RFR: 8279508: Auto-vectorize Math.round API [v2]

[Sandhya Viswanathan]

On Wed, 19 Jan 2022 17:38:25 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

>> Summary of changes:
>> - Intrinsify Math.round(float) and Math.round(double) APIs.
>> - Extend auto-vectorizer to infer vector operations on encountering scalar IR nodes for above intrinsics.
>> - Test creation using new IR testing framework.
>> 
>> Following are the performance number of a JMH micro included with the patch 
>> 
>> Test System: Intel(R) Xeon(R) Platinum 8380 CPU @ 2.30GHz (Icelake Server)
>> 
>>   |   | BASELINE AVX2 | WithOpt AVX2 | Gain (opt/baseline) | Baseline AVX3 | Withopt AVX3 | Gain (opt/baseline)
>> -- | -- | -- | -- | -- | -- | -- | --
>> Benchmark | ARRAYLEN | Score (ops/ms) | Score (ops/ms) |   | Score (ops/ms) | Score (ops/ms) |  
>> FpRoundingBenchmark.test_round_double | 1024 | 518.532 | 1364.066 | 2.630630318 | 512.908 | 4292.11 | 8.368186887
>> FpRoundingBenchmark.test_round_double | 2048 | 270.137 | 830.986 | 3.076165057 | 273.159 | 2459.116 | 9.002507697
>> FpRoundingBenchmark.test_round_float | 1024 | 752.436 | 7780.905 | 10.34095259 | 752.49 | 9506.694 | 12.63364829
>> FpRoundingBenchmark.test_round_float | 2048 | 389.499 | 4113.046 | 10.55983712 | 389.63 | 4863.673 | 12.48279907
>> 
>> Kindly review and share your feedback.
>> 
>> Best Regards,
>> Jatin
>
> Jatin Bhateja has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8279508: Adding a test for scalar intrinsification.

The JVM currently initializes the x86 mxcsr to round to nearest even, see below in stubGenerator_x86_64.cpp:
    // Round to nearest (even), 64-bit mode, exceptions masked
    StubRoutines::x86::_mxcsr_std = 0x1F80;
The above works for Math.rint which is specified to be round to nearest even.
Please see:
https://www.intel.com/content/www/us/en/developer/articles/technical/intel-sdm.html : section 4.8.4

The rounding mode needed for Math.round is round to positive infinity which needs a different x86 mxcsr initialization(0x5F80).

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PR: https://git.openjdk.java.net/jdk/pull/7094