infinite loop detect in fixed_point_rewrite [pr] (#11038)

test/unit/test_graph_rewrite.py

@@ -299,6 +299,15 @@ class TestRecurse(unittest.TestCase):
     with self.assertRaises(RuntimeError):
       graph_rewrite(a, pm)
 
+  def test_inf_loop_bottom_up(self):
+    a = UOp.variable('a', 0, 10)
+    pm = PatternMatcher([
+      (UPat(Ops.DEFINE_VAR, name="x"), lambda x: x.replace(op=Ops.DEFINE_REG)),
+      (UPat(Ops.DEFINE_REG, name="x"), lambda x: x.replace(op=Ops.DEFINE_VAR)),
+    ])
+    with self.assertRaises(RuntimeError):
+      graph_rewrite(a, pm, bottom_up=True)
+
 def bidir_append(ctx, x, b): ctx.append((x.arg if x.op is Ops.CONST else "+", b))
 class TestBidirectional(unittest.TestCase):
   def test_simple(self):

tinygrad/uop/ops.py

@@ -733,7 +733,11 @@ class PatternMatcher:
   def fixed_point_rewrite(self, uop:UOp, ctx=None) -> UOp:
     # apply rewrite rules until a fixed point is reached. may return `uop` itself if PatternMatcher doesn't match
     new_n: UOp|None = uop
-    while new_n is not None: last_n, new_n = new_n, self.rewrite(new_n, ctx)
+    seen = set()
+    while new_n is not None:
+      if new_n in seen: raise RuntimeError("infinite loop in fixed_point_rewrite")
+      seen.add(new_n)
+      last_n, new_n = new_n, self.rewrite(new_n, ctx)
     return last_n
 
 # *** non-blocking UOp tracker ***