Among all the modifications, this new byacc also solves a 14 year old bug [1] PR: bin/23254 [1] Submitted by: marka@nominum.com [1] MFC after: 3 weeks
81 lines
3.9 KiB
Plaintext
81 lines
3.9 KiB
Plaintext
Tom Shields, March 17, 2014
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NOTE: README.BTYACC needs to be re-written (or another README file created)
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to explain how the extensions were adopted into the byacc-20140101 baseline.
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bytacc changes adopted; see README.BTYACC for details.
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Changes to the skeleton to send debugging output to stderr.
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Changes to the types of the grammar tables from 'short' to 'int' if
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MAXTABLE is defined to be > MAXSHORT for generation of large grammar tables
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for large grammars. This can be set with the invocation of configure
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using the --with-max-table-size=N option
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Changes to add syntactic sugaring for definition and semantic processing
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of inherited attributes associated with non-terminal symbols on the rhs of
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rules. Inherited attributes are implemented as {}-actions, and as such may
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introduce reduce/reduce conflicts. The advantage over manual {}-actions is
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that checking is performed on the number and types of inherited attributes,
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and the positions on the semantic stack are automatically calculated.
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Identical actions produced by the inherited attribute syntax are collapsed
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into a single action, reducing reduce/reduce conflicts.
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The %type specification is extended to define the types of inherited
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attributes associated with non-terminal symbols on the lhs of rules, in
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addition to its original purpose in yacc for specifying the type of the
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symbol itself on the parser's value stack. In order to permit the two
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aspects to be used independently, the <type> specification following
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%type is optional.
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Added a mechanism to the parser skeleton for computing and propagating
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the text position of grammar symbols if %locations directive is present in
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the grammar specificaion, or if yacc is invoked with the -L flag. Includes
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support for "@$" and "@N" (for N an integer) in actions to refer to symbol
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locations.
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Added a bison compatible %destructor directive to define a memory
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management mechanism to the parser skeleton that enables cleanup of semantic
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values and text positions associated with the parsing stack that are
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automatically discarded during error processing.
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The following is only available if configure is invoked with the
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--enable-btyacc=yes:
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Changes to enable backtracking with semantic disambiguation, if yacc is
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invoked with the -B flag. A yacc parser disambiguates shift/reduce conflicts
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by choosing to shift, and reduce/reduce conflicts by reducing using the
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earliest grammar rule (in the grammar specification). A btyacc parser handles
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a conflict by remembering the current parse point and entering into trial parse
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mode. In trial parse mode, the parser tries each of the conflicting options
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until the first successs or until all options are exhausted. If the parser
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runs into an error while in trial parse mode, it backtracks to the most recent
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conflict point and tries the next alternative. If the parser finds a
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successful parse, it backtracks to the point where it first entered trial parse
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mode and continues with normal parsing, using the action at the start of the
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successful trial path. Success in trial parse mode is achieved either by
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successfully reducing the start symbol at the end of input or by invocation of
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YYVALID (or YYVALID_NESTED). Semantic disambiguation is supported within
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[]-actions, which are always executed, with invocation of YYERROR to terminate
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a trial path or YYVALID (and YYVALID_NESTED). Yacc {}-actions are only
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executed during normal parsing.
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btyacc changes deferred:
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------------------------
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The -S command line flag to enable use of an alternate parser skeleton,
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read from an external file.
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Preprocessor statements: %define, %ifdef/%endif, %include.
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btyacc changes rejected:
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------------------------
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The yyerror_detailed() option for error messages is replaced by the
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byacc feature for specifying options paramters to yyerror().
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C++-specific features in the parser skeleton.
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