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Re: Re: When do you function?

by ameoba (Initiate)
on Dec 27, 2000 at 18:50 UTC ( #48428=note: print w/ replies, xml ) Need Help??

in reply to Re: When do you function?
in thread When do you function?

I'm relatively new to perl (OK, a complete newbie), but after looking at the same program written in C, I got much different results. 330ms and 350ms user time. Is there a way in perl to force inlining of functions? Or maybe this just points out a place for improvements in the compiler... (which, I understand, due to the interpreted nature of perl, is neccessarily minimal, but would this be a quick-fix job?)
Comment on Re: Re: When do you function?
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Benchmarks and compilation
by Falkkin (Chaplain) on Dec 27, 2000 at 20:54 UTC
    Did you test the performance of the perl programs (vs. equivalent C code) on your machine? Chances are relatively decent that your machine is better than my clunky 133 MHz Pentium.

    Fact correction: Perl is a compiled language, actually (well, as compiled as Java, anyhow). The perl program works by taking in your source file, and compiling it in several stages.

    Stage 1: the compile phase. In this phase, Perl converts your program into a data structure called a "parse tree". If the compiler sees a BEGIN block or any "use" or "no" declarations, it quickly hands those off for evaluation by the interpreter.
    Stage 2: Code generation (optional). If you're running one of the compiler backend modules (such as B::Bytecode, B::C, or B::CC), the compiler then outputs Perl bytecodes (much like a Java .class file) or a standalone chunk of (very odd-looking) C code. These code-generators are all highly experimental at the present.
    Stage 3: Parse-tree reconstruction. If you did stage 2, stage 3 remakes the parse tree out of the Perl bytecodes or C opcodes. This speeds up execution, because running the bytecodes as-is owuld be a performance hit.
    Stage 4: The execution phase. The interpreter takes the parse tree and executes it.

    This is Perl compilation in a nutshell... read Chapter 18 of the 3rd edition of Programming Perl for a more in-depth analysis. For many tasks (especially simple ones such as these) Perl will be slower than C, because C is basically a more-portable form of assembly language, and assembly language (once actually assembled) works with raw hardware, and is hence as about fast as you can get.

    Another difference between Perl and a native C app that may affect performance is the fact that Perl has its own stack (actually, it has several stacks) as opposed to a C program, which is likely to just use the system stack.

Inlining functions
by mirod (Canon) on Dec 27, 2000 at 20:52 UTC

    I add my voice to this: the should be a way to inline functions and method calls.

    Granted you can use the pre-processor (perl -P) to inline functions but this does not work for method calls. This is really bad when designing OO Perl, where I find myself using straight hash access ($o->{field}) instead of accessors ($o->field) for some often-called methods (or writing painfull and risky kludges), which makes maintenance much harder

    I am actually very surprised this is not even a Perl 6 RFC, I would think that this is a simple (I would think it is quite easy to implement) way to enhance speed or maintainability of OO Perl programs.

      Well, since it'd probably be just an attribute on a subroutine, and we already have those, there's nothing really to RFC. {grin}

      Ideally, the Perl compiler would just recognize when a subroutine qualifies as an open-coded version, and "do the right thing". I'd prefer that. Just like how certain subroutines are recognized as "constant" now, and pre-evaluated.

      -- Randal L. Schwartz, Perl hacker

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