UPSTREAM PR #18610: ggml webgpu: initial flashattention implementation

ggml/src/ggml-webgpu/ggml-webgpu-shader-lib.hpp

@@ -0,0 +1,166 @@
+#ifndef GGML_WEBGPU_SHADER_LIB_HPP
+#define GGML_WEBGPU_SHADER_LIB_HPP
+
+#include "ggml.h"
+#include "pre_wgsl.hpp"
+
+#include <string>
+#include <vector>
+
+#define GGML_WEBGPU_F16_SIZE_BYTES                   2
+#define GGML_WEBGPU_FLASH_ATTN_PREFERRED_KV_SG_TILES 8u
+#define GGML_WEBGPU_FLASH_ATTN_PREFERRED_WG_SIZE     128u
+// Matches GGML_PAD(..., 256) in src/llama-context.cpp for KV cache sizing.
+#define GGML_WEBGPU_KV_SEQ_PAD                       256u
+
+struct ggml_webgpu_flash_attn_shader_lib_context {
+    ggml_type kv_type;
+    uint32_t  head_dim_qk;
+    uint32_t  head_dim_v;
+    bool      kv_direct;
+    bool      has_mask;
+    bool      has_sinks;
+    bool      uses_logit_softcap;
+    uint32_t  sg_mat_m;
+    uint32_t  sg_mat_n;
+    uint32_t  sg_mat_k;
+    size_t    wg_mem_limit_bytes;
+    uint32_t  max_subgroup_size;
+};
+
+struct ggml_webgpu_flash_attn_shader_decisions {
+    uint32_t q_tile  = 0;
+    uint32_t kv_tile = 0;
+    uint32_t wg_size = 0;
+};
+
+struct ggml_webgpu_processed_shader {
+    std::string                             wgsl;
+    std::string                             variant;
+    ggml_webgpu_flash_attn_shader_decisions decisions;
+};
+
+// This is exposed because it's necessary in supports_op
+inline size_t ggml_webgpu_flash_attn_wg_mem_bytes(uint32_t q_tile,
+                                                  uint32_t kv_tile,
+                                                  uint32_t head_dim_qk,
+                                                  uint32_t head_dim_v,
+                                                  bool     has_mask,
+                                                  bool     kv_direct) {
+    const uint32_t max_head_dim = std::max(head_dim_qk, head_dim_v);
+    size_t         elems        = 0;
+    elems += q_tile * head_dim_qk;        // q_shmem
+    if (!kv_direct) {
+        elems += kv_tile * max_head_dim;  // kv_shmem
+    }
+    elems += q_tile * head_dim_v;         // o_shmem
+    if (has_mask) {
+        elems += q_tile * kv_tile;        // mask_shmem
+    }
+    elems += q_tile * kv_tile;            // inter_shmem
+    elems += q_tile;                      // row_max_shmem
+    elems += q_tile;                      // exp_sum_shmem
+    return elems * GGML_WEBGPU_F16_SIZE_BYTES;
+}
+
+static uint32_t ggml_webgpu_flash_attn_max_kv_tile(const ggml_webgpu_flash_attn_shader_lib_context & context) {
+    const size_t limit_bytes  = context.wg_mem_limit_bytes;
+    const size_t q_tile       = context.sg_mat_m;
+    const size_t base_q_bytes = (context.head_dim_qk + context.head_dim_v + 2) * q_tile * GGML_WEBGPU_F16_SIZE_BYTES;
+    size_t       bytes_per_kv = 0;
+    if (!context.kv_direct) {
+        bytes_per_kv += std::max(context.head_dim_qk, context.head_dim_v);
+    }
+    if (context.has_mask) {
+        bytes_per_kv += q_tile;
+    }
+    bytes_per_kv += q_tile;
+    bytes_per_kv *= GGML_WEBGPU_F16_SIZE_BYTES;
+    const uint32_t max_kv_tile = (limit_bytes - base_q_bytes) / bytes_per_kv;
+    return (max_kv_tile / context.sg_mat_n) * context.sg_mat_n;
+}
+
+inline ggml_webgpu_processed_shader ggml_webgpu_preprocess_flash_attn_shader(
+    pre_wgsl::Preprocessor &                          preprocessor,
+    const char *                                      shader_src,
+    const ggml_webgpu_flash_attn_shader_lib_context & context) {
+    std::vector<std::string> defines;
+    std::string              variant = "flash_attn";
+
+    switch (context.kv_type) {
+        case GGML_TYPE_F32:
+            defines.push_back("KV_F32");
+            break;
+        case GGML_TYPE_F16:
+            defines.push_back("KV_F16");
+            break;
+        case GGML_TYPE_Q4_0:
+            defines.push_back("KV_Q4_0");
+            break;
+        case GGML_TYPE_Q8_0:
+            defines.push_back("KV_Q8_0");
+            break;
+        default:
+            GGML_ABORT("Unsupported KV type for flash attention shader");
+    }
+    variant += std::string("_") + ggml_type_name(context.kv_type);
+
+    if (context.has_mask) {
+        defines.push_back("MASK");
+        variant += "_mask";
+    }
+    if (context.has_sinks) {
+        defines.push_back("SINKS");
+        variant += "_sinks";
+    }
+    if (context.uses_logit_softcap) {
+        defines.push_back("LOGIT_SOFTCAP");
+        variant += "_lgsc";
+    }
+
+    if (context.kv_direct) {
+        defines.push_back("KV_DIRECT");
+        variant += "_kvdirect";
+    }
+
+    defines.push_back(std::string("HEAD_DIM_QK=") + std::to_string(context.head_dim_qk));
+    variant += std::string("_hsqk") + std::to_string(context.head_dim_qk);
+
+    defines.push_back(std::string("HEAD_DIM_V=") + std::to_string(context.head_dim_v));
+    variant += std::string("_hsv") + std::to_string(context.head_dim_v);
+
+    // For now these are not part of the variant name
+    defines.push_back(std::string("SG_MAT_M=") + std::to_string(context.sg_mat_m));
+    defines.push_back(std::string("SG_MAT_N=") + std::to_string(context.sg_mat_n));
+    defines.push_back(std::string("SG_MAT_K=") + std::to_string(context.sg_mat_k));
+
+    // Add chosen Q/KV tile sizes
+    uint32_t q_tile  = context.sg_mat_m;
+    uint32_t kv_tile = std::min(ggml_webgpu_flash_attn_max_kv_tile(context),
+                                context.sg_mat_n * GGML_WEBGPU_FLASH_ATTN_PREFERRED_KV_SG_TILES);
+    if (context.kv_direct)  {
+        GGML_ASSERT(kv_tile <= GGML_WEBGPU_KV_SEQ_PAD);
+        // Avoids having to use bounds-checks and decreasing performance for direct KV loads
+        while (GGML_WEBGPU_KV_SEQ_PAD % kv_tile != 0) {
+            kv_tile -= context.sg_mat_n;
+        }
+    }
+
+    defines.push_back(std::string("Q_TILE=") + std::to_string(q_tile));
+    defines.push_back(std::string("KV_TILE=") + std::to_string(kv_tile));
+
+    // workgroup size
+    uint32_t wg_size = std::max(context.max_subgroup_size, GGML_WEBGPU_FLASH_ATTN_PREFERRED_WG_SIZE);
+
+    defines.push_back(std::string("WG_SIZE=") + std::to_string(wg_size));
+
+    ggml_webgpu_processed_shader result;
+    result.wgsl              = preprocessor.preprocess(shader_src, defines);
+    result.variant           = variant;
+    result.decisions.q_tile  = q_tile;
+    result.decisions.kv_tile = kv_tile;
+    result.decisions.wg_size = wg_size;
+    return result;
+}
+
+#endif  // GGML_WEBGPU_SHADER_LIB_HPP