Re: Performance impact of "redundant" rules

[Sylvain Schmitz]

Frans Englich wrote:
> Sometimes I write grammar constructs like this:
>
> StringLiteral: STRING_LITERAL
>
> and use StringLiteral in subsequent rules, instead of STRING_LITERAL directly. 
> The reason is readability, and to stay consistent with an EBNF specification.
>
> I wonder, does this cause a negative performance impact? When I compile the 
> parser with extra debug output I see that it actually performs a 
> STRING_LITERAL --> StringLiteral reduction.

These rules are called "unit rules" in the literature; much effort was 
devoted to wiping them out automatically from LR parsers since they 
account for a non-negligible space and time overhead (an old paper by 
Joliat mentionned a 47% time improvement when getting rid of them).  If 
you want to learn more about these techniques, the chapter on LR 
optimization in Sippu and Soisalon-Soininen's _Parsing_Theory_ seems 
rather exhaustive.

--
  Sylvain