Clang+LLVM

Article: Bulldozer Benchmarks, What Do They Mean?
By: ? (0xe2.0x9a.0x9b.delete@this.gmail.com), April 15, 2011 10:11 pm
Room: Moderated Discussions
Linus Torvalds (torvalds@linux-foundation.org) on 4/15/11 wrote:
---------------------------
>? (0xe2.0x9a.0x9b@gmail.com) on 4/15/11 wrote:
>>
>>If the language is big (like C/C++ for example), it is more efficient to write
>>a new language-specific parser generator - instead of writing a parser by hand in an imperative language.
>
>Hey, I'm ok with that

You do? I expected you wouldn't, and maybe would accompany your arguments with a couple of insults.

>- if you write your own language-
>specific parser generator, you can avoid just about all
>the problems with the generic parser generators. I even
>said that in one of posts.

Yes. I remember that. The question I asked was whether you have actually done such a sort of thing (using the C preprocessor, like you are mentioning below, does not really count here).

>That said, for most cases, I still disagree with you that
>you even need one. Parsing is generally not the
>really "hard" part (although I do agree that it can be a
>lot of work to get right, partly because of performance
>issues, partly because you do want to give reasonable
>error messages).

I didn't write that programming a parser by hand is hard. It is not: you simply stick to the rule that each grammar-rule maps to a new C/C++/etc function. That is easy, especially if you put the grammar-rule into a comment just above the function declaration. But after a while of doing that, you realize that a non-trivial part of the code is repetitive, and that it would be good to know whether the grammar is sane.

An other effect is that if the project is an open source one, the generator will prevent people from submitting erroneous parsing code. It means less work for the ones who are checking the correctness of the submitted code.

Also worth mentioning is that the generator can be written in a language that is more productive for the task. The generator can be written in C++/Java/Python/etc, while all the rest of the code and the generated parser uses C.

In addition, the execution speed of the generator has an *inverse* relation to the execution speed of the generated parser: the slower the generator (because of more advanced algorithms), the faster the generated parser. If you think about this, this a very interesting property. If the programmer is writing the parser by hand (no parser generator), it simply means that the programmer has taken the role of the generator. There are two major problems when a human is the parser generator: the inverse relation still applies, and the human functions as an ultra-slow and error-prone interpreter for executing the just mentioned advanced algorithms.

>In fact, in the project I worked on, I just used the C
>preprocessor as a simple "parser generator" for the
>most mind-numbing repeated code. It's not pretty, but
>it sure isn't complicated. The C parsing rules are not
>complicated enough to really merit anything more (and
>I really only did "C with extensions", not C++).
>
>So yes, I do agree that a lot of hand-written parsers
>should be table-driven too, and have "generated" contend.
>
>But I don't even consider that a "parser generator", if
>it only generates one specialized parser. Then it's just
>regular programming.
>
>Linus

Yes, it is regular programming.

But I would venture that there are also other reasons (than the language-specificity itself) why a language-specific parser generator will not be used in other projects. One such reason is that the generator is not a separate package (.rpm or whatever).
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New article: Bulldozer benchmark analysisDavid Kanter2011/03/30 02:27 AM
  New article: Bulldozer benchmark analysisNaN2011/03/30 07:47 AM
    New article: Bulldozer benchmark analysisDavid Kanter2011/03/30 09:25 AM
      New article: Bulldozer benchmark analysisdc2011/03/30 12:07 PM
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                  Post removedDavid Kanter2011/04/02 12:39 PM
          New article: Bulldozer benchmark analysisanonymous2011/04/04 05:08 AM
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                                          U+269B?2011/04/13 03:52 AM
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                                    Imperative compilers (and the lack of it)Azazel2011/04/12 08:23 PM
                                Imperative compilers (and the lack of it)none2011/04/10 06:00 AM
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