Why is D slower than LuaJIT?

[Gary Whatmore]

Andreas Mayer Wrote:

> To see what performance advantage D would give me over using a scripting language, I made a small benchmark. It consists of this code:
> 
> >    auto L = iota(0.0, 10000000.0);
> >    auto L2 = map!"a / 2"(L);
> >    auto L3 = map!"a + 2"(L2);
> >    auto V = reduce!"a + b"(L3);

First note: this is a synthetic toy benchmark. Take it with a grain of salt. It represent in no way the true state of D.

> 
> It runs in 281 ms on my computer.
> 
> The same code in Lua (using LuaJIT) runs in 23 ms.

Your mp3 player or file system was doing stuff while executing the benchmark. You probably don't know how to run the test many times and use the average/minimum result for both languages. For example D does not have JIT startup cost so take the minimum result for D, JIT has varying startup speed so take the average or slowest result for Luajit. Compare these. More fair for native code D.

> That's about 10 times faster. I would have expected D to be faster. Did I do something wrong?
> 
> The first Lua version uses a simplified design. I thought maybe that is unfair to ranges, which are more complicated. You could argue ranges have more features and do more work. To make it fair, I made a second Lua version of the above benchmark that emulates ranges. It is still 29 ms fast.
> 
> The full D version is here: http://pastebin.com/R5AGHyPx
> The Lua version: http://pastebin.com/Sa7rp6uz
> Lua version that emulates ranges: http://pastebin.com/eAKMSWyr
> 
> Could someone help me solving this mystery?

My guesses are:

1) you didn't even test this and didn't use optimizations. -> User error
2) whenever doing benchmarks you must compare the competing stuff against all D compilers, cut and paste the object code of different compilers and manually build the fastest executable.
3) you didn't use inline assembler or profiler for D
4) you were using unstable Phobos functions. There is no doubt the final Phobos 2.0 will beat Luajit. D *is* a compiler statical language, Luajit just a joke.
5) you were using old d runtime garbage collector. One fellow here made a precise state of the art GC which beats even Java's 20 year old GC and C#. Patch your dmd to use this instead.

Not intending to start a religious war but if your native code runs slower than *JIT* code, you're doing something wrong. D will always beat JIT. Lua is also a joke language, D is for high performance servers and operating systems. In the worst case, disassemble the luajit program, steal its codes and write it using inline assembler in D. D must win these performance battles. It's technically superior.

> 
> Or is D, unlike I thought, not suitable for high performance computing? What should I do?

It is. D a) takes time to mature and b) you didn't fully utilize the compiler.