From 05d0ced1aa6c1e685e3228e7c43aab3fbb1d88c3 Mon Sep 17 00:00:00 2001 From: Nick Bowler Date: Sun, 23 Jul 2023 10:55:40 -0400 Subject: [PATCH] Solve Bison conflicts using precedence declarations. We can use a precedence declaration to directly inform Bison of how to resolve the specific conflict at hand, rather than just saying "it is ok to have N conflicts." There is no significant change to the generated output but this way keeps things more straightforward when modifying the parser. --- src/parse.y | 33 ++++++++++++++++++++------------- 1 file changed, 20 insertions(+), 13 deletions(-) diff --git a/src/parse.y b/src/parse.y index fd5776e..ec7ea1b 100644 --- a/src/parse.y +++ b/src/parse.y @@ -265,11 +265,9 @@ static struct cdecl *insert_identifier(struct cdecl *decl, char *ident) %type english english_declaration %type english_parameter -/* - * Harmless shift/reduce conflicts in english_parameter. See comments below - * for more details. - */ -%expect 2 +/* Precedence declaration to avoid conflict in english_parameter; see below. */ +%right T_TYPE +%right T_IDENT %% @@ -460,7 +458,12 @@ english: T_DECLARE T_IDENT T_AS english_declaration { $$ = $2; } -storage_func_specs: { $$ = NULL; } | declspec_simple storage_func_specs { +/* + * We use a precedence declaration to prefer shifting an identifier + * over reducing this empty rule; see below. + */ +storage_func_specs: %prec T_TYPE { $$ = NULL; } +storage_func_specs: declspec_simple storage_func_specs { ALLOC_STRUCT($$, struct cdecl_declspec, .type = $1, .next = $2); @@ -539,14 +542,18 @@ null_decl: { } /* - * There is a small shift/reduce conflict here. An unadorned identifier - * as the first thing in the parameter might be a typedef name deep in the - * first english_declaration (thus empty storage_func_specs and empty - * english_declarator need to be reduced) or it might be the identifier - * before the "as" (thus the identifier should be shifted). + * There is a shift/reduce conflict here when an identifier appears as the + * first token. The conflict is between shifting T_IDENT, or reducing the + * empty production for storage_func_specs (cf. english_declaration). + * + * - In either case, if we reduce, we won't match T_IDENT T_AS since the + * stack now has the extra storage_func_specs nonterminal symbol. + * - And if we shift, we won't match english_declaration since it is + * too late to add storage_func_specs to the stack. * - * The typedef name conflict is the only issue, so treating it as a special - * case makes the shift harmless. + * The only valid input affected by the conflict is a simple type names, + * possibly followed by qualifiers. So the conflict is adequately resolved + * by shifting, so long as we have a special-case reduction to handle this. */ english_parameter: english_declaration | typedef_name_qual null_decl { ALLOC_STRUCT($$, struct cdecl, -- 2.43.2