metal : add top-k radix implementation (#28073)

Assisted-by: DeepSeek-v4-Flash-0731

ggml/src/ggml-metal/ggml-metal-ops.cpp

@@ -5091,7 +5091,9 @@ int ggml_metal_op_argsort(ggml_metal_op_t ctx, int idx) {
     return 1;
 }
 
-int ggml_metal_op_top_k(ggml_metal_op_t ctx, int idx) {
+// bitonic-sort + merge fallback: efficient when k is small and there are few rows,
+// where the single-workgroup-per-row radix-select cannot reach enough parallelism
+static void ggml_metal_op_top_k_bitonic(ggml_metal_op_t ctx, int idx) {
     ggml_tensor * op = ctx->node(idx);
 
     ggml_metal_library_t lib = ctx->lib;
@@ -5199,6 +5201,74 @@ int ggml_metal_op_top_k(ggml_metal_op_t ctx, int idx) {
 
         len <<= 1;
     }
+}
+
+// radix-select: one workgroup per row. Maps each float to an order-preserving unsigned
+// key, finds the k-th largest via 4 radix-8 histogram passes, then compacts the top-k
+// indices. Fast for large k and/or many rows.
+static void ggml_metal_op_top_k_radix(ggml_metal_op_t ctx, int idx) {
+    ggml_tensor * op = ctx->node(idx);
+
+    ggml_metal_library_t lib = ctx->lib;
+    ggml_metal_encoder_t enc = ctx->enc;
+
+    GGML_ASSERT(ggml_is_contiguous_rows(op->src[0]));
+
+    GGML_TENSOR_LOCALS( int32_t, ne0, op->src[0], ne);
+    GGML_TENSOR_LOCALS(uint64_t, nb0, op->src[0], nb);
+
+    auto pipeline = ggml_metal_library_get_pipeline_top_k_radix(lib, op);
+
+    // one workgroup per row; radix-select the k-th largest value
+    const int nth = std::min(1024, ggml_metal_pipeline_max_theads_per_threadgroup(pipeline));
+
+    ggml_metal_kargs_top_k args = {
+        /*.ne00  =*/ ne00,
+        /*.ne01  =*/ ne01,
+        /*.ne02  =*/ ne02,
+        /*.ne03  =*/ ne03,
+        /*.nb01  =*/ nb01,
+        /*.nb02  =*/ nb02,
+        /*.nb03  =*/ nb03,
+        /*.top_k =*/ (int32_t) op->ne[0],
+    };
+
+    // shared memory: 256-entry histogram + bucket/above scalars + output counter
+    const size_t smem_histo  = GGML_PAD(256*sizeof(uint32_t), 16);
+    const size_t smem_bucket = GGML_PAD(    sizeof(uint32_t), 16);
+    const size_t smem_above  = GGML_PAD(    sizeof(uint32_t), 16);
+    const size_t smem_out    = GGML_PAD(    sizeof(uint32_t), 16);
+
+    ggml_metal_encoder_set_pipeline(enc, pipeline);
+    ggml_metal_encoder_set_bytes   (enc, &args, sizeof(args), 0);
+    ggml_metal_encoder_set_buffer  (enc, ggml_metal_get_buffer_id(op->src[0]), 1);
+    ggml_metal_encoder_set_buffer  (enc, ggml_metal_get_buffer_id(op),         2);
+
+    ggml_metal_encoder_set_threadgroup_memory_size(enc, smem_histo,  0);
+    ggml_metal_encoder_set_threadgroup_memory_size(enc, smem_bucket, 1);
+    ggml_metal_encoder_set_threadgroup_memory_size(enc, smem_above,  2);
+    ggml_metal_encoder_set_threadgroup_memory_size(enc, smem_out,    3);
+
+    ggml_metal_encoder_dispatch_threadgroups(enc, ne01, ne02, ne03, nth, 1, 1);
+}
+
+int ggml_metal_op_top_k(ggml_metal_op_t ctx, int idx) {
+    ggml_tensor * op = ctx->node(idx);
+
+    // radix-select has a fixed single-workgroup-per-row cost (~50-60us) that is only
+    // amortized for long rows, many rows, or a large k; otherwise the bitonic path wins
+    const int ncols = op->src[0]->ne[0];
+    const int k     = op->ne[0];
+    const int nrows = ggml_nrows(op->src[0]);
+
+    const bool use_radix =
+        ncols > 2048 && (k > 64 || (nrows > 4 && ncols >= 8192));
+
+    if (use_radix) {
+        ggml_metal_op_top_k_radix(ctx, idx);
+    } else {
+        ggml_metal_op_top_k_bitonic(ctx, idx);
+    }
 
     return 1;
 }