Clang+LLVM

Article: Bulldozer Benchmarks, What Do They Mean?
By: Rob Thorpe (rthorpe.delete@this.robertthorpeconsulting.com), April 14, 2011 12:33 pm
Room: Moderated Discussions
Linus Torvalds (torvalds@linux-foundation.org) on 4/13/11 wrote:
---------------------------
>I suspect it's one of those problems that aren't all
>that difficult in theory, but in practice you end up
>fighting all the silly impedance issues because the
>tools expect one model, and mixing generated code with
>the non-generated gets ugly really quickly.

As far as I can see all you're saying here is that "there must be problems" but you haven't actually tried it or read or talked to anyone that has.

I'm not sure that it's really so difficult. I agree that by experience a software engineer would say "this tool is made for this, so there will probably be problems using it in a slightly different way". I agree that's a good principle. But, I'm not sure that it applies in this specific situation.

I've written a short bison parser that takes it's token stream from structures in memory rather than a file. Nothing as complex as C++ or even C. But, from doing that I'm not really sure that there are any big problems.

Bison type parser generators are quite conceptually simple. To a bison parser the lexer is a function that returns the next token (both it's type and semantic value). Those things then disappear within Bison's generated code and cannot be touched. Then the information comes out the other side when Bison executes the actions specified along with the grammar. The semantic information is then fed back to the user's code at that time.

There's nothing in this structure that prevents parsing from data structures in memory or makes it particularly difficult.

>But that is seldom the case. It's certainly not the case
>for C (much less C++). C is close enough to context-free
>that you can do things within the confines of something
>like bison without it being too painful, but when
>you get to C++, you're going to be in real pain.

It's probably impossible for an LALR parser generator to parse C++, it's too complicated. But these days GLR/Tomita parser generators are available, Bison is capable of GLR and so are a few others.

GCC stopped using Bison for parsing C before Bison became capable of GLR.

>You might as well just write the parser yourself instead.
>Usually using a trivial recursive-descent model, which is
>dead simple, rather than even trying to do anything fancy.

Recursive descent for a language that's LL(1) is very straightforward and elegant. Recursive descent for an LL(k) language requires multiple-token lookahead or backtracking. It's as unpleasant as LR type parsing in my opinion.

The GCC C++ parser uses both lookahead and backtracking. A comment in the GCC C++ parser (cp/parser.c) source file says:

The parser is of the standard recursive-descent variety. Upcoming
tokens in the token stream are examined in order to determine which
production to use when parsing a non-terminal. Some C++ constructs
require arbitrary look ahead to disambiguate. For example, it is
impossible, in the general case, to tell whether a statement is an
expression or declaration without scanning the entire statement.
Therefore, the parser is capable of "parsing tentatively." When the
parser is not sure what construct comes next, it enters this mode.
Then, while we attempt to parse the construct, the parser queues up
error messages, rather than issuing them immediately, and saves the
tokens it consumes. If the construct is parsed successfully, the
parser "commits", i.e., it issues any queued error messages and
the tokens that were being preserved are permanently discarded.
If, however, the construct is not parsed successfully, the parser
rolls back its state completely so that it can resume parsing using
a different alternative."

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TopicPosted ByDate
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
      New article: Bulldozer benchmark analysisFoo_2011/03/30 12:17 PM
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                  Post removedDavid Kanter2011/04/02 12:39 PM
          New article: Bulldozer benchmark analysisanonymous2011/04/04 05:08 AM
      New article: Bulldozer benchmark analysisinterested2011/04/01 03:28 PM
        New article: Bulldozer benchmark analysisDavid Kanter2011/04/01 03:53 PM
          New article: Bulldozer benchmark analysisNaN2011/04/02 07:12 AM
            New article: Bulldozer benchmark analysisDavid Kanter2011/04/02 09:05 AM
              New article: Bulldozer benchmark analysisNaN2011/04/02 09:24 AM
                New article: Bulldozer benchmark analysisDavid Kanter2011/04/02 12:36 PM
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                                Imperative compilers (and the lack of it)?2011/04/10 12:43 AM
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                                          U+269B?2011/04/13 03:52 AM
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                                              U+269BMichael S2011/04/20 09:11 AM
                                                U+269BHeath Provost2011/04/20 10:26 AM
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                                                                        We actually upgraded to 2008 R2anonymous2011/04/24 08:44 PM
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                                                                            File size optimizationsjrl2011/04/25 05:52 PM
                                                                          File size optimizationsDavid Kanter2011/04/25 07:49 AM
                                                                            File size optimizationsRicardo B2011/04/25 08:22 AM
                                                                              ContinuationRicardo B2011/04/25 08:25 AM
                                                                              File size optimizationsChristian Packmann2011/04/25 09:56 AM
                                                                                File size optimizationsEduardoS2011/04/25 11:57 AM
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                                                                              File size optimizationsEduardoS2011/04/25 12:57 PM
                                                                            File size optimizationsjrl2011/04/25 05:54 PM
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                                                                          File size optimizationsHeath Provost2011/04/25 02:02 PM
                                    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
                                  Imperative compilers (and the lack of it)dmsc2011/04/10 06:39 AM
                                  Imperative compilers (and the lack of it)EduardoS2011/04/10 12:55 PM
                                compiler failure rateshobold2011/04/10 10:55 AM
                                  compiler failure rates?2011/04/10 11:13 PM
                                    compiler failure rateshobold2011/04/11 05:07 AM
                                Imperative compilers (and the lack of it)EduardoS2011/04/10 12:55 PM
                                  Imperative compilers (and the lack of it)?2011/04/10 11:13 PM
                                    Imperative compilers (and the lack of it)Konrad Schwarz2011/04/14 01:28 AM
                                      Imperative compilers (and the lack of it)gallier22011/04/14 04:47 AM
                                      Imperative compilers (and the lack of it)EduardoS2011/04/14 02:04 PM
                                        Imperative compilers (and the lack of it)Konrad Schwarz2011/04/15 03:43 AM
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                                  Hard register problemsMax2011/04/11 03:05 PM
                                    Hard register problemsRob Thorpe2011/04/11 03:28 PM
                                Imperative compilers (and the lack of it)gallier22011/04/11 09:26 AM
                                  I wonder why.j2011/04/11 10:39 PM
                                    I wonder why.?2011/04/11 11:44 PM
                                Imperative compilers (and the lack of it)nksingh2011/04/12 11:00 PM
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                                AMD's Open64 compiler ?none2011/04/10 07:23 AM
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    New article: Bulldozer benchmark analysisNaN2011/03/30 05:24 PM
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            AMD's memory controllers are fineanon2011/04/05 04:52 PM
              AMD's memory controllers are fineVincent Diepeveen2011/04/06 01:36 AM
                AMD's memory controllers are fineanon2011/04/06 06:59 AM
                  AMD's memory controllers are fineVincent Diepeveen2011/04/07 02:15 PM
                    AMD's memory controllers are fineVincent Diepeveen2011/04/07 02:27 PM
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                    AMD's memory controllers are fineanon2011/04/07 02:54 PM
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      a waste of timeDavid Kanter2011/04/09 06:56 PM
        a waste of timehorsefly2011/04/10 05:13 AM
          InvestorsDavid Kanter2011/04/10 06:27 AM
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