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