RFR: JDK-8270147: Increase stride size allowing unrolling more loops [v2]

[Radoslaw Smogura]

On Sat, 10 Jul 2021 15:05:09 GMT, Radoslaw Smogura <github.com+7535718+rsmogura at openjdk.org> wrote:

>> # Description
>> 
>> This increase allowed stride size for loop unrolling to almost maximum possible
>> value which is around `max_jint /  2 - 2`,
>> so the value which will prevent overflow when stride is doubled in C2.
>> 
>> The motivation of this change is discussion and research about unrolling
>> vector (SIMD) loops. In such a case stride size depends on elements size
>> and machine size of vector for AVX256 and int stride size is 8,
>> and loop unroll happens. However short vectors will not cause loop unroll.
>> 
>> 
>>     for (int i = 0; i < SPECIES.loopBound(longSize); i += SPECIES.length() /* 8 for int, 16 for short */ ) {
>>       var v = ShortVector.fromByteBuffer(SPECIES, srcBufferHeap, i << 1, ByteOrder.nativeOrder());
>>       v.intoByteBuffer(dstBufferHeap, i << 1, ByteOrder.nativeOrder());
>>     }
>> 
>> 
>> # Notes
>> Stride size was decreased some time ago https://github.com/openjdk/panama-foreign/commit/2683d5390bd58683ae13bdd8582127c308d8fd04
>> 
>> The exact reasons for this are not known for me (over unroll of some loops?).
>> 
>> Original thread https://mail.openjdk.java.net/pipermail/panama-dev/2021-June/014310.html
>
> Radoslaw Smogura has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Drop whitespace, again.
>  - Set maximum stride size to be 1 << 2 or max vector size
>    
>    Keep overflow test if stride can overflow when doubling
>    Drop diagnostic flag

Hi Vladimir & thank you for comments.

I really don't understand the limit `1<< 2` for initial stride size, as loop unroll count is determined by LoopMaxUnroll.

I adjusted code to your suggestions, right now `initial_stride_sz = MAX2(1<<2, Matcher::max_vector_size(T_BYTE) / 2)`

I dropped the JVM flag controlling what procedure should be used to determine max stride size, is it ok?

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PR: https://git.openjdk.java.net/jdk/pull/4658