--- gforth/Attic/gforth.ds 1994/10/24 19:15:57 1.1 +++ gforth/Attic/gforth.ds 1995/01/24 17:31:22 1.7 @@ -1,9 +1,9 @@ \input texinfo @c -*-texinfo-*- @comment The source is gforth.ds, from which gforth.texi is generated @comment %**start of header (This is for running Texinfo on a region.) -@setfilename gforth-info +@setfilename gforth.info @settitle GNU Forth Manual -@setchapternewpage odd +@comment @setchapternewpage odd @comment %**end of header (This is for running Texinfo on a region.) @ifinfo @@ -15,13 +15,13 @@ Copyright @copyright{} 1994 GNU Forth De this manual provided the copyright notice and this permission notice are preserved on all copies. - @ignore +@ignore Permission is granted to process this file through TeX and print the results, provided the printed document carries a copying permission notice identical to this one except for the removal of this paragraph (this paragraph not being relevant to the printed manual). - @end ignore +@end ignore Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "Distribution" and "General Public License" are @@ -77,19 +77,19 @@ personal machines. This manual correspon @end ifinfo @menu -* License:: -* Goals:: About the GNU Forth Project -* Other Books:: Things you might want to read -* Invocation:: Starting GNU Forth -* Words:: Forth words available in GNU Forth -* ANS conformance:: Implementation-defined options etc. -* Model:: The abstract machine of GNU Forth -* Emacs and GForth:: The GForth Mode -* Internals:: Implementation details -* Bugs:: How to report them -* Pedigree:: Ancestors of GNU Forth -* Word Index:: An item for each Forth word -* Node Index:: An item for each node +* License:: +* Goals:: About the GNU Forth Project +* Other Books:: Things you might want to read +* Invocation:: Starting GNU Forth +* Words:: Forth words available in GNU Forth +* ANS conformance:: Implementation-defined options etc. +* Model:: The abstract machine of GNU Forth +* Emacs and GForth:: The GForth Mode +* Internals:: Implementation details +* Bugs:: How to report them +* Pedigree:: Ancestors of GNU Forth +* Word Index:: An item for each Forth word +* Node Index:: An item for each node @end menu @node License, Goals, Top, Top @@ -253,22 +253,23 @@ the user initialization file @file{.gfor option @code{--no-rc} is given; this file is first searched in @file{.}, then in @file{~}, then in the normal path (see above). -@node Words, , Invocation, Top +@node Words, ANS conformance, Invocation, Top @chapter Forth Words @menu -* Notation:: -* Arithmetic:: -* Stack Manipulation:: -* Memory access:: -* Control Structures:: -* Local Variables:: -* Defining Words:: -* Vocabularies:: -* Files:: -* Blocks:: -* Other I/O:: -* Programming Tools:: +* Notation:: +* Arithmetic:: +* Stack Manipulation:: +* Memory access:: +* Control Structures:: +* Locals:: +* Defining Words:: +* Wordlists:: +* Files:: +* Blocks:: +* Other I/O:: +* Programming Tools:: +* Threading Words:: @end menu @node Notation, Arithmetic, Words, Words @@ -277,10 +278,10 @@ then in @file{~}, then in the normal pat The Forth words are described in this section in the glossary notation that has become a de-facto standard for Forth texts, i.e. -@quotation +@format @var{word} @var{Stack effect} @var{wordset} @var{pronunciation} +@end format @var{Description} -@end quotation @table @var @item word @@ -314,11 +315,10 @@ wordsets. Words that are not defined in A description of the behaviour of the word. @end table -The name of a stack item corresponds in the following way with its type: +The type of a stack item is specified by the character(s) the name +starts with: @table @code -@item name starts with -Type @item f Bool, i.e. @code{false} or @code{true}. @item c @@ -353,7 +353,7 @@ Wordlist ID, same size as Cell Pointer to a name structure @end table -@node Arithmetic, , Notation, Words +@node Arithmetic, Stack Manipulation, Notation, Words @section Arithmetic Forth arithmetic is not checked, i.e., you will not hear about integer overflow on addition or multiplication, you may hear about division by @@ -363,8 +363,17 @@ corresponds to @code{2 1 -}. Forth offer operators. If you perform division with potentially negative operands, you do not want to use @code{/} or @code{/mod} with its undefined behaviour, but rather @code{fm/mod} or @code{sm/mod} (probably the -former). +former, @pxref{Mixed precision}). +@menu +* Single precision:: +* Bitwise operations:: +* Mixed precision:: operations with single and double-cell integers +* Double precision:: Double-cell integer arithmetic +* Floating Point:: +@end menu + +@node Single precision, Bitwise operations, Arithmetic, Arithmetic @subsection Single precision doc-+ doc-- @@ -377,6 +386,7 @@ doc-abs doc-min doc-max +@node Bitwise operations, Mixed precision, Single precision, Arithmetic @subsection Bitwise operations doc-and doc-or @@ -385,6 +395,7 @@ doc-invert doc-2* doc-2/ +@node Mixed precision, Double precision, Bitwise operations, Arithmetic @subsection Mixed precision doc-m+ doc-*/ @@ -396,6 +407,7 @@ doc-um/mod doc-fm/mod doc-sm/rem +@node Double precision, Floating Point, Mixed precision, Arithmetic @subsection Double precision doc-d+ doc-d- @@ -404,7 +416,56 @@ doc-dabs doc-dmin doc-dmax -@node Stack Manipulation,,, +@node Floating Point, , Double precision, Arithmetic +@subsection Floating Point + +Angles in floating point operations are given in radians (a full circle +has 2 pi radians). Note, that gforth has a separate floating point +stack, but we use the unified notation. + +Floating point numbers have a number of unpleasant surprises for the +unwary (e.g., floating point addition is not associative) and even a few +for the wary. You should not use them unless you know what you are doing +or you don't care that the results you get are totally bogus. If you +want to learn about the problems of floating point numbers (and how to +avoid them), you might start with @cite{David (?) Goldberg, What Every +Computer Scientist Should Know About Floating-Point Arithmetic, ACM +Computing Surveys 23(1):5@minus{}48, March 1991}. + +doc-f+ +doc-f- +doc-f* +doc-f/ +doc-fnegate +doc-fabs +doc-fmax +doc-fmin +doc-floor +doc-fround +doc-f** +doc-fsqrt +doc-fexp +doc-fexpm1 +doc-fln +doc-flnp1 +doc-flog +doc-falog +doc-fsin +doc-fcos +doc-fsincos +doc-ftan +doc-fasin +doc-facos +doc-fatan +doc-fatan2 +doc-fsinh +doc-fcosh +doc-ftanh +doc-fasinh +doc-facosh +doc-fatanh + +@node Stack Manipulation, Memory access, Arithmetic, Words @section Stack Manipulation gforth has a data stack (aka parameter stack) for characters, cells, @@ -417,13 +478,25 @@ theoretically keep floating point number additional difficulty, you don't know how many cells a floating point number takes. It is reportedly possible to write words in a way that they work also for a unified stack model, but we do not recommend trying -it. Also, a Forth system is allowed to keep the local variables on the +it. Instead, just say that your program has an environmental dependency +on a separate FP stack. + +Also, a Forth system is allowed to keep the local variables on the return stack. This is reasonable, as local variables usually eliminate the need to use the return stack explicitly. So, if you want to produce a standard complying program and if you are using local variables in a word, forget about return stack manipulations in that word (see the standard document for the exact rules). +@menu +* Data stack:: +* Floating point stack:: +* Return stack:: +* Locals stack:: +* Stack pointer manipulation:: +@end menu + +@node Data stack, Floating point stack, Stack Manipulation, Stack Manipulation @subsection Data stack doc-drop doc-nip @@ -444,6 +517,7 @@ doc-2tuck doc-2swap doc-2rot +@node Floating point stack, Return stack, Data stack, Stack Manipulation @subsection Floating point stack doc-fdrop doc-fnip @@ -453,6 +527,7 @@ doc-ftuck doc-fswap doc-frot +@node Return stack, Locals stack, Floating point stack, Stack Manipulation @subsection Return stack doc->r doc-r> @@ -463,8 +538,10 @@ doc-2r> doc-2r@ doc-2rdrop +@node Locals stack, Stack pointer manipulation, Return stack, Stack Manipulation @subsection Locals stack +@node Stack pointer manipulation, , Locals stack, Stack Manipulation @subsection Stack pointer manipulation doc-sp@ doc-sp! @@ -475,9 +552,16 @@ doc-rp! doc-lp@ doc-lp! -@node Memory access +@node Memory access, Control Structures, Stack Manipulation, Words @section Memory access +@menu +* Stack-Memory transfers:: +* Address arithmetic:: +* Memory block access:: +@end menu + +@node Stack-Memory transfers, Address arithmetic, Memory access, Memory access @subsection Stack-Memory transfers doc-@ @@ -494,6 +578,7 @@ doc-sf! doc-df@ doc-df! +@node Address arithmetic, Memory block access, Stack-Memory transfers, Memory access @subsection Address arithmetic ANS Forth does not specify the sizes of the data types. Instead, it @@ -542,6 +627,7 @@ doc-dfalign doc-dfaligned doc-address-unit-bits +@node Memory block access, , Address arithmetic, Memory access @subsection Memory block access doc-move @@ -555,7 +641,7 @@ doc-cmove> doc-fill doc-blank -@node Control Structures +@node Control Structures, Locals, Memory access, Words @section Control Structures Control structures in Forth cannot be used in interpret state, only in @@ -563,6 +649,16 @@ compile state, i.e., in a colon definiti limitation, but have not seen a satisfying way around it yet, although many schemes have been proposed. +@menu +* Selection:: +* Simple Loops:: +* Counted Loops:: +* Arbitrary control structures:: +* Calls and returns:: +* Exception Handling:: +@end menu + +@node Selection, Simple Loops, Control Structures, Control Structures @subsection Selection @example @@ -581,7 +677,7 @@ ELSE ENDIF @end example -You can use @code{THEN} instead of {ENDIF}. Indeed, @code{THEN} is +You can use @code{THEN} instead of @code{ENDIF}. Indeed, @code{THEN} is standard, and @code{ENDIF} is not, although it is quite popular. We recommend using @code{ENDIF}, because it is less confusing for people who also know other languages (and is not prone to reinforcing negative @@ -608,7 +704,7 @@ can avoid using @code{?dup}. CASE @var{n1} OF @var{code1} ENDOF @var{n2} OF @var{code2} ENDOF - @dots + @dots{} ENDCASE @end example @@ -617,6 +713,7 @@ Executes the first @var{codei}, where th the last @code{ENDOF}. It may use @var{n}, which is on top of the stack, but must not consume it. +@node Simple Loops, Counted Loops, Selection, Control Structures @subsection Simple Loops @example @@ -648,6 +745,7 @@ AGAIN This is an endless loop. +@node Counted Loops, Arbitrary control structures, Simple Loops, Control Structures @subsection Counted Loops The basic counted loop is: @@ -689,27 +787,27 @@ There are several variations on the coun 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.: -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: --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 @var{n}. One alternative is @code{@var{n} S+LOOP}, where the negative case behaves symmetrical to the positive case: --2 0 ?DO i . -1 +LOOP prints 0 -1 +@code{-2 0 ?DO i . -1 S+LOOP} prints @code{0 -1} --1 0 ?DO i . -1 +LOOP prints 0 +@code{-1 0 ?DO i . -1 S+LOOP} prints @code{0} - 0 0 ?DO i . -1 +LOOP prints nothing +@code{ 0 0 ?DO i . -1 S+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 Forth standard. @@ -734,10 +832,1216 @@ iterates @var{n+1} times; @code{i} produ and ending with 0. Other Forth systems may behave differently, even if they support @code{FOR} loops. -@node Locals +@node Arbitrary control structures, Calls and returns, Counted Loops, Control Structures +@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). + +doc-if +doc-ahead +doc-then +doc-begin +doc-until +doc-again +doc-cs-pick +doc-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: + +doc-else +doc-while +doc-repeat + +Counted loop words constitute a separate group of words: + +doc-?do +doc-do +doc-for +doc-loop +doc-s+loop +doc-+loop +doc-next +doc-leave +doc-?leave +doc-unloop +doc-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 definition (@code{LOOP} etc. compile an @code{UNLOOP} on the +fall-through path). Also, you have to ensure that all @code{LEAVE}s are +resolved (by using one of the loop-ending words or @code{DONE}). + +Another group of control structure words are + +doc-case +doc-endcase +doc-of +doc-endof + +@i{case-sys} and @i{of-sys} cannot be processed using @code{cs-pick} and +@code{cs-roll}. + +@subsubsection Programming Style + +In order to ensure readability we recommend that you do not create +arbitrary control structures directly, but define new control structure +words for the control structure you want and use these words in your +program. + +E.g., instead of writing + +@example +begin + ... +if [ 1 cs-roll ] + ... +again then +@end example + +we recommend defining control structure words, e.g., + +@example +: while ( dest -- orig dest ) + POSTPONE if + 1 cs-roll ; immediate + +: repeat ( orig dest -- ) + POSTPONE again + POSTPONE then ; immediate +@end example + +and then using these to create the control structure: + +@example +begin + ... +while + ... +repeat +@end example + +That's much easier to read, isn't it? Of course, @code{BEGIN} and +@code{WHILE} are predefined, so in this example it would not be +necessary to define them. + +@node Calls and returns, Exception Handling, Arbitrary control structures, Control Structures +@subsection Calls and returns + +A definition can be called simply be writing the name of the +definition. When the end of the definition is reached, it returns. An earlier return can be forced using + +doc-exit + +Don't forget to clean up the return stack and @code{UNLOOP} any +outstanding @code{?DO}...@code{LOOP}s before @code{EXIT}ing. The +primitive compiled by @code{EXIT} is + +doc-;s + +@node Exception Handling, , Calls and returns, Control Structures +@subsection Exception Handling + +doc-catch +doc-throw + +@node Locals, Defining Words, Control Structures, Words @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). + +@menu +* gforth locals:: +* ANS Forth locals:: +@end menu + +@node gforth locals, ANS Forth locals, Locals, Locals +@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: + +@menu +* Where are locals visible by name?:: +* How long do locals live? :: +* Programming Style:: +* Implementation:: +@end menu + +@node Where are locals visible by name?, How long do locals live?, gforth locals, gforth locals +@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 +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: +@example +AHEAD +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 +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. + +doc-assume-live + +E.g., +@example +@{ x @} +AHEAD +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 + +@node How long do locals live?, Programming Style, Where are locals visible by name?, gforth locals +@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). + +@node Programming Style, Implementation, How long do locals live?, gforth locals +@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 @ref{gforth locals} 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 +: strcmp @{ addr1 u1 addr2 u2 -- n @} + u1 u2 min 0 + ?do + addr1 c@ addr2 c@ - ?dup + if + unloop exit + then + addr1 char+ TO addr1 + addr2 char+ TO addr2 + loop + 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 +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 +: strcmp @{ addr1 u1 addr2 u2 -- n @} + addr1 addr2 + 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. + +@node Implementation, , Programming Style, gforth locals +@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-laddr# +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. (@pxref{Wordlists}). 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} +@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+!#} 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 +: +@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 @var{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. + + +@node ANS Forth locals, , gforth locals, Locals +@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. + +@node Defining Words, Wordlists, Locals, Words +@section Defining Words + +@node Values, , Defining Words, Defining Words +@subsection Values + +@node Wordlists, Files, Defining Words, Words +@section Wordlists + +@node Files, Blocks, Wordlists, Words +@section Files + +@node Blocks, Other I/O, Files, Words +@section Blocks + +@node Other I/O, Programming Tools, Blocks, Words +@section Other I/O + +@node Programming Tools, Threading Words, Other I/O, Words +@section Programming Tools + +@menu +* Debugging:: Simple and quick. +* Assertions:: Making your programs self-checking. +@end menu + +@node Debugging, Assertions, Programming Tools, Programming Tools +@subsection Debugging + +The simple debugging aids provided in @file{debugging.fs} +are meant to support a different style of debugging than the +tracing/stepping debuggers used in languages with long turn-around +times. + +A much better (faster) way in fast-compilig languages is to add +printing code at well-selected places, let the program run, look at +the output, see where things went wrong, add more printing code, etc., +until the bug is found. + +The word @code{~~} is easy to insert. It just prints debugging +information (by default the source location and the stack contents). It +is also easy to remove (@kbd{C-x ~} in the Emacs Forth mode to +query-replace them with nothing). The deferred words +@code{printdebugdata} and @code{printdebugline} control the output of +@code{~~}. The default source location output format works well with +Emacs' compilation mode, so you can step through the program at the +source level using @kbd{C-x `} (the advantage over a stepping debugger +is that you can step in any direction and you know where the crash has +happened or where the strange data has occurred). + +Note that the default actions clobber the contents of the pictured +numeric output string, so you should not use @code{~~}, e.g., between +@code{<#} and @code{#>}. + +doc-~~ +doc-printdebugdata +doc-printdebugline + +@node Assertions, , Debugging, Programming Tools +@subsection Assertions + +It is a good idea to make your programs self-checking, in particular, if +you use an assumption (e.g., that a certain field of a data structure is +never zero) that may become wrong during maintenance. GForth supports +assertions for this purpose. They are used like this: + +@example +assert( @var{flag} ) +@end example + +The code between @code{assert(} and @code{)} should compute a flag, that +should be true if everything is alright and false otherwise. It should +not change anything else on the stack. The overall stack effect of the +assertion is @code{( -- )}. E.g. + +@example +assert( 1 1 + 2 = ) \ what we learn in school +assert( dup 0<> ) \ assert that the top of stack is not zero +assert( false ) \ this code should not be reached +@end example + +The need for assertions is different at different times. During +debugging, we want more checking, in production we sometimes care more +for speed. Therefore, assertions can be turned off, i.e., the assertion +becomes a comment. Depending on the importance of an assertion and the +time it takes to check it, you may want to turn off some assertions and +keep others turned on. GForth provides several levels of assertions for +this purpose: + +doc-assert0( +doc-assert1( +doc-assert2( +doc-assert3( +doc-assert( +doc-) + +@code{Assert(} is the same as @code{assert1(}. The variable +@code{assert-level} specifies the highest assertions that are turned +on. I.e., at the default @code{assert-level} of one, @code{assert0(} and +@code{assert1(} assertions perform checking, while @code{assert2(} and +@code{assert3(} assertions are treated as comments. + +Note that the @code{assert-level} is evaluated at compile-time, not at +run-time. I.e., you cannot turn assertions on or off at run-time, you +have to set the @code{assert-level} appropriately before compiling a +piece of code. You can compile several pieces of code at several +@code{assert-level}s (e.g., a trusted library at level 1 and newly +written code at level 3). + +doc-assert-level + +If an assertion fails, a message compatible with Emacs' compilation mode +is produced and the execution is aborted (currently with @code{ABORT"}. +If there is interest, we will introduce a special throw code. But if you +intend to @code{catch} a specific condition, using @code{throw} is +probably more appropriate than an assertion). + +@node Threading Words, , Programming Tools, Words +@section Threading Words + +These words provide access to code addresses and other threading stuff +in gforth (and, possibly, other interpretive Forths). It more or less +abstracts away the differences between direct and indirect threading +(and, for direct threading, the machine dependences). However, at +present this wordset is still inclomplete. It is also pretty low-level; +some day it will hopefully be made unnecessary by an internals words set +that abstracts implementation details away completely. + +doc->code-address +doc->does-code +doc-code-address! +doc-does-code! +doc-does-handler! +doc-/does-handler + +@node ANS conformance, Model, Words, Top +@chapter ANS conformance + +@node Model, Emacs and GForth, ANS conformance, Top +@chapter Model + +@node Emacs and GForth, Internals, Model, Top +@chapter Emacs and GForth + +GForth comes with @file{gforth.el}, an improved version of +@file{forth.el} by Goran Rydqvist (icluded in the TILE package). The +improvements are a better (but still not perfect) handling of +indentation. I have also added comment paragraph filling (@kbd{M-q}), +commenting (@kbd{C-x \}) and uncommenting (@kbd{C-x |}) regions and +removing debugging tracers (@kbd{C-x ~}). I left the stuff I do not use +alone, even though some of it only makes sense for TILE. To get a +description of these features, enter Forth mode and type @kbd{C-h m}. + +In addition, GForth supports Emacs quite well: The source code locations +given in error messages, debugging output (from @code{~~}) and failed +assertion messages are in the right format for Emacs' compilation mode +(@pxref{Compilation, , Running Compilations under Emacs, emacs, Emacs +Manual}) so the source location corresponding to an error or other +message is only a few keystrokes away (@kbd{C-x `} for the next error, +@kbd{C-c C-c} for the error under the cursor). + +Also, if you @code{include} @file{etags.fs}, a new @file{TAGS} file +(@pxref{Tags, , Tags Tables, emacs, Emacs Manual}) will be produced that +contains the definitions of all words defined afterwards. You can then +find the source for a word using @kbd{M-.}. Note that emacs can use +several tags files at the same time (e.g., one for the gforth sources +and one for your program). + +To get all these benefits, add the following lines to your @file{.emacs} +file: + +@example +(autoload 'forth-mode "gforth.el") +(setq auto-mode-alist (cons '("\\.fs\\'" . forth-mode) auto-mode-alist)) +@end example + +@node Internals, Bugs, Emacs and GForth, Top +@chapter Internals + +Reading this section is not necessary for programming with gforth. It +should be helpful for finding your way in the gforth sources. + +@menu +* Portability:: +* Threading:: +* Primitives:: +* System Architecture:: +@end menu + +@node Portability, Threading, Internals, Internals +@section Portability + +One of the main goals of the effort is availability across a wide range +of personal machines. fig-Forth, and, to a lesser extent, F83, achieved +this goal by manually coding the engine in assembly language for several +then-popular processors. This approach is very labor-intensive and the +results are short-lived due to progress in computer architecture. + +Others have avoided this problem by coding in C, e.g., Mitch Bradley +(cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is +particularly popular for UNIX-based Forths due to the large variety of +architectures of UNIX machines. Unfortunately an implementation in C +does not mix well with the goals of efficiency and with using +traditional techniques: Indirect or direct threading cannot be expressed +in C, and switch threading, the fastest technique available in C, is +significantly slower. Another problem with C is that it's very +cumbersome to express double integer arithmetic. + +Fortunately, there is a portable language that does not have these +limitations: GNU C, the version of C processed by the GNU C compiler +(@pxref{C Extensions, , Extensions to the C Language Family, gcc.info, +GNU C Manual}). Its labels as values feature (@pxref{Labels as Values, , +Labels as Values, gcc.info, GNU C Manual}) makes direct and indirect +threading possible, its @code{long long} type (@pxref{Long Long, , +Double-Word Integers, gcc.info, GNU C Manual}) corresponds to Forths +double numbers. GNU C is available for free on all important (and many +unimportant) UNIX machines, VMS, 80386s running MS-DOS, the Amiga, and +the Atari ST, so a Forth written in GNU C can run on all these +machines@footnote{Due to Apple's look-and-feel lawsuit it is not +available on the Mac (@pxref{Boycott, , Protect Your Freedom---Fight +``Look And Feel'', gcc.info, GNU C Manual}).}. + +Writing in a portable language has the reputation of producing code that +is slower than assembly. For our Forth engine we repeatedly looked at +the code produced by the compiler and eliminated most compiler-induced +inefficiencies by appropriate changes in the source-code. + +However, register allocation cannot be portably influenced by the +programmer, leading to some inefficiencies on register-starved +machines. We use explicit register declarations (@pxref{Explicit Reg +Vars, , Variables in Specified Registers, gcc.info, GNU C Manual}) to +improve the speed on some machines. They are turned on by using the +@code{gcc} switch @code{-DFORCE_REG}. Unfortunately, this feature not +only depends on the machine, but also on the compiler version: On some +machines some compiler versions produce incorrect code when certain +explicit register declarations are used. So by default +@code{-DFORCE_REG} is not used. + +@node Threading, Primitives, Portability, Internals +@section Threading + +GNU C's labels as values extension (available since @code{gcc-2.0}, +@pxref{Labels as Values, , Labels as Values, gcc.info, GNU C Manual}) +makes it possible to take the address of @var{label} by writing +@code{&&@var{label}}. This address can then be used in a statement like +@code{goto *@var{address}}. I.e., @code{goto *&&x} is the same as +@code{goto x}. + +With this feature an indirect threaded NEXT looks like: +@example +cfa = *ip++; +ca = *cfa; +goto *ca; +@end example +For those unfamiliar with the names: @code{ip} is the Forth instruction +pointer; the @code{cfa} (code-field address) corresponds to ANS Forths +execution token and points to the code field of the next word to be +executed; The @code{ca} (code address) fetched from there points to some +executable code, e.g., a primitive or the colon definition handler +@code{docol}. + +Direct threading is even simpler: +@example +ca = *ip++; +goto *ca; +@end example + +Of course we have packaged the whole thing neatly in macros called +@code{NEXT} and @code{NEXT1} (the part of NEXT after fetching the cfa). + +@menu +* Scheduling:: +* Direct or Indirect Threaded?:: +* DOES>:: +@end menu + +@node Scheduling, Direct or Indirect Threaded?, Threading, Threading +@subsection Scheduling + +There is a little complication: Pipelined and superscalar processors, +i.e., RISC and some modern CISC machines can process independent +instructions while waiting for the results of an instruction. The +compiler usually reorders (schedules) the instructions in a way that +achieves good usage of these delay slots. However, on our first tries +the compiler did not do well on scheduling primitives. E.g., for +@code{+} implemented as +@example +n=sp[0]+sp[1]; +sp++; +sp[0]=n; +NEXT; +@end example +the NEXT comes strictly after the other code, i.e., there is nearly no +scheduling. After a little thought the problem becomes clear: The +compiler cannot know that sp and ip point to different addresses (and +the version of @code{gcc} we used would not know it even if it was +possible), so it could not move the load of the cfa above the store to +the TOS. Indeed the pointers could be the same, if code on or very near +the top of stack were executed. In the interest of speed we chose to +forbid this probably unused ``feature'' and helped the compiler in +scheduling: NEXT is divided into the loading part (@code{NEXT_P1}) and +the goto part (@code{NEXT_P2}). @code{+} now looks like: +@example +n=sp[0]+sp[1]; +sp++; +NEXT_P1; +sp[0]=n; +NEXT_P2; +@end example +This can be scheduled optimally by the compiler. + +This division can be turned off with the switch @code{-DCISC_NEXT}. This +switch is on by default on machines that do not profit from scheduling +(e.g., the 80386), in order to preserve registers. + +@node Direct or Indirect Threaded?, DOES>, Scheduling, Threading +@subsection Direct or Indirect Threaded? + +Both! After packaging the nasty details in macro definitions we +realized that we could switch between direct and indirect threading by +simply setting a compilation flag (@code{-DDIRECT_THREADED}) and +defining a few machine-specific macros for the direct-threading case. +On the Forth level we also offer access words that hide the +differences between the threading methods (@pxref{Threading Words}). + +Indirect threading is implemented completely +machine-independently. Direct threading needs routines for creating +jumps to the executable code (e.g. to docol or dodoes). These routines +are inherently machine-dependent, but they do not amount to many source +lines. I.e., even porting direct threading to a new machine is a small +effort. + +@node DOES>, , Direct or Indirect Threaded?, Threading +@subsection DOES> +One of the most complex parts of a Forth engine is @code{dodoes}, i.e., +the chunk of code executed by every word defined by a +@code{CREATE}...@code{DOES>} pair. The main problem here is: How to find +the Forth code to be executed, i.e. the code after the @code{DOES>} (the +DOES-code)? There are two solutions: + +In fig-Forth the code field points directly to the dodoes and the +DOES-code address is stored in the cell after the code address +(i.e. at cfa cell+). It may seem that this solution is illegal in the +Forth-79 and all later standards, because in fig-Forth this address +lies in the body (which is illegal in these standards). However, by +making the code field larger for all words this solution becomes legal +again. We use this approach for the indirect threaded version. Leaving +a cell unused in most words is a bit wasteful, but on the machines we +are targetting this is hardly a problem. The other reason for having a +code field size of two cells is to avoid having different image files +for direct and indirect threaded systems (@pxref{System Architecture}). + +The other approach is that the code field points or jumps to the cell +after @code{DOES}. In this variant there is a jump to @code{dodoes} at +this address. @code{dodoes} can then get the DOES-code address by +computing the code address, i.e., the address of the jump to dodoes, +and add the length of that jump field. A variant of this is to have a +call to @code{dodoes} after the @code{DOES>}; then the return address +(which can be found in the return register on RISCs) is the DOES-code +address. Since the two cells available in the code field are usually +used up by the jump to the code address in direct threading, we use +this approach for direct threading. We did not want to add another +cell to the code field. + +@node Primitives, System Architecture, Threading, Internals +@section Primitives + +@menu +* Automatic Generation:: +* TOS Optimization:: +* Produced code:: +@end menu + +@node Automatic Generation, TOS Optimization, Primitives, Primitives +@subsection Automatic Generation + +Since the primitives are implemented in a portable language, there is no +longer any need to minimize the number of primitives. On the contrary, +having many primitives is an advantage: speed. In order to reduce the +number of errors in primitives and to make programming them easier, we +provide a tool, the primitive generator (@file{prims2x.fs}), that +automatically generates most (and sometimes all) of the C code for a +primitive from the stack effect notation. The source for a primitive +has the following form: + +@format +@var{Forth-name} @var{stack-effect} @var{category} [@var{pronounc.}] +[@code{""}@var{glossary entry}@code{""}] +@var{C code} +[@code{:} +@var{Forth code}] +@end format + +The items in brackets are optional. The category and glossary fields +are there for generating the documentation, the Forth code is there +for manual implementations on machines without GNU C. E.g., the source +for the primitive @code{+} is: +@example ++ n1 n2 -- n core plus +n = n1+n2; +@end example + +This looks like a specification, but in fact @code{n = n1+n2} is C +code. Our primitive generation tool extracts a lot of information from +the stack effect notations@footnote{We use a one-stack notation, even +though we have separate data and floating-point stacks; The separate +notation can be generated easily from the unified notation.}: The number +of items popped from and pushed on the stack, their type, and by what +name they are referred to in the C code. It then generates a C code +prelude and postlude for each primitive. The final C code for @code{+} +looks like this: + +@example +I_plus: /* + ( n1 n2 -- n ) */ /* label, stack effect */ +/* */ /* documentation */ +@{ +DEF_CA /* definition of variable ca (indirect threading) */ +Cell n1; /* definitions of variables */ +Cell n2; +Cell n; +n1 = (Cell) sp[1]; /* input */ +n2 = (Cell) TOS; +sp += 1; /* stack adjustment */ +NAME("+") /* debugging output (with -DDEBUG) */ +@{ +n = n1+n2; /* C code taken from the source */ +@} +NEXT_P1; /* NEXT part 1 */ +TOS = (Cell)n; /* output */ +NEXT_P2; /* NEXT part 2 */ +@} +@end example + +This looks long and inefficient, but the GNU C compiler optimizes quite +well and produces optimal code for @code{+} on, e.g., the R3000 and the +HP RISC machines: Defining the @code{n}s does not produce any code, and +using them as intermediate storage also adds no cost. + +There are also other optimizations, that are not illustrated by this +example: Assignments between simple variables are usually for free (copy +propagation). If one of the stack items is not used by the primitive +(e.g. in @code{drop}), the compiler eliminates the load from the stack +(dead code elimination). On the other hand, there are some things that +the compiler does not do, therefore they are performed by +@file{prims2x.fs}: The compiler does not optimize code away that stores +a stack item to the place where it just came from (e.g., @code{over}). + +While programming a primitive is usually easy, there are a few cases +where the programmer has to take the actions of the generator into +account, most notably @code{?dup}, but also words that do not (always) +fall through to NEXT. + +@node TOS Optimization, Produced code, Automatic Generation, Primitives +@subsection TOS Optimization + +An important optimization for stack machine emulators, e.g., Forth +engines, is keeping one or more of the top stack items in +registers. If a word has the stack effect @var{in1}...@var{inx} @code{--} +@var{out1}...@var{outy}, keeping the top @var{n} items in registers +@itemize +@item +is better than keeping @var{n-1} items, if @var{x>=n} and @var{y>=n}, +due to fewer loads from and stores to the stack. +@item is slower than keeping @var{n-1} items, if @var{x<>y} and @var{x