Re: small Bison optimization: remove single character literal tokens

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: John Naylor <john(dot)naylor(at)2ndquadrant(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: small Bison optimization: remove single character literal tokens
Date: 2019-12-16 15:23:09
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John Naylor <john(dot)naylor(at)2ndquadrant(dot)com> writes:
> Traditionally, when Bison generates the header file, it starts
> numbering named tokens at 258, so that numbers below 256 can be used
> for character literals. Then, during parsing, the external token
> number or character literal is mapped to Bison's internal token number
> via the yytranslate[] array.
> The newly-released Bison 3.5 has the option "%define api.token.raw",
> which causes Bison to write out the same ("raw") token numbers it
> would use internally, and thus skip building the yytranslate[] array
> as well as the code to access it. To make use of this, there cannot be
> any single character literals in the grammar, otherwise Bison will
> refuse to build.
> Attached is a draft patch to make the core grammar forward-compatible
> with this option by using FULL_NAMES for all valid single character
> tokens. Benchmarking raw parsing with "%define api.token.raw" enabled
> shows ~1.7-1.9% improvement compared with not setting the option. Not
> much, but doing one less array access per token reduces cache
> pollution and saves a few kB of binary size as well.

TBH, I'm having a hard time getting excited about this. It seems like
you've just moved the mapping from point A to point B, that is, in
place of a lookup in the grammar you have to have the lexer translate
ASCII characters to something else. I'm not sure that's an improvement
at all. And I'm really unexcited about applying a patch that's this
invasive in order to chase a very small improvement ... especially a
very small improvement that we can't even have anytime soon.

> It'll be years before Bison 3.5 is common in the wild,

It'll be *decades* before we'd consider requiring it, really, unless
there are truly striking improvements unrelated to this point.

regards, tom lane

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