BEGIN}, in particular, for normal
   @code{BEGIN}...@code{WHILE}...@code{REPEAT} and
   @code{BEGIN}...@code{UNTIL} loops and it is implemented in our
   compiler. When the branch to the @code{BEGIN} is finally generated by
   @code{AGAIN} or @code{UNTIL}, the compiler checks the guess and
   warns the user if it was too optimisitic:
     { x }
     \ x ? 
   [ 1 cs-roll ] THEN
   @end example
   Here, @code{x} lives only until the @code{BEGIN}, but the compiler
   optimistically assumes that it lives until the @code{THEN}. It notices
   this difference when it compiles the @code{UNTIL} and issues a
   warning. The user can avoid the warning, and make sure that @code{x}
   is not used in the wrong area by using explicit scoping:
     { x }
   [ 1 cs-roll ] THEN
   @end example
   Since the guess is optimistic, there will be no spurious error messages
   about undefined locals.
   If the @code{BEGIN} is not reachable from above (e.g., after
   @code{AHEAD} or @code{EXIT}), the compiler cannot even make an
   optimistic guess, as the locals visible after the @code{BEGIN} may be
   defined later. Therefore, the compiler assumes that no locals are
   visible after the @code{BEGIN}. However, the useer can use
   @code{ASSUME-LIVE} to make the compiler assume that the same locals are
   visible at the BEGIN as at the point where the item was created.
   { x }
   [ 1 CS-ROLL ] THEN
   @end example
   Other cases where the locals are defined before the @code{BEGIN} can be
   handled by inserting an appropriate @code{CS-ROLL} before the
   @code{ASSUME-LIVE} (and changing the control-flow stack manipulation
   behind the @code{ASSUME-LIVE}).
   Cases where locals are defined after the @code{BEGIN} (but should be
   visible immediately after the @code{BEGIN}) can only be handled by
   rearranging the loop. E.g., the ``most insidious'' example above can be
   arranged into:
     { x }
     ... 0=
   @end example
   @subsubsection How long do locals live?
   The right answer for the lifetime question would be: A local lives at
   least as long as it can be accessed. For a value-flavoured local this
   means: until the end of its visibility. However, a variable-flavoured
   local could be accessed through its address far beyond its visibility
   scope. Ultimately, this would mean that such locals would have to be
   garbage collected. Since this entails un-Forth-like implementation
   complexities, I adopted the same cowardly solution as some other
   languages (e.g., C): The local lives only as long as it is visible;
   afterwards its address is invalid (and programs that access it
   afterwards are erroneous).
   @subsubsection Programming Style
   The freedom to define locals anywhere has the potential to change
   programming styles dramatically. In particular, the need to use the
   return stack for intermediate storage vanishes. Moreover, all stack
   manipulations (except @code{PICK}s and @code{ROLL}s with run-time
   determined arguments) can be eliminated: If the stack items are in the
   wrong order, just write a locals definition for all of them; then
   write the items in the order you want.
   This seems a little far-fetched and eliminating stack manipulations is
   unlikely to become a conscious programming objective. Still, the
   number of stack manipulations will be reduced dramatically if local
   variables are used liberally (e.g., compare @code{max} in \sect{misc}
   with a traditional implementation of @code{max}).
   This shows one potential benefit of locals: making Forth programs more
   readable. Of course, this benefit will only be realized if the
   programmers continue to honour the principle of factoring instead of
   using the added latitude to make the words longer.
   Using @code{TO} can and should be avoided.  Without @code{TO},
   every value-flavoured local has only a single assignment and many
   advantages of functional languages apply to Forth. I.e., programs are
   easier to analyse, to optimize and to read: It is clear from the
   definition what the local stands for, it does not turn into something
   different later.
   E.g., a definition using @code{TO} might look like this:
   : strcmp @{ addr1 u1 addr2 u2 -- n @}
    u1 u2 min 0
      addr1 c@ addr2 c@ - ?dup
        unloop exit
      addr1 char+ TO addr1
      addr2 char+ TO addr2
    u1 u2 - ;
   @end example
   Here, @code{TO} is used to update @code{addr1} and @code{addr2} at
   every loop iteration. @code{strcmp} is a typical example of the
   readability problems of using @code{TO}. When you start reading
   @code{strcmp}, you think that @code{addr1} refers to the start of the
   string. Only near the end of the loop you realize that it is something
   This can be avoided by defining two locals at the start of the loop that
   are initialized with the right value for the current iteration.
   : strcmp @{ addr1 u1 addr2 u2 -- n @}
    addr1 addr2
    u1 u2 min 0 
    ?do @{ s1 s2 @}
      s1 c@ s2 c@ - ?dup 
        unloop exit
      s1 char+ s2 char+
    u1 u2 - ;
   @end example
   Here it is clear from the start that @code{s1} has a different value
   in every loop iteration.
   @subsubsection Implementation
   GNU Forth uses an extra locals stack. The most compelling reason for
   this is that the return stack is not float-aligned; using an extra stack
   also eliminates the problems and restrictions of using the return stack
   as locals stack. Like the other stacks, the locals stack grows toward
   lower addresses. A few primitives allow an efficient implementation:
   In addition to these primitives, some specializations of these
   primitives for commonly occurring inline arguments are provided for
   efficiency reasons, e.g., @code{@@local0} as specialization of
   @code{@@local#} for the inline argument 0. The following compiling words
   compile the right specialized version, or the general version, as
   Combinations of conditional branches and @code{lp+!#} like
   @code{?branch-lp+!#} (the locals pointer is only changed if the branch
   is taken) are provided for efficiency and correctness in loops.
   A special area in the dictionary space is reserved for keeping the
   local variable names. @code{@{} switches the dictionary pointer to this
   area and @code{@}} switches it back and generates the locals
   initializing code. @code{W:} etc.@ are normal defining words. This
   special area is cleared at the start of every colon definition.
   A special feature of GNU Forths dictionary is used to implement the
   definition of locals without type specifiers: every wordlist (aka
   vocabulary) has its own methods for searching
   etc. (@xref{dictionary}). For the present purpose we defined a wordlist
   with a special search method: When it is searched for a word, it
   actually creates that word using @code{W:}. @code{@{} changes the search
   order to first search the wordlist containing @code{@}}, @code{W:} etc.,
   and then the wordlist for defining locals without type specifiers.
   The lifetime rules support a stack discipline within a colon
   definition: The lifetime of a local is either nested with other locals
   lifetimes or it does not overlap them.
   At @code{BEGIN}, @code{IF}, and @code{AHEAD} no code for locals stack
   pointer manipulation is generated. Between control structure words
   locals definitions can push locals onto the locals stack. @code{AGAIN}
   is the simplest of the other three control flow words. It has to
   restore the locals stack depth of the corresponding @code{BEGIN}
   before branching. The code looks like this:
   @code{lp+!#} current-locals-size @minus{} dest-locals-size
   @code{branch} <begin>
   @end format
   @code{UNTIL} is a little more complicated: If it branches back, it
   must adjust the stack just like @code{AGAIN}. But if it falls through,
   the locals stack must not be changed. The compiler generates the
   following code:
   @code{?branch-lp+!#} <begin> current-locals-size @minus{} dest-locals-size
   @end format
   The locals stack pointer is only adjusted if the branch is taken.
   @code{THEN} can produce somewhat inefficient code:
   @code{lp+!#} current-locals-size @minus{} orig-locals-size
   <orig target>:
   @code{lp+!#} orig-locals-size @minus{} new-locals-size
   @end format
   The second @code{lp+!#} adjusts the locals stack pointer from the
   level at the {\em orig} point to the level after the @code{THEN}. The
   first @code{lp+!#} adjusts the locals stack pointer from the current
   level to the level at the orig point, so the complete effect is an
   adjustment from the current level to the right level after the
   In a conventional Forth implementation a dest control-flow stack entry
   is just the target address and an orig entry is just the address to be
   patched. Our locals implementation adds a wordlist to every orig or dest
   item. It is the list of locals visible (or assumed visible) at the point
   described by the entry. Our implementation also adds a tag to identify
   the kind of entry, in particular to differentiate between live and dead
   (reachable and unreachable) orig entries.
   A few unusual operations have to be performed on locals wordlists:
   Several features of our locals wordlist implementation make these
   operations easy to implement: The locals wordlists are organised as
   linked lists; the tails of these lists are shared, if the lists
   contain some of the same locals; and the address of a name is greater
   than the address of the names behind it in the list.
   Another important implementation detail is the variable
   @code{dead-code}. It is used by @code{BEGIN} and @code{THEN} to
   determine if they can be reached directly or only through the branch
   that they resolve. @code{dead-code} is set by @code{UNREACHABLE},
   @code{AHEAD}, @code{EXIT} etc., and cleared at the start of a colon
   definition, by @code{BEGIN} and usually by @code{THEN}.
   Counted loops are similar to other loops in most respects, but
   @code{LEAVE} requires special attention: It performs basically the same
   service as @code{AHEAD}, but it does not create a control-flow stack
   entry. Therefore the information has to be stored elsewhere;
   traditionally, the information was stored in the target fields of the
   branches created by the @code{LEAVE}s, by organizing these fields into a
   linked list. Unfortunately, this clever trick does not provide enough
   space for storing our extended control flow information. Therefore, we
   introduce another stack, the leave stack. It contains the control-flow
   stack entries for all unresolved @code{LEAVE}s.
   Local names are kept until the end of the colon definition, even if
   they are no longer visible in any control-flow path. In a few cases
   this may lead to increased space needs for the locals name area, but
   usually less than reclaiming this space would cost in code size.
   @subsection ANS Forth locals
   The ANS Forth locals wordset does not define a syntax for locals, but
   words that make it possible to define various syntaxes. One of the
   possible syntaxes is a subset of the syntax we used in the gforth locals
   wordset, i.e.:
   @{ local1 local2 ... -- comment @}
   @end example
   @{ local1 local2 ... @}
   @end example
   The order of the locals corresponds to the order in a stack comment. The
   restrictions are:
   @itemize @bullet
   Locals can only be cell-sized values (no type specifers are allowed).
   Locals can be defined only outside control structures.
   Locals can interfere with explicit usage of the return stack. For the
   exact (and long) rules, see the standard. If you don't use return stack
   accessing words in a definition using locals, you will we all right. The
   purpose of this rule is to make locals implementation on the return
   stack easier.
   The whole definition must be in one line.
   @end itemize
   Locals defined in this way behave like @code{VALUE}s
   (@xref{values}). I.e., they are initialized from the stack. Using their
   name produces their value. Their value can be changed using @code{TO}.
   Since this syntax is supported by gforth directly, you need not do
   anything to use it. If you want to port a program using this syntax to
   another ANS Forth system, use @file{anslocal.fs} to implement the syntax
   on the other system.
   Note that a syntax shown in the standard, section A.13 looks
   similar, but is quite different in having the order of locals
   reversed. Beware!
   The ANS Forth locals wordset itself consists of the following word
   The ANS Forth locals extension wordset defines a syntax, but it is so
   awful that we strongly recommend not to use it. We have implemented this
   syntax to make porting to gforth easy, but do not document it here. The
   problem with this syntax is that the locals are defined in an order
   reversed with respect to the standard stack comment notation, making
   programs harder to read, and easier to misread and miswrite. The only
   merit of this syntax is that it is easy to implement using the ANS Forth
   locals wordset.
 @contents  @contents
 @bye  @bye

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