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    SubjectRe: [PATCH 2/3] netfilter : 3 patches to boost ip_tables performance
    On Thu, Sep 22, 2005 at 03:05:50PM +0200, Eric Dumazet wrote:
    (...)
    > It was necessary to get the best code with gcc-3.4.4 on i386 and
    > gcc-4.0.1 on x86_64
    >
    > For example :
    >
    > bool1 = FWINV(ret != 0, IPT_INV_VIA_OUT);
    > if (bool1) {
    >
    > gives a better code than :
    >
    > if (FWINV(ret != 0, IPT_INV_VIA_OUT)) {
    >
    > (one less conditional branch)
    >
    > Dont ask me why, it is shocking but true :(

    I also noticed many times that gcc's optimization of "if (complex condition)"
    is rather poor and it's often better to put it in a variable before. I even
    remember that if you use an intermediate variable, it can often generate a
    CMOV instruction on processors which support it, while it produces cond tests
    and jumps without the variable. Generally speaking, if you want fast code,
    you have to write it as a long sequence of small instructions, just as if
    you were writing assembly. As you said, shocking but true.

    BTW, cheers for your optimizations !

    Regards,
    Willy

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