sycl: fuse mul_mat(gate) + mul_mat(up) + GLU for q4_K dense FFN (#26779)

Measured on Arc Pro B70 (Battlemage, Level Zero), llama-bench -r 20, two
interleaved rounds, tg128:

    qwen2.5-3B-Instruct Q4_K_M    154.18 -> 158.53 t/s   +2.8%
    gemma-2-2b-it Q4_K_M          162.45 -> 165.62 t/s   +2.0%

llama-batched-bench on qwen2.5-3B, S_TG by batch size:

      B=1   142.72 -> 147.57 t/s    +3.4%
      B=2   243.72 -> 268.26 t/s   +10.1%
      B=4   359.58 -> 398.02 t/s   +10.7%
      B=8   449.75 -> 505.63 t/s   +12.4%

ggml/src/ggml-sycl/ggml-sycl.cpp

@@ -4561,6 +4561,66 @@ static void ggml_sycl_mul_mat(ggml_backend_sycl_context & ctx, const ggml_tensor
     }
 }
 
+// Fused dense-FFN mat-vec for the {mul_mat(gate), mul_mat(up), GLU} subgraph at node_idx.
+// Returns false if it declined, in which case the caller runs the three nodes normally.
+static bool ggml_sycl_mul_mat_glu_mmvq_fused(ggml_backend_sycl_context & ctx, ggml_cgraph * cgraph, int node_idx) {
+    if (!ggml_sycl_can_fuse(cgraph, node_idx, { GGML_OP_MUL_MAT, GGML_OP_MUL_MAT, GGML_OP_GLU }, {})) {
+        return false;
+    }
+
+    ggml_tensor *       glu  = cgraph->nodes[node_idx + 2];
+    ggml_tensor *       gate = glu->src[0];
+    ggml_tensor *       up   = glu->src[1];
+    const ggml_tensor * wu   = up->src[0];
+    const ggml_tensor * wg   = gate->src[0];
+    const ggml_tensor * act  = up->src[1];
+
+    // this writes glu->data directly rather than the per-device row slices that
+    // ggml_sycl_op_mul_mat() stitches back together, so it cannot serve split weights
+    if (ggml_backend_buffer_is_sycl_split(wu->buffer) || ggml_backend_buffer_is_sycl_split(wg->buffer)) {
+        return false;
+    }
+
+    // with DMMV prioritised the unfused path would not have gone through mmvq at all
+    if (g_ggml_sycl_prioritize_dmmv) {
+        return false;
+    }
+
+    // install the reorder (SoA) layout the fused kernel needs, as the unfused mmvq path would;
+    // a no-op once done. after the bail checks so a declined op does not pay for it.
+    opt_for_reorder(&ctx, wu, act, up, mul_mat_algo::MMVQ);
+    opt_for_reorder(&ctx, wg, act, gate, mul_mat_algo::MMVQ);
+
+    const auto * extra_u = static_cast<const ggml_tensor_extra_gpu *>(wu->extra);
+    const auto * extra_g = static_cast<const ggml_tensor_extra_gpu *>(wg->extra);
+    if (!extra_u || !extra_g || !extra_u->optimized_feature.reorder || !extra_g->optimized_feature.reorder) {
+        return false;
+    }
+
+    // log the up mat-mul: glu's own srcs are the two intermediates the fusion never materialises
+    scope_op_debug_print scope_dbg_print(__func__, up, /*num_src=*/2, " : fused with gate + GLU");
+
+    const int64_t ne00 = wu->ne[0];
+    const int64_t ne11 = act->ne[1];
+
+    const queue_ptr stream           = ctx.stream();
+    const int       src1_padded_cols = GGML_PAD((int) ne00, MATRIX_ROW_PADDING);
+
+    // one activation, quantized once and fully consumed into src1_ddq before the GEMV on this
+    // in-order queue, so glu->data aliasing the dead activation needs no memory-range check
+    ggml_sycl_pool_alloc<char> src1_q8_alloc(ctx.pool(),
+                                             (size_t) ne11 * src1_padded_cols * sizeof(block_q8_1) / QK8_1);
+    char *                     src1_ddq = src1_q8_alloc.get();
+
+    quantize_row_q8_1_sycl<quantize_and_reorder_q8_1_soa>((const float *) act->data, src1_ddq, (int) ne00, (int) ne11,
+                                                          src1_padded_cols, stream);
+
+    return ggml_sycl_mul_mat_vec_q_glu_reorder(wu->type, ggml_get_glu_op(glu), wu->data, wg->data, src1_ddq,
+                                               (float *) glu->data, (int) ne00, (int) wu->ne[1], (int) ne11,
+                                               /*stride_col_y_bytes=*/src1_padded_cols * (int) sizeof(block_q8_1) /
+                                                   QK8_1,
+                                               /*stride_col_dst=*/(int) glu->ne[0], stream);
+}
 
 __dpct_inline__ static void k_copy_src1_to_contiguous(
     const char *__restrict__ src1_original, char *__restrict__ src1_contiguous,
@@ -5591,6 +5651,11 @@ static void ggml_backend_sycl_graph_compute_impl(ggml_backend_sycl_context * syc
             continue;
         }
 
+        if (node->op == GGML_OP_MUL_MAT && ggml_sycl_mul_mat_glu_mmvq_fused(*sycl_ctx, cgraph, i)) {
+            i += 2;
+            continue;
+        }
+
         bool ok = ggml_sycl_compute_forward(*sycl_ctx, node);
         if (!ok) {
             GGML_LOG_ERROR("%s: error: op not supported %s (%s)\n", __func__, node->name, ggml_op_name(node->op));