D for scientific computing

[H. S. Teoh]

On Thu, Jan 24, 2013 at 03:18:01PM -0800, Walter Bright wrote:
> On 1/24/2013 1:13 PM, H. S. Teoh wrote:
> >On Thu, Jan 24, 2013 at 12:15:07PM -0800, Walter Bright wrote:
> >>On 1/24/2013 8:36 AM, H. S. Teoh wrote:
> >>>Nevertheless, I also have made the same observation that code
> >>>produced by gdc consistently outperforms code produced by dmd.
> >>>Usually by about 20-30%, sometimes as much as 50-60%, IME. That's a
> >>>pretty big discrepancy for me, esp. when I'm doing compute-intensive
> >>>geometric computations.
> >>
> >>Do you mean floating point code? 32 or 64 bit?
> >
> >Floating-point, 64-bit, tested on dmd -O vs. gdc -O3.
> 
> Next, are you using floats, doubles, or reals?

Both reals and floats. Well, let's get some real measurements. Here's a
quick run-through of various test programs I have lying around:

Test program #1 (iterating 2-variable function over grid), uses reals:
- Test case with n=400:
	Using DMD:	~8 seconds (consistently)
	Using GDC:	~6 seconds (consistently)
	* So the DMD version is 33% slower than the GDC version.
	  (That is, 8/6*100 = 133%, so 33% slower.)

- Test case with n=600:
	Using DMD:	~27 seconds (consistently)
	Using GDC:	~19 seconds (consistently)
	* So the DMD version is 42% slower than the GDC version.


Test program #2 (terrain generation simulator), uses floats:
(The running time of this one depends on the RNG, so I fixed the seed
value in order to make a fair comparison.)
- Test case with seed=380170304, n=20 with water & wind simulation:
	Using DMD:	~10 seconds (consistently)
	Using GDC:	~7 seconds (consistently)
	* So the DMD version is 42% slower than the GDC version.

- Test case with seed=380170304, n=25 with water & wind simulation:
	Using DMD:	~14 seconds (consistently)
	Using GDC:	~9 seconds (consistently)
	* So the DMD version is 55% slower than the GDC version.


Test program #3 (enumeration of coordinates of n-dimensional polytopes),
uses reals:
- All permutations and changes of sign of <1,2,3,4,5,6,7>:
	Using DMD:	~4 seconds (consistently)
	Using GDC:	~3 seconds (consistently)
	* So the DMD version is 33% slower than the GDC version.

- All permutations and changes of sign of <1,2,3,4,5,6,7,7>:
	Using DMD:	~41 seconds (consistently)
	Using GDC:	~27 seconds (consistently)
	* So the DMD version is 51% slower than the GDC version.

- Even permutations and all changes of sign of <1,2,3,4,5,6,7,8>:
	Using DMD:	~40 seconds (consistently)
	Using GDC:	~27 seconds (consistently)
	* So the DMD version is 48% slower than the GDC version.


All test programs were compiled with dmd -O for the DMD version, and gdc
-O3 for the GDC version. The source code is unchanged between the two
compilers, and there are no version()'s that depend on a particular
compiler. The measurements stated above are averages of about 3-4 runs.

As you can see, the performance difference is between the two is pretty
clear.  I'm pretty sure this isn't only because of floating point
operations, because the above test programs all use a lot of inner
loops, and GDC does some pretty sophisticated loop unrolling and other
such optimizations.


T

-- 
Two wrongs don't make a right; but three rights do make a left...