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

[Marco Leise]

Am Wed, 13 Feb 2013 15:00:21 +0100
schrieb Joseph Rushton Wakeling <joseph.wakeling at webdrake.net>:

> 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]

Ok, I get pretty much the same numbers as before with:
  ldmd2 -O -inline -release
It's even a bit faster than my loooong command line.
Do these numbers tell us, that there are such huge differences
in the handling of floating point value between different
AMD64 CPUs? I can't quite make a rhyme of it yet.
What version of LLVM are you using, mine is 3.1. 3.0 is
minimum and 3.2 is recommended for LDC2.

> 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.

_THAT_ I can reproduce with GDC! :

D code serial with dimension 32768 ...
  using floats Total time: 24.415 [sec]
  using doubles Total time: 23.268 [sec]
  using reals Total time: 25.168 [sec]

It's the exact same pattern.

-- 
Marco