Performance of loops

[Chris via Digitalmars-d]

I tested the performance of three types of loops (see code 
below). It turns out that the fastest loop is the "plainLoop". 
Unless my examples are completely screwed up, the difference 
between "plainLoop" and the other two loops is gigantic (e.g.):

9 ms, 149 μs, and 4 hnsecs  // foreach (const ref w)
9 ms, 77 μs, and 8 hnsecs   // foreach (ref w)
1 ms, 183 μs, and 6 hnsecs  // foreach (w)
with -release -inline -O -boundscheck=off

8 ms, 492 μs, and 3 hnsecs
8 ms, 287 μs, and 1 hnsec
341 μs and 2 hnsecs

[compiler dmd v2.067.0, Linux Ubuntu, 64bit]


import std.datetime : benchmark, Duration;
import std.string : format;
import std.conv : to;
import std.stdio : writeln;

enum {
  string[] words = ["Hello", "world", "Ola", "mundo"],
}

void main() {
  auto result = benchmark!(constLoop, refLoop, 
plainLoop)(100_000);
  writeln(to!Duration(result[0]));
  writeln(to!Duration(result[1]));
  writeln(to!Duration(result[2]));

}

void constLoop() {
  size_t cnt;
  foreach (const ref w; words)
    cnt += w.length;
}

void refLoop() {
  size_t cnt;
  foreach (ref w; words)
    cnt += w.length;
}

void plainLoop() {
  size_t cnt;
  foreach (w; words)
    cnt += w.length;
}