RFR: 8366444: Add support for add/mul reduction operations for Float16

[Xiaohong Gong]

On Fri, 26 Sep 2025 12:00:31 GMT, Bhavana Kilambi <bkilambi at openjdk.org> wrote:

> This patch adds mid-end support for vectorized add/mul reduction operations for half floats. It also includes backend aarch64 support for these operations. Only vectorization support through autovectorization is added as VectorAPI currently does not support Float16 vector species.
> 
> Both add and mul reduction vectorized through autovectorization mandate the implementation to be strictly ordered. The following is how each of these reductions is implemented for different aarch64 targets -
> 
> **For AddReduction :**
> On Neon only targets (UseSVE = 0): Generates scalarized additions using the scalar `fadd` instruction for both 8B and 16B vector lengths. This is because Neon does not provide a direct instruction for computing strictly ordered floating point add reduction.
> 
> On SVE targets (UseSVE > 0): Generates the `fadda` instruction which computes add reduction for floating point in strict order.
> 
> **For MulReduction :**
> Both Neon and SVE do not provide a direct instruction for computing strictly ordered floating point multiply reduction. For vector lengths of 8B and 16B, a scalarized sequence of scalar `fmul` instructions is generated and multiply reduction for vector lengths > 16B is not supported.
> 
> Below is the performance of the two newly added microbenchmarks in `Float16OperationsBenchmark.java` tested on three different aarch64 machines and with varying `MaxVectorSize` -
> 
> Note: On all machines, the score (ops/ms) is compared with the master branch without this patch which generates a sequence of loads (`ldrsh`) to load the FP16 value into an FPR and a scalar `fadd/fmul` to add/multiply the loaded value to the running sum/product. The ratios given below are the ratios between the throughput with this patch and the throughput without this patch.
> Ratio > 1 indicates the performance with this patch is better than the master branch.
> 
> **N1 (UseSVE = 0, max vector length = 16B):**
> 
> Benchmark         vectorDim  Mode   Cnt  8B     16B
> ReductionAddFP16  256        thrpt  9    1.41   1.40
> ReductionAddFP16  512        thrpt  9    1.41   1.41
> ReductionAddFP16  1024       thrpt  9    1.43   1.40
> ReductionAddFP16  2048       thrpt  9    1.43   1.40
> ReductionMulFP16  256        thrpt  9    1.22   1.22
> ReductionMulFP16  512        thrpt  9    1.21   1.23
> ReductionMulFP16  1024       thrpt  9    1.21   1.22
> ReductionMulFP16  2048       thrpt  9    1.20   1.22
> 
> 
> On N1, the scalarized sequence of `fadd/fmul` are generated for both `MaxVectorSize` of 8B and 16B for add reduction ...

src/hotspot/cpu/aarch64/aarch64_vector.ad line 272:

> 270:         if (length_in_bytes > 16 || !is_feat_fp16_supported()) {
> 271:           return false;
> 272:         }

Reductions with `length_in_bytes < 8` should also be skipped. Because such operations are not supported now, while the IRs with 32-bit vector size might exist, right?

src/hotspot/cpu/aarch64/aarch64_vector.ad line 3427:

> 3425:              (!VM_Version::use_neon_for_vector(Matcher::vector_length_in_bytes(n->in(2))) ||
> 3426:               n->as_Reduction()->requires_strict_order())) ||
> 3427:             (Matcher::vector_element_basic_type(n->in(2)) == T_SHORT && UseSVE > 0));

`UseSVE > 0` is a requirement for all cases. I suggest separate rule for `AddReductionVHF`. The predicate is much more complex now.

src/hotspot/cpu/aarch64/c2_MacroAssembler_aarch64.cpp line 1900:

> 1898:           fmulh(dst, dst, vtmp);
> 1899:           ins(vtmp, H, vsrc, 0, 7);
> 1900:           fmulh(dst, dst, vtmp);

Do you know why the performance is not improved significantly for multiply reduction? Seems instructions between different `ins` instructions will have a data-dependence, which is not expected? Could you please use other instructions instead of clear the register `vtmp` before `ins` and check the performance changes. 

Note that a clear of `mov` such as `MOVI Vd.2D, #0` has zero cost from V2's guide.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27526#discussion_r2386942508
PR Review Comment: https://git.openjdk.org/jdk/pull/27526#discussion_r2386995748
PR Review Comment: https://git.openjdk.org/jdk/pull/27526#discussion_r2387053858