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yacc.c: also define a symbol number for the empty token

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This is not only cleaner, it also protects us from mixing signed
values (YYEMPTY is #defined as -2) with unsigned types (the
yysymbol_type_t enum is typically compiled as a small unsigned).
For instance GCC 9:

    input.c: In function 'yyparse':
    input.c:1107:7: error: conversion to 'unsigned int' from 'int'
                           may change the sign of the result
                           [-Werror=sign-conversion]
     1107 |   yyn += yytoken;
          |       ^~
    input.c:1107:10: error: conversion to 'int' from 'unsigned int'
                            may change the sign of the result
                            [-Werror=sign-conversion]
     1107 |   yyn += yytoken;
          |          ^~~~~~~
    input.c:1108:47: error: comparison of integer expressions of
                            different signedness:
                            'yytype_int8' {aka 'const signed char'} and
                            'yysymbol_type_t' {aka 'enum yysymbol_type_t'}
                            [-Werror=sign-compare]
     1108 |   if (yyn < 0 || YYLAST < yyn || yycheck[yyn] != yytoken)
          |                                               ^~
    input.c:702:25: error: operand of ?: changes signedness from 'int'
                           to 'unsigned int' due to unsignedness of
                           other operand [-Werror=sign-compare]
      702 | #define YYEMPTY         (-2)
          |                         ^~~~
    input.c:1220:33: note: in expansion of macro 'YYEMPTY'
     1220 |   yytoken = yychar == YYEMPTY ? YYEMPTY : YYTRANSLATE (yychar);
          |                                 ^~~~~~~
    input.c:1220:41: error: unsigned conversion from 'int' to
                            'unsigned int' changes value
                            from '-2' to '4294967294'
                            [-Werror=sign-conversion]
     1220 |   yytoken = yychar == YYEMPTY ? YYEMPTY : YYTRANSLATE (yychar);
          |                                         ^

Eventually, it might be interesting to move away from -2 (which is the
only possible negative symbol number) and use the next available
number, to save bits.  We could actually even simply use "0" and shift
the rest, which would allow to write "!yytoken" to mean really
"yytoken != YYEMPTY".

* data/skeletons/c.m4 (b4_declare_symbol_enum): Define YYSYMBOL_YYEMPTY.
* data/skeletons/yacc.c: Use it.

* src/parse-gram.y (yyreport_syntax_error): Use YYSYMBOL_YYEMPTY, not
YYEMPTY, when dealing with a symbol.

* tests/regression.at: Adjust.
This commit is contained in:
Akim Demaille
2020-03-28 15:37:09 +01:00
parent 00c80bc96c
commit f3c18c8e80
6 changed files with 25 additions and 5 deletions
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6
data/skeletons/yacc.c
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@@ -1348,7 +1348,7 @@ yysyntax_error_arguments (const yyparse_context_t *yyctx,
one exception: it will still contain any token that will not be
accepted due to an error action in a later state.]])[
*/
if (yyctx->yytoken != YYEMPTY)
if (yyctx->yytoken != YYSYMBOL_YYEMPTY)
{
int yyn;]b4_lac_if([[
YYDPRINTF ((stderr, "Constructing syntax error message\n"));]])[
@@ -1576,7 +1576,7 @@ yyparse (]m4_ifset([b4_parse_param], [b4_formals(b4_parse_param)], [void])[)]])[
/* The return value of yyparse. */
int yyresult;
/* Lookahead token as an internal (translated) token number. */
yysymbol_type_t yytoken = 0;
yysymbol_type_t yytoken = YYSYMBOL_YYEMPTY;
/* The variables used to return semantic value and location from the
action routines. */
YYSTYPE yyval;]b4_locations_if([[
@@ -1889,7 +1889,7 @@ yyreduce:
yyerrlab:
/* Make sure we have latest lookahead translation. See comments at
user semantic actions for why this is necessary. */
yytoken = yychar == YYEMPTY ? YYEMPTY : YYTRANSLATE (yychar);
yytoken = yychar == YYEMPTY ? YYSYMBOL_YYEMPTY : YYTRANSLATE (yychar);
/* If not already recovering from an error, report this error. */
if (!yyerrstatus)