Re: RFC: generate the default semantic action

[Akim Demaille]

> Le 14 oct. 2018 à 18:13, Hans Åberg <address@hidden> a écrit :
> 
> POSIX probably got it from there through copy-and-paste, then.

Right.

> So rules with empty RHS do not have any default value. What happen with 
> Bison’s default value in such a case, that is, what does it get when there is 
> no $1?

Garbage.

Except in C++ with variants, where $$ is only default constructed.

yacc.c:

> yyreduce:
>   /* yyn is the number of a rule to reduce with.  */
>   yylen = yyr2[yyn];
> 
>   /* If YYLEN is nonzero, implement the default value of the action:
>      '$$ = $1'.
> 
>      Otherwise, the following line sets YYVAL to garbage.
>      This behavior is undocumented and Bison
>      users should not rely upon it.  Assigning to YYVAL
>      unconditionally makes the parser a bit smaller, and it avoids a
>      GCC warning that YYVAL may be used uninitialized.  */
>   yyval = yyvsp[1-yylen];

glr.c:

> /* Default (constant) value used for initialization for null
>    right-hand sides.  Unlike the standard yacc.c template, here we set
>    the default value of $$ to a zeroed-out value.  Since the default
>    value is undefined, this behavior is technically correct.  */
> static YYSTYPE yyval_default;
…
>   if (yyrhslen == 0)
>     *yyvalp = yyval_default;
>   else
>     *yyvalp = yyvsp[YYFILL 
> (1-yyrhslen)].yystate.yysemantics.yysval;]b4_locations_if([[


lalr1.cc:

>   yyreduce:
>     yylen = yyr2_[yyn];
>     {
>       stack_symbol_type yylhs;
>       yylhs.state = yy_lr_goto_state_ (yystack_[yylen].state, 
> yyr1_[yyn]);]b4_variant_if([
>       /* Variants are always initialized to an empty instance of the
>          correct type. The default '$$ = $1' action is NOT applied
>          when using variants.  */
>       b4_symbol_variant(address@hidden@}]], [yylhs.value], [build])], [
>       /* If YYLEN is nonzero, implement the default value of the
>          action: '$$ = $1'.  Otherwise, use the top of the stack.
> 
>          Otherwise, the following line sets YYLHS.VALUE to garbage.
>          This behavior is undocumented and Bison users should not rely
>          upon it.  */
>       if (yylen)
>         yylhs.value = address@hidden - address@hidden;
>       else
>         yylhs.value = address@hidden@}.value;])[

lalr1.java:

>     /* If YYLEN is nonzero, implement the default value of the action:
>        '$$ = $1'.  Otherwise, use the top of the stack.
> 
>        Otherwise, the following line sets YYVAL to garbage.
>        This behavior is undocumented and Bison
>        users should not rely upon it.  */
>     if (yylen > 0)
>       yyval = yystack.valueAt (yylen - 1);
>     else
>       yyval = yystack.valueAt (0);