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digitalmars.D.bugs - [Issue 9917] New: Excessive overhead for small-size vector ops

http://d.puremagic.com/issues/show_bug.cgi?id=9917

           Summary: Excessive overhead for small-size vector ops
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: bearophile_hugs eml.cc


--- Comment #0 from bearophile_hugs eml.cc 2013-04-10 19:38:48 PDT ---
I suggest to inline with normal code the vector ops among arrays that are
statically known to be quite short.

The following two benchmarks compute the number of Pythagorean triples and show
the performance difference between using a sum among ulong[2] and doing the
same with a struct that defined opOpAssign("+").

Runtime first version about 5.8 seconds, runtime second version about 0.7
seconds.


// First version - - - - - - - - - - 
import std.stdio;

ulong[2] tri(in ulong lim, in ulong a=3, in ulong b=4, in ulong c=5) pure {
  immutable l = a + b + c;
  if (l > lim)
    return [0, 0];
  ulong[2] r = [1, lim / l];
  r[] += tri(lim,  a - 2*b + 2*c,  2*a - b + 2*c,  2*a - 2*b + 3*c)[];
  r[] += tri(lim,  a + 2*b + 2*c,  2*a + b + 2*c,  2*a + 2*b + 3*c)[];
  r[] += tri(lim, -a + 2*b + 2*c, -2*a + b + 2*c, -2*a + 2*b + 3*c)[];
  return r;
}

void main() {
  tri(10 ^^ 8).writeln;
}
//- - - - - - - - - - - - - - -



// Second version - - - - - - - - - - 
import std.stdio;

struct Pair {
    ulong x, y;
    void opOpAssign(string op="+")(in Pair p2) pure nothrow {
        x += p2.x;
        y += p2.y;
    }
}

Pair tri(in ulong lim, in ulong a=3, in ulong b=4, in ulong c=5) pure {
  immutable l = a + b + c;
  if (l > lim)
    return Pair();
  auto r = Pair(1, lim / l);
  r += tri(lim,  a - 2*b + 2*c,  2*a - b + 2*c,  2*a - 2*b + 3*c);
  r += tri(lim,  a + 2*b + 2*c,  2*a + b + 2*c,  2*a + 2*b + 3*c);
  r += tri(lim, -a + 2*b + 2*c, -2*a + b + 2*c, -2*a + 2*b + 3*c);
  return r;
}

void main() {
  tri(10 ^^ 8).writeln;
}
//- - - - - - - - - - - - - - -

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