metal : single allocation of encode_async block (#9747)

* Single allocation of encode_async block with non-ARC capture in ggml-metal.m

* Moving Block_release to the deallocation code

* Release encode block when re-setting encoding buffer count if needed

* Update ggml/src/ggml-metal.m

---------

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

ggml/src/ggml-metal.m

@@ -239,8 +239,6 @@ struct ggml_backend_metal_context {
     struct ggml_cgraph * gf;
 
     // the callback given to the thread pool
-    // TODO: ideally, this should be created once, utilizing the command buffer state above
-    //       for some reason, doing it like this leads to a crash
     void (^encode_async)(size_t ith);
 
     // n_cb command buffers + 1 used by the main thread
@@ -683,6 +681,8 @@ static void ggml_metal_free(struct ggml_backend_metal_context * ctx) {
         [ctx->kernels[i].pipeline release];
     }
 
+    Block_release(ctx->encode_async);
+
     [ctx->queue release];
     [ctx->device release];
 
@@ -3000,46 +3000,6 @@ static enum ggml_status ggml_metal_graph_compute(
             }
         }
 
-        // TODO: how to avoid this allocation? I tried initializing it in ggml_backend_metal_set_n_cb but it crashes.
-        ctx->encode_async = ^(size_t iter) {
-            const int cb_idx = iter;
-            const int n_cb_l = ctx->n_cb;
-
-            const int n_nodes_0 = ctx->n_nodes_0;
-            const int n_nodes_1 = ctx->n_nodes_1;
-
-            const int n_nodes_per_cb = ctx->n_nodes_per_cb;
-
-            id<MTLCommandBuffer> command_buffer  = ctx->command_buffers[cb_idx];
-            id<MTLComputeCommandEncoder> encoder = [command_buffer computeCommandEncoder];
-
-            int node_start = 0;
-            int node_end   = n_nodes_0;
-
-            if (cb_idx < n_cb_l) {
-                node_start = n_nodes_0 + (                                         (cb_idx + 0) * n_nodes_per_cb);
-                node_end   = n_nodes_0 + (MIN((cb_idx == n_cb_l - 1) ? n_nodes_1 : (cb_idx + 1) * n_nodes_per_cb, n_nodes_1));
-            }
-
-            for (int idx = node_start; idx < node_end; ++idx) {
-                if (should_capture) {
-                    [encoder pushDebugGroup:[NSString stringWithCString:ggml_op_desc(ggml_graph_node(gf, idx)) encoding:NSUTF8StringEncoding]];
-                }
-
-                ggml_metal_encode_node(ctx, idx, encoder);
-
-                if (should_capture) {
-                    [encoder popDebugGroup];
-                }
-            }
-
-            [encoder endEncoding];
-
-            if (cb_idx < 2 || ctx->abort_callback == NULL) {
-                [command_buffer commit];
-            }
-        };
-
         // the main thread commits the first few commands immediately
         // command_buffer[n_cb]
         {
@@ -3468,10 +3428,50 @@ static void ggml_backend_metal_set_n_cb(ggml_backend_t backend, int n_cb) {
         }
     }
 
-    // TODO: setting encode_async here causes crash during the next ggml_metal_graph_compute call. why?
-    //ctx->encode_async = ^(size_t iter) {
-    //    ...
-    //};
+    if (ctx->encode_async) {
+        Block_release(ctx->encode_async);
+    }
+
+    ctx->encode_async = Block_copy(^(size_t iter) {
+        const int cb_idx = iter;
+        const int n_cb_l = ctx->n_cb;
+
+        const int n_nodes_0 = ctx->n_nodes_0;
+        const int n_nodes_1 = ctx->n_nodes_1;
+
+        const int n_nodes_per_cb = ctx->n_nodes_per_cb;
+
+        id<MTLCommandBuffer> command_buffer  = ctx->command_buffers[cb_idx];
+        id<MTLComputeCommandEncoder> encoder = [command_buffer computeCommandEncoder];
+
+        int node_start = 0;
+        int node_end   = n_nodes_0;
+
+        if (cb_idx < n_cb_l) {
+            node_start = n_nodes_0 + (                                         (cb_idx + 0) * n_nodes_per_cb);
+            node_end   = n_nodes_0 + (MIN((cb_idx == n_cb_l - 1) ? n_nodes_1 : (cb_idx + 1) * n_nodes_per_cb, n_nodes_1));
+        }
+
+        const bool should_capture = ctx->capture_next_compute;
+
+        for (int idx = node_start; idx < node_end; ++idx) {
+            if (should_capture) {
+                [encoder pushDebugGroup:[NSString stringWithCString:ggml_op_desc(ggml_graph_node(ctx->gf, idx)) encoding:NSUTF8StringEncoding]];
+            }
+
+            ggml_metal_encode_node(ctx, idx, encoder);
+
+            if (should_capture) {
+                [encoder popDebugGroup];
+            }
+        }
+
+        [encoder endEncoding];
+
+        if (cb_idx < 2 || ctx->abort_callback == NULL) {
+            [command_buffer commit];
+        }
+    });
 }
 
 static struct ggml_backend_i ggml_backend_metal_i = {