rangeify: detect and assert cycles (#12405)

* rangeify: assert cycles

* rng=2

* any

.github/workflows/test.yml

@@ -532,10 +532,8 @@ jobs:
         llvm: "true"
     - name: Test CPU=1 RANGEIFY=1
       # TODO: add more passing tests here
-      # rangeify diamond cycle gives the wrong answer
       run: |
         CPU=1 CPU_LLVM=0 RANGEIFY=1 python3 -m pytest -n auto --durations 20 \
-          -k "not test_assign_diamond_cycle" \
           test/test_tiny.py test/test_rangeify.py test/test_ops.py test/test_symbolic_ops.py test/test_symbolic_jit.py test/test_tensor_variable.py \
           test/test_outerworld_range.py test/test_randomness.py test/test_nn.py test/test_arange.py test/test_tensor.py test/test_optim.py \
           test/test_setitem.py test/test_assign.py test/test_multitensor.py

tinygrad/schedule/rangeify.py

@@ -603,7 +603,14 @@ def renumber_range(ctx:LocalAddBufferContext, r:UOp):
   ctx.range += 1
   return ret
 
+def find_bufs(x:UOp):
+  idxs = [s for s in x.toposort(gate=lambda x: x.op is not Ops.ASSIGN) if s.op is Ops.INDEX]
+  read_from: dict[UOp, Ops] = {}
+  if any((buf:=idx.as_buf()).op is Ops.BUFFER and read_from.setdefault(buf, op:=idx.src[0].op) is not op for idx in idxs):
+    raise RuntimeError(f"cycle detected while indexing {buf}")
+
 to_define_global = PatternMatcher([
+  (UPat(Ops.STORE, name="x"), find_bufs),
   (UPat(Ops.BUFFER, name="buf"), debuf),
   (UPat(Ops.BIND, name="b"), unbind_kernel),
   (UPat((Ops.ASSIGN, Ops.MSTACK, Ops.MSELECT), name="assign"), handle_assign),