[clang] [clang] Implement __builtin_stdc_rotate_left, __builtin_stdc_rotate_right (PR #160259)

[Erich Keane via cfe-commits]

================
@@ -3680,6 +3680,41 @@ the arguments. Both arguments and the result have the bitwidth specified
 by the name of the builtin. These builtins can be used within constant
 expressions.
 
+``__builtin_stdc_rotate_left`` and ``__builtin_stdc_rotate_right``
+------------------------------------------------------------------
+
+**Syntax**:
+
+.. code-block:: c
+
+    T __builtin_stdc_rotate_left(T value, count)
+    T __builtin_stdc_rotate_right(T value, count)
+
+where ``T`` is any unsigned integer type and ``count`` is any integer type.
+
+**Description**:
+
+These builtins rotate the bits in ``value`` by ``count`` positions. The
+``__builtin_stdc_rotate_left`` builtin rotates bits to the left, while
+``__builtin_stdc_rotate_right`` rotates bits to the right. The first
+argument (``value``) must be an unsigned integer type, including ``_BitInt`` types.
+The second argument (``count``) can be any integer type. The rotation count is
+normalized modulo the bit-width of the value being rotated, with negative
+counts converted to equivalent positive rotations (e.g., rotating left
+by ``-1`` is equivalent to rotating left by ``BitWidth-1``). These builtins can
+be used within constant expressions.
+
+**Example of use**:
+
+.. code-block:: c
+
+  unsigned char rotated_left = __builtin_stdc_rotate_left((unsigned char)0xB1, 3);
+  unsigned int rotated_right = __builtin_stdc_rotate_right(0x12345678U, 8);
+
+  unsigned char neg_rotate = __builtin_stdc_rotate_left((unsigned char)0xB1, -1);
+
+  unsigned _BitInt(17) rotated_odd = __builtin_stdc_rotate_left(0x1ABCD, 5);
----------------
erichkeane wrote:

See this comment, I'm concerned about this example still.  can you explain why this conversion for the return is possible when the RHS is going to result in an unsigned int?

https://github.com/llvm/llvm-project/pull/160259