fix(tracing): do not process already visited tracers

- some topologies triggered a bug where a tracer was visited several times
- this created duplicate edges which caused problems later in the code
- simply skip already visited tracers
- add a check to see that all edges are indeed unique

concrete/common/tracing/tracing_helpers.py

@@ -6,7 +6,7 @@ from typing import Callable, Dict, Iterable, OrderedDict, Set, Type
 import networkx as nx
 from networkx.algorithms.dag import is_directed_acyclic_graph
 
-from ..debugging.custom_assert import custom_assert
+from ..debugging.custom_assert import assert_true, custom_assert
 from ..representation.intermediate import Input
 from ..values import BaseValue
 from .base_tracer import BaseTracer
@@ -108,6 +108,8 @@ def create_graph_from_output_tracers(
         # use dict as ordered set
         next_tracers: Dict[BaseTracer, None] = {}
         for tracer in current_tracers:
+            if tracer in visited_tracers:
+                continue
             current_ir_node = tracer.traced_computation
             graph.add_node(current_ir_node)
 
@@ -124,4 +126,12 @@ def create_graph_from_output_tracers(
 
     custom_assert(is_directed_acyclic_graph(graph))
 
+    # Check each edge is unique
+    unique_edges = set(
+        (pred, succ, tuple((k, v) for k, v in edge_data.items()))
+        for pred, succ, edge_data in graph.edges(data=True)
+    )
+    number_of_edges = len(graph.edges)
+    assert_true(len(unique_edges) == number_of_edges)
+
     return graph

tests/numpy/test_compile.py

@@ -38,6 +38,20 @@ def small_fused_table(x):
     return (10 * (numpy.cos(x + 1) + 1)).astype(numpy.uint32)
 
 
+def complicated_topology(x):
+    """Mix x in an intricated way."""
+    intermediate = x
+    x_p_1 = intermediate + 1
+    x_p_2 = intermediate + 2
+    x_p_3 = x_p_1 + x_p_2
+    return (
+        x_p_3.astype(numpy.int32),
+        x_p_2.astype(numpy.int32),
+        (x_p_2 + 3).astype(numpy.int32),
+        x_p_3.astype(numpy.int32) + 67,
+    )
+
+
 @pytest.mark.parametrize(
     "function,input_ranges,list_of_arg_names",
     [
@@ -55,6 +69,7 @@ def small_fused_table(x):
             ["x", "y"],
             marks=pytest.mark.xfail(strict=True, raises=ValueError),
         ),
+        pytest.param(complicated_topology, ((0, 10),), ["x"]),
     ],
 )
 def test_compile_function_multiple_outputs(function, input_ranges, list_of_arg_names):