no casted const from sub and div [pr] (#17543)

test/unit/test_dtype_weak.py

@@ -76,6 +76,11 @@ class TestWeakPromotion(unittest.TestCase):
       committed = graph_rewrite((UOp.const(1).cast(dtypes.int32) + UOp.const(1.0)).cast(dtypes.float32), pm_lower_index_dtype, ctx={})
     self.assertEqual([u.dtype for u in committed.toposort() if u.op is Ops.ADD], [dtypes.float32])
 
+  def test_div_sub_operand_kept_weak(self):
+    a = Tensor.empty(4, dtype=dtypes.float32)
+    for t in (a / 1, a - 0):
+      self.assertEqual(t.uop.src[1].dtype, dtypes.weakfloat)
+
   def test_cast_weak_expression_commits_at_cast_floor(self):
     # the floor never narrows: a cast BELOW the default does not pull the compute width down with it
     with Context(DEFAULT_FLOAT=dtypes.float32):

tinygrad/mixin/elementwise.py

@@ -115,7 +115,8 @@ class ElementwiseMixin(CreationMixin):
     ```
     """
     a, b = self._broadcasted(x, reverse)
-    return a + (-b)
+    # alu, not +: _broadcasted already promoted these, and a second promote would cast -b (only a bare weak CONST is kept weak)
+    return a.alu(Ops.ADD, -b)
 
   def mul(self, x: Self | ConstType, reverse: bool = False) -> Self:
     """
@@ -245,8 +246,9 @@ class ElementwiseMixin(CreationMixin):
     if dtypes.is_int(a.dtype) and dtypes.is_int(b.dtype):
       if rounding_mode == "trunc": return a.alu(Ops.CDIV, b)
       if rounding_mode == "floor": return a.alu(Ops.FLOORDIV, b)
-      a = a.cast(dtypes.default_float)
-    d = a * b.reciprocal()
+    if dtypes.is_int(a.dtype) or a.dtype == dtypes.bool: a = a.cast(dtypes.default_float)
+    # alu, not *: _broadcasted already promoted these, and a second promote would cast 1/b (only a bare weak CONST is kept weak)
+    d = a.alu(Ops.MUL, b.reciprocal())
     if rounding_mode is None: return d
     if rounding_mode == "trunc": return d.trunc()
     if rounding_mode == "floor": return d.floor()