Finding large difference b/w execution time of c++ and D codes for same problem

[Joseph Rushton Wakeling]

On 02/13/2013 02:26 PM, Marco Leise wrote:
> I compiled with LDC2 and these are the results:
>
> D code serial with dimension 32768 ...
>    using floats  Total time: 13.399 [sec]
>    using doubles Total time:  9.429 [sec]
>    using reals   Total time:  8.909 [sec] // <- !!!
>
> You get both, 50% more speed and more precision!

Compiling with ldmd2 -O -inline -release on 64-bit Ubuntu, latest from-GitHub 
LDC, LLVM 3.2:

   D code serial with dimension 32768 ...
     using floats Total time: 4.751 [sec]
     using doubles Total time: 4.362 [sec]
     using reals Total time: 5.95 [sec]

Using double is indeed marginally faster than float, but real is slower than both.

What's disturbing is that when compiled instead with gdmd -O -inline -release 
the code is dramatically slower:

   D code serial with dimension 32768 ...
     using floats Total time: 22.108 [sec]
     using doubles Total time: 21.203 [sec]
     using reals Total time: 23.717 [sec]

It's the first time I've encountered such a dramatic difference between GDC and 
LDC, and I'm wondering whether it's down to a bug or some change between D 
releases 2.060 and 2.061.