RFR: 8276673: Optimize abs operations in C2 compiler [v4]

[Jie Fu]

On Wed, 15 Dec 2021 15:02:56 GMT, Fei Gao <fgao at openjdk.org> wrote:

>>> The PR optimizes abs operations in the C2 middle end. Can I have your review please?
>> 
>> So what's the performance data before and after this patch?
>> Does it also benefit on x86?
>> 
>> It would be better to provide a jmh micro benchmark.
>> Thanks.
>
>> > The PR optimizes abs operations in the C2 middle end. Can I have your review please?
>> 
>> So what's the performance data before and after this patch? Does it also benefit on x86?
>> 
>> It would be better to provide a jmh micro benchmark. Thanks.
> 
> Thanks, @DamonFool . Yes, it's supposed to benefit all archs. 
> For example, here is the performance data on x86.
> 
> Before the patch:
> Benchmark                 (seed)   Mode  Cnt       Score       Error   Units
> MathBench.absConstantInt       0  thrpt    5  291960.380 ± 10724.572  ops/ms
> 
> After the patch:
> Benchmark                 (seed)   Mode  Cnt       Score      Error   Units
> MathBench.absConstantInt       0  thrpt    5  336271.533 ± 3778.210  ops/ms
> 
> The jmh micro benchmark testcase has been added in the latest commit.

Hi @fg1417 ,

Thanks for your update.

Now I see that you are trying to optimize the following three abs() patterns:
 1) Math.abs(-38)
 2) (char) Math.abs((char) c)
 3) Math.abs(0 - x)

But did you see these code patterns in real programs?
I'm a bit worried that we just improve the complexity of C2 with (almost) no performance gain in the real world.
Thanks.

-------------

PR: https://git.openjdk.java.net/jdk/pull/6755