Fix SubgraphMatch for case of no anchor found

ghstack-source-id: 43e81f4ad4c9ca4903a82a91191f8900f34ef56e
Pull Request resolved: #86421

test/test_fx_passes.py

@@ -664,6 +664,27 @@ def pattern(a, a_dtype, a_scale, b, b_dtype, b_scale, out_scale):
         TestCase(False, False, 1),
     ]
 
+class NoAnchorFound:
+    # This test case is for pattern where no matching anchor is found in the target graph
+    # `anchor` is the starting point of the pattern matching, it's usually the boundary returning nodes
+    @staticmethod
+    def forward(x):
+        x = x + 1
+        return x
+
+    @staticmethod
+    def pattern(a):
+        b1 = a.relu()
+        return b1
+
+    test_cases = [
+        # match_output, match_placeholder, num_matches
+        TestCase(False, False, 0),
+        TestCase(True, False, 0),
+        TestCase(False, True, 0),
+        TestCase(True, True, 0)
+    ]
+
 @instantiate_parametrized_tests
 class TestFXMatcherUtils(JitTestCase):
 
@@ -683,6 +704,7 @@ class TestFXMatcherUtils(JitTestCase):
         MultipleOutputsHorizontalPattern,
         MultiOutputWithWithInvalidMatches,
         QuantizationFp8Pattern,
+        NoAnchorFound,
     ])
     def test_subgraph_matcher(self, test_model):
 

torch/fx/passes/utils/matcher_utils.py

@@ -7,10 +7,26 @@
 import torch.utils._pytree as pytree
 from typing import Dict, List, Set, Any
 import logging
+import os
 
 __all__ = ['SubgraphMatcher', 'InternalMatch']
 
-logger = logging.getLogger(__name__)
+# Set`PYTORCH_MATCHER_LOGLEVEL=INFO` to see debug logs
+def _init_logger():
+    logger = logging.getLogger(__name__)
+
+    level = os.environ.get('PYTORCH_MATCHER_LOGLEVEL', 'WARNING').upper()
+    logger.setLevel(level)
+    console = logging.StreamHandler()
+    formatter = logging.Formatter("%(filename)s > %(message)s")
+    console.setFormatter(formatter)
+    console.setLevel(level)
+    # add the handlers to the logger
+    logger.addHandler(console)
+    logger.propagate = False
+    return logger
+
+logger = _init_logger()
 
 @compatibility(is_backward_compatible=False)
 @dataclass
@@ -126,7 +142,7 @@ def _remove_overlapping_matches(self, matches: List[InternalMatch]) -> List[Inte
         return non_overlapping_matches
 
     def _match_args(self, pn: Any, gn: Any, match: InternalMatch) -> bool:
-        assert not(isinstance(pn, Node) and isinstance(gn, Node)), "pn and gn cannot both be Node"
+        assert not (isinstance(pn, Node) and isinstance(gn, Node)), "pn and gn cannot both be Node"
 
         if isinstance(pn, Node) and not isinstance(gn, Node):
             if pn.op == "placeholder":
@@ -247,6 +263,9 @@ def match(self, graph: Graph) -> List[InternalMatch]:
                 if self._nodes_are_equal(pattern_anchor, node):
                     match_candidates[pattern_anchor].append(node)
         match_candidates_list = list(match_candidates.items())
+
+        logger.info(f"Initial match_candidates_list: {match_candidates_list}\n")
+
         matches: List[InternalMatch] = []
 
         def backtracking(anchor_index, match):
@@ -275,7 +294,8 @@ def backtracking(anchor_index, match):
                 match = copy.copy(saved_match)
 
         match = InternalMatch(anchors=self.pattern_anchors)
-        backtracking(0, match)
+        if match_candidates_list:
+            backtracking(0, match)
 
         # filter out the matches where the subgraph is not fully_contained
         before = len(matches)
@@ -302,4 +322,6 @@ def backtracking(anchor_index, match):
             if before != after:
                 logger.info(f"Filtered out {before - after} matches because matched subgraphs are overlapping")
 
+        logger.info(f"Matches returned: {matches}")
+
         return matches