X-Git-Url: https://git.proxmox.com/?p=mirror_edk2.git;a=blobdiff_plain;f=Tools%2FCCode%2FSource%2FPccts%2FCHANGES_SUMMARY.txt;fp=Tools%2FCCode%2FSource%2FPccts%2FCHANGES_SUMMARY.txt;h=0000000000000000000000000000000000000000;hp=91defae169c4e7f5c3f059ae52130b94715a9e77;hb=808def96aa4589fba9c2d0ea55837754a3b7a4f7;hpb=9216450d1143056a50a5f916984a2d7faf590488 diff --git a/Tools/CCode/Source/Pccts/CHANGES_SUMMARY.txt b/Tools/CCode/Source/Pccts/CHANGES_SUMMARY.txt deleted file mode 100644 index 91defae169..0000000000 --- a/Tools/CCode/Source/Pccts/CHANGES_SUMMARY.txt +++ /dev/null @@ -1,2049 +0,0 @@ -====================================================================== - - CHANGES_SUMMARY.TXT - - A QUICK overview of changes from 1.33 in reverse order - - A summary of additions rather than bug fixes and minor code changes. - - Numbers refer to items in CHANGES_FROM_133*.TXT - which may contain additional information. - - DISCLAIMER - - The software and these notes are provided "as is". They may include - typographical or technical errors and their authors disclaims all - liability of any kind or nature for damages due to error, fault, - defect, or deficiency regardless of cause. All warranties of any - kind, either express or implied, including, but not limited to, the - implied warranties of merchantability and fitness for a particular - purpose are disclaimed. - -====================================================================== - -#258. You can specify a user-defined base class for your parser - - The base class must constructor must have a signature similar to - that of ANTLRParser. - -#253. Generation of block preamble (-preamble and -preamble_first) - - The antlr option -preamble causes antlr to insert the code - BLOCK_PREAMBLE at the start of each rule and block. - - The antlr option -preamble_first is similar, but inserts the - code BLOCK_PREAMBLE_FIRST(PreambleFirst_123) where the symbol - PreambleFirst_123 is equivalent to the first set defined by - the #FirstSetSymbol described in Item #248. - -#248. Generate symbol for first set of an alternative - - rr : #FirstSetSymbol(rr_FirstSet) ( Foo | Bar ) ; - -#216. Defer token fetch for C++ mode - - When the ANTLRParser class is built with the pre-processor option - ZZDEFER_FETCH defined, the fetch of new tokens by consume() is deferred - until LA(i) or LT(i) is called. - -#215. Use reset() to reset DLGLexerBase -#188. Added pccts/h/DLG_stream_input.h -#180. Added ANTLRParser::getEofToken() -#173. -glms for Microsoft style filenames with -gl -#170. Suppression for predicates with lookahead depth >1 - - Consider the following grammar with -ck 2 and the predicate in rule - "a" with depth 2: - - r1 : (ab)* "@" - ; - - ab : a - | b - ; - - a : (A B)? => <>? A B C - ; - - b : A B C - ; - - Normally, the predicate would be hoisted into rule r1 in order to - determine whether to call rule "ab". However it should *not* be - hoisted because, even if p is false, there is a valid alternative - in rule b. With "-mrhoistk on" the predicate will be suppressed. - - If "-info p" command line option is present the following information - will appear in the generated code: - - while ( (LA(1)==A) - #if 0 - - Part (or all) of predicate with depth > 1 suppressed by alternative - without predicate - - pred << p(LATEXT(2))>>? - depth=k=2 ("=>" guard) rule a line 8 t1.g - tree context: - (root = A - B - ) - - The token sequence which is suppressed: ( A B ) - The sequence of references which generate that sequence of tokens: - - 1 to ab r1/1 line 1 t1.g - 2 ab ab/1 line 4 t1.g - 3 to b ab/2 line 5 t1.g - 4 b b/1 line 11 t1.g - 5 #token A b/1 line 11 t1.g - 6 #token B b/1 line 11 t1.g - - #endif - - A slightly more complicated example: - - r1 : (ab)* "@" - ; - - ab : a - | b - ; - - a : (A B)? => <>? (A B | D E) - ; - - b : <>? D E - ; - - - In this case, the sequence (D E) in rule "a" which lies behind - the guard is used to suppress the predicate with context (D E) - in rule b. - - while ( (LA(1)==A || LA(1)==D) - #if 0 - - Part (or all) of predicate with depth > 1 suppressed by alternative - without predicate - - pred << q(LATEXT(2))>>? - depth=k=2 rule b line 11 t2.g - tree context: - (root = D - E - ) - - The token sequence which is suppressed: ( D E ) - The sequence of references which generate that sequence of tokens: - - 1 to ab r1/1 line 1 t2.g - 2 ab ab/1 line 4 t2.g - 3 to a ab/1 line 4 t2.g - 4 a a/1 line 8 t2.g - 5 #token D a/1 line 8 t2.g - 6 #token E a/1 line 8 t2.g - - #endif - && - #if 0 - - pred << p(LATEXT(2))>>? - depth=k=2 ("=>" guard) rule a line 8 t2.g - tree context: - (root = A - B - ) - - #endif - - (! ( LA(1)==A && LA(2)==B ) || p(LATEXT(2)) ) { - ab(); - ... - -#165. (Changed in MR13) option -newAST - - To create ASTs from an ANTLRTokenPtr antlr usually calls - "new AST(ANTLRTokenPtr)". This option generates a call - to "newAST(ANTLRTokenPtr)" instead. This allows a user - to define a parser member function to create an AST object. - -#161. (Changed in MR13) Switch -gxt inhibits generation of tokens.h - -#158. (Changed in MR13) #header causes problem for pre-processors - - A user who runs the C pre-processor on antlr source suggested - that another syntax be allowed. With MR13 such directives - such as #header, #pragma, etc. may be written as "\#header", - "\#pragma", etc. For escaping pre-processor directives inside - a #header use something like the following: - - \#header - << - \#include - >> - -#155. (Changed in MR13) Context behind predicates can suppress - - With -mrhoist enabled the context behind a guarded predicate can - be used to suppress other predicates. Consider the following grammar: - - r0 : (r1)+; - - r1 : rp - | rq - ; - rp : <

>? B ; - rq : (A)? => <>? (A|B); - - In earlier versions both predicates "p" and "q" would be hoisted into - rule r0. With MR12c predicate p is suppressed because the context which - follows predicate q includes "B" which can "cover" predicate "p". In - other words, in trying to decide in r0 whether to call r1, it doesn't - really matter whether p is false or true because, either way, there is - a valid choice within r1. - -#154. (Changed in MR13) Making hoist suppression explicit using <> - - A common error, even among experienced pccts users, is to code - an init-action to inhibit hoisting rather than a leading action. - An init-action does not inhibit hoisting. - - This was coded: - - rule1 : <<;>> rule2 - - This is what was meant: - - rule1 : <<;>> <<;>> rule2 - - With MR13, the user can code: - - rule1 : <<;>> <> rule2 - - The following will give an error message: - - rule1 : <> rule2 - - If the <> appears as an init-action rather than a leading - action an error message is issued. The meaning of an init-action - containing "nohoist" is unclear: does it apply to just one - alternative or to all alternatives ? - -#151a. Addition of ANTLRParser::getLexer(), ANTLRTokenStream::getLexer() - - You must manually cast the ANTLRTokenStream to your program's - lexer class. Because the name of the lexer's class is not fixed. - Thus it is impossible to incorporate it into the DLGLexerBase - class. - -#151b.(Changed in MR12) ParserBlackBox member getLexer() - -#150. (Changed in MR12) syntaxErrCount and lexErrCount now public - -#149. (Changed in MR12) antlr option -info o (letter o for orphan) - - If there is more than one rule which is not referenced by any - other rule then all such rules are listed. This is useful for - alerting one to rules which are not used, but which can still - contribute to ambiguity. - -#148. (Changed in MR11) #token names appearing in zztokens,token_tbl - - One can write: - - #token Plus ("+") "\+" - #token RP ("(") "\(" - #token COM ("comment begin") "/\*" - - The string in parenthesis will be used in syntax error messages. - -#146. (Changed in MR11) Option -treport for locating "difficult" alts - - It can be difficult to determine which alternatives are causing - pccts to work hard to resolve an ambiguity. In some cases the - ambiguity is successfully resolved after much CPU time so there - is no message at all. - - A rough measure of the amount of work being peformed which is - independent of the CPU speed and system load is the number of - tnodes created. Using "-info t" gives information about the - total number of tnodes created and the peak number of tnodes. - - Tree Nodes: peak 1300k created 1416k lost 0 - - It also puts in the generated C or C++ file the number of tnodes - created for a rule (at the end of the rule). However this - information is not sufficient to locate the alternatives within - a rule which are causing the creation of tnodes. - - Using: - - antlr -treport 100000 .... - - causes antlr to list on stdout any alternatives which require the - creation of more than 100,000 tnodes, along with the lookahead sets - for those alternatives. - - The following is a trivial case from the ansi.g grammar which shows - the format of the report. This report might be of more interest - in cases where 1,000,000 tuples were created to resolve the ambiguity. - - ------------------------------------------------------------------------- - There were 0 tuples whose ambiguity could not be resolved - by full lookahead - There were 157 tnodes created to resolve ambiguity between: - - Choice 1: statement/2 line 475 file ansi.g - Choice 2: statement/3 line 476 file ansi.g - - Intersection of lookahead[1] sets: - - IDENTIFIER - - Intersection of lookahead[2] sets: - - LPARENTHESIS COLON AMPERSAND MINUS - STAR PLUSPLUS MINUSMINUS ONESCOMPLEMENT - NOT SIZEOF OCTALINT DECIMALINT - HEXADECIMALINT FLOATONE FLOATTWO IDENTIFIER - STRING CHARACTER - ------------------------------------------------------------------------- - -#143. (Changed in MR11) Optional ";" at end of #token statement - - Fixes problem of: - - #token X "x" - - << - parser action - >> - - Being confused with: - - #token X "x" <> - -#142. (Changed in MR11) class BufFileInput subclass of DLGInputStream - - Alexey Demakov (demakov@kazbek.ispras.ru) has supplied class - BufFileInput derived from DLGInputStream which provides a - function lookahead(char *string) to test characters in the - input stream more than one character ahead. - The class is located in pccts/h/BufFileInput.* of the kit. - -#140. #pred to define predicates - - +---------------------------------------------------+ - | Note: Assume "-prc on" for this entire discussion | - +---------------------------------------------------+ - - A problem with predicates is that each one is regarded as - unique and capable of disambiguating cases where two - alternatives have identical lookahead. For example: - - rule : <>? A - | <>? A - ; - - will not cause any error messages or warnings to be issued - by earlier versions of pccts. To compare the text of the - predicates is an incomplete solution. - - In 1.33MR11 I am introducing the #pred statement in order to - solve some problems with predicates. The #pred statement allows - one to give a symbolic name to a "predicate literal" or a - "predicate expression" in order to refer to it in other predicate - expressions or in the rules of the grammar. - - The predicate literal associated with a predicate symbol is C - or C++ code which can be used to test the condition. A - predicate expression defines a predicate symbol in terms of other - predicate symbols using "!", "&&", and "||". A predicate symbol - can be defined in terms of a predicate literal, a predicate - expression, or *both*. - - When a predicate symbol is defined with both a predicate literal - and a predicate expression, the predicate literal is used to generate - code, but the predicate expression is used to check for two - alternatives with identical predicates in both alternatives. - - Here are some examples of #pred statements: - - #pred IsLabel <>? - #pred IsLocalVar <>? - #pred IsGlobalVar <>? - #pred IsVar <>? IsLocalVar || IsGlobalVar - #pred IsScoped <>? IsLabel || IsLocalVar - - I hope that the use of EBNF notation to describe the syntax of the - #pred statement will not cause problems for my readers (joke). - - predStatement : "#pred" - CapitalizedName - ( - "<>?" - | "<>?" predOrExpr - | predOrExpr - ) - ; - - predOrExpr : predAndExpr ( "||" predAndExpr ) * ; - - predAndExpr : predPrimary ( "&&" predPrimary ) * ; - - predPrimary : CapitalizedName - | "!" predPrimary - | "(" predOrExpr ")" - ; - - What is the purpose of this nonsense ? - - To understand how predicate symbols help, you need to realize that - predicate symbols are used in two different ways with two different - goals. - - a. Allow simplification of predicates which have been combined - during predicate hoisting. - - b. Allow recognition of identical predicates which can't disambiguate - alternatives with common lookahead. - - First we will discuss goal (a). Consider the following rule: - - rule0: rule1 - | ID - | ... - ; - - rule1: rule2 - | rule3 - ; - - rule2: <>? ID ; - rule3: <>? ID ; - - When the predicates in rule2 and rule3 are combined by hoisting - to create a prediction expression for rule1 the result is: - - if ( LA(1)==ID - && ( isX(LATEXT(1) || !isX(LATEXT(1) ) ) { rule1(); ... - - This is inefficient, but more importantly, can lead to false - assumptions that the predicate expression distinguishes the rule1 - alternative with some other alternative with lookahead ID. In - MR11 one can write: - - #pred IsX <>? - - ... - - rule2: <>? ID ; - rule3: <>? ID ; - - During hoisting MR11 recognizes this as a special case and - eliminates the predicates. The result is a prediction - expression like the following: - - if ( LA(1)==ID ) { rule1(); ... - - Please note that the following cases which appear to be equivalent - *cannot* be simplified by MR11 during hoisting because the hoisting - logic only checks for a "!" in the predicate action, not in the - predicate expression for a predicate symbol. - - *Not* equivalent and is not simplified during hoisting: - - #pred IsX <>? - #pred NotX <>? - ... - rule2: <>? ID ; - rule3: <>? ID ; - - *Not* equivalent and is not simplified during hoisting: - - #pred IsX <>? - #pred NotX !IsX - ... - rule2: <>? ID ; - rule3: <>? ID ; - - Now we will discuss goal (b). - - When antlr discovers that there is a lookahead ambiguity between - two alternatives it attempts to resolve the ambiguity by searching - for predicates in both alternatives. In the past any predicate - would do, even if the same one appeared in both alternatives: - - rule: <>? X - | <>? X - ; - - The #pred statement is a start towards solving this problem. - During ambiguity resolution (*not* predicate hoisting) the - predicates for the two alternatives are expanded and compared. - Consider the following example: - - #pred Upper <>? - #pred Lower <>? - #pred Alpha <>? Upper || Lower - - rule0: rule1 - | <>? ID - ; - - rule1: - | rule2 - | rule3 - ... - ; - - rule2: <>? ID; - rule3: <>? ID; - - The definition of #pred Alpha expresses: - - a. to test the predicate use the C code "isAlpha(LATEXT(1))" - - b. to analyze the predicate use the information that - Alpha is equivalent to the union of Upper and Lower, - - During ambiguity resolution the definition of Alpha is expanded - into "Upper || Lower" and compared with the predicate in the other - alternative, which is also "Upper || Lower". Because they are - identical MR11 will report a problem. - - ------------------------------------------------------------------------- - t10.g, line 5: warning: the predicates used to disambiguate rule rule0 - (file t10.g alt 1 line 5 and alt 2 line 6) - are identical when compared without context and may have no - resolving power for some lookahead sequences. - ------------------------------------------------------------------------- - - If you use the "-info p" option the output file will contain: - - +----------------------------------------------------------------------+ - |#if 0 | - | | - |The following predicates are identical when compared without | - | lookahead context information. For some ambiguous lookahead | - | sequences they may not have any power to resolve the ambiguity. | - | | - |Choice 1: rule0/1 alt 1 line 5 file t10.g | - | | - | The original predicate for choice 1 with available context | - | information: | - | | - | OR expr | - | | - | pred << Upper>>? | - | depth=k=1 rule rule2 line 14 t10.g | - | set context: | - | ID | - | | - | pred << Lower>>? | - | depth=k=1 rule rule3 line 15 t10.g | - | set context: | - | ID | - | | - | The predicate for choice 1 after expansion (but without context | - | information): | - | | - | OR expr | - | | - | pred << isUpper(LATEXT(1))>>? | - | depth=k=1 rule line 1 t10.g | - | | - | pred << isLower(LATEXT(1))>>? | - | depth=k=1 rule line 2 t10.g | - | | - | | - |Choice 2: rule0/2 alt 2 line 6 file t10.g | - | | - | The original predicate for choice 2 with available context | - | information: | - | | - | pred << Alpha>>? | - | depth=k=1 rule rule0 line 6 t10.g | - | set context: | - | ID | - | | - | The predicate for choice 2 after expansion (but without context | - | information): | - | | - | OR expr | - | | - | pred << isUpper(LATEXT(1))>>? | - | depth=k=1 rule line 1 t10.g | - | | - | pred << isLower(LATEXT(1))>>? | - | depth=k=1 rule line 2 t10.g | - | | - | | - |#endif | - +----------------------------------------------------------------------+ - - The comparison of the predicates for the two alternatives takes - place without context information, which means that in some cases - the predicates will be considered identical even though they operate - on disjoint lookahead sets. Consider: - - #pred Alpha - - rule1: <>? ID - | <>? Label - ; - - Because the comparison of predicates takes place without context - these will be considered identical. The reason for comparing - without context is that otherwise it would be necessary to re-evaluate - the entire predicate expression for each possible lookahead sequence. - This would require more code to be written and more CPU time during - grammar analysis, and it is not yet clear whether anyone will even make - use of the new #pred facility. - - A temporary workaround might be to use different #pred statements - for predicates you know have different context. This would avoid - extraneous warnings. - - The above example might be termed a "false positive". Comparison - without context will also lead to "false negatives". Consider the - following example: - - #pred Alpha - #pred Beta - - rule1: <>? A - | rule2 - ; - - rule2: <>? A - | <>? B - ; - - The predicate used for alt 2 of rule1 is (Alpha || Beta). This - appears to be different than the predicate Alpha used for alt1. - However, the context of Beta is B. Thus when the lookahead is A - Beta will have no resolving power and Alpha will be used for both - alternatives. Using the same predicate for both alternatives isn't - very helpful, but this will not be detected with 1.33MR11. - - To properly handle this the predicate expression would have to be - evaluated for each distinct lookahead context. - - To determine whether two predicate expressions are identical is - difficult. The routine may fail to identify identical predicates. - - The #pred feature also compares predicates to see if a choice between - alternatives which is resolved by a predicate which makes the second - choice unreachable. Consider the following example: - - #pred A <>? - #pred B <>? - #pred A_or_B A || B - - r : s - | t - ; - s : <>? ID - ; - t : <>? ID - ; - - ---------------------------------------------------------------------------- - t11.g, line 5: warning: the predicate used to disambiguate the - first choice of rule r - (file t11.g alt 1 line 5 and alt 2 line 6) - appears to "cover" the second predicate when compared without context. - The second predicate may have no resolving power for some lookahead - sequences. - ---------------------------------------------------------------------------- - -#132. (Changed in 1.33MR11) Recognition of identical predicates in alts - - Prior to 1.33MR11, there would be no ambiguity warning when the - very same predicate was used to disambiguate both alternatives: - - test: ref B - | ref C - ; - - ref : <>? A - - In 1.33MR11 this will cause the warning: - - warning: the predicates used to disambiguate rule test - (file v98.g alt 1 line 1 and alt 2 line 2) - are identical and have no resolving power - - ----------------- Note ----------------- - - This is different than the following case - - test: <>? A B - | <>? A C - ; - - In this case there are two distinct predicates - which have exactly the same text. In the first - example there are two references to the same - predicate. The problem represented by this - grammar will be addressed later. - - -#127. (Changed in 1.33MR11) - - Count Syntax Errors Count DLG Errors - ------------------- ---------------- - - C++ mode ANTLRParser:: DLGLexerBase:: - syntaxErrCount lexErrCount - C mode zzSyntaxErrCount zzLexErrCount - - The C mode variables are global and initialized to 0. - They are *not* reset to 0 automatically when antlr is - restarted. - - The C++ mode variables are public. They are initialized - to 0 by the constructors. They are *not* reset to 0 by the - ANTLRParser::init() method. - - Suggested by Reinier van den Born (reinier@vnet.ibm.com). - -#126. (Changed in 1.33MR11) Addition of #first <<...>> - - The #first <<...>> inserts the specified text in the output - files before any other #include statements required by pccts. - The only things before the #first text are comments and - a #define ANTLR_VERSION. - - Requested by and Esa Pulkkinen (esap@cs.tut.fi) and Alexin - Zoltan (alexin@inf.u-szeged.hu). - -#124. A Note on the New "&&" Style Guarded Predicates - - I've been asked several times, "What is the difference between - the old "=>" style guard predicates and the new style "&&" guard - predicates, and how do you choose one over the other" ? - - The main difference is that the "=>" does not apply the - predicate if the context guard doesn't match, whereas - the && form always does. What is the significance ? - - If you have a predicate which is not on the "leading edge" - it is cannot be hoisted. Suppose you need a predicate that - looks at LA(2). You must introduce it manually. The - classic example is: - - castExpr : - LP typeName RP - | .... - ; - - typeName : <>? ID - | STRUCT ID - ; - - The problem is that isTypeName() isn't on the leading edge - of typeName, so it won't be hoisted into castExpr to help - make a decision on which production to choose. - - The *first* attempt to fix it is this: - - castExpr : - <>? - LP typeName RP - | .... - ; - - Unfortunately, this won't work because it ignores - the problem of STRUCT. The solution is to apply - isTypeName() in castExpr if LA(2) is an ID and - don't apply it when LA(2) is STRUCT: - - castExpr : - (LP ID)? => <>? - LP typeName RP - | .... - ; - - In conclusion, the "=>" style guarded predicate is - useful when: - - a. the tokens required for the predicate - are not on the leading edge - b. there are alternatives in the expression - selected by the predicate for which the - predicate is inappropriate - - If (b) were false, then one could use a simple - predicate (assuming "-prc on"): - - castExpr : - <>? - LP typeName RP - | .... - ; - - typeName : <>? ID - ; - - So, when do you use the "&&" style guarded predicate ? - - The new-style "&&" predicate should always be used with - predicate context. The context guard is in ADDITION to - the automatically computed context. Thus it useful for - predicates which depend on the token type for reasons - other than context. - - The following example is contributed by Reinier van den Born - (reinier@vnet.ibm.com). - - +-------------------------------------------------------------------------+ - | This grammar has two ways to call functions: | - | | - | - a "standard" call syntax with parens and comma separated args | - | - a shell command like syntax (no parens and spacing separated args) | - | | - | The former also allows a variable to hold the name of the function, | - | the latter can also be used to call external commands. | - | | - | The grammar (simplified) looks like this: | - | | - | fun_call : ID "(" { expr ("," expr)* } ")" | - | /* ID is function name */ | - | | "@" ID "(" { expr ("," expr)* } ")" | - | /* ID is var containing fun name */ | - | ; | - | | - | command : ID expr* /* ID is function name */ | - | | path expr* /* path is external command name */ | - | ; | - | | - | path : ID /* left out slashes and such */ | - | | "@" ID /* ID is environment var */ | - | ; | - | | - | expr : .... | - | | "(" expr ")"; | - | | - | call : fun_call | - | | command | - | ; | - | | - | Obviously the call is wildly ambiguous. This is more or less how this | - | is to be resolved: | - | | - | A call begins with an ID or an @ followed by an ID. | - | | - | If it is an ID and if it is an ext. command name -> command | - | if followed by a paren -> fun_call | - | otherwise -> command | - | | - | If it is an @ and if the ID is a var name -> fun_call | - | otherwise -> command | - | | - | One can implement these rules quite neatly using && predicates: | - | | - | call : ("@" ID)? && <>? fun_call | - | | (ID)? && <>? command | - | | (ID "(")? fun_call | - | | command | - | ; | - | | - | This can be done better, so it is not an ideal example, but it | - | conveys the principle. | - +-------------------------------------------------------------------------+ - -#122. (Changed in 1.33MR11) Member functions to reset DLG in C++ mode - - void DLGFileReset(FILE *f) { input = f; found_eof = 0; } - void DLGStringReset(DLGChar *s) { input = s; p = &input[0]; } - - Supplied by R.A. Nelson (cowboy@VNET.IBM.COM) - -#119. (Changed in 1.33MR11) Ambiguity aid for grammars - - The user can ask for additional information on ambiguities reported - by antlr to stdout. At the moment, only one ambiguity report can - be created in an antlr run. - - This feature is enabled using the "-aa" (Ambiguity Aid) option. - - The following options control the reporting of ambiguities: - - -aa ruleName Selects reporting by name of rule - -aa lineNumber Selects reporting by line number - (file name not compared) - - -aam Selects "multiple" reporting for a token - in the intersection set of the - alternatives. - - For instance, the token ID may appear dozens - of times in various paths as the program - explores the rules which are reachable from - the point of an ambiguity. With option -aam - every possible path the search program - encounters is reported. - - Without -aam only the first encounter is - reported. This may result in incomplete - information, but the information may be - sufficient and much shorter. - - -aad depth Selects the depth of the search. - The default value is 1. - - The number of paths to be searched, and the - size of the report can grow geometrically - with the -ck value if a full search for all - contributions to the source of the ambiguity - is explored. - - The depth represents the number of tokens - in the lookahead set which are matched against - the set of ambiguous tokens. A depth of 1 - means that the search stops when a lookahead - sequence of just one token is matched. - - A k=1 ck=6 grammar might generate 5,000 items - in a report if a full depth 6 search is made - with the Ambiguity Aid. The source of the - problem may be in the first token and obscured - by the volume of data - I hesitate to call - it information. - - When the user selects a depth > 1, the search - is first performed at depth=1 for both - alternatives, then depth=2 for both alternatives, - etc. - - Sample output for rule grammar in antlr.g itself: - - +---------------------------------------------------------------------+ - | Ambiguity Aid | - | | - | Choice 1: grammar/70 line 632 file a.g | - | Choice 2: grammar/82 line 644 file a.g | - | | - | Intersection of lookahead[1] sets: | - | | - | "\}" "class" "#errclass" "#tokclass" | - | | - | Choice:1 Depth:1 Group:1 ("#errclass") | - | 1 in (...)* block grammar/70 line 632 a.g | - | 2 to error grammar/73 line 635 a.g | - | 3 error error/1 line 894 a.g | - | 4 #token "#errclass" error/2 line 895 a.g | - | | - | Choice:1 Depth:1 Group:2 ("#tokclass") | - | 2 to tclass grammar/74 line 636 a.g | - | 3 tclass tclass/1 line 937 a.g | - | 4 #token "#tokclass" tclass/2 line 938 a.g | - | | - | Choice:1 Depth:1 Group:3 ("class") | - | 2 to class_def grammar/75 line 637 a.g | - | 3 class_def class_def/1 line 669 a.g | - | 4 #token "class" class_def/3 line 671 a.g | - | | - | Choice:1 Depth:1 Group:4 ("\}") | - | 2 #token "\}" grammar/76 line 638 a.g | - | | - | Choice:2 Depth:1 Group:5 ("#errclass") | - | 1 in (...)* block grammar/83 line 645 a.g | - | 2 to error grammar/93 line 655 a.g | - | 3 error error/1 line 894 a.g | - | 4 #token "#errclass" error/2 line 895 a.g | - | | - | Choice:2 Depth:1 Group:6 ("#tokclass") | - | 2 to tclass grammar/94 line 656 a.g | - | 3 tclass tclass/1 line 937 a.g | - | 4 #token "#tokclass" tclass/2 line 938 a.g | - | | - | Choice:2 Depth:1 Group:7 ("class") | - | 2 to class_def grammar/95 line 657 a.g | - | 3 class_def class_def/1 line 669 a.g | - | 4 #token "class" class_def/3 line 671 a.g | - | | - | Choice:2 Depth:1 Group:8 ("\}") | - | 2 #token "\}" grammar/96 line 658 a.g | - +---------------------------------------------------------------------+ - - For a linear lookahead set ambiguity (where k=1 or for k>1 but - when all lookahead sets [i] with i>? A ; - c : A ; - - Prior to 1.33MR10 the code generated for "start" would resemble: - - while { - if (LA(1)==A && - (!LA(1)==A || isUpper())) { - a(); - } - }; - - This code is wrong because it makes rule "c" unreachable from - "start". The essence of the problem is that antlr fails to - recognize that there can be a valid alternative within "a" even - when the predicate <>? is false. - - In 1.33MR10 with -mrhoist the hoisting of the predicate into - "start" is suppressed because it recognizes that "c" can - cover all the cases where the predicate is false: - - while { - if (LA(1)==A) { - a(); - } - }; - - With the antlr "-info p" switch the user will receive information - about the predicate suppression in the generated file: - - -------------------------------------------------------------- - #if 0 - - Hoisting of predicate suppressed by alternative without predicate. - The alt without the predicate includes all cases where - the predicate is false. - - WITH predicate: line 7 v1.g - WITHOUT predicate: line 7 v1.g - - The context set for the predicate: - - A - - The lookahead set for the alt WITHOUT the semantic predicate: - - A - - The predicate: - - pred << isUpper(LATEXT(1))>>? - depth=k=1 rule b line 9 v1.g - set context: - A - tree context: null - - Chain of referenced rules: - - #0 in rule start (line 5 v1.g) to rule a - #1 in rule a (line 7 v1.g) - - #endif - -------------------------------------------------------------- - - A predicate can be suppressed by a combination of alternatives - which, taken together, cover a predicate: - - start : (a)* "@" ; - - a : b | ca | cb | cc ; - - b : <>? ( A | B | C ) ; - - ca : A ; - cb : B ; - cc : C ; - - Consider a more complex example in which "c" covers only part of - a predicate: - - start : (a)* "@" ; - - a : b - | c - ; - - b : <>? - ( A - | X - ); - - c : A - ; - - Prior to 1.33MR10 the code generated for "start" would resemble: - - while { - if ( (LA(1)==A || LA(1)==X) && - (! (LA(1)==A || LA(1)==X) || isUpper()) { - a(); - } - }; - - With 1.33MR10 and -mrhoist the predicate context is restricted to - the non-covered lookahead. The code resembles: - - while { - if ( (LA(1)==A || LA(1)==X) && - (! (LA(1)==X) || isUpper()) { - a(); - } - }; - - With the antlr "-info p" switch the user will receive information - about the predicate restriction in the generated file: - - -------------------------------------------------------------- - #if 0 - - Restricting the context of a predicate because of overlap - in the lookahead set between the alternative with the - semantic predicate and one without - Without this restriction the alternative without the predicate - could not be reached when input matched the context of the - predicate and the predicate was false. - - WITH predicate: line 11 v4.g - WITHOUT predicate: line 12 v4.g - - The original context set for the predicate: - - A X - - The lookahead set for the alt WITHOUT the semantic predicate: - - A - - The intersection of the two sets - - A - - The original predicate: - - pred << isUpper(LATEXT(1))>>? - depth=k=1 rule b line 15 v4.g - set context: - A X - tree context: null - - The new (modified) form of the predicate: - - pred << isUpper(LATEXT(1))>>? - depth=k=1 rule b line 15 v4.g - set context: - X - tree context: null - - #endif - -------------------------------------------------------------- - - The bad news about -mrhoist: - - (a) -mrhoist does not analyze predicates with lookahead - depth > 1. - - (b) -mrhoist does not look past a guarded predicate to - find context which might cover other predicates. - - For these cases you might want to use syntactic predicates. - When a semantic predicate fails during guess mode the guess - fails and the next alternative is tried. - - Limitation (a) is illustrated by the following example: - - start : (stmt)* EOF ; - - stmt : cast - | expr - ; - cast : <>? LP ID RP ; - - expr : LP ID RP ; - - This is not much different from the first example, except that - it requires two tokens of lookahead context to determine what - to do. This predicate is NOT suppressed because the current version - is unable to handle predicates with depth > 1. - - A predicate can be combined with other predicates during hoisting. - In those cases the depth=1 predicates are still handled. Thus, - in the following example the isUpper() predicate will be suppressed - by line #4 when hoisted from "bizarre" into "start", but will still - be present in "bizarre" in order to predict "stmt". - - start : (bizarre)* EOF ; // #1 - // #2 - bizarre : stmt // #3 - | A // #4 - ; - - stmt : cast - | expr - ; - - cast : <>? LP ID RP ; - - expr : LP ID RP ; - | <>? A - - Limitation (b) is illustrated by the following example of a - context guarded predicate: - - rule : (A)? <

>? // #1 - (A // #2 - |B // #3 - ) // #4 - | <> B // #5 - ; - - Recall that this means that when the lookahead is NOT A then - the predicate "p" is ignored and it attempts to match "A|B". - Ideally, the "B" at line #3 should suppress predicate "q". - However, the current version does not attempt to look past - the guard predicate to find context which might suppress other - predicates. - - In some cases -mrhoist will lead to the reporting of ambiguities - which were not visible before: - - start : (a)* "@"; - a : bc | d; - bc : b | c ; - - b : <>? A; - c : A ; - - d : A ; - - In this case there is a true ambiguity in "a" between "bc" and "d" - which can both match "A". Without -mrhoist the predicate in "b" - is hoisted into "a" and there is no ambiguity reported. However, - with -mrhoist, the predicate in "b" is suppressed by "c" (as it - should be) making the ambiguity in "a" apparent. - - The motivations for these changes were hoisting problems reported - by Reinier van den Born (reinier@vnet.ibm.com) and several others. - -#113. (Changed in 1.33MR10) new context guarded pred: (g)? && <

>? expr - - The existing context guarded predicate: - - rule : (guard)? => <

>? expr - | next_alternative - ; - - generates code which resembles: - - if (lookahead(expr) && (!guard || pred)) { - expr() - } else .... - - This is not suitable for some applications because it allows - expr() to be invoked when the predicate is false. This is - intentional because it is meant to mimic automatically computed - predicate context. - - The new context guarded predicate uses the guard information - differently because it has a different goal. Consider: - - rule : (guard)? && <

>? expr - | next_alternative - ; - - The new style of context guarded predicate is equivalent to: - - rule : <>? expr - | next_alternative - ; - - It generates code which resembles: - - if (lookahead(expr) && guard && pred) { - expr(); - } else ... - - Both forms of guarded predicates severely restrict the form of - the context guard: it can contain no rule references, no - (...)*, no (...)+, and no {...}. It may contain token and - token class references, and alternation ("|"). - - Addition for 1.33MR11: in the token expression all tokens must - be at the same height of the token tree: - - (A ( B | C))? && ... is ok (all height 2) - (A ( B | ))? && ... is not ok (some 1, some 2) - (A B C D | E F G H)? && ... is ok (all height 4) - (A B C D | E )? && ... is not ok (some 4, some 1) - - This restriction is required in order to properly compute the lookahead - set for expressions like: - - rule1 : (A B C)? && <>? rule2 ; - rule2 : (A|X) (B|Y) (C|Z); - - This addition was suggested by Rienier van den Born (reinier@vnet.ibm.com) - -#109. (Changed in 1.33MR10) improved trace information - - The quality of the trace information provided by the "-gd" - switch has been improved significantly. Here is an example - of the output from a test program. It shows the rule name, - the first token of lookahead, the call depth, and the guess - status: - - exit rule gusxx {"?"} depth 2 - enter rule gusxx {"?"} depth 2 - enter rule gus1 {"o"} depth 3 guessing - guess done - returning to rule gus1 {"o"} at depth 3 - (guess mode continues - an enclosing guess is still active) - guess done - returning to rule gus1 {"Z"} at depth 3 - (guess mode continues - an enclosing guess is still active) - exit rule gus1 {"Z"} depth 3 guessing - guess done - returning to rule gusxx {"o"} at depth 2 (guess mode ends) - enter rule gus1 {"o"} depth 3 - guess done - returning to rule gus1 {"o"} at depth 3 (guess mode ends) - guess done - returning to rule gus1 {"Z"} at depth 3 (guess mode ends) - exit rule gus1 {"Z"} depth 3 - line 1: syntax error at "Z" missing SC - ... - - Rule trace reporting is controlled by the value of the integer - [zz]traceOptionValue: when it is positive tracing is enabled, - otherwise it is disabled. Tracing during guess mode is controlled - by the value of the integer [zz]traceGuessOptionValue. When - it is positive AND [zz]traceOptionValue is positive rule trace - is reported in guess mode. - - The values of [zz]traceOptionValue and [zz]traceGuessOptionValue - can be adjusted by subroutine calls listed below. - - Depending on the presence or absence of the antlr -gd switch - the variable [zz]traceOptionValueDefault is set to 0 or 1. When - the parser is initialized or [zz]traceReset() is called the - value of [zz]traceOptionValueDefault is copied to [zz]traceOptionValue. - The value of [zz]traceGuessOptionValue is always initialzed to 1, - but, as noted earlier, nothing will be reported unless - [zz]traceOptionValue is also positive. - - When the parser state is saved/restored the value of the trace - variables are also saved/restored. If a restore causes a change in - reporting behavior from on to off or vice versa this will be reported. - - When the -gd option is selected, the macro "#define zzTRACE_RULES" - is added to appropriate output files. - - C++ mode - -------- - int traceOption(int delta) - int traceGuessOption(int delta) - void traceReset() - int traceOptionValueDefault - - C mode - -------- - int zzTraceOption(int delta) - int zzTraceGuessOption(int delta) - void zzTraceReset() - int zzTraceOptionValueDefault - - The argument "delta" is added to the traceOptionValue. To - turn on trace when inside a particular rule one: - - rule : <> - ( - rest-of-rule - ) - <> - ; /* fail clause */ <> - - One can use the same idea to turn *off* tracing within a - rule by using a delta of (-1). - - An improvement in the rule trace was suggested by Sramji - Ramanathan (ps@kumaran.com). - -#108. A Note on Deallocation of Variables Allocated in Guess Mode - - NOTE - ------------------------------------------------------ - This mechanism only works for heap allocated variables - ------------------------------------------------------ - - The rewrite of the trace provides the machinery necessary - to properly free variables or undo actions following a - failed guess. - - The macro zzUSER_GUESS_HOOK(guessSeq,zzrv) is expanded - as part of the zzGUESS macro. When a guess is opened - the value of zzrv is 0. When a longjmp() is executed to - undo the guess, the value of zzrv will be 1. - - The macro zzUSER_GUESS_DONE_HOOK(guessSeq) is expanded - as part of the zzGUESS_DONE macro. This is executed - whether the guess succeeds or fails as part of closing - the guess. - - The guessSeq is a sequence number which is assigned to each - guess and is incremented by 1 for each guess which becomes - active. It is needed by the user to associate the start of - a guess with the failure and/or completion (closing) of a - guess. - - Guesses are nested. They must be closed in the reverse - of the order that they are opened. - - In order to free memory used by a variable during a guess - a user must write a routine which can be called to - register the variable along with the current guess sequence - number provided by the zzUSER_GUESS_HOOK macro. If the guess - fails, all variables tagged with the corresponding guess - sequence number should be released. This is ugly, but - it would require a major rewrite of antlr 1.33 to use - some mechanism other than setjmp()/longjmp(). - - The order of calls for a *successful* guess would be: - - zzUSER_GUESS_HOOK(guessSeq,0); - zzUSER_GUESS_DONE_HOOK(guessSeq); - - The order of calls for a *failed* guess would be: - - zzUSER_GUESS_HOOK(guessSeq,0); - zzUSER_GUESS_HOOK(guessSeq,1); - zzUSER_GUESS_DONE_HOOK(guessSeq); - - The default definitions of these macros are empty strings. - - Here is an example in C++ mode. The zzUSER_GUESS_HOOK and - zzUSER_GUESS_DONE_HOOK macros and myGuessHook() routine - can be used without change in both C and C++ versions. - - ---------------------------------------------------------------------- - << - - #include "AToken.h" - - typedef ANTLRCommonToken ANTLRToken; - - #include "DLGLexer.h" - - int main() { - - { - DLGFileInput in(stdin); - DLGLexer lexer(&in,2000); - ANTLRTokenBuffer pipe(&lexer,1); - ANTLRCommonToken aToken; - P parser(&pipe); - - lexer.setToken(&aToken); - parser.init(); - parser.start(); - }; - - fclose(stdin); - fclose(stdout); - return 0; - } - - >> - - << - char *s=NULL; - - #undef zzUSER_GUESS_HOOK - #define zzUSER_GUESS_HOOK(guessSeq,zzrv) myGuessHook(guessSeq,zzrv); - #undef zzUSER_GUESS_DONE_HOOK - #define zzUSER_GUESS_DONE_HOOK(guessSeq) myGuessHook(guessSeq,2); - - void myGuessHook(int guessSeq,int zzrv) { - if (zzrv == 0) { - fprintf(stderr,"User hook: starting guess #%d\n",guessSeq); - } else if (zzrv == 1) { - free (s); - s=NULL; - fprintf(stderr,"User hook: failed guess #%d\n",guessSeq); - } else if (zzrv == 2) { - free (s); - s=NULL; - fprintf(stderr,"User hook: ending guess #%d\n",guessSeq); - }; - } - - >> - - #token A "a" - #token "[\t \ \n]" <> - - class P { - - start : (top)+ - ; - - top : (which) ? <> - | other <> - ; <> - - which : which2 - ; - - which2 : which3 - ; - which3 - : (label)? <> - | (global)? <> - | (exclamation)? <> - ; - - label : <getText());>> A ":" ; - - global : <getText());>> A "::" ; - - exclamation : <getText());>> A "!" ; - - other : <getText());>> "other" ; - - } - ---------------------------------------------------------------------- - - This is a silly example, but illustrates the idea. For the input - "a ::" with tracing enabled the output begins: - - ---------------------------------------------------------------------- - enter rule "start" depth 1 - enter rule "top" depth 2 - User hook: starting guess #1 - enter rule "which" depth 3 guessing - enter rule "which2" depth 4 guessing - enter rule "which3" depth 5 guessing - User hook: starting guess #2 - enter rule "label" depth 6 guessing - guess failed - User hook: failed guess #2 - guess done - returning to rule "which3" at depth 5 (guess mode continues - - an enclosing guess is still active) - User hook: ending guess #2 - User hook: starting guess #3 - enter rule "global" depth 6 guessing - exit rule "global" depth 6 guessing - guess done - returning to rule "which3" at depth 5 (guess mode continues - - an enclosing guess is still active) - User hook: ending guess #3 - enter rule "global" depth 6 guessing - exit rule "global" depth 6 guessing - exit rule "which3" depth 5 guessing - exit rule "which2" depth 4 guessing - exit rule "which" depth 3 guessing - guess done - returning to rule "top" at depth 2 (guess mode ends) - User hook: ending guess #1 - enter rule "which" depth 3 - ..... - ---------------------------------------------------------------------- - - Remember: - - (a) Only init-actions are executed during guess mode. - (b) A rule can be invoked multiple times during guess mode. - (c) If the guess succeeds the rule will be called once more - without guess mode so that normal actions will be executed. - This means that the init-action might need to distinguish - between guess mode and non-guess mode using the variable - [zz]guessing. - -#101. (Changed in 1.33MR10) antlr -info command line switch - - -info - - p - extra predicate information in generated file - - t - information about tnode use: - at the end of each rule in generated file - summary on stderr at end of program - - m - monitor progress - prints name of each rule as it is started - flushes output at start of each rule - - f - first/follow set information to stdout - - 0 - no operation (added in 1.33MR11) - - The options may be combined and may appear in any order. - For example: - - antlr -info ptm -CC -gt -mrhoist on mygrammar.g - -#100a. (Changed in 1.33MR10) Predicate tree simplification - - When the same predicates can be referenced in more than one - alternative of a block large predicate trees can be formed. - - The difference that these optimizations make is so dramatic - that I have decided to use it even when -mrhoist is not selected. - - Consider the following grammar: - - start : ( all )* ; - - all : a - | d - | e - | f - ; - - a : c A B - | c A C - ; - - c : <>? - ; - - d : <>? B C - ; - - e : <>? B C - ; - - f : e X Y - ; - - In rule "a" there is a reference to rule "c" in both alternatives. - The length of the predicate AAA is k=2 and it can be followed in - alternative 1 only by (A B) while in alternative 2 it can be - followed only by (A C). Thus they do not have identical context. - - In rule "all" the alternatives which refer to rules "e" and "f" allow - elimination of the duplicate reference to predicate CCC. - - The table below summarized the kind of simplification performed by - 1.33MR10. In the table, X and Y stand for single predicates - (not trees). - - (OR X (OR Y (OR Z))) => (OR X Y Z) - (AND X (AND Y (AND Z))) => (AND X Y Z) - - (OR X (... (OR X Y) ... )) => (OR X (... Y ... )) - (AND X (... (AND X Y) ... )) => (AND X (... Y ... )) - (OR X (... (AND X Y) ... )) => (OR X (... ... )) - (AND X (... (OR X Y) ... )) => (AND X (... ... )) - - (AND X) => X - (OR X) => X - - In a test with a complex grammar for a real application, a predicate - tree with six OR nodes and 12 leaves was reduced to "(OR X Y Z)". - - In 1.33MR10 there is a greater effort to release memory used - by predicates once they are no longer in use. - -#100b. (Changed in 1.33MR10) Suppression of extra predicate tests - - The following optimizations require that -mrhoist be selected. - - It is relatively easy to optimize the code generated for predicate - gates when they are of the form: - - (AND X Y Z ...) - or (OR X Y Z ...) - - where X, Y, Z, and "..." represent individual predicates (leaves) not - predicate trees. - - If the predicate is an AND the contexts of the X, Y, Z, etc. are - ANDed together to create a single Tree context for the group and - context tests for the individual predicates are suppressed: - - -------------------------------------------------- - Note: This was incorrect. The contexts should be - ORed together. This has been fixed. A more - complete description is available in item #152. - --------------------------------------------------- - - Optimization 1: (AND X Y Z ...) - - Suppose the context for Xtest is LA(1)==LP and the context for - Ytest is LA(1)==LP && LA(2)==ID. - - Without the optimization the code would resemble: - - if (lookaheadContext && - !(LA(1)==LP && LA(1)==LP && LA(2)==ID) || - ( (! LA(1)==LP || Xtest) && - (! (LA(1)==LP || LA(2)==ID) || Xtest) - )) {... - - With the -mrhoist optimization the code would resemble: - - if (lookaheadContext && - ! (LA(1)==LP && LA(2)==ID) || (Xtest && Ytest) {... - - Optimization 2: (OR X Y Z ...) with identical contexts - - Suppose the context for Xtest is LA(1)==ID and for Ytest - the context is also LA(1)==ID. - - Without the optimization the code would resemble: - - if (lookaheadContext && - ! (LA(1)==ID || LA(1)==ID) || - (LA(1)==ID && Xtest) || - (LA(1)==ID && Ytest) {... - - With the -mrhoist optimization the code would resemble: - - if (lookaheadContext && - (! LA(1)==ID) || (Xtest || Ytest) {... - - Optimization 3: (OR X Y Z ...) with distinct contexts - - Suppose the context for Xtest is LA(1)==ID and for Ytest - the context is LA(1)==LP. - - Without the optimization the code would resemble: - - if (lookaheadContext && - ! (LA(1)==ID || LA(1)==LP) || - (LA(1)==ID && Xtest) || - (LA(1)==LP && Ytest) {... - - With the -mrhoist optimization the code would resemble: - - if (lookaheadContext && - (zzpf=0, - (LA(1)==ID && (zzpf=1) && Xtest) || - (LA(1)==LP && (zzpf=1) && Ytest) || - !zzpf) { - - These may appear to be of similar complexity at first, - but the non-optimized version contains two tests of each - context while the optimized version contains only one - such test, as well as eliminating some of the inverted - logic (" !(...) || "). - - Optimization 4: Computation of predicate gate trees - - When generating code for the gates of predicate expressions - antlr 1.33 vanilla uses a recursive procedure to generate - "&&" and "||" expressions for testing the lookahead. As each - layer of the predicate tree is exposed a new set of "&&" and - "||" expressions on the lookahead are generated. In many - cases the lookahead being tested has already been tested. - - With -mrhoist a lookahead tree is computed for the entire - lookahead expression. This means that predicates with identical - context or context which is a subset of another predicate's - context disappear. - - This is especially important for predicates formed by rules - like the following: - - uppperCaseVowel : <>? vowel; - vowel: : <>? LETTERS; - - These predicates are combined using AND since both must be - satisfied for rule upperCaseVowel. They have identical - context which makes this optimization very effective. - - The affect of Items #100a and #100b together can be dramatic. In - a very large (but real world) grammar one particular predicate - expression was reduced from an (unreadable) 50 predicate leaves, - 195 LA(1) terms, and 5500 characters to an (easily comprehensible) - 3 predicate leaves (all different) and a *single* LA(1) term. - -#98. (Changed in 1.33MR10) Option "-info p" - - When the user selects option "-info p" the program will generate - detailed information about predicates. If the user selects - "-mrhoist on" additional detail will be provided explaining - the promotion and suppression of predicates. The output is part - of the generated file and sandwiched between #if 0/#endif statements. - - Consider the following k=1 grammar: - - start : ( all ) * ; - - all : ( a - | b - ) - ; - - a : c B - ; - - c : <>? - | B - ; - - b : <>? X - ; - - Below is an excerpt of the output for rule "start" for the three - predicate options (off, on, and maintenance release style hoisting). - - For those who do not wish to use the "-mrhoist on" option for code - generation the option can be used in a "diagnostic" mode to provide - valuable information: - - a. where one should insert null actions to inhibit hoisting - b. a chain of rule references which shows where predicates are - being hoisted - - ====================================================================== - Example of "-info p" with "-mrhoist on" - ====================================================================== - #if 0 - - Hoisting of predicate suppressed by alternative without predicate. - The alt without the predicate includes all cases where the - predicate is false. - - WITH predicate: line 11 v36.g - WITHOUT predicate: line 12 v36.g - - The context set for the predicate: - - B - - The lookahead set for alt WITHOUT the semantic predicate: - - B - - The predicate: - - pred << LATEXT(1)>>? depth=k=1 rule c line 11 v36.g - - set context: - B - tree context: null - - Chain of referenced rules: - - #0 in rule start (line 1 v36.g) to rule all - #1 in rule all (line 3 v36.g) to rule a - #2 in rule a (line 8 v36.g) to rule c - #3 in rule c (line 11 v36.g) - - #endif - && - #if 0 - - pred << LATEXT(1)>>? depth=k=1 rule b line 15 v36.g - - set context: - X - tree context: null - - #endif - ====================================================================== - Example of "-info p" with the default -prc setting ( "-prc off") - ====================================================================== - #if 0 - - OR - pred << LATEXT(1)>>? depth=k=1 rule c line 11 v36.g - - set context: - nil - tree context: null - - pred << LATEXT(1)>>? depth=k=1 rule b line 15 v36.g - - set context: - nil - tree context: null - - #endif - ====================================================================== - Example of "-info p" with "-prc on" and "-mrhoist off" - ====================================================================== - #if 0 - - OR - pred << LATEXT(1)>>? depth=k=1 rule c line 11 v36.g - - set context: - B - tree context: null - - pred << LATEXT(1)>>? depth=k=1 rule b line 15 v36.g - - set context: - X - tree context: null - - #endif - ====================================================================== - -#60. (Changed in 1.33MR7) Major changes to exception handling - - There were significant problems in the handling of exceptions - in 1.33 vanilla. The general problem is that it can only - process one level of exception handler. For example, a named - exception handler, an exception handler for an alternative, or - an exception for a subrule always went to the rule's exception - handler if there was no "catch" which matched the exception. - - In 1.33MR7 the exception handlers properly "nest". If an - exception handler does not have a matching "catch" then the - nextmost outer exception handler is checked for an appropriate - "catch" clause, and so on until an exception handler with an - appropriate "catch" is found. - - There are still undesirable features in the way exception - handlers are implemented, but I do not have time to fix them - at the moment: - - The exception handlers for alternatives are outside the - block containing the alternative. This makes it impossible - to access variables declared in a block or to resume the - parse by "falling through". The parse can still be easily - resumed in other ways, but not in the most natural fashion. - - This results in an inconsistentcy between named exception - handlers and exception handlers for alternatives. When - an exception handler for an alternative "falls through" - it goes to the nextmost outer handler - not the "normal - action". - - A major difference between 1.33MR7 and 1.33 vanilla is - the default action after an exception is caught: - - 1.33 Vanilla - ------------ - In 1.33 vanilla the signal value is set to zero ("NoSignal") - and the code drops through to the code following the exception. - For named exception handlers this is the "normal action". - For alternative exception handlers this is the rule's handler. - - 1.33MR7 - ------- - In 1.33MR7 the signal value is NOT automatically set to zero. - - There are two cases: - - For named exception handlers: if the signal value has been - set to zero the code drops through to the "normal action". - - For all other cases the code branches to the nextmost outer - exception handler until it reaches the handler for the rule. - - The following macros have been defined for convenience: - - C/C++ Mode Name - -------------------- - (zz)suppressSignal - set signal & return signal arg to 0 ("NoSignal") - (zz)setSignal(intValue) - set signal & return signal arg to some value - (zz)exportSignal - copy the signal value to the return signal arg - - I'm not sure why PCCTS make a distinction between the local - signal value and the return signal argument, but I'm loathe - to change the code. The burden of copying the local signal - value to the return signal argument can be given to the - default signal handler, I suppose. - -#53. (Explanation for 1.33MR6) What happens after an exception is caught ? - - The Book is silent about what happens after an exception - is caught. - - The following code fragment prints "Error Action" followed - by "Normal Action". - - test : Word ex:Number <> - exception[ex] - catch NoViableAlt: - <> - ; - - The reason for "Normal Action" is that the normal flow of the - program after a user-written exception handler is to "drop through". - In the case of an exception handler for a rule this results in - the exection of a "return" statement. In the case of an - exception handler attached to an alternative, rule, or token - this is the code that would have executed had there been no - exception. - - The user can achieve the desired result by using a "return" - statement. - - test : Word ex:Number <> - exception[ex] - catch NoViableAlt: - <> - ; - - The most powerful mechanism for recovery from parse errors - in pccts is syntactic predicates because they provide - backtracking. Exceptions allow "return", "break", - "consumeUntil(...)", "goto _handler", "goto _fail", and - changing the _signal value. - -#41. (Added in 1.33MR6) antlr -stdout - - Using "antlr -stdout ..." forces the text that would - normally go to the grammar.c or grammar.cpp file to - stdout. - -#40. (Added in 1.33MR6) antlr -tab to change tab stops - - Using "antlr -tab number ..." changes the tab stops - for the grammar.c or grammar.cpp file. The number - must be between 0 and 8. Using 0 gives tab characters, - values between 1 and 8 give the appropriate number of - space characters. - -#34. (Added to 1.33MR1) Add public DLGLexerBase::set_line(int newValue) - - Previously there was no public function for changing the line - number maintained by the lexer. - -#28. (Added to 1.33MR1) More control over DLG header - - Version 1.33MR1 adds the following directives to PCCTS - for C++ mode: - - #lexprefix <> - - Adds source code to the DLGLexer.h file - after the #include "DLexerBase.h" but - before the start of the class definition. - - #lexmember <> - - Adds source code to the DLGLexer.h file - as part of the DLGLexer class body. It - appears immediately after the start of - the class and a "public: statement. -