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RE: [hackers-il] Summary of a Bash Lecture

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  • Chen Shapira
    ... And the ! command to use the history
    Message 1 of 59 , Oct 30, 2000
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      > simple readline quickies:
      >
      > arrows - history
      > ^-r find command in history by name
      >
      > readline is NOT shell

      And the ! command to use the history
    • Nadav Har'El
      ... The obvious answer is that all languages can be optimized the same: if the optimizer is smart enough, and given a program in C that can somehow be run
      Message 59 of 59 , Nov 15, 2000
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        On Wed, Nov 15, 2000, Chen Shapira wrote about "RE: [hackers-il] Summary of a Bash Lecture":
        > BTW. Is there any knowledge about a theoretical limit of how well a language
        > can be optimised "by design"?
        >
        > Meaning: a program written in c can be optimised for speed until it takes x
        > seconds to run and no farther, while the same program in pascal can be
        > optimised until it takes y seconds to run and no farther, where x!=y?

        The "obvious" answer is that all languages can be optimized the same: if the
        optimizer is smart enough, and given a program in C that can somehow be run
        faster if written in Fortran, then the super-smart optimizer can convert the
        program to Fortran as its first step.

        But in real life, this is not so simple. Consider the following zsh script:

        let i=0
        CMD="let i++"

        if [[ `date` = "never" ]]
        then
        CMD="let i+=2"
        fi

        for j in a b c
        do
        eval $CMD
        done
        print $i


        This can be optimized (if the optimizer is smart enough) to
        if [[ `date` = "never" ]]
        then
        print 6
        else
        print 3
        fi

        But if you realise that date can never return "never" (sounds like a James
        Bond movie...), then it can be further optimized to
        print 3

        This looks like a contrived example, and it is, but it shows a point: a
        smart enough optimizer can take code in Zsh, Perl, C++, or whatever, and
        optimize it into the best possible assembly-language code (i.e., that no
        other code that does the same thing runs faster). But how do you define
        "doing the same thing"? Instead of running the "date" program, the shell
        compiler/optimizer could call ctime() directly. But what if the installed
        date program has some strange side-effects (e.g., changing the access time
        of the file "/bin/date"!) that the program relies on? Maybe the whole point
        of the above program was to be a "touch -a /bin/date" clone, and the "let i=.."
        stuff, not the `date` stuff, needs to be optimized out?

        The problem is that different languages let you do pretty high-level things
        that each has dozens of implications and/or side-effects, so keeping all these
        side-effects may mean we can't do any optimization at all...

        As another example, consider a Tcl/Tk program (I hope you know what I'm
        talking about). In Tcl/Tk, the callbacks of widgets are written in the
        interpreted language Tcl; Because of this fact you (the programmer) can
        change widget bindings on the fly, and other crazy stuff like that. And
        all this interpreting is slow. It is obvious that it is possible to write
        a different program in (say) C/Xlib, that does exactly the same thing, but
        quicker, but making this transformation (also known as "optimization" - the
        distinction between a "compiler" and "optimizer" in this context is a little
        fuzzy) is nearly impossible in practice... Also, does this C/Xlib really does
        the *same* thing? The Tcl/Tk program might be able to read widget bindings
        from a user's script file, and the C/Xlib will not... So it's not exactly the
        same thing - who decides if this optimization is a good enough approximation?




        --
        Nadav Har'El | Wednesday, Nov 15 2000, 17 Heshvan 5761
        nyh@... |-----------------------------------------
        Phone: +972-53-245868, ICQ 13349191 |Willpower: The ability to eat only one
        http://nadav.harel.org.il |salted peanut.
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