ggml : add ggml_backend_op_alloc_size_may_expand, use it in RPC (#27960)

some backends (Metal, SYCL, WebGPU) require additional memory for
fleeting data for certain ops, which is reflected in their
get_alloc_size implementations.

add ggml_backend_op_alloc_size_may_expand() to the backend utils,
listing these ops, and assert in ggml_backend_buft_get_alloc_size
that a backend expanding the alloc size of a compute op only does so
for ops listed in the helper.

use the helper in the RPC backend to decide whether to query the
remote server for the actual alloc size, instead of a hardcoded list.

Assisted-by: pi:llama.cpp/Qwen3.8-27B

ggml/include/ggml-backend.h

@@ -424,6 +424,10 @@ extern "C" {
     // Compare the output of two backends
     GGML_API bool ggml_backend_compare_graph_backend(ggml_backend_t backend1, ggml_backend_t backend2, struct ggml_cgraph * graph, ggml_backend_eval_callback callback, void * user_data, struct ggml_tensor const * const * test_nodes, size_t num_test_nodes);
 
+    // returns true for ops that may require additional memory for fleeting data on some backends,
+    // i.e. the backend's get_alloc_size may return more than ggml_nbytes for the output tensor
+    GGML_API bool ggml_backend_op_alloc_size_may_expand(enum ggml_op op);
+
     // Tensor initialization
     GGML_API enum ggml_status ggml_backend_tensor_alloc(ggml_backend_buffer_t buffer, struct ggml_tensor * tensor, void * addr);
     GGML_API enum ggml_status ggml_backend_view_init(struct ggml_tensor * tensor);

ggml/src/ggml-backend.cpp

@@ -65,6 +65,13 @@ size_t ggml_backend_buft_get_alloc_size(ggml_backend_buffer_type_t buft, const s
     if (buft->iface.get_alloc_size) {
         size_t size = buft->iface.get_alloc_size(buft, tensor);
         assert(size >= ggml_nbytes(tensor));
+
+        // [TAG_ALLOC_SIZE_EXPAND]
+        // if you hit this assert, update ggml_backend_op_alloc_size_may_expand() accordingly
+        GGML_ASSERT(size <= ggml_nbytes(tensor) ||
+                    ggml_op_is_empty(tensor->op) ||
+                    ggml_backend_op_alloc_size_may_expand(tensor->op));
+
         return size;
     }
     return ggml_nbytes(tensor);
@@ -2101,6 +2108,22 @@ ggml_backend_t ggml_backend_sched_get_tensor_backend(ggml_backend_sched_t sched,
 
 // utils
 
+// [TAG_ALLOC_SIZE_EXPAND]
+// returns true for ops that may require additional memory for fleeting data on some backends,
+// i.e. the backend's get_alloc_size may return more than ggml_nbytes for the output tensor
+bool ggml_backend_op_alloc_size_may_expand(enum ggml_op op) {
+    switch (op) {
+        case GGML_OP_FLASH_ATTN_EXT:
+        case GGML_OP_MUL_MAT_ID:
+        case GGML_OP_CUMSUM:
+        case GGML_OP_ARGSORT:
+        case GGML_OP_TOP_K:
+            return true;
+        default:
+            return false;
+    }
+}
+
 enum ggml_status ggml_backend_view_init(struct ggml_tensor * tensor) {
     GGML_ASSERT(tensor);
     GGML_ASSERT(tensor->buffer == NULL);

ggml/src/ggml-rpc/ggml-rpc.cpp

@@ -826,10 +826,10 @@ static size_t ggml_backend_rpc_buffer_type_get_alloc_size(ggml_backend_buffer_ty
     // See comments in init_tensor.
     rpc_get |= ggml_is_quantized(tensor->type) && (tensor->ne[0] % 512 != 0) && (tensor->view_src == nullptr);
 
-    // ops that require additional memory for fleeting data on certain backends
+    // [TAG_ALLOC_SIZE_EXPAND]
+    // ops that may require additional memory for fleeting data on certain backends
     // ref: https://github.com/ggml-org/llama.cpp/pull/15966
-    rpc_get |= tensor->op == GGML_OP_FLASH_ATTN_EXT;
-    rpc_get |= tensor->op == GGML_OP_MUL_MAT_ID;
+    rpc_get |= ggml_backend_op_alloc_size_may_expand(tensor->op);
 
     if (rpc_get) {
         ggml_backend_rpc_buffer_type_context * buft_ctx = (ggml_backend_rpc_buffer_type_context *)buft->context;