ggml : add comment

ggml/include/ggml.h

@@ -2578,10 +2578,32 @@ extern "C" {
         struct ggml_tensor *  grad,
         struct ggml_tensor *  sgd_params); // alpha, weight decay
 
+    // build forward mutiple tensors and select one of them for computing
+    // this is useful for creating graphs that have constant topology but compute different things based on the input
+    // ref: https://github.com/ggml-org/llama.cpp/pull/18550
     //
-    // automatic differentiation
+    // nodes:
+    //   | - build forward into the graph but do not compute
+    //   c - build forward into the graph and compute
+    //
+    //    |  |  ...  c  ...  |
+    //    |  |  ...  c  ...  |
+    //    |  |  ...  c  ...  |
+    //   [0  1  ... idx ...  n-1]        <-- ggml_build_forward_select(..., n, idx)
+    //               c
+    //               c
+    //
+    // example:
+    //   struct ggml_tensor * curs[3];
+    //
+    //   curs[0]  = compute0(...);
+    //   curs[1]  = compute1(...);
+    //   curs[2]  = compute2(...);
+    //
+    //   int idx = select_branch(some_input);
+    //
+    //   struct ggml_tensor * out = ggml_build_forward_select(cgraph, curs, 3, idx);
     //
-
     GGML_API struct ggml_tensor * ggml_build_forward_select(
             struct ggml_cgraph  * cgraph,
             struct ggml_tensor ** tensors,

ggml/src/ggml.c

@@ -6741,7 +6741,7 @@ static size_t ggml_visit_parents_graph(struct ggml_cgraph * cgraph, struct ggml_
             // update the compute flag regardless
             for (int i = 0; i < GGML_MAX_SRC; ++i) {
                 struct ggml_tensor * src = node->src[i];
-                if (src && (src->flags & GGML_TENSOR_FLAG_COMPUTE) == 0) {
+                if (src && ((src->flags & GGML_TENSOR_FLAG_COMPUTE) == 0)) {
                     ggml_visit_parents_graph(cgraph, src, true);
                 }
             }