http://d.puremagic.com/issues/show_bug.cgi?id=255

           Summary: Odd performance difference w/ complex doubles.
           Product: D
           Version: 0.162
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla@digitalmars.com
        ReportedBy: godaves@yahoo.com


Version foo runs 5x slower depending on which scope the complex double variables are declared in.

import std.stdio, std.string;
void main(char[][] args)
{
  char  bit_num = 0, byte_acc = 0;
  const int iter = 50;
  const double lim = 2.0 * 2.0;
version(foo)
{
  cdouble Z, C;
}

  int n = atoi(args[1]);

  writefln("P4\n%d %d",n,n);

  for(int y=0; y<n; y++)
    for(int x=0; x<n; x++)
    {
version(foo)
{}
else
{
       cdouble Z,C;
}
       Z = 0 + 0i;
       C = 2*cast(double)x/n - 1.5 + 2i*cast(double)y/n - 1i;

       for(int i = 0; i < iter && norm(Z) <= lim; i++)
         Z = Z*Z + C;

       byte_acc = (byte_acc << 1) | ((norm(Z) > lim) ? 0x00:0x01);

       bit_num++;
       if(bit_num == 8)
       {
           putc(byte_acc,stdout);
           bit_num = byte_acc = 0;
       }
       else if(x == n-1)
       {
           byte_acc  <<= (8-n%8);
           putc(byte_acc,stdout);
           bit_num = byte_acc = 0;
       }
    }
}

double norm(cdouble C)
{
    return C.re*C.re + C.im*C.im;
}


-- 

http://d.puremagic.com/issues/show_bug.cgi?id=255





------- Comment #1 from jpelcis@gmail.com  2006-07-16 21:11 -------
I used the following code to time it:

long getCount () {
        asm {
                naked;
                rdtsc;
                ret;
        }
}

and had the first and last lines of main check the time.  I got a performance difference, but it was under 10%.  What parameter were you passing and how were you testing the time?


-- 

http://d.puremagic.com/issues/show_bug.cgi?id=255





------- Comment #2 from godaves@yahoo.com  2006-07-16 22:53 -------
I timed it (the exact code posted) on Linux/P4 and Win32/AMD64 using an external timer in both cases, and in both cases the difference was as large over several runs, about 5x. I was using 1000 as n.

When I added your timer function the difference virtually disappeared (with it running at the better time once your internal timer was added).

Sure looks like an alignment issue, which can be transitive and sensitive to seemingly unrelated changes in the code elsewhere (which makes it all the more frustrating). I've run into this with math.pow() too.


-- 

http://d.puremagic.com/issues/show_bug.cgi?id=255





------- Comment #3 from baryluk@mpi.int.pl  2007-06-21 11:31 -------
Timings (n=1000, Athlon 1GHz) of code from comment #0 (zero changes)

dmd-1.015: 1.95 sec
dmd-1.015 -version=foo: 2.03 sec

dmd-1.015 -O -inline: 1.03 sec
dmd-1.015 -O -inline -version=foo: 1.60 sec

using bash builtin function time. optimised version without foo is better becouse local variables can be properly alligned on the stack and access to them can be optimised.


-- 

http://d.puremagic.com/issues/show_bug.cgi?id=255


Andrei Alexandrescu <andrei@metalanguage.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|                            |andrei@metalanguage.com
         AssignedTo|nobody@puremagic.com        |bugzilla@digitalmars.com


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