swscale/ops: add ff_sws_op_list_is_noop()

And use it in ff_sws_compile_pass() instead of hard-coding the check there.
This check will become more sophisticated in the following commits.

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Niklas Haas <git@haasn.dev>

libswscale/graph.c

@@ -515,15 +515,7 @@ static int add_convert_pass(SwsGraph *graph, SwsFormat src, SwsFormat dst,
     av_log(ctx, AV_LOG_DEBUG, "Unoptimized operation list:\n");
     ff_sws_op_list_print(ctx, AV_LOG_DEBUG, ops);
     av_log(ctx, AV_LOG_DEBUG, "Optimized operation list:\n");
-
     ff_sws_op_list_optimize(ops);
-    if (ops->num_ops == 0) {
-        av_log(ctx, AV_LOG_VERBOSE, "  optimized into memcpy\n");
-        ff_sws_op_list_free(&ops);
-        *output = input;
-        return 0;
-    }
-
     ff_sws_op_list_print(ctx, AV_LOG_VERBOSE, ops);
 
     ret = ff_sws_compile_pass(graph, ops, 0, dst, input, output);

libswscale/ops.c

@@ -542,6 +542,11 @@ int ff_sws_op_list_append(SwsOpList *ops, SwsOp *op)
     return ff_sws_op_list_insert_at(ops, ops->num_ops, op);
 }
 
+bool ff_sws_op_list_is_noop(const SwsOpList *ops)
+{
+    return !ops->num_ops;
+}
+
 int ff_sws_op_list_max_size(const SwsOpList *ops)
 {
     int max_size = 0;
@@ -1040,6 +1045,12 @@ int ff_sws_compile_pass(SwsGraph *graph, SwsOpList *ops, int flags, SwsFormat ds
     SwsPass *pass;
     int ret;
 
+    /* Check if the whole operation graph is an end-to-end no-op */
+    if (ff_sws_op_list_is_noop(ops)) {
+        *output = input;
+        return 0;
+    }
+
     if (ops->num_ops < 2) {
         av_log(ctx, AV_LOG_ERROR, "Need at least two operations.\n");
         return AVERROR(EINVAL);

libswscale/ops.h

@@ -242,6 +242,12 @@ void ff_sws_op_list_free(SwsOpList **ops);
  */
 SwsOpList *ff_sws_op_list_duplicate(const SwsOpList *ops);
 
+/**
+ * Returns whether an op list represents a true no-op operation, i.e. may be
+ * eliminated entirely from an execution graph.
+ */
+bool ff_sws_op_list_is_noop(const SwsOpList *ops);
+
 /**
  * Returns the size of the largest pixel type used in `ops`.
  */