Thread: question on optimization

  1. #1
    Just because ygfperson's Avatar
    Join Date
    Jan 2002

    question on optimization

    as far as i know, a c++ compiler inputs c++ code, optimizes, converts it to assembly code, optimizes again, and then assembles into opcodes.

    how does optimization work? here's some code:
    a = b;
    c = b;
    b = a;  //swaps b and c
    b = b + 1;
    in assembly it's probably something close to:
    xchg ebx, ecx;
    inc ebx;
    does the optimizer just search for pre-defined patterns of logic? does it search for any kind of redundancy? how effective are the optimizations?

  2. #2
    Registered User
    Join Date
    Sep 2001
    i always thought that the compiler outputted optimized code .... also, theres one little state that you forgot, the code is turned into and object once compiled. then its linked.

  3. #3
    Just because ygfperson's Avatar
    Join Date
    Jan 2002
    let me rephrase:
    when optimizations are done, does the compiler search for redundancies of any kind, or just look for patterns of predefined redundant logic?

  4. #4
    Registered User
    Join Date
    Sep 2001
    maybe a combination of both ...?

  5. #5
    Guest Sebastiani's Avatar
    Join Date
    Aug 2001
    Waterloo, Texas
    The compiler optimizes the code AFTER it has been converted to assembly. And the method/degree of optimization depends on the compiler, so it's hard to define what/how the optimization is being performed. I always turn off optimization for the simple fact that I would rather do it myself - hey, if I write ****ty/slow code, that's my fault! See, if the compiler optimizes erroneously (ie: inside a thread, etc) then you might have some mysterious bugs on your hands! Nonetheless, the 'volatile' keyword is there to protect against these things from happening to optimized code (usually...)
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
        return std::pow
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;

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