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* data/glr.c, data/lalr1.cc, data/yacc.c: When YYABORT was

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invoked, yydestruct the lookahead.
* tests/calc.at (Calculator $1): Update the expected lengths of
traces: there is an added line for the discarded lookahead.
* doc/bison.texinfo (Destructor Decl): Some rewording.
Define "discarded" symbols.
This commit is contained in:
Akim Demaille
2004-09-06 07:48:20 +00:00
parent 0fe1f06d36
commit e757bb10bc
6 changed files with 64 additions and 27 deletions
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9
ChangeLog
View File

@@ -1,3 +1,12 @@
2004-09-03 Akim Demaille <akim@epita.fr>
* data/glr.c, data/lalr1.cc, data/yacc.c: When YYABORT was
invoked, yydestruct the lookahead.
* tests/calc.at (Calculator $1): Update the expected lengths of
traces: there is an added line for the discarded lookahead.
* doc/bison.texinfo (Destructor Decl): Some rewording.
Define "discarded" symbols.
2004-09-02 Akim Demaille <akim@epita.fr>
* data/lalr1.cc (translate_, destruct_): No reason to be static.

8
data/glr.c
View File

@@ -1916,7 +1916,13 @@ yyrecoverSyntaxError (yyGLRStack* yystack,
yyposn = yystack.yytops.yystates[0]->yyposn;
}
yyDone:
;
/* On YYABORT, free the lookahead. */
if (yystack.yyerrflag == 1 && yytoken != YYEMPTY)
{
YY_SYMBOL_PRINT ("Error: discarding lookahead",
yytoken, yylvalp, yyllocp);
yydestruct (yytoken, yylvalp]b4_location_if([, yyllocp])[);
}
yyfreeGLRStack (&yystack);
return yystack.yyerrflag;

4
data/lalr1.cc
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@@ -714,6 +714,10 @@ yyacceptlab:
/* Abort. */
yyabortlab:
/* Free the lookahead. */
YY_SYMBOL_PRINT ("Error: discarding lookahead", ilooka_, &value, &location);
destruct_ (ilooka_, &value, &location);
looka_ = empty_;
return 1;
}

3
data/yacc.c
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@@ -1209,6 +1209,9 @@ yyacceptlab:
| yyabortlab -- YYABORT comes here. |
`-----------------------------------*/
yyabortlab:
YY_SYMBOL_PRINT ("Error: discarding lookahead", yytoken, &yylval, &yylloc);
yydestruct (yytoken, &yylval]b4_location_if([, &yylloc])[);
yychar = YYEMPTY;
yyresult = 1;
goto yyreturn;

55
doc/bison.texinfo
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@@ -787,7 +787,7 @@ are possible---either locally redefining @samp{a}, or using the
value of @samp{a} from the outer scope. So this approach cannot
work.
A simple solution to this problem is to declare the parser to
A simple solution to this problem is to declare the parser to
use the @acronym{GLR} algorithm.
When the @acronym{GLR} parser reaches the critical state, it
merely splits into two branches and pursues both syntax rules
@@ -871,7 +871,7 @@ type t = (a) .. b;
The parser can be turned into a @acronym{GLR} parser, while also telling Bison
to be silent about the one known reduce/reduce conflict, by
adding these two declarations to the Bison input file (before the first
adding these two declarations to the Bison input file (before the first
@samp{%%}):
@example
@@ -893,7 +893,7 @@ Bison to make sure that @acronym{GLR} splitting is only done where it is
intended. A @acronym{GLR} parser splitting inadvertently may cause
problems less obvious than an @acronym{LALR} parser statically choosing the
wrong alternative in a conflict.
Second, consider interactions with the lexer (@pxref{Semantic Tokens})
Second, consider interactions with the lexer (@pxref{Semantic Tokens})
with great care. Since a split parser consumes tokens
without performing any actions during the split, the lexer cannot
obtain information via parser actions. Some cases of
@@ -977,20 +977,20 @@ parses as either an @code{expr} or a @code{stmt}
@samp{x} as an @code{ID}).
Bison detects this as a reduce/reduce conflict between the rules
@code{expr : ID} and @code{declarator : ID}, which it cannot resolve at the
time it encounters @code{x} in the example above. Since this is a
@acronym{GLR} parser, it therefore splits the problem into two parses, one for
time it encounters @code{x} in the example above. Since this is a
@acronym{GLR} parser, it therefore splits the problem into two parses, one for
each choice of resolving the reduce/reduce conflict.
Unlike the example from the previous section (@pxref{Simple GLR Parsers}),
however, neither of these parses ``dies,'' because the grammar as it stands is
ambiguous. One of the parsers eventually reduces @code{stmt : expr ';'} and
the other reduces @code{stmt : decl}, after which both parsers are in an
identical state: they've seen @samp{prog stmt} and have the same unprocessed
input remaining. We say that these parses have @dfn{merged.}
ambiguous. One of the parsers eventually reduces @code{stmt : expr ';'} and
the other reduces @code{stmt : decl}, after which both parsers are in an
identical state: they've seen @samp{prog stmt} and have the same unprocessed
input remaining. We say that these parses have @dfn{merged.}
At this point, the @acronym{GLR} parser requires a specification in the
grammar of how to choose between the competing parses.
In the example above, the two @code{%dprec}
declarations specify that Bison is to give precedence
declarations specify that Bison is to give precedence
to the parse that interprets the example as a
@code{decl}, which implies that @code{x} is a declarator.
The parser therefore prints
@@ -1007,7 +1007,7 @@ T (x) + y;
@end example
@noindent
This is another example of using @acronym{GLR} to parse an unambiguous
This is another example of using @acronym{GLR} to parse an unambiguous
construct, as shown in the previous section (@pxref{Simple GLR Parsers}).
Here, there is no ambiguity (this cannot be parsed as a declaration).
However, at the time the Bison parser encounters @code{x}, it does not
@@ -1066,7 +1066,7 @@ as both an @code{expr} and a @code{decl}, and prints
@end example
Bison requires that all of the
productions that participate in any particular merge have identical
productions that participate in any particular merge have identical
@samp{%merge} clauses. Otherwise, the ambiguity would be unresolvable,
and the parser will report an error during any parse that results in
the offending merge.
@@ -3734,14 +3734,13 @@ terminal symbol. All kinds of token declarations allow
@cindex freeing discarded symbols
@findex %destructor
Some symbols can be discarded by the parser, typically during error
recovery (@pxref{Error Recovery}). Basically, during error recovery,
embarrassing symbols already pushed on the stack, and embarrassing
tokens coming from the rest of the file are thrown away until the parser
falls on its feet. If these symbols convey heap based information, this
memory is lost. While this behavior is tolerable for batch parsers,
such as in compilers, it is unacceptable for parsers that can
possibility ``never end'' such as shells, or implementations of
Some symbols can be discarded by the parser. For instance, during error
recovery (@pxref{Error Recovery}), embarrassing symbols already pushed
on the stack, and embarrassing tokens coming from the rest of the file
are thrown away until the parser falls on its feet. If these symbols
convey heap based information, this memory is lost. While this behavior
can be tolerable for batch parsers, such as in compilers, it is not for
possibly ``never ending'' parsers such as shells, or implementations of
communication protocols.
The @code{%destructor} directive allows for the definition of code that
@@ -3794,6 +3793,22 @@ typeless: string; // $$ = $1 does not apply; $1 is destroyed.
typefull: string; // $$ = $1 applies, $1 is not destroyed.
@end smallexample
@sp 1
@cindex discarded symbols
@dfn{Discarded symbols} are the following:
@itemize
@item
stacked symbols popped during the first phase of error recovery,
@item
incoming terminals during the second phase of error recovery,
@item
the current lookahead when the parser aborts (either via an explicit
call to @code{YYABORT}, or as a consequence of a failed error recovery).
@end itemize
@node Expect Decl
@subsection Suppressing Conflict Warnings
@cindex suppressing conflict warnings

12
tests/calc.at
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@@ -466,21 +466,21 @@ _AT_CHECK_CALC([$1],
[486])
# Some syntax errors.
_AT_CHECK_CALC_ERROR([$1], [1], [0 0], [11],
_AT_CHECK_CALC_ERROR([$1], [1], [0 0], [12],
[1.2: syntax error, unexpected "number"])
_AT_CHECK_CALC_ERROR([$1], [1], [1//2], [15],
_AT_CHECK_CALC_ERROR([$1], [1], [1//2], [16],
[1.2: syntax error, unexpected '/', expecting "number" or '-' or '(' or '!'])
_AT_CHECK_CALC_ERROR([$1], [1], [error], [4],
_AT_CHECK_CALC_ERROR([$1], [1], [error], [5],
[1.0: syntax error, unexpected $undefined])
_AT_CHECK_CALC_ERROR([$1], [1], [1 = 2 = 3], [22],
_AT_CHECK_CALC_ERROR([$1], [1], [1 = 2 = 3], [23],
[1.6: syntax error, unexpected '='])
_AT_CHECK_CALC_ERROR([$1], [1],
[
+1],
[14],
[15],
[2.0: syntax error, unexpected '+'])
# Exercise error messages with EOF: work on an empty file.
_AT_CHECK_CALC_ERROR([$1], [1], [/dev/null], [4],
_AT_CHECK_CALC_ERROR([$1], [1], [/dev/null], [5],
[1.0: syntax error, unexpected "end of input"])
# Exercise the error token: without it, we die at the first error,