RFR: 8210416: [linux] Poor StrictMath performance due to non-optimized compilation

[David Holmes]

Hi Severin,

On 12/09/2018 5:48 PM, Severin Gehwolf wrote:
> Hi David,
> 
> On Wed, 2018-09-12 at 13:57 +1000, David Holmes wrote:
>> Hi Severin,
>>
>> Sorry I'm a bit confused now.
>>
>> Originally for ppc/s390x/aarch64 the optimization level for fdlibm was
>> HIGH. But now it will be LOW due to:
>>
>>     45 ifneq ($(FDLIBM_CFLAGS), )
>>     46   BUILD_LIBFDLIBM_OPTIMIZATION := LOW
>>
>> as those platforms will use gcc/clang with support for -ffp-contract and
>> so FDLIBM_CFLAGS will not be empty.
>>
>> ??
> 
> Correct. That was done based on Andrew Haley's comment here:
> http://mail.openjdk.java.net/pipermail/build-dev/2018-September/023160.html
> 
> I've done some measurements with -O2 and -O3. -O2 is sufficient for a
> 2.75 speedup for sin/cos on ppc64le as compared to the -O0 baseline. On
> the other hand the speedup of -O3 as compared to -O2 is only 1.05 for
> sin/cos on ppc64le.
> 
> Since the difference between them were not huge, I've used -O2, a.k.a
> LOW. See also:
> http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8210416/microbenchmarks_results/
> 
> To me it seems -O2 is sufficiently good. Note that HIGH == -O3 and LOW
> == -O2 on gcc arches. Does that clear things up?

Yes that explains the missing -O2 - thanks.

But I don't understand why the optimization setting is being tied to the 
availability of the -ffp-contract flag?

Thanks,
David

> FWIW, I'm happy to change it back to HIGH if there are concerns. See
> also Gustavo's reply here for some more data points:
> http://mail.openjdk.java.net/pipermail/build-dev/2018-September/023193.html
> 
> Thanks,
> Severin
> 
>> David
>>
>> On 12/09/2018 2:14 AM, Severin Gehwolf wrote:
>>> Hi Erik,
>>>
>>> Thanks for the review!
>>>
>>> On Tue, 2018-09-11 at 08:57 -0700, Erik Joelsson wrote:
>>>> Hello Severin,
>>>>
>>>> Even if using the macro, I still think you need to add a condition on
>>>> the compiler types where the switch can be reasonably expected to exist.
>>>
>>> How about this?
>>> http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8210416/webrev.05/
>>>
>>> Thanks,
>>> Severin
>>>
>>>> On 2018-09-11 05:02, Severin Gehwolf wrote:
>>>>> On Mon, 2018-09-10 at 09:29 -0700, Erik Joelsson wrote:
>>>>>> I see. I was not aware of that issue, but we clearly need to file a bug
>>>>>> for it and fix it. In this case I think it's fine to us the macro however.
>>>>>
>>>>> OK. Update webrev, which now uses FLAGS_COMPILER_CHECK_ARGUMENTS.
>>>>>
>>>>> http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8210416/webrev.04/
>>>>>
>>>>> Micro-benchmark results from running [1] for x86_64 and ppc64le are
>>>>> here (-O2 is sufficient it seems):
>>>>>
>>>>> http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8210416/microbenchmarks_results/
>>>>>
>>>>> More thoughts?
>>>>>
>>>>> Thanks,
>>>>> Severin
>>>>>
>>>>> [1] https://github.com/gromero/strictmath/
>>>>>
>>>>
>>>>
>