[clang] [clang][CodeGen] Add TBAA for bit-field accesses (PR #226806)

[Bruno De Fraine via cfe-commits]

brunodf-gf wrote:

> My idea is to use omnipotent char for the whole bit-field storage unit, but keep the struct path, offset and size. Fields in the same storage unit would still alias, while separate units could be distinguished by the struct path. Does this make sense, or is there some language rule I am missing here?

OK. So where something like:

```
struct S {
  int a;
  int b;
};
```

Gives a hierarchy like:

```mermaid
flowchart LR
  int --- char
  SA[S::a] --- int
  SB[S::b] --- int
```

You propose that something like:

```
struct B {
   // offset 0
   int a : 2;
   int b : 2;
   char : 0;
   // offset 1
   int c : 2;
   // offset 4
   int d;
};
```

Gets a hierarchy:

```mermaid
flowchart LR
  int --- char
  B0["B+0"] --- char
  B1["B+1"] --- char
  BD["B::d"] --- int
```

Where tag `B+0` is used for accessing the storage of `B::a` and `B::b`, and tag `B+1` is used for accessing the storage of `B::c`. Do I understand that correctly?

The implication would be that TBAA considers a bit-field access `B+0` disjunct from any other access except direct `char` access (but including other bit-field access such as `B+1`). I think that should be OK in the language (bit-fields are not even addressable objects). Tagging @rjmccall for an expert opinion.

https://github.com/llvm/llvm-project/pull/226806