gf.c: make edge de-duplication a caller responsibility

On my system, this saves ~500 ms when loading CairoMakie (and all
dependent packages)

Author: topolarity

src/gf.c

@@ -1997,6 +1997,9 @@ JL_DLLEXPORT void jl_method_instance_add_backedge(jl_method_instance_t *callee,
             jl_gc_wb(callee, backedges);
         }
         else {
+#ifndef JL_NDEBUG
+            // It is the caller's (inference's) responsibility to de-duplicate edges. Here we are only
+            // checking its work.
             size_t i = 0, l = jl_array_nrows(backedges);
             for (i = 0; i < l; i++) {
                 // optimized version of while (i < l) i = get_next_edge(callee->backedges, i, &invokeTypes, &mi);
@@ -2012,6 +2015,8 @@ JL_DLLEXPORT void jl_method_instance_add_backedge(jl_method_instance_t *callee,
                     break;
                 }
             }
+            assert(!found && "duplicate back-edge registered");
+#endif
         }
         if (!found)
             push_edge(backedges, invokesig, caller);
@@ -2037,14 +2042,20 @@ JL_DLLEXPORT void jl_method_table_add_backedge(jl_methtable_t *mt, jl_value_t *t
         else {
             // check if the edge is already present and avoid adding a duplicate
             size_t i, l = jl_array_nrows(mt->backedges);
+#ifndef JL_NDEBUG
+            // It is the caller's (inference's) responsibility to de-duplicate edges. Here we are only
+            // checking its work.
+            int found = 0;
             for (i = 1; i < l; i += 2) {
                 if (jl_array_ptr_ref(mt->backedges, i) == (jl_value_t*)caller) {
                     if (jl_types_equal(jl_array_ptr_ref(mt->backedges, i - 1), typ)) {
-                        JL_UNLOCK(&mt->writelock);
-                        return;
+                        found = 1;
+                        break;
                     }
                 }
             }
+            assert(!found && "duplicate back-edge registered");
+#endif
             // reuse an already cached instance of this type, if possible
             // TODO: use jl_cache_type_(tt) like cache_method does, instead of this linear scan?
             for (i = 1; i < l; i += 2) {