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multistart: use b4_accept instead of action post-processing

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For each start symbol, generate a parsing function with a richer
return value than the usual of yyparse.  Reserve a place for the
returned semantic value, in order to avoid having to pass a pointer as
argument to "return" that value.  This also makes the call to the
parsing function independent of whether a given start-symbol is typed.

For instance, if the grammar file contains:

    %type <int> expression
    %start input expression

(so "input" is valueless) we get

    typedef struct
    {
      int yystatus;
    } yyparse_input_t;

    yyparse_input_t yyparse_input (void);

    typedef struct
    {
      int yyvalue;
      int yystatus;
    } yyparse_expression_t;

    yyparse_expression_t yyparse_expression (void);

This commit also changes the implementation of the parser termination:
when there are multiple start symbols, it is the initial rules that
explicitly YYACCEPT.  They do that after having exported the
start-symbol's value (if it is typed):

  switch (yyn)
    {
  case 1: /* $accept: YY_EXPRESSION expression $end  */
  { ((*yyvalue).TOK_expression) = (yyvsp[-1].TOK_expression); YYACCEPT; }
    break;

  case 2: /* $accept: YY_INPUT input $end  */
  { YYACCEPT; }
    break;

I have tried several ways to deal with termination, and this is the
one that appears the best one to me.  It is also the most natural.

* src/scan-code.h, src/scan-code.l (obstack_for_actions): New.
* src/reader.c (grammar_rule_check_and_complete): Generate the actions
of the rules for each start symbol.

* data/skeletons/bison.m4 (b4_symbol_slot): New, with safer semantics
than type and type_tag.
* data/skeletons/yacc.c (b4_accept): New.
Generates the body of the action of the start rules.
(_b4_declare_sub_yyparse): For each start symbol define a dedicated
return type for its parsing function.
Adjust the declaration of its parsing function.
(_b4_define_sub_yyparse): Adjust the definition of the function.

* examples/c/lexcalc/parse.y: Check the case of valueless symbols.
* examples/c/lexcalc/lexcalc.test: Check start symbols.
This commit is contained in:
Akim Demaille
2020-07-05 08:00:20 +02:00
parent a6805bb8d9
commit d9cf99b6a5
10 changed files with 146 additions and 43 deletions
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14
data/skeletons/bison.m4
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@@ -465,6 +465,19 @@ m4_case([$1],
# but are S_YYEMPTY and symbol_kind::S_YYEMPTY in C++.
m4_copy([b4_symbol_kind_base], [b4_symbol_kind])
# b4_symbol_slot(NUM)
# -------------------
# The name of union member that contains the value of these symbols.
# Currently, we are messy, this should actually be type_tag, but type_tag
# has several meanings.
m4_define([b4_symbol_slot],
[m4_case(b4_percent_define_get([[api.value.type]]),
[union], [b4_symbol([$1], [type_tag])],
[variant], [b4_symbol([$1], [type_tag])],
[b4_symbol([$1], [type])])])
# b4_symbol(NUM, FIELD)
# ---------------------
# Fetch FIELD of symbol #NUM (or "orig NUM"). Fail if undefined.
@@ -475,6 +488,7 @@ m4_define([b4_symbol],
[id], [b4_symbol_token_kind([$1])],
[kind_base], [b4_symbol_kind_base([$1])],
[kind], [b4_symbol_kind([$1])],
[slot], [b4_symbol_slot([$1])],
[_b4_symbol($@)])])