Suboptimal code generation in C2?

[Vitaly Davidovich]

Great, thank you guys.

sent from my phone
On Sep 23, 2015 9:09 PM, "Vladimir Kozlov" <vladimir.kozlov at oracle.com>
wrote:

> Could be loop predicates or C2 splits loop (pre-main-post) before we
> collapsed it. But I agree that we could do better.
>
> Aleksey, thank you for filing RFE.
>
> Thanks,
> Vladimir
>
> On 9/24/15 3:04 AM, Vitaly Davidovich wrote:
>
>> Thanks Aleksey.
>>
>> The cmov is probably a non-issue here since both dependencies are in
>> registers.  The arithmetic dance, jumps, and register allocation seems
>> off though.  For instance, esi is cleared with xor then it jumps to code
>> that adds edx to esi, then it moves esi into eax - this could be
>> replaced with moving edx into eax.
>>
>> sent from my phone
>>
>> 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
>> <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
>>
>>
>> sent from my phone
>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/attachments/20150923/bec4c84e/attachment-0001.html>