4.4BSD/usr/src/old/yacc/yaccdiffs
Yacc Differences
This document gives a short list of differences between the
new Yacc and earlier Yaccs.
_B_u_g_s _F_i_x_e_d
1. There was a bug which caused Yacc to silently steal
away in the night if an action had mismatched '' in it;
this is fixed.
2. A number of table size overflow conditions used to be
checked incorrectly or not at all; this is now better.
3. A bug which suppressed the printing of some rules with
empty RHS's on the y.output file has been fixed.
_S_p_e_e_d_u_p_s, _S_h_r_i_n_k_s, _a_n_d _D_i_d_d_l_e_s
1. The old optimizer (-o) flag is now the default in Yacc.
At the same time, the Yacc process itself has been sped
up; the result is that Yacc takes about the same or
slightly longer on short inputs, but is much faster on
long inputs.
2. The optimized parsers produced by Yacc are likely to be
2-3 times faster and 1-2k bytes smaller than the old
ones, for medium/large grammars. The time to parse is
now essentially independent of the grammar size; it
used to grow as the size of the grammar did.
3. The y.output file has been considerably reformatted, to
make it easier to read. The old "goto" table is gone;
the goto's for nonterminal symbols are now printed in
the states where they occur. Rules which can be
reduced in a state have their rule number printed after
them, in (). This makes it much easier to interpret
the "reduce n" actions. The message "same as n" has
been removed; duplicate states are in fact duplicated,
saving shuffling and cross-referencing.
4. Various table sizes are somewhat bigger.
5. The form feed character, and the construction '\f', are
now recognized; form feed is ignored (=whitespace) on
input.
January 14, 1977
- 2 -
6. The arrays "yysterm" and "yysnter" are no longer pro-
duced on output; they were little used, and took up a
surprising amount of space in the parser.
7. Rules in the input which are not reduced are now com-
plained about; they may represent unreachable parts of
the grammar, botched precedence, or duplicate rules.
As with conflicts, a summary complaint, "n rules not
reduced", appears at the terminal; more information is
on the y.output file.
_N_e_w _F_e_a_t_u_r_e_s
1. The actions are now copied into the middle of the
parser, rather than being gathered into a separate rou-
tine. It's faster. Also, you can return a value from
yyparse (and stop parsing...) by saying `return(x);' in
an action. There are macros which simulate various
interesting parsing actions:
YYERROR causes the parser to behave as if a syntax
error had been encountered (i.e., do error recovery)
YYACCEPT causes a return from yyparse with a value of 0
YYABORT causes a return from yyparse with a value of 1
2. The repositioning of the actions may cause scope prob-
lems for some people who include lexical analyzers in
funny places. This can probably be avoided by using
another new feature: the `-d' option. Invoking Yacc
with the -d option causes the #defines generated by
Yacc to be written out onto a file called "y.tab.h",
(as well as on the "y.tab.c" file). This can then be
included as desired in lexical analyzers, etc.
3. Actions are now permitted within rules; for such
actions, $$, $1, $2, etc. continue to have their usual
meanings. An error message is returned if any $n
refers to a value lying to the right of the action in
the rule. These internal actions are assumed to return
a value, which is accessed through the $n mechanism.
In the y.output file, the actions are referred to by
created nonterminal names of the form $$nnn. All
actions within rules are assumed to be distinct. If
some actions are the same, Yacc might report
reduce/reduce conflicts which could be resolved by
explicitly identifying identical actions; does anyone
have a good idea for a syntax to do this? The = sign
may now be omitted in action constructions of the form
={ ... }.
January 14, 1977
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4. As a result of the rearrangement of rules, people who
thought they knew what $1 really turned into, and wrote
programs which referred to yypv[1], etc., are in trou-
ble. See Steve Johnson if you are really suffering.
January 14, 1977