Re: scanner/parser minimization

On 02.03.2013 17:09, Tom Lane wrote:
> Greg Stark<stark@mit.edu>  writes:
>> Regarding yytransition I think the problem is we're using flex to
>> implement keyword recognition which is usually not what it's used for.
>> Usually people use flex to handle syntax things like quoting and
>> numeric formats. All identifiers are handled by flex as equivalent.
>> Then the last step in the scanner for identifiers is to look up the
>> identifier in a hash table and return the keyword token if it's a
>> keyword. That would massively simplify the scanner tables.
>
> Uh ... no.  I haven't looked into why the flex tables are so large,
> but this theory is just wrong.  See ScanKeywordLookup().

Interestingly, the yy_transition array generated by flex used to be much
smaller:

8.3: 22072 elements
8.4: 62623 elements
master: 64535 elements

The big jump between 8.3 and 8.4 was caused by introduction of the
unicode escapes: U&'foo' [UESCAPE 'x'] . And in particular, the "error
rule" for the UESCAPE, which we use to avoid backtracking.

I experimented with a patch that uses two extra flex states to shorten
the error rules, see attached. The idea is that after lexing a unicode
literal like "U&'foo'", you enter a new state, in which you check
whether an "UESCAPE 'x'" follows. This slashes the size of the array to
36581 elements.

- Heikki