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

[Andy Kaylor via cfe-commits]

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@@ -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
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andykaylor wrote:

This comment appears to be stale. There's only one array generated, and the bitfield members are represented by two !u8i variables. This is what the incubator generates, but classic codegen generates an i16 for the bitfield members.

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