Accumulation is naturally carried in the FMA addend, which is why the existing `FmaVF/FmaVD` x86 AD rules tie the destination to the addend and emit the 231 form (dst = m1*m2 + dst), leaving both multiplicands intact. The Float16 rules `vector_fma_HF_reg/vector_fma_HF_mem` in x86.ad instead tied the destination to a multiplicand and emitted `vfmadd132ph` (dst = dst*src + addend).
In a register-blocked reduction (the canonical FP16 GEMM/dot-product micro-kernel) each multiplicand is reused across several accumulators within a k-step, so the 132 form cannot keep an accumulator live in its own register and forces C2 to emit vmovdqu register-register copies per k-step for a tile of T accumulators, increasing register pressure and throttling throughput. The added pressure also makes the allocator overflow the ZMM register file sooner, so at larger tiles these copies become actual stack spills/reloads rather than register-register moves — and unlike register-register moves (which several microarchitectures eliminate at rename), stack traffic cannot be elided and costs real load/store bandwidth. This change reworks the two FmaVHF rules to match the addend as the destination and emit vfmadd231ph, matching the FmaVF/FmaVD behaviour. Following are the performance number of include micro benchmark on x86 target with AVX512-FP16 feature (Granite Rapids at 2.5GHz fixed frequency) Baseline:- Benchmark (K) Mode Cnt Score Error Units Float16VectorFMAAccumBenchmark.fmaAccum24 256 thrpt 2 401.999 ops/ms Float16VectorFMAAccumBenchmark.fmaAccum24 1024 thrpt 2 92.296 ops/ms Withopt:- Benchmark (K) Mode Cnt Score Error Units Float16VectorFMAAccumBenchmark.fmaAccum24 256 thrpt 2 550.271 ops/ms Float16VectorFMAAccumBenchmark.fmaAccum24 1024 thrpt 2 139.446 ops/ms Kindly review and share your feedback. Best Regards, Jatin --------- - [x] I confirm that I make this contribution in accordance with the [OpenJDK Interim AI Policy](https://openjdk.org/legal/ai). ------------- Commit messages: - 8387213: Optimize Float16Vector.fma JIT sequence for x86 AVX512-FP16 targets Changes: https://git.openjdk.org/jdk/pull/31724/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=31724&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8387213 Stats: 160 lines in 4 files changed: 154 ins; 0 del; 6 mod Patch: https://git.openjdk.org/jdk/pull/31724.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/31724/head:pull/31724 PR: https://git.openjdk.org/jdk/pull/31724
