Support symbolic slice with no start [pr] (#10775)

* add symbolic slice with no start

* reshape the test

* step must be int

* just add a cast...

* more cast...

test/test_symbolic_ops.py

@@ -175,6 +175,15 @@ class TestSymbolicOps(unittest.TestCase):
       expected = a[3:5, i:i+2].numpy()
       np.testing.assert_allclose(symbolic, expected, atol=1e-6, rtol=1e-6)
 
+  def test_slice_no_start(self):
+    for i in range(1, 5):
+      vi = Variable("i", 1, 10).bind(i)
+      a = Tensor.rand(7, 11)
+      symbolic = a[3:5, :vi:1].reshape(2,i)
+      symbolic = symbolic.numpy()
+      expected = a[3:5, :i:1].numpy()
+      np.testing.assert_allclose(symbolic, expected, atol=1e-6, rtol=1e-6)
+
   def test_expand_padded(self):
     for i in range(1, 5):
       vi = Variable("i", 1, 10).bind(i)

tinygrad/tensor.py

@@ -1143,19 +1143,19 @@ class Tensor(MathTrait):
           boundary = [index, index+1] if index >= 0 else [index+size, index+size+1]
         case slice():
           if index.step == 0: raise ValueError(f"{index=} cannot have 0 as step")
-          if all(s is None for s in (index.start,index.stop,index.step)):
-            boundary, stride = [0, size], 1
-          elif all(isinstance(s, int) or s is None for s in (index.start,index.stop,index.step)):
+          start, stop = 0 if index.start is None else index.start, size if index.stop is None else index.stop
+          step = 1 if index.step is None else index.step
+          boundary, stride = [start, stop], step
+          if all(isinstance(s, int) for s in (start,stop,step)):
             # handle int slicing
             *boundary, stride = index.indices(cast(SupportsIndex, size))
             if stride * (boundary[1] - boundary[0]) < 0: boundary = [0, 0]
             elif stride < 0: boundary = [boundary[1] + 1, boundary[0] + 1]
             # update size for slice
             size = ceildiv((boundary[1] - boundary[0]), abs(stride))
-          elif index.step is None and all(isinstance(s,(int,UOp))for s in (index.start,index.stop)) and resolve((index.stop-index.start) > 0, False):
+          elif (step == 1) and isinstance(step, int) and all(isinstance(s,(int,UOp)) for s in (start, stop)) and resolve((stop-start) > 0, False):
             # simple symbolic slice
-            boundary = [index.start, index.stop]
-            size = (index.stop - index.start).ssimplify()
+            size = cast(UOp|int, cast(UOp, (stop - start)).ssimplify())
           else: raise TypeError(f"slice {index=} is not supported")
         case None: pass # do nothing
         case _: raise IndexError(f"{type(index).__name__} indexing is not supported")