Performance of loops

[Chris via Digitalmars-d]

On Friday, 24 April 2015 at 11:39:14 UTC, John Colvin wrote:
> On Friday, 24 April 2015 at 11:00:23 UTC, Chris wrote:
>> 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;
>> }
>
> dmd's optimiser isn't great. GDC creates identical assembly for 
> all three functions.
>
> Rule of thumb: if you want high performance, use GDC or LDC.

Yeah, I figure. However, the gap is a serious issue for dmd, even 
if it's only a reference compiler.