[Issue 6658] New: Slow short array equality

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=6658

           Summary: Slow short array equality
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Keywords: performance
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2011-09-12 16:20:16 PDT ---
Equality comparisons between very short arrays is quite slow in D/DMD. When the
arrays are both fixed-sized and short, I suggest to inline item comparisons
instead of calling a runtime function, to speed up the code significantly.

A benchmark:


bool eq1(T, size_t N)(const ref T[N] a, const ref T[N] b) pure nothrow {
    return a == b;
}

bool eq2(T, size_t N)(const ref T[N] a, const ref T[N] b) pure nothrow {
    foreach (size_t i; 0 .. a.length)
        if (a[i] != b[i])
            return false;
    return true;
}

void main() {
    import std.stdio, std.random;
    enum size_t N = 3;
    enum size_t M = 100;
    rndGen.seed(1);

    int[N][M] data;
    foreach (ref d; data)
        foreach (ref x; d)
            x = uniform(0, 4);

    int total;
    foreach (i; 0 .. 20_000)
        foreach (j; 0 .. M)
            foreach (k; 0 .. M)
                static if (1)
                    total += eq1(data[j], data[k]); // 11.5 seconds
                else
                    total += eq2(data[j], data[k]); // 2.45 seconds

    writeln(total);
}

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