Suboptimal code generation in C2?

[Aleksey Shipilev]

Hi,

On 09/23/2015 08:49 PM, Vitaly Davidovich wrote:
> Consider this simple method:
> 
>     private static int test(final int ops, int start) {
>         for (int i = 0; i < ops; ++i) {
>             start++;
>         }
>         return start;
>     }

> test(int, int):
> leal(%rsi,%rdi), %edx
> movl%esi, %eax
> testl%edi, %edi
> cmovg%edx, %eax
> ret
> 
> Any ideas on why C2 doesn't generate roughly the same thing? Clang 3.7
> and ICC 13.0.1 are almost the same as the GCC output above.

Yup, could be better:
  https://bugs.openjdk.java.net/browse/JDK-8137049

I am not entirely sure conditional move is better than a branch in this
particular case, since $ops is probably always non-negative in practice.
HotSpot generates a branch there, which is good. The arithmetic dance is
indeed not very clean -- I'd speculate that's a leftover from loop peeling.

Thanks,
-Aleksey


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