• d-bugmail puremagic.com Apr 18 2008
• d-bugmail puremagic.com Apr 18 2008

http://d.puremagic.com/issues/show_bug.cgi?id=2008

           Summary: Poor optimization of functions with ref parameters
           Product: D
           Version: 1.028
          Platform: PC
        OS/Version: Windows
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla digitalmars.com
        ReportedBy: aarti interia.pl


Functions with ref parameters can be up to 2x slower than functions which
parameters are not passed by reference. It is especially visible for programs
which are compiled with all optimizations on:
-O
-inline
-release

Test program:
------------------------

char peek_ref(ref char[] a) {
    return a[0];
}

char peek_noref(char[] a) {
    return a[0];
}


ulong rdtsc() {
    asm {
        naked;
        rdtsc;
        ret;
    }
}

void main() {
    char[] tst = "test string";
    char c;
    long starttime;

    writef("Direct: ");
    starttime = rdtsc;
    for(int i=0; i<1000; i++)
        c = tst[0];
    writefln(c, " - time: ", rdtsc - starttime);

    writef("With ref: ");
    starttime = rdtsc;
    for(int i=0; i<1000; i++)
        c = tst.peek_ref();
    writefln(c, " - time: ", rdtsc - starttime);

    writef("Without ref: ");
    starttime = rdtsc;
    for(int i=0; i<1000; i++)
        c = tst.peek_noref();
    writefln(c, " - time: ", rdtsc - starttime);
}

----------------------


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http://d.puremagic.com/issues/show_bug.cgi?id=2008





------- Comment #1 from wbaxter gmail.com  2008-04-18 18:22 -------
I believe the problem is that ref parameters disable inlining in DMD.  Or so I
think I heard somewhere before.


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