RFR: 8275275: AArch64: Fix performance regression after auto-vectorization on NEON

[Xiaohong Gong]

On Tue, 6 Sep 2022 03:13:25 GMT, Fei Gao <fgao at openjdk.org> wrote:

> For some vector opcodes, there are no corresponding AArch64 NEON
> instructions but supporting them benefits vector API. Some of
> this kind of opcodes are also used by superword for auto-
> vectorization and here is the list:
> 
> VectorCastD2I, VectorCastL2F
> MulVL
> AddReductionVI/L/F/D
> MulReductionVI/L/F/D
> AndReductionV, OrReductionV, XorReductionV
> 
> 
> We did some micro-benchmark performance tests on NEON and found
> that some of listed opcodes hurt the performance of loops after
> auto-vectorization, but others don't.
> 
> This patch disables those opcodes for superword, which have
> obvious performance regressions after auto-vectorization on
> NEON. Besides, one jtreg test case, where IR nodes are checked,
> is added in the patch to protect the code against change by
> mistake in the future.
> 
> Here is the performance data before and after the patch on NEON.
> 
> Benchmark       length  Mode  Cnt   Before    After     Units
> AddReductionVD   1024   thrpt  15   450.830   548.001   ops/ms
> AddReductionVF   1024   thrpt  15   514.468   548.013   ops/ms
> MulReductionVD   1024   thrpt  15   405.613   499.531   ops/ms
> MulReductionVF   1024   thrpt  15   451.292   495.061   ops/ms
> 
> Note:
> Because superword doesn't vectorize reductions unconnected with
> other vector packs, the benchmark function for Add/Mul
> reduction is like:
> 
> //  private double[] da, db;
> //  private double dresult;
>   public void AddReductionVD() {
>     double result = 1;
>     for (int i = startIndex; i < length; i++) {
>       result += (da[i] + db[i]);
>     }
>     dresult += result;
>   }
> 
> 
> Specially, vector multiply long has been implemented but disabled
> for both vector API and superword. Out of the same reason, the
> patch re-enables MulVL on NEON for Vector API but still disables
> it for superword. The performance uplift on vector API is ~12.8x
> on my local.
> 
> Benchmark          length  Mode  Cnt  Before   After    Units
> Long128Vector.MUL   1024   thrpt  10  55.015   760.593  ops/ms
> MulVL(superword)    1024   thrpt  10  907.788  907.805  ops/ms
> 
> Note:
> The superword benchmark function is:
> 
> //  private long[] in1, in2, res;
>   public void MulVL() {
>     for (int i = 0; i < length; i++) {
>       res[i] = in1[i] * in2[i];
>     }
>   }
> 
> The Vector API benchmark case is from:
> https://github.com/openjdk/panama-vector/blob/vectorIntrinsics/test/micro/org/openjdk/bench/jdk/incubator/vector/operation/Long128Vector.java#L190

src/hotspot/cpu/aarch64/aarch64_vector_ad.m4 line 146:

> 144:     // Fail fast, otherwise fall through to common vector_size_supported() check.
> 145:     switch (opcode) {
> 146:       case Op_MulVL:

Enabling `MulVL` for vector api is great. Thanks for doing this! However, this might break several match rules like https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/aarch64_vector.ad#L2025 and the `vmls`. The assertion in line-2035 might fail if this rule is matched for a long vector and runs on hardwares that do not support sve. One way to fix is adding the predicate to these rules to skip the long vector type for neon. Thanks!

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PR: https://git.openjdk.org/jdk/pull/10175