xserv/csrc/embedding/rope.cu

#include <cuda_bf16.h>
#include <math.h>
#include "../common.cuh"

// RoPE: Rotary Position Embedding, using the Qwen/Llama rotate_half layout.
// For each dimension i in the first half at position `pos`:
//   y[i]            = x[i] * cos - x[i + half_dim] * sin
//   y[i + half_dim] = x[i + half_dim] * cos + x[i] * sin
// where cos/sin come from precomputed cos_cache/sin_cache.
//
// cos_cache[pos][i] = cos(pos * freq[i])
// sin_cache[pos][i] = sin(pos * freq[i])
// freq[i] = 1.0 / (theta ^ (2i / head_dim))

// Apply RoPE in-place to Q or K tensor.
// x shape: [num_tokens, num_heads, head_dim]
// cos_cache, sin_cache shape: [max_seq_len, head_dim/2]
// positions: [num_tokens] — the position index for each token

__global__ void rope_f32(
    float* __restrict__ x,                    // [num_tokens, num_heads, head_dim]
    const float* __restrict__ cos_cache,      // [max_seq_len, half_dim]
    const float* __restrict__ sin_cache,      // [max_seq_len, half_dim]
    const int* __restrict__ positions,        // [num_tokens]
    int num_heads, int head_dim
) {
    int token_idx = blockIdx.x;
    int head_idx = blockIdx.y;
    int half_dim = head_dim / 2;
    int pair_idx = threadIdx.x;  // which pair (0..half_dim)

    if (pair_idx >= half_dim) return;

    int pos = positions[token_idx];
    float cos_val = cos_cache[pos * half_dim + pair_idx];
    float sin_val = sin_cache[pos * half_dim + pair_idx];

    int base = (token_idx * num_heads + head_idx) * head_dim;
    float x0 = x[base + pair_idx];
    float x1 = x[base + pair_idx + half_dim];

    x[base + pair_idx]            = x0 * cos_val - x1 * sin_val;
    x[base + pair_idx + half_dim] = x1 * cos_val + x0 * sin_val;
}

__global__ void rope_bf16(
    __nv_bfloat16* __restrict__ x,
    const float* __restrict__ cos_cache,
    const float* __restrict__ sin_cache,
    const int* __restrict__ positions,
    int num_heads, int head_dim
) {
    int token_idx = blockIdx.x;
    int head_idx = blockIdx.y;
    int half_dim = head_dim / 2;
    int pair_idx = threadIdx.x;

    if (pair_idx >= half_dim) return;

    int pos = positions[token_idx];
    float cos_val = cos_cache[pos * half_dim + pair_idx];
    float sin_val = sin_cache[pos * half_dim + pair_idx];

    int base = (token_idx * num_heads + head_idx) * head_dim;
    float x0 = __bfloat162float(x[base + pair_idx]);
    float x1 = __bfloat162float(x[base + pair_idx + half_dim]);

    x[base + pair_idx]            = __float2bfloat16(x0 * cos_val - x1 * sin_val);
    x[base + pair_idx + half_dim] = __float2bfloat16(x1 * cos_val + x0 * sin_val);
}

// Precompute cos/sin cache on GPU
__global__ void compute_rope_cache(
    float* __restrict__ cos_cache,  // [max_seq_len, half_dim]
    float* __restrict__ sin_cache,
    int max_seq_len, int half_dim, float theta
) {
    int pos = blockIdx.x;
    int i = threadIdx.x;
    if (i >= half_dim) return;

    float freq = 1.0f / powf(theta, (float)(2 * i) / (float)(2 * half_dim));
    float angle = (float)pos * freq;
    cos_cache[pos * half_dim + i] = cosf(angle);
    sin_cache[pos * half_dim + i] = sinf(angle);
}

extern "C" {

void launch_rope_f32(void* x, const void* cos_cache, const void* sin_cache,
                     const void* positions, int num_tokens, int num_heads,
                     int head_dim, void* stream) {
    dim3 grid(num_tokens, num_heads);
    int block = head_dim / 2;
    rope_f32<<<grid, block, 0, (cudaStream_t)stream>>>(
        (float*)x, (const float*)cos_cache, (const float*)sin_cache,
        (const int*)positions, num_heads, head_dim);
    CUDA_CHECK_LAST_ERROR();
}

void launch_rope_bf16(void* x, const void* cos_cache, const void* sin_cache,
                      const void* positions, int num_tokens, int num_heads,
                      int head_dim, void* stream) {
    dim3 grid(num_tokens, num_heads);
    int block = head_dim / 2;
    rope_bf16<<<grid, block, 0, (cudaStream_t)stream>>>(
        (__nv_bfloat16*)x, (const float*)cos_cache, (const float*)sin_cache,
        (const int*)positions, num_heads, head_dim);
    CUDA_CHECK_LAST_ERROR();
}

void launch_compute_rope_cache(void* cos_cache, void* sin_cache,
                               int max_seq_len, int half_dim, float theta,
                               void* stream) {
    compute_rope_cache<<<max_seq_len, half_dim, 0, (cudaStream_t)stream>>>(
        (float*)cos_cache, (float*)sin_cache, max_seq_len, half_dim, theta);
    CUDA_CHECK_LAST_ERROR();
}

}