Re: parser performance, was Popularity of compiler tools, was LRgen

> [My understanding is that GCC switched to a hand-written parser
> because of the difficulty of parsing the awful C++ grammar with
> anything other than hand-written hacks. The new parser may be a
> little faster but that wasn't a big issue, since parse time is never a
> bottleneck in a compiler. -John]


I want to disagree with our esteemed moderator a little bit. Parsing
time is not a bottleneck when optimizing. But the speed of the
compiler matters more when not optimizing, and in that case the parser
can indeed be a bottleneck. When compiling C++ with gcc with a lot of
header files, the parsing time can be up to 50% of the total
compilation time when not optimizing.


Ian
[Are you including tokenizing in the 50%? Lexers often do take a lot
of time, since they have to do something to each character. But once
the lexer has shrunk the input from a stream of characters to a stream
of tokens, the parser rarely takes an appreciable amount of time.
Opinions vary about the relative performance of DFA lexers vs ad-hoc
hand written ones, which I think means that the implementation is more
important than the technique. -John]

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