b10891

vulkan: fall back to shared-memory reduction for dmmv on PowerVR (#28341)

The Imagination proprietary Vulkan compiler returns VK_ERROR_UNKNOWN from
vkCreateComputePipelines for every dequant mul_mat_vec shader built with the
subgroup-only reduction that requires a subgroup size >= 16. That covers the
k-quants, the i-quants, TQ2_0, MXFP4 and NVFP4. ggml rethrows, so the first
generated token of any such model kills the process.

Reproduced on a Pixel 11 Pro (PowerVR C-Series CXTP-48-1536 MC1, driver
1.662.3024, subgroup size 128, min 32, max 128). The failure is independent of
subgroup size: 32, 64 and 128 all fail, as does dropping the full-subgroups
flag and the required-subgroup-size pNext. The legacy quants, which use the
plain subgroup reduction, compile and run fine.

The shared-memory reduction variant compiles and matches the CPU reference for
q2_K, q3_K, q4_K, q5_K and q6_K. The hybrid variant also compiles but costs
27% of token throughput (3.78 vs 5.20 t/s on Qwen3.5-2B-Q4_K_M).

Website:

• https://llama.app

Attestations:

• https://github.com/ggml-org/llama.cpp/attestations/46549009

macOS/iOS:

• macOS Apple Silicon (arm64)
• macOS Apple Silicon (arm64, KleidiAI enabled) DISABLED
• macOS Intel (x64)
• iOS XCFramework

Linux:

• Ubuntu x64 (CPU)
• Ubuntu arm64 (CPU)
• Ubuntu s390x (CPU)
• Ubuntu x64 (Vulkan)
• Ubuntu arm64 (Vulkan)
• Ubuntu x64 (ROCm 10.0)
• Ubuntu x64 (OpenVINO)
• Ubuntu x64 (SYCL FP32)
• Ubuntu x64 (SYCL FP16)

Android:

• Android arm64 (CPU)

Windows:

• Windows x64 (CPU)
• Windows arm64 (CPU)
• Windows arm64 (OpenCL Adreno)
• Windows x64 (CUDA 12) - CUDA 12.4 DLLs
• Windows x64 (CUDA 13) - CUDA 13.3 DLLs
• Windows arm64 (CUDA 13) (preview) - CUDA 13.4 DLLs
• Windows x64 (Vulkan)
• Windows x64 (OpenVINO)
• Windows x64 (SYCL)
• Windows x64 (ROCm 10.0)

openEuler:

• DISABLED
• openEuler x86 (310p)
• openEuler x86 (910b, ACL Graph)
• openEuler aarch64 (310p)
• openEuler aarch64 (910b, ACL Graph)

UI:

• UI