[Issue 12175] New: More efficient very common case for std.algorithm.sum

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=12175

           Summary: More efficient very common case for std.algorithm.sum
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2014-02-15 02:53:30 PST ---
The most common code path of std.algorithm.sum contains:

+    Result seed = 0;
+    return reduce!"a + b"(seed, r);


I suggest to add a "static if" that tests if the input is an array (or a random
access range) and in such case instead of reduce to use a more efficient loop
like this:

immutable lenR = r.length;
for (int i = 2; i < lenR; i += 2) {
    total1 += r[i];
    total2 += r[i + 1];
}
return total1 + total2;


This is more efficient on most modern CPUs.

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