RFR (M): 8143353: Update for x86 sin and cos in the math lib

Deshpande, Vivek R vivek.r.deshpande at intel.com
Thu Nov 26 01:16:18 UTC 2015

Hi Vladimir

Please find the webrev with your suggested updates attached with the mail.
We will update it in the jbs entry soon.
Please let me know if it needs further changes.


-----Original Message-----
From: Deshpande, Vivek R 
Sent: Tuesday, November 24, 2015 10:22 AM
To: 'joe darcy'; Vladimir Kozlov
Cc: Viswanathan, Sandhya; Berg, Michael C; hotspot compiler
Subject: RE: RFR (M): 8143353: Update for x86 sin and cos in the math lib

HI Vladimir, Joe

I have done the jtreg tests in hotspot and tests from jdk you have mentioned. It passed those tests.
The ~4x gain is with XX:+UnlockDiagnosticVMOptions -XX:DisableIntrinsic=_dsin/_dcos over without that option.
The performance gain is 3.2x over base jdk, that is over current fsin/fcos intrinsic. This gain is more realistic.

Could I get those tests around the boundary values. Would WorstCaseTests.java jtreg test in jdk test those ?
If yes, then it has passed those boundary cases.

I would work on adding either diagnostic flag or just one flag for libm and send out the webrev soon.


-----Original Message-----
From: joe darcy [mailto:joe.darcy at oracle.com]
Sent: Monday, November 23, 2015 6:28 PM
To: Vladimir Kozlov; Deshpande, Vivek R
Cc: Viswanathan, Sandhya; Berg, Michael C; hotspot compiler
Subject: Re: RFR (M): 8143353: Update for x86 sin and cos in the math lib


Just getting added to the thread..

On 11/23/2015 5:13 PM, Vladimir Kozlov wrote:
> Thank you, for explanation, Vivek.
> Please, run jdk/test/java/lang/Math/ jtreg tests in addition to 
> Hotspot tests.
> On 11/23/15 12:24 PM, Deshpande, Vivek R wrote:
>> Hi Vladimir
>> The result we obtain with LIBM are within +/- 1ulp from StrictMath 
>> result and not exact result. So I added the flag to switch between 
>> Quick explanation:
>> This is what we observed with comparison to HPA Library
>> (http://www.nongnu.org/hpalib/) explained with an example.
>> LIBM Observed Math result=0.19457293629570213 (4596178249117717083L) 
>> (StrictMath - 1ulp) Required result should be = 0.19457293629570216
>> (4596178249117717084L) (StrictMath result) or 0.1945729362957022
>> (4596178249117717085L) (StrictMath + 1ulp.) This means HPA library 
>> result is between the above two values and Exact result would be 
>> pretty close to it.
>> So here StrictMath result is less than quad-precision result, Math 
>> result should be StrictMath or StrictMath + 1ulp and not StrictMath - 
>> 1ulp, according to our test.
> Note, java.lang.Math allows to have 1ulp off (in both direction, I
> think) and it should be consistent for Interpreter and code generated 
> by JIT compilers:
> http://docs.oracle.com/javase/7/docs/api/java/lang/Math.html#sin%28dou
> ble%29

That interpretation of the spec is not quite right. For the Math methods with a 1/2 ulp error bound, the floating-point result closest to the exact result must be returned. For the methods with a 1 ulp error bound, either of the floating-point result bracketing the true result can be returned, subject to the monotonicity constraints of the specification of the particular method.

>> I have done the experiments with XX:+UnlockDiagnosticVMOptions 
>> -XX:DisableIntrinsic=_dsin and XX:+UnlockDiagnosticVMOptions 
>> -XX:DisableIntrinsic=_dcos. With this option, the interpreter would 
>> go through LIBM and C1 and c2 through FDLIBM.
>> If we want to disable LIBM completely, we need the flags 
>> -XX:+UseLibmSinIntrinsic and -XX:+UseLibmCosIntrinsic.
> I was thinking about using existing
> DirectiveSet::is_intrinsic_disabled() and 
> vmIntrinsics::is_disabled_by_flags(). You need to add additional 
> versions of functions which accept intrinsic ID instead of methodHandle.
> If you still want to use flags make them diagnostic.
> Or have one flag for all LIBM intrinsics -XX:+UseLibmIntrinsic.
>> Also the performance gain ~4x is with XX:+UnlockDiagnosticVMOptions 
>> -XX:DisableIntrinsic=_dsin/_dcos.
> You confused me here. So you get 4x when only Interpreter use LIBM 
> code and compilers use FDLIB?

Just to be clear, are you comparing the new code to FDLIBM (StrictMath) or to the existing fsin/fcos instrinsics (Math)?

I'm part way through porting the FDLIBM code to Java (JDK-8134780: Port fdlibm to Java), which is providing a significant speed boost to the StrictMath methods that have been ported.

I find the current patch *insufficient* as-is in terms of its testing. 
For example, part of patch says

# For sin

+//     This means that the main path is actually only taken for
+//     2^-252 <= |X| < 90112.

# For cos

+//     This means that the main path is actually only taken for
+//     2^-252 <= |X| < 90112.

If nothing else, there are no tests at around those boundary values, which is unacceptable. There should also be some tests of values of interest to the algorithm in question.



> Thanks,
> Vladimir
>> Let me know your thoughts on this. I would answer more questions and 
>> give more data if needed.
>> Regards,
>> Vivek
>> -----Original Message-----
>> From: Vladimir Kozlov [mailto:vladimir.kozlov at oracle.com]
>> Sent: Monday, November 23, 2015 10:37 AM
>> To: Deshpande, Vivek R; hotspot-compiler-dev at openjdk.java.net
>> Cc: Viswanathan, Sandhya
>> Subject: Re: RFR (M): 8143353: Update for x86 sin and cos in the math 
>> lib
>> On 11/20/15 12:22 PM, Vladimir Kozlov wrote:
>>> What is the reason you decided to add new flags? exp() and log() 
>>> changes did not have flags.
>>> It would be interesting to see what happens if you disable 
>>> intrinsics using existing flag, for example:
>>>    -XX:+UnlockDiagnosticVMOptions -XX:DisableIntrinsic=_dexp
>> Hi Vivek,
>> I want to point that you can do this experiment later. We can file 
>> bugs and fixed them after FC.
>> For now, please, answer my question about flags only. This is the 
>> only thing holding it from push.
>> Thanks,
>> Vladimir
>>> Thanks,
>>> Vladimir
>>> On 11/20/15 12:03 PM, Deshpande, Vivek R wrote:
>>>> Hi all
>>>> I would like to contribute a patch which optimizes Math.sin() and
>>>> Math.cos() for 64 and 32 bit X86 architecture using Intel LIBM
>>>>    implementation.
>>>> The improvement gives ~4.25x gain over base for both sin and cos.
>>>> The option to use the optimizations are -XX:+UseLibmSinIntrinsic 
>>>> and -XX:+UseLibmCosIntrinsic.
>>>> Could you please review and sponsor this patch.
>>>> Bug-id:
>>>> https://bugs.openjdk.java.net/browse/JDK-8143353
>>>> webrev:
>>>> http://cr.openjdk.java.net/~mcberg/8143353/webrev.01/
>>>> Thanks and regards,
>>>> Vivek

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