Add support for sym_ite (#111440)

This PR supports sym_ite. This is useful for converting SymBool to SymInt in e.g. #109916. Internally, it uses sympy.Piecewise. We cannot use sympy.ITE because it expects the arguments and output all to be boolean type but we want return SymInt type when converting a SymBool to SymInt. So we use sympy.Piecewise to denote the symbolic relationship.

Note that this pr uses the range analysis for sympy.Piecewise implemented in https://github.com/pytorch/pytorch/blob/main/torch/utils/_sympy/value_ranges.py.

Test Plan:
See added test.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/111440
Approved by: https://github.com/ezyang

c10/core/SymNodeImpl.h

@@ -97,6 +97,9 @@ class C10_API SymNodeImpl : public c10::intrusive_ptr_target {
   virtual SymNode sym_not() {
     TORCH_CHECK(false, "NYI");
   };
+  virtual SymNode sym_ite(const SymNode& then_val, const SymNode& else_val) {
+    TORCH_CHECK(false, "NYI");
+  };
   // NB: self is ignored here, only the arguments are used
   virtual SymNode is_contiguous(
       ArrayRef<SymNode> sizes,

docs/source/conf.py

@@ -967,6 +967,7 @@ coverage_ignore_functions = [
     "parallel_and",
     "parallel_or",
     "sym_sqrt",
+    "sym_ite",
     "sympy_is_channels_last_contiguous_2d",
     "sympy_is_channels_last_contiguous_3d",
     "sympy_is_channels_last_strides_2d",

docs/source/torch.rst

@@ -706,6 +706,7 @@ Symbolic Numbers
     sym_max
     sym_min
     sym_not
+    sym_ite
 
 Export Path
 -------------

test/test_dynamic_shapes.py

@@ -423,6 +423,50 @@ class TestPySymInt(TestCase):
         self.assertIsInstance(r, torch.SymInt, msg=type(r))
         self.assertExpectedInline(str(shape_env.guards[1][0]), """Eq(3*s0, 15)""")
 
+    def test_sym_ite(self):
+        shape_env = ShapeEnv()
+        t = create_symint(shape_env, 5)
+        f = create_symint(shape_env, 4)
+        b1 = True
+        r1 = torch.sym_ite(b1, t, f)
+        self.assertTrue(r1 is t)
+        b2 = False
+        r2 = torch.sym_ite(b2, t, f)
+        self.assertTrue(r2 is f)
+        b3 = t == 5
+        r3 = torch.sym_ite(b3, t, f)
+        self.assertEqual(len(shape_env.guards), 0)
+        self.assertEqual(r3, 5)
+        self.assertEqual(type(t), type(r3))
+        self.assertExpectedInline(str(shape_env.guards[0][0]), """Eq(Piecewise((s0, Eq(s0, 5)), (s1, True)), 5)""")
+        b4 = f == 5
+        r4 = torch.sym_ite(b4, t, f)
+        self.assertEqual(len(shape_env.guards), 1)
+        self.assertEqual(r4, 4)
+        self.assertEqual(type(f), type(r4))
+        self.assertExpectedInline(str(shape_env.guards[1][0]), """Eq(Piecewise((s0, Eq(s1, 5)), (s1, True)), 4)""")
+
+    def test_tracing_sym_ite(self):
+        def f(x):
+            b = x.shape[0] == 5
+            ret = torch.sym_ite(b, x.shape[0], x.shape[1])
+            return ret
+
+        gm = make_fx(f, tracing_mode="symbolic")(torch.ones(4, 5))
+        self.assertEqual(len(gm.shape_env.guards), 0)
+        self.assertExpectedInline(gm.code.strip(), """\
+def forward(self, x_1):
+    sym_size = torch.ops.aten.sym_size(x_1, 0)
+    eq = sym_size == 5
+    sym_size_1 = torch.ops.aten.sym_size(x_1, 1);  x_1 = None
+    sym_ite = torch.sym_ite(eq, sym_size, sym_size_1);  eq = sym_size = sym_size_1 = None
+    return sym_ite""")
+        r1 = gm(torch.ones(4, 5))
+        self.assertIsInstance(r1, int)
+        self.assertEqual(r1, 5)
+        r2 = gm(torch.ones(5, 4))
+        self.assertIsInstance(r2, int)
+        self.assertEqual(r2, 5)
 
     def test_int_conversion(self):
         shape_env = ShapeEnv()
@@ -684,7 +728,8 @@ class TestSymNumberMagicMethods(TestCase):
 
     @parametrize("fn", list(symbolic_shapes.magic_methods.keys()))
     def test_bool_method(self, fn):
-        if fn not in symbolic_shapes.bool_magic_methods:
+        # sym_ite has its own tests
+        if fn not in symbolic_shapes.bool_magic_methods or fn == "sym_ite":
             self.skipTest(f"{fn} is non-bool")
 
         is_unary_fn = fn in symbolic_shapes.unary_magic_methods

torch/__init__.py

@@ -55,7 +55,7 @@ __all__ = [
     'set_float32_matmul_precision', 'get_float32_matmul_precision',
     'set_warn_always', 'is_warn_always_enabled', 'SymInt', 'SymFloat',
     'SymBool', 'sym_not', 'unravel_index',
-    'sym_int', 'sym_float', 'sym_max', 'sym_min', 'compile', 'vmap',
+    'sym_int', 'sym_float', 'sym_max', 'sym_min', 'sym_ite', 'compile', 'vmap',
     'export', 'autocast', 'cond',
 ]
 
@@ -390,6 +390,9 @@ class SymBool:
     def __sym_not__(self) -> "SymBool":
         raise AssertionError("type stub not overridden")
 
+    def __sym_ite__(self, then_val, else_val):
+        raise AssertionError("type stub not overridden")
+
     def __eq__(self, other) -> builtins.bool:
         raise AssertionError("type stub not overridden")
 
@@ -456,6 +459,12 @@ def sym_min(a, b):
         return b.__sym_min__(a)
     return builtins.min(a, b)  # type: ignore[operator]
 
+def sym_ite(b, t, f):
+    assert isinstance(b, (SymBool, builtins.bool)) and type(t) == type(f)
+    if isinstance(b, SymBool):
+        return b.__sym_ite__(t, f)
+    return t if b else f
+
 # Check to see if we can load C extensions, and if not provide some guidance
 # on what the problem might be.
 try:

torch/csrc/utils/python_symnode.h

@@ -145,6 +145,19 @@ class PythonSymNodeImpl : public c10::SymNodeImpl {
     return getPyObj().attr("str")().cast<std::string>();
   }
 
+  c10::SymNode dispatch_sym_ite_(
+      const char* fname,
+      const c10::SymNode& other,
+      const c10::SymNode& third) {
+    auto pother = dynamic_cast<PythonSymNodeImpl*>(other.get());
+    auto pthird = dynamic_cast<PythonSymNodeImpl*>(third.get());
+    TORCH_CHECK(pother);
+    TORCH_CHECK(pthird);
+    py::gil_scoped_acquire acquire;
+    auto r = getPyObj().attr(fname)(pother->getPyObj(), pthird->getPyObj());
+    return c10::make_intrusive<PythonSymNodeImpl>(r);
+  }
+
   c10::SymNode dispatch_common_(const char* fname, const c10::SymNode& other) {
     auto pother = dynamic_cast<PythonSymNodeImpl*>(other.get());
     TORCH_CHECK(pother);
@@ -226,6 +239,11 @@ class PythonSymNodeImpl : public c10::SymNodeImpl {
     return dispatch_common_(__func__, other);
   }
 
+  c10::SymNode sym_ite(const c10::SymNode& other, const c10::SymNode& third)
+      override {
+    return dispatch_sym_ite_(__func__, other, third);
+  }
+
   c10::SymNode sym_not() override {
     return dispatch_common_(__func__);
   }

torch/fx/experimental/symbolic_shapes.py

@@ -35,6 +35,7 @@ from torch import (  # noqa: F401
     sym_max,
     sym_min,
     sym_not,
+    sym_ite,
     SymBool,
     SymFloat,
     SymInt,
@@ -962,6 +963,9 @@ class SymNode:
     def sym_max(self, other) -> "SymNode":  # noqa: F811
         return self._sym_max(other)  # type: ignore[attr-defined]
 
+    def sym_ite(self, then_val, else_val) -> "SymNode":
+        return self._sym_ite(then_val, else_val)
+
     def sym_sqrt(self) -> "SymNode":  # noqa: F811
         return self._sym_sqrt()  # type: ignore[attr-defined]
 
@@ -1181,6 +1185,7 @@ magic_methods = {
     'neg': lambda a: -a,
     'sym_min': lambda a, b: sympy.Min(a, b),
     'sym_max': lambda a, b: sympy.Max(a, b),
+    'sym_ite': lambda a, t, f: sympy.Piecewise((t, a), (f, True)),
     'sym_sqrt': lambda a: sympy.sqrt(a),
     'abs': lambda a: sympy.Abs(a),
 }
@@ -1318,13 +1323,13 @@ unary_magic_methods = {
 
 # Most methods are only registered on SymInt and SymFloat
 # Some methods are only be registered on SymBool
-only_bool_magic_methods = {"and", "or", "sym_not"}
+only_bool_magic_methods = {"and", "or", "sym_not", "sym_ite"}
 # Methods that are also on SymBool, in addition to on SymInt and SymFloat
 also_bool_magic_methods = {"eq"}
 bool_magic_methods = only_bool_magic_methods | also_bool_magic_methods
 
 magic_methods_on_math = {"ceil", "floor"}
-magic_methods_on_submodule = {"sym_float", "sym_sqrt", "sym_min", "sym_max", "sym_not"}
+magic_methods_on_submodule = {"sym_float", "sym_sqrt", "sym_min", "sym_max", "sym_not", "sym_ite"}
 magic_methods_on_operator_with_trailing_underscore = {"and", "or"}
 
 def method_to_operator(method):
@@ -1463,6 +1468,36 @@ def _make_node_magic(method, func):
 
     if method in unary_magic_methods:
         setattr(SymNode, f"_{method_attr}", unary_magic_impl)
+    elif method == "sym_ite":
+
+        def sym_ite_impl(pred_node, then_node, else_node):
+            out_hint = then_node.hint if pred_node.hint else else_node.hint
+            if SYM_FUNCTION_MODE:
+                return to_node(
+                    pred_node,
+                    _handle_sym_dispatch(
+                        sym_ite,
+                        (wrap_node(pred_node), wrap_node(then_node), wrap_node(else_node)), {}
+                    )
+                )
+
+            try:
+                out = func(pred_node.expr, then_node.expr, else_node.expr)
+            except Exception:
+                log.warning("failed to eval %s(%s, %s, %s)", method, pred_node.expr, then_node.expr, else_node.expr)
+                raise
+
+            out = safe_expand(out)
+            fx_node, _ = pred_node.shape_env.create_fx_call_function(
+                sym_ite,
+                (
+                    pred_node.fx_node,
+                    then_node.fx_node,
+                    else_node.fx_node
+                )
+            )
+            return SymNode(out, pred_node.shape_env, then_node.pytype, out_hint, fx_node=fx_node)
+        setattr(SymNode, f"_{method_attr}", sym_ite_impl)
     else:
         setattr(SymNode, f"_{method_attr}", binary_magic_impl)
 
@@ -1602,6 +1637,19 @@ def _make_user_magic(method, user_type):
 
     if method in unary_magic_methods:
         setattr(user_type, f"__{method}__", unary_magic_impl)
+    elif method == "sym_ite":
+
+        def sym_ite_magic_impl(pred, then_val, else_val):
+            pred_node = pred.node
+            then_node = to_node(pred_node, then_val)
+            else_node = to_node(pred_node, else_val)
+            if then_node is NotImplemented or else_node is NotImplemented:
+                return NotImplemented
+            assert isinstance(then_node, SymNode) and isinstance(else_node, SymNode) and then_node.pytype == else_node.pytype
+            ret = wrap_node(getattr(pred.node, method_attr)(then_node, else_node))
+            return get_constant(ret) if ret.node.is_constant() else ret
+
+        setattr(user_type, f"__{method}__", sym_ite_magic_impl)
     else:
         setattr(user_type, f"__{method}__", binary_magic_impl)
         if method in reflectable_magic_methods:

torch/fx/experimental/validator.py

@@ -276,6 +276,7 @@ try:
             torch.sym_float: lift(ops.to_real),
             torch.sym_max: lift(ops.max),
             torch.sym_min: lift(ops.min),
+            torch.sym_ite: lift(lambda b, t, f: t if b else f),
             sym_sqrt: lift(ops.sqrt),
             # Not lifted because we only use this function as a
             # marker for adding the expression as validator input.

torch/overrides.py

@@ -221,6 +221,7 @@ def get_ignored_functions() -> Set[Callable]:
         torch.sym_max,
         torch.sym_min,
         torch.sym_not,
+        torch.sym_ite,
         torch.sym_constrain_range,
         torch.sym_constrain_range_for_size,
         torch.tril_indices,