On Fri, 24 Jul 2026 05:03:32 GMT, Jatin Bhateja <[email protected]> wrote:
>> 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). > > Jatin Bhateja has updated the pull request incrementally with one additional > commit since the last revision: > > Review comments resolution Hi @mhaessig , @TobiHartmann , @iwanowww , we need another approval to transition this to ready for integration state. ------------- PR Comment: https://git.openjdk.org/jdk/pull/31724#issuecomment-5087262927
