RFR: 8280510: AArch64: Vectorize operations with loop induction variable [v4]

[Tobias Hartmann]

On Tue, 26 Apr 2022 04:53:49 GMT, Pengfei Li <pli at openjdk.org> wrote:

>> AArch64 has SVE instruction of populating incrementing indices into an
>> SVE vector register. With this we can vectorize some operations in loop
>> with the induction variable operand, such as below.
>> 
>>   for (int i = 0; i < count; i++) {
>>     b[i] = a[i] * i;
>>   }
>> 
>> This patch enables the vectorization of operations with loop induction
>> variable by extending current scope of C2 superword vectorizable packs.
>> Before this patch, any scalar input node in a vectorizable pack must be
>> an out-of-loop invariant. This patch takes the induction variable input
>> as consideration. It allows the input to be the iv phi node or phi plus
>> its index offset, and creates a `PopulateIndexNode` to generate a vector
>> filled with incrementing indices. On AArch64 SVE, final generated code
>> for above loop expression is like below.
>> 
>>   add     x12, x16, x10
>>   add     x12, x12, #0x10
>>   ld1w    {z16.s}, p7/z, [x12]
>>   index   z17.s, w1, #1
>>   mul     z17.s, p7/m, z17.s, z16.s
>>   add     x10, x17, x10
>>   add     x10, x10, #0x10
>>   st1w    {z17.s}, p7, [x10]
>> 
>> As there is no populating index instruction on AArch64 NEON or other
>> platforms like x86, a function named `is_populate_index_supported()` is
>> created in the VectorNode class for the backend support check.
>> 
>> Jtreg hotspot::hotspot_all_no_apps, jdk::tier1~3 and langtools::tier1
>> are tested and no issue is found. Hotspot jtreg has existing tests in
>> `compiler/c2/cr7192963/Test*Vect.java` covering this kind of use cases so
>> no new jtreg is created within this patch. A new JMH is created in this
>> patch and tested on a 512-bit SVE machine. Below test result shows the
>> performance can be significantly improved in some cases.
>> 
>>   Benchmark                       Performance
>>   IndexVector.exprWithIndex1            ~7.7x
>>   IndexVector.exprWithIndex2           ~13.3x
>>   IndexVector.indexArrayFill            ~5.7x
>
> Pengfei Li has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Address comments and align AD file code

Looks good to me too. Have you considered adding a IR verification test?

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

Marked as reviewed by thartmann (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/7491