### 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).

if
then
begin
until
again
cs-pick
cs-roll

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
words.

Some standard control structure words are built from these words:

else
while
repeat

Counted loop words constitute a separate group of words:

?do
do
for
loop
s+loop
+loop
next
leave
?leave
unloop
undo

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

case
endcase
of
endof

@i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and
@code{cs-roll}.

@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).

@subsection gforth locals

Locals can be defined with

@example
@{ local1 local2 ... -- comment @}
@end example
or
@example
@{ local1 local2 ... @}
@end example

E.g.,
@example
: max @{ n1 n2 -- n3 @}
n1 n2 > if
n1
else
n2
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:

@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+ ;
@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:

@example
: 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
@code{SCOPE}...@code{ENDSCOPE}.

doc-scope
doc-endscope

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

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:
@example
BEGIN
x
[ 1 CS-ROLL ] THEN
{ x }
...
UNTIL
@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:
@example
IF
{ x }
BEGIN
\ x ?
[ 1 cs-roll ] THEN
...
UNTIL
@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:
@example
IF
SCOPE
{ x }
ENDSCOPE
BEGIN
[ 1 cs-roll ] THEN
...
UNTIL
@end example

Since the guess is optimistic, there will be no spurious error messages

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.

doc-assume-live

E.g.,
@example
{ x }
ASSUME-LIVE
BEGIN
x
[ 1 CS-ROLL ] THEN
...
UNTIL
@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:
@example
BEGIN
{ x }
... 0=
WHILE
x
REPEAT
@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:
@example
u1 u2 min 0
?do
if
unloop exit
then
loop
u1 u2 - ;
@end example
every loop iteration. @code{strcmp} is a typical example of the
@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
else.

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.
@example
u1 u2 min 0
?do @{ s1 s2 @}
s1 c@ s2 c@ - ?dup
if
unloop exit
then
s1 char+ s2 char+
loop
2drop
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:

doc-@local#
doc-f@local#
doc-lp+!#
doc-lp!
doc->l
doc-f>l

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
appropriate:

doc-compile-@@local
doc-compile-f@@local
doc-compile-lp+!

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:
@format
@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:
@format
@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:
@format
@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
@code{THEN}.

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:

doc-common-list
doc-sub-list?
doc-list-size

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.:

@example
@{ local1 local2 ... -- comment @}
@end example
or
@example
@{ local1 local2 ... @}
@end example

The order of the locals corresponds to the order in a stack comment. The
restrictions are:

@itemize @bullet
@item
Locals can only be cell-sized values (no type specifers are allowed).
@item
Locals can be defined only outside control structures.
@item
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.
@item
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

doc-(local)

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

 Removed from v.1.1 changed lines Added in v.1.2

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