Why Java (server VM) is faster than D?

[Adam D. Ruppe via Digitalmars-d]

On Monday, 3 August 2015 at 16:50:42 UTC, John Colvin wrote:
> Making SubFoo a final class and test take SubFoo gives a >10x 
> speedup for me.

Right, gdc and ldc will the the aggressive inlining and local 
data optimizations automatically once it is able to devirtualize 
the calls (at least when you use the -O flags).

dmd, however, even with -inline, doesn't make the local copy of 
the variable - it disassembles to this:

08098740 <_D1l4testFC1l6SubFooiZi>:
  8098740:       55                      push   ebp
  8098741:       8b ec                   mov    ebp,esp
  8098743:       89 c1                   mov    ecx,eax
  8098745:       53                      push   ebx
  8098746:       31 d2                   xor    edx,edx
  8098748:       8b 5d 08                mov    ebx,DWORD PTR 
[ebp+0x8]
  809874b:       56                      push   esi
  809874c:       85 c9                   test   ecx,ecx
  809874e:       7e 0f                   jle    809875f 
<_D1l4testFC1l6SubFooiZi+0x1f>
  8098750:       8b 43 08                mov    eax,DWORD PTR 
[ebx+0x8]
  8098753:       8d 74 40 01             lea    esi,[eax+eax*2+0x1]
  8098757:       42                      inc    edx
  8098758:       89 73 08                mov    DWORD PTR 
[ebx+0x8],esi
  809875b:       39 ca                   cmp    edx,ecx
  809875d:       7c f1                   jl     8098750 
<_D1l4testFC1l6SubFooiZi+0x10>
  809875f:       8b 43 08                mov    eax,DWORD PTR 
[ebx+0x8]
  8098762:       5e                      pop    esi
  8098763:       5b                      pop    ebx
  8098764:       5d                      pop    ebp
  8098765:       c2 04 00                ret    0x4



There's no call in there, but there is still indirect memory 
access for the variable, so it doesn't get the caching benefits 
of the stack.



It isn't news that dmd's optimizer is pretty bad next to.... 
well, pretty much everyone else nowdays, whether gdc, ldc, or 
Java, but it is sometimes nice to take a look at why.



The biggest magic of Java IMO here is being CPU cache friendly!