sycl: initialize flash-attention implementation

ye-NX · safranowith · ye-NX · commit fc0e0413bcfb · 2025-11-03T14:50:15.000+02:00
Co-authored-by: safranowith &lt;bsh155762@gmail.com&gt;
Co-authored-by: ye-NX &lt;y8703470@gmail.com&gt;
diff --git a/ggml/src/ggml-sycl/CMakeLists.txt b/ggml/src/ggml-sycl/CMakeLists.txt
@@ -27,6 +27,15 @@ file(GLOB   GGML_HEADERS_SYCL "*.hpp")
 file(GLOB   GGML_SOURCES_SYCL "*.cpp")
 target_sources(ggml-sycl PRIVATE ${GGML_HEADERS_SYCL} ${GGML_SOURCES_SYCL})
 
+# Include flash-attn sources (SYCL optimized flash attention implementation)
+file(GLOB GGML_HEADERS_SYCL_FLASH "flash-attn/*.h" "flash-attn/*.hpp")
+file(GLOB GGML_SOURCES_SYCL_FLASH "flash-attn/*.cpp" "flash-attn/*.c")
+target_sources(ggml-sycl PRIVATE ${GGML_HEADERS_SYCL_FLASH} ${GGML_SOURCES_SYCL_FLASH})
+
+# Also include kernel headers under flash-attn/kernels
+file(GLOB GGML_HEADERS_SYCL_FLASH_KERNELS "flash-attn/kernels/*.h" "flash-attn/kernels/*.hpp")
+target_sources(ggml-sycl PRIVATE ${GGML_HEADERS_SYCL_FLASH_KERNELS})
+
 if (WIN32)
     # To generate a Visual Studio solution, using Intel C++ Compiler for ggml-sycl is mandatory
     if( ${CMAKE_GENERATOR} MATCHES "Visual Studio" AND NOT (${CMAKE_GENERATOR_TOOLSET} MATCHES "Intel C"))
diff --git a/ggml/src/ggml-sycl/flash-attn/flash-attn-sycl.cpp b/ggml/src/ggml-sycl/flash-attn/flash-attn-sycl.cpp
@@ -0,0 +1,98 @@
+#include "flash-attn-sycl.h"
+
+#include "kernels/flash-attn-kernel.h"
+
+#include <cmath>
+#include <cstring>
+#include <limits>
+#include <sycl/sycl.hpp>
+
+#define FLASH_ATTN_BR_MAX 32
+#define FLASH_ATTN_BC_MAX 32
+
+// Flash Attention: https://arxiv.org/abs/2205.14135
+void ggml_sycl_op_flash_attn(ggml_backend_sycl_context & ctx, ggml_tensor * dst) {
+    const ggml_tensor * Q    = dst->src[0];
+    const ggml_tensor * K    = dst->src[1];
+    const ggml_tensor * V    = dst->src[2];
+    const ggml_tensor * mask = dst->src[3];
+
+    GGML_ASSERT(Q != nullptr);
+    GGML_ASSERT(K != nullptr);
+    GGML_ASSERT(V != nullptr);
+    GGML_ASSERT(dst != nullptr);
+
+    if (Q->type != GGML_TYPE_F32 || K->type != GGML_TYPE_F32 || V->type != GGML_TYPE_F32 || dst->type != GGML_TYPE_F32) {
+        fprintf(stderr, "[SYCL] FLASH-ATTENTION: tensor type not supported (Q=%d, K=%d, V=%d, dst=%d)\n", Q->type, K->type, V->type, dst->type);
+        return;
+    }
+
+    const float * Q_d   = (const float *) Q->data;
+    const float * K_d   = (const float *) K->data;
+    const float * V_d   = (const float *) V->data;
+    float *       dst_d = (float *) dst->data;
+
+    dpct::queue_ptr stream = ctx.stream();
+
+    const int64_t d = Q->ne[0];
+    const int64_t N = Q->ne[1];
+
+    float scale;
+    float max_bias;
+    float logit_softcap;
+    
+    std::memcpy(&scale, (const float *) dst->op_params + 0, sizeof(float));
+    std::memcpy(&max_bias, (const float *) dst->op_params + 1, sizeof(float));
+    std::memcpy(&logit_softcap, (const float *) dst->op_params + 2, sizeof(float));
+
+    const bool masked = (mask != nullptr);
+
+    const int Br = std::min((int) FLASH_ATTN_BR_MAX, (int) N);
+    const int Bc = std::min((int) FLASH_ATTN_BC_MAX, (int) N);
+
+    const int Tr = (N + Br - 1) / Br;
+    const int Tc = (N + Bc - 1) / Bc;
+
+    float * l_d = (float *) sycl::malloc_device(N * sizeof(float), *stream);
+    float * m_d = (float *) sycl::malloc_device(N * sizeof(float), *stream);
+
+    stream->fill(l_d, 0.0f, N);
+    stream->fill(m_d, -std::numeric_limits<float>::infinity(), N);
+    stream->fill(dst_d, 0.0f, N * d);
+    stream->wait();
+
+    for (int j = 0; j < Tc; ++j) {
+        stream->submit([&](sycl::handler & cgh) {
+            cgh.parallel_for(sycl::range<1>(Tr), [=](sycl::id<1> idx) {
+                const int i = idx[0];
+                flash_attn_tiled_kernel<FLASH_ATTN_BR_MAX, FLASH_ATTN_BC_MAX>(Q_d, K_d, V_d, dst_d, l_d, m_d, i, j, Br,
+                                                                              Bc, N, d, masked, scale);
+            });
+        });
+    }
+
+    stream->wait();
+
+    sycl::free(l_d, *stream);
+    sycl::free(m_d, *stream);
+}
+
+bool ggml_sycl_flash_attn_ext_supported(const ggml_tensor * dst) {
+    const ggml_tensor * Q = dst->src[0];
+    const ggml_tensor * K = dst->src[1];
+    const ggml_tensor * V = dst->src[2];
+
+    if (Q == nullptr || K == nullptr || V == nullptr) {
+        return false;
+    }
+
+    if (Q->type != GGML_TYPE_F32 || K->type != GGML_TYPE_F32 || V->type != GGML_TYPE_F32) {
+        return false;
+    }
+
+    if (dst->type != GGML_TYPE_F32) {
+        return false;
+    }
+
+    return true;
+}
diff --git a/ggml/src/ggml-sycl/flash-attn/flash-attn-sycl.h b/ggml/src/ggml-sycl/flash-attn/flash-attn-sycl.h
@@ -0,0 +1,10 @@
+#pragma once
+
+#include "../common.hpp"
+
+// Flash attention operation for SYCL backend
+// This implements the Flash Attention algorithm optimized for SYCL devices
+void ggml_sycl_op_flash_attn(ggml_backend_sycl_context & ctx, ggml_tensor * dst);
+
+// Check if flash attention is supported for given tensor
+bool ggml_sycl_flash_attn_ext_supported(const ggml_tensor * dst);
diff --git a/ggml/src/ggml-sycl/flash-attn/kernels/flash-attn-kernel.h b/ggml/src/ggml-sycl/flash-attn/kernels/flash-attn-kernel.h
@@ -0,0 +1,108 @@
+#pragma once
+
+#include <sycl/sycl.hpp>
+
+template <int Br_MAX = 32, int Bc_MAX = 32>
+inline void flash_attn_tiled_kernel(const float * Q,
+                                    const float * K,
+                                    const float * V,
+                                    float *       O,
+                                    float *       l,
+                                    float *       m,
+                                    const int     i_block,
+                                    const int     j_block,
+                                    const int     Br,
+                                    const int     Bc,
+                                    const int     N,
+                                    const int     d,
+                                    const bool    masked,
+                                    const float   scale) {
+    const int i_start = i_block * Br;
+    const int j_start = j_block * Bc;
+
+    float S[Br_MAX][Bc_MAX];
+    float P[Br_MAX][Bc_MAX];
+    float m_local[Br_MAX];
+    float l_local[Br_MAX];
+
+    for (int qi = 0; qi < Br; ++qi) {
+        const int q_row = i_start + qi;
+        if (q_row >= N) {
+            continue;
+        }
+
+        for (int kj = 0; kj < Bc; ++kj) {
+            const int k_row = j_start + kj;
+            if (k_row >= N) {
+                S[qi][kj] = -INFINITY;
+                continue;
+            }
+
+            if (masked && k_row > q_row) {
+                S[qi][kj] = -INFINITY;
+                continue;
+            }
+
+            float score = 0.0f;
+            for (int k = 0; k < d; ++k) {
+                score += Q[q_row * d + k] * K[k_row * d + k];
+            }
+            S[qi][kj] = score * scale;
+        }
+    }
+
+    for (int qi = 0; qi < Br; ++qi) {
+        const int q_row = i_start + qi;
+        if (q_row >= N) {
+            continue;
+        }
+
+        m_local[qi] = -INFINITY;
+        for (int kj = 0; kj < Bc; ++kj) {
+            if (j_start + kj < N) {
+                m_local[qi] = sycl::fmax(m_local[qi], S[qi][kj]);
+            }
+        }
+
+        l_local[qi] = 0.0f;
+        for (int kj = 0; kj < Bc; ++kj) {
+            if (j_start + kj < N && !sycl::isinf(S[qi][kj])) {
+                P[qi][kj] = sycl::exp(S[qi][kj] - m_local[qi]);
+                l_local[qi] += P[qi][kj];
+            } else {
+                P[qi][kj] = 0.0f;
+            }
+        }
+    }
+
+    for (int qi = 0; qi < Br; ++qi) {
+        const int q_row = i_start + qi;
+        if (q_row >= N) {
+            continue;
+        }
+
+        const float m_old = m[q_row];
+        const float m_new = sycl::fmax(m_old, m_local[qi]);
+        const float l_old = l[q_row];
+        const float l_new = sycl::exp(m_old - m_new) * l_old + sycl::exp(m_local[qi] - m_new) * l_local[qi];
+
+        const float correction_old = sycl::exp(m_old - m_new);
+        const float correction_new = sycl::exp(m_local[qi] - m_new);
+
+        for (int k = 0; k < d; ++k) {
+            float pv = 0.0f;
+            for (int kj = 0; kj < Bc; ++kj) {
+                const int v_row = j_start + kj;
+                if (v_row < N) {
+                    pv += P[qi][kj] * V[v_row * d + k];
+                }
+            }
+
+            const int o_idx = q_row * d + k;
+            O[o_idx]        = (correction_old * O[o_idx] + correction_new * pv) / l_new;
+        }
+
+        l[q_row] = l_new;
+        m[q_row] = m_new;
+    }
+}
diff --git a/ggml/src/ggml-sycl/ggml-sycl.cpp b/ggml/src/ggml-sycl/ggml-sycl.cpp
@@ -3839,6 +3839,9 @@ static bool ggml_sycl_compute_forward(ggml_backend_sycl_context & ctx, struct gg
         case GGML_OP_ARGSORT:
             ggml_sycl_argsort(ctx, dst);
             break;
+        case GGML_OP_FLASH_ATTN_EXT:
+            ggml_sycl_op_flash_attn(ctx, dst);
+            break;
         case GGML_OP_TIMESTEP_EMBEDDING:
             ggml_sycl_op_timestep_embedding(ctx, dst);
             break;
@@ -4501,6 +4504,8 @@ static bool ggml_backend_sycl_device_supports_op(ggml_backend_dev_t dev, const g
         case GGML_OP_MEAN:
         case GGML_OP_ARGSORT:
             return ggml_is_contiguous(op->src[0]);
+        case GGML_OP_FLASH_ATTN_EXT:
+            return ggml_sycl_flash_attn_ext_supported(op);
         case GGML_OP_POOL_2D:
         case GGML_OP_ACC:
             return true;
