[llvm] [IR] Add initial support for the byte type (PR #178666)

[Nikita Popov via llvm-commits]

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@@ -13010,15 +13050,36 @@ is always a *no-op cast* because no bits change with this
 conversion. The conversion is done as if the ``value`` had been stored
 to memory and read back as type ``ty2``. Pointer (or vector of
 pointers) types may only be converted to other pointer (or vector of
-pointers) types with the same address space through this instruction.
-To convert pointers to other types, use the :ref:`inttoptr <i_inttoptr>`
-or :ref:`ptrtoint <i_ptrtoint>` instructions first.
+pointers) types with the same address space or byte (or vector of bytes) types
+through this instruction. To convert pointers to other types, use the
+:ref:`inttoptr <i_inttoptr>` or :ref:`ptrtoint <i_ptrtoint>` instructions first.
 
 There is a caveat for bitcasts involving vector types in relation to
 endianness. For example ``bitcast <2 x i8> <value> to i16`` puts element zero
 of the vector in the least significant bits of the i16 for little-endian while
 element zero ends up in the most significant bits for big-endian.
 
+If ``value`` is of the :ref:`byte type <t_byte>`:
+
+* If ``value`` contains at least one ``poison`` bit, the cast result is
+  ``poison``.
----------------
nikic wrote:

This isn't quite right for casts to vector types, where only the lanes with the poison bits would be poison.

https://github.com/llvm/llvm-project/pull/178666