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Re: lexer speed, was Bison

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Hans-Peter Diettrich schrieb:
>> [Compilers spend a lot of time in the lexer, because that's the only
>> phase that has to look at the input one character at a time. -John]
>
> When the source code resides in a memory buffer, the time for reading
> e.g. the characters of an identifier (in the lexer) is neglectable vs.
> the time spent in lookup and entering the identifier into a symbol table
> (in the parser).
>
> Even if a lexer reads single characters from a file, most OSs maintain
> their own file buffer, so that little overhead is added over the
> program-buffered solution.
>
> I really would like to see some current benchmarks about the behaviour
> of current compilers and systems.
>
> DoDi
> [The benchmarks I did were a while ago, but they showed a large
> fraction of time in the lexer.  I wouldn't disagree that building the
> symbol table is slow, but figure out some estimate of the ratio of
> the number of characters in a source file to the number of tokens,
> and that is a rough estimate of how much slower the lexer will be
> than the parser. I agree that some current analyses would be useful.
> -John]

Your estimation suggests that the benchmarked parser does not much
more than syntax checking the tokens. In this case I agree that a
parser automaton does less work (state transitions) than a lexer
automaton (or equivalents), and that detailed benchmarking is not
required to proof this fact.

I still disagree that "Compilers spend a lot of time in the lexer".
My point is that a real *compiler* does not normally spend significant
time in neither the lexer nor the parser[1]. In so far it's okay to
"profile your compiler to see where it's spending its time and fix
what needs to be fixed", as you said :-)


[1] The timing depends heavily on what tasks are assigned to the parser
itself. Is code generation part of the parser, or part of a subsequent
(optimization...) stage? What about symbol table (scopes) management and
lookup, and an eventual preprocessor which IMO in an C compiler consumes
more time than the lexer and parser altogether? But further discussion
of these academic issues doesn't help in spotting the real bottlenecks
of any concrete compiler ;-)

DoDi
[Perhaps you could do some experiments and tell us whether it confirms
your intuition.  Like I said, when I profiled some compilers, I was
surprised to see how much time they spent grinding through the
characters in the input. -John]

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Bison determinis​tic LALR(1) parser for Java/C++ (kind of complex langauge) without 'lexar hack' support hsad005@gmail.com - 2012-08-17 11:22 -0700
  Re: Bison determinis​tic LALR(1) parser for Java/C++ (kind of complex langauge) without 'lexar hack' support Hans-Peter Diettrich <DrDiettrich1@aol.com> - 2012-08-18 10:13 +0100
    Re: lexer speed, was Bison Hans-Peter Diettrich <DrDiettrich1@aol.com> - 2012-08-20 01:01 +0100
      Re: lexer speed, was Bison Hans-Peter Diettrich <DrDiettrich1@aol.com> - 2012-08-20 16:14 +0100
      Re: lexer speed, was Bison BGB <cr88192@hotmail.com> - 2012-08-20 14:14 -0500
        Re: lexer speed, was Bison Hans-Peter Diettrich <DrDiettrich1@aol.com> - 2012-08-21 07:40 +0100
      Re: lexer speed, was Bison "BartC" <bc@freeuk.com> - 2012-08-21 17:39 +0100
    Re: Bison determinis​tic LALR(1) parser for Java/C++ (kind of complex langauge) without 'lexar hack' support anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2012-08-20 13:35 +0000
      Re: Bison determinis​tic LALR(1) parser for Java/C++ (kind of complex langauge) without 'lexar hack' support BGB <cr88192@hotmail.com> - 2012-08-21 14:45 -0500
        Re: Bison determinis​tic LALR(1) parser for Java/C++ (kind of complex langauge) without 'lexar hack' support "BartC" <bc@freeuk.com> - 2012-08-22 14:04 +0100
          Re: Bison determinis​tic LALR(1) parser for Java/C++ (kind of complex langauge) without 'lexar hack' support BGB <cr88192@hotmail.com> - 2012-08-26 19:37 -0500
            Bison deterministic LALR parser for Java/C++ "BartC" <bc@freeuk.com> - 2012-08-29 22:03 +0100
              speeding up C recompilation, was Re: Bison deterministic LALR BGB <cr88192@hotmail.com> - 2012-09-04 13:45 -0500
              Re: C include handling, was Bison deterministic LALR Marco van de Voort <marcov@toad.stack.nl> - 2012-09-05 08:40 +0000
  Re: Bison determinis​tic LALR(1) parser for Java/C++ (kind of complex langauge) without 'lexar hack' support hsad005@gmail.com - 2012-08-18 02:09 -0700

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