[CIR][CodeGen] Discrete bitfield layout

[tommymcm]

Minimized test case:

struct {
  signed : 0
} __attribute((__ms_struct__)) A;

Failed assertions:
https://github.com/llvm/clangir/blob/aeac352c9de907fcbb7adde22bbe4a7cfa3105be/clang/lib/CIR/CodeGen/CIRRecordLayoutBuilder.cpp#L517C5-L517C21

Notes:
The Microsoft bitfield layout rule allocates discrete storage units of the field's formal type and only combines adjacent fields of the same formal type. We want to emit a layout with these discrete storage units instead of combining them into a continuous run.

[Andres-Salamanca]

Hi @tommymcm, I previously worked on upstreaming bitfields and added features like volatile bitfields for AAPCS and FineGrainedBitfieldAccess. I’m going to backport those changes and then start working on this new issue.

[tommymcm]

Hi @tommymcm, I previously worked on upstreaming bitfields and added features like volatile bitfields for AAPCS and FineGrainedBitfieldAccess. I’m going to backport those changes and then start working on this new issue.

Sounds good, thanks Andres!

[Andres-Salamanca]

• [CIR] Backport bitfield changes from upstream #1840
• [CIR] Backport volatile bit-fields #1857