makarna/pkg/backend/cuda/cuda_dequant_q4k.cu

#include "cuda_common.cuh"

// ============================================================
// Q4_K Dequantization Kernel
// 256 elements per block, 128 bytes of qs (2 elements per byte)
// Complex scale unpacking
// ============================================================
__global__ void dequant_q4k_kernel(const BlockQ4_K* blocks, float* out, int numBlocks) {
    int blockIdx_q = blockIdx.x;
    int elemIdx = threadIdx.x; // 0-255
    
    if (blockIdx_q >= numBlocks) return;
    
    const BlockQ4_K* b = &blocks[blockIdx_q];
    float d = fp16_to_fp32(b->d);
    float dmin = fp16_to_fp32(b->dmin);
    
    // Unpack scales and mins
    unsigned char sc[8], m[8];
    #pragma unroll
    for (int j = 0; j < 4; j++) {
        sc[j] = b->scales[j] & 63;
        m[j] = b->scales[j + 4] & 63;
    }
    #pragma unroll
    for (int j = 4; j < 8; j++) {
        sc[j] = (b->scales[j + 4] & 0xF) | ((b->scales[j - 4] >> 6) << 4);
        m[j] = (b->scales[j + 4] >> 4) | ((b->scales[j] >> 6) << 4);
    }
    
    // Which sub-block and position
    int subBlock = elemIdx / 32;
    int subPos = elemIdx % 32;
    int qsIdx = (subBlock / 2) * 32 + subPos;
    
    unsigned char qs = b->qs[qsIdx];
    int val = (subBlock % 2 == 0) ? (qs & 0xF) : (qs >> 4);
    
    float scale = d * (float)sc[subBlock];
    float minVal = dmin * (float)m[subBlock];
    
    out[blockIdx_q * 256 + elemIdx] = (float)val * scale - minVal;
}

int cuda_dequant_q4k(const void* blocks, float* out, int numBlocks) {
    if (numBlocks == 0) return 0;
    dequant_q4k_kernel<<<numBlocks, 256>>>((const BlockQ4_K*)blocks, out, numBlocks);
    CHECK_CUDA(cudaGetLastError());
    return 0;
}