Re: {maint} yacc.c: initialize yylval in pure-parser mode

[Paul Eggert]

On 09/28/2012 12:22 AM, Akim Demaille wrote:
> That's not true: the test case I sent _does_ use that
> initial yylval, yet GCC does not complain. 

Ouch.  I looked into that, and it's a GCC bug.
I filed a bug report here:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54804

In the meantime, here is a similar grammar that
does not run afoul of the GCC bug, i.e., GCC correctly
reports that yylval is uninitialized:

%pure-parser
%code
{
  #define YYSTYPE double
  extern void report_error (const char *);
  extern void handle_value (int);
  static void yyerror (const char *msg)
  {
    report_error (msg);
  }

  static int yylex (YYSTYPE *lvalp)
  {
    static const char *input = "a";
    return *input++;
  }

  int main (void)
  {
    return yyparse ();
  }
}

%token 'a';
%%
exp: 'a' { handle_value ($1); };


Compile it this way, with Bison master:

bison bar.y
gcc -S -Wall -O2 bar.tab.c

I get the correct warning:

bar.tab.c: In function 'yyparse':
bar.tab.c:1133:12: warning: 'yylval' may be used uninitialized in this function 
[-Wuninitialized]

If we alter Bison to always initialize yylval,
we'll suppress this correct warning.  On the other hand, if
we use the patch I'm proposing, the warning should
still be generated.