Fix strict aliasing rule violation in bitwise_binary_op (pytorch#66194)

Summary:
Fixes pytorch#66119

Failure on ARM Neoverse N1 before this PR:
```
======================================================================
FAIL: test_bitwise_ops_cpu_int16 (__main__.TestBinaryUfuncsCPU)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/opt/pytorch/pytorch/torch/testing/_internal/common_device_type.py", line 373, in instantiated_test
    result = test(self, **param_kwargs)
  File "test_binary_ufuncs.py", line 315, in test_bitwise_ops
    self.assertEqual(op(a, b), op(a_np, b_np))
  File "/opt/pytorch/pytorch/torch/testing/_internal/common_utils.py", line 1633, in assertEqual
    self.assertEqual(
  File "/opt/pytorch/pytorch/torch/testing/_internal/common_utils.py", line 1611, in assertEqual
    super().assertTrue(result, msg=self._get_assert_msg(msg, debug_msg=debug_msg))
AssertionError: False is not true : Tensors failed to compare as equal!Found 176 different element(s) (out of 225), with the greatest difference of 21850 (-21846 vs. 4) occuring at index (0, 2).

======================================================================
FAIL: test_bitwise_ops_cpu_int32 (__main__.TestBinaryUfuncsCPU)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/opt/pytorch/pytorch/torch/testing/_internal/common_device_type.py", line 373, in instantiated_test
    result = test(self, **param_kwargs)
  File "test_binary_ufuncs.py", line 315, in test_bitwise_ops
    self.assertEqual(op(a, b), op(a_np, b_np))
  File "/opt/pytorch/pytorch/torch/testing/_internal/common_utils.py", line 1633, in assertEqual
    self.assertEqual(
  File "/opt/pytorch/pytorch/torch/testing/_internal/common_utils.py", line 1611, in assertEqual
    super().assertTrue(result, msg=self._get_assert_msg(msg, debug_msg=debug_msg))
AssertionError: False is not true : Tensors failed to compare as equal!Found 188 different element(s) (out of 225), with the greatest difference of 1335341061 (-1335341056 vs. 5) occuring at index (14, 8).

----------------------------------------------------------------------
```
which passes now.

CC malfet ezyang

Pull Request resolved: pytorch#66194

Reviewed By: dagitses, bdhirsh, ngimel

Differential Revision: D31430274

Pulled By: malfet

fbshipit-source-id: bcf1c9d584c02eff328dd5b1f7af064fac5942c9

aten/src/ATen/cpu/vec/vec_base.h

@@ -147,6 +147,10 @@ struct Vectorized {
   inline operator T*() {
     return values;
   }
+  // Return the values as char* for type punning
+  auto as_bytes() const -> const char* {
+    return reinterpret_cast<const char*>(values);
+  }
   template <int64_t mask_>
   static Vectorized<T> blend(const Vectorized<T>& a, const Vectorized<T>& b) {
     int64_t mask = mask_;
@@ -736,15 +740,33 @@ inline Vectorized<T> operator^(const Vectorized<T>& a, const Vectorized<T>& b) {
 
 #else
 
+template <typename T>
+auto load(char const* data) -> T {
+  T ret;
+  std::memcpy(&ret, data, sizeof(ret));
+  return ret;
+}
+
 template<class T, typename Op>
 static inline Vectorized<T> bitwise_binary_op(const Vectorized<T> &a, const Vectorized<T> &b, Op op) {
   static constexpr uint32_t element_no = VECTOR_WIDTH / sizeof(intmax_t);
   __at_align__ intmax_t buffer[element_no];
-  const intmax_t *a_ptr = reinterpret_cast<const intmax_t*>((const T*) a);
-  const intmax_t *b_ptr = reinterpret_cast<const intmax_t*>((const T*) b);
-  for (uint32_t i = 0U; i < element_no; ++ i) {
-    buffer[i] = op(a_ptr[i], b_ptr[i]);
-  }
+  static_assert(VECTOR_WIDTH % sizeof(intmax_t) == 0, "VECTOR_WIDTH not a multiple of sizeof(intmax_t)");
+  static_assert(sizeof(buffer) == sizeof(Vectorized<T>), "sizeof(buffer) must match sizeof(Vectorized<T>)");
+  // We should be using memcpy in order to respect the strict aliasing rule
+  // see: https://github.com/pytorch/pytorch/issues/66119
+  // Using char* is defined in the C11 standard 6.5 Expression paragraph 7
+  // (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf)
+  const auto* a_data = a.as_bytes();
+  const auto* b_data = b.as_bytes();
+  // load each intmax_t chunk and process; increase pointers by sizeof(intmax_t)
+  for (auto& out : buffer) {
+    out = op(load<intmax_t>(a_data), load<intmax_t>(b_data));
+    a_data += sizeof(intmax_t);
+    b_data += sizeof(intmax_t);
+  }
+  assert(a_data == a.as_bytes() + sizeof(a));
+  assert(b_data == b.as_bytes() + sizeof(b));
   return Vectorized<T>::loadu(buffer);
 }