[clang] [CIR] Add initial support for bitfields in structs (PR #142041)

[via cfe-commits]

================
@@ -14,6 +14,105 @@
 
 namespace clang::CIRGen {
 
+/// Record with information about how a bitfield should be accessed. This is
+/// very similar to what LLVM codegen does, once CIR evolves it's possible we
+/// can use a more higher level representation.
+///
+/// Often we lay out a sequence of bitfields as a contiguous sequence of bits.
+/// When the AST record layout does this, we represent it in CIR as a
+/// `!cir.record` type, which directly reflects the structure's layout,
+/// including bitfield packing and padding, using CIR types such as
+/// `!cir.bool`, `!s8i`, `!u16i`.
+///
+/// To access a particular bitfield in CIR, we use the operations
+/// `cir.get_bitfield` (`GetBitfieldOp`) or `cir.set_bitfield`
+/// (`SetBitfieldOp`). These operations rely on the `bitfield_info`
+/// attribute, which provides detailed metadata required for access,
+/// such as the size and offset of the bitfield, the type and size of
+/// the underlying storage, and whether the value is signed.
+/// The CIRGenRecordLayout also has a bitFields map which encodes which
+/// byte-sequence this bitfield falls within. Let's assume the following C
+/// struct:
+///
+///   struct S {
+///     char a, b, c;
+///     unsigned bits : 3;
+///     unsigned more_bits : 4;
+///     unsigned still_more_bits : 7;
+///   };
+///
+/// This will end up as the following cir.record. The first array is the
+/// bitfield, and the second is the padding out to a 4-byte alignment.
+///
+///   !rec_S = !cir.record<struct "S" padded {!s8i, !s8i, !s8i, !u8i, !u8i,
+///   !cir.array<!u8i x 3>}>
+///
+/// When generating code to access more_bits, we'll generate something
+/// essentially like this:
+///
+///   #bfi_more_bits = #cir.bitfield_info<name = "more_bits", storage_type =
+///   !u16i, size = 4, offset = 3, is_signed = false>
+///
+///   cir.func @store_field() {
+///     %0 = cir.alloca !rec_S, !cir.ptr<!rec_S>, ["s"] {alignment = 4 : i64}
+///     %1 = cir.const #cir.int<2> : !s32i
+///     %2 = cir.cast(integral, %1 : !s32i), !u32i
+///     %3 = cir.get_member %0[4] {name = "more_bits"} : !cir.ptr<!rec_S> ->
+///     !cir.ptr<!u16i>
----------------
Andres-Salamanca wrote:

sorry, when I was writing this, I was running some tests and forgot to revert the struct back to match what the comment was describing. Thanks for catching that

https://github.com/llvm/llvm-project/pull/142041