sycl: FWHT for widths above 512 and Kronecker sizes (port of upstream #28254, #29243) - #302
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…T support (ggml-org#28016) (ggml-org#28254) * Reapply "sycl : add Kronecker product FWHT support for sizes 384, 640, 768, 12…" (ggml-org#28184) This reverts commit c845263. * tests : fix unused variable M in test-backend-ops * tests: fix trailing space error and isolate kronecker tests for sycl backend only (cherry picked from commit 4d91760)
The SYCL FWHT covers 64 to 512 via the standard butterfly network, plus 384/640/768/1280 via the Kronecker/Paley construction added separately in Hadamard hint can produce (1024, 2048, 4096, 8192); those still fall through to the default case and run as a dense GEMM against the materialized rotation tensor, correct but O(n^2) instead of O(n log n). fwht_kernel_wide runs one row per work-group instead of per sub-group, so each work-item keeps N/NT values rather than N/WARP_SIZE. Butterflies below the sub-group width still shuffle; those up to the work-group width go through work-group local memory; the rest stay in registers. Same butterfly and sign convention as the existing narrow kernel. ggml's SYCL backend registration (dpct::dev_mgr) unconditionally requires a GPU-labeled platform to exist and throws before any op-level test can run, so test-backend-ops could not be exercised on this box (a GPU-less pod) even via the CPU device. Verified instead with a standalone harness: the same kernel body run through a real SYCL CPU device (Intel oneAPI DPC++ 2026.1, OpenCL CPU backend), checked against an independent recursive-doubling Hadamard reference, cross-validated by first running the existing unmodified narrow kernel through the identical harness and confirming it passes (rules out a reference-convention bug before trusting a pass on the new code). Random-input results for all four widths, single- and multi-row: N=1024 NT=256 rows=1 max_abs_err=1.7e-07 max_rel_err=4.9e-04 PASS N=2048 NT=256 rows=1 max_abs_err=1.9e-07 max_rel_err=2.0e-04 PASS N=4096 NT=256 rows=1 max_abs_err=2.0e-07 max_rel_err=1.4e-04 PASS N=8192 NT=256 rows=1 max_abs_err=2.5e-07 max_rel_err=3.8e-03 PASS N=1024 NT=256 rows=7 max_abs_err=2.4e-07 max_rel_err=1.0e-03 PASS N=2048 NT=256 rows=5 max_abs_err=3.0e-07 max_rel_err=9.4e-04 PASS N=4096 NT=256 rows=3 max_abs_err=2.7e-07 max_rel_err=1.7e-03 PASS N=8192 NT=256 rows=2 max_abs_err=2.5e-07 max_rel_err=1.9e-03 PASS This covers the kernel algorithm itself; it does not exercise the ggml dispatch/supports_op integration end to end, which needs a real GPU (or a SYCL GPU plugin) to get past backend registration. test-backend-ops build is verified: fwht.cpp recompiles with zero warnings as part of ggml-sycl. (cherry picked from commit c829670)
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What
Brings two merged upstream SYCL FWHT changes to
prism, cherry-picked with-x:Why
On
prismthe SYCL FWHT dispatch stops at 512. Any Hadamard-hinted matmul wider than that falls back to a dense GEMM against the materialized rotation, which is O(n^2) per row instead of O(n log n). CUDA, Metal and Vulkan onprismalready cover these widths, so SYCL was the only backend left on the dense path for them.How
ggml/src/ggml-sycl/fwht.cppis now byte-identical to upstream at c829670. The only manual change is the conflict resolution intests/test-backend-ops.cpp: the upstream SYCL-only Kronecker cases are added next to the existingprismsigned-FWHT and SwiGLU cases, and both are kept.Testing
git diff --checkclean; test-backend-ops builds on CPU with the merged test file.