adams381 wrote:
I've been struggling with this representation. The field description is the
right place to look, and `#cir.bitfield_info` is the natural home for all of
it. However, that cannot be reached from here. It is an operand of
`cir.get_bitfield` and `cir.set_bitfield`, so it exists only where a field is
accessed. CallConvLowering classifies a signature and sees the record type
alone, and the type carries no equivalent: a member is marked `bitfield` and
has a storage type, but no name, width, offset or signedness. `bitfield_info`
itself already carries `storage_type`, `size` and `offset`, so the declared
type really is the only piece missing from it.
The equivalent that is reachable is to put it on the member description in the
record type. `StructType` and `UnionType` already carry a `member_kinds` array
parallel to the member types. We could add a third array that carries the
extent per member, printed on the kind:
```mlir
!rec_S = !cir.struct<"S" {bitfield<11> !u64i, bitfield<4> !u16i, data
!u32i}>
```
against today's
```mlir
!rec_S = !cir.struct<"S" {bitfield !u64i, bitfield
!cir.array<!cir.array<!u8i x 3> x 0>, bitfield !u16i, bitfield
!cir.array<!cir.array<!u8i x 2> x 0>, data !u32i}>
```
`bitfield<11> !u64i` reads as an 8-byte access unit whose bit-fields were
declared with types extending to 11 bytes. Absent means nothing outruns the
unit, so records without such a unit print exactly as they do now.
This fixes the problem that a zero-length member is still a member, so it
reaches the LLVM struct type and shifts member indices:
```mlir
%struct.S = type { i64, [0 x i8], i16, [0 x i8], i32 } // -fclangir
%struct.S = type { i64, i16, i32 } // classic
```
That divergence is why this patch has to split CHECK prefixes in
`bitfields.cpp` and move `cir.get_member` indices in `paren-list-agg-init.cpp`.
An extent on the member description adds no member, so the lowered struct
matches classic again and most of the test churn here disappears.
I can implement the extent parameter as a prereq for this PR and then rebase
when that lands.
https://github.com/llvm/llvm-project/pull/220069
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