• via Digitalmars-d-bugs (38/38) Jul 20 2014 https://issues.dlang.org/show_bug.cgi?id=13170

via Digitalmars-d-bugs <digitalmars-d-bugs puremagic.com> writes:

https://issues.dlang.org/show_bug.cgi?id=13170

          Issue ID: 13170
           Summary: Optimizer - improve branch prediction for final switch
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: DMD
          Assignee: nobody puremagic.com
          Reporter: bugzilla digitalmars.com

void A();
void B();
void C();
void D();
void E();

void test(int i)
{
    while (1)
    {
        final switch (i)
        {
            case 1: A(); break;
            case 2: B(); break;
            case 3: C(); break;
            case 4: D(); break;
            case 5: E(); break;
        }
        ++i;
    }
}

Instead of using one dispatch at the start of the loop, put the dispatch at
each break;, which will produce much better branch prediction.

See also:

http://eli.thegreenplace.net/2012/07/12/computed-goto-for-efficient-dispatch-tables/

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