- **Explanation**: Fixes a crash with key paths on 32-bit platforms reproducible for types that have 16-byte alignment.
The intended bit layout of `ComputedArgumentSize` in `KeyPath` on 32-bit is:
```
┌───────┬───────────┐
│ bits │ field │
├───────┼───────────┤
│ 0–27 │ size │
├───────┼───────────┤
│ 28–29 │ padding │
├───────┼───────────┤
│ 30–31 │ alignment │
└───────┴───────────┘
```
Currently, `alignmentMask = 0x6000_0000`, i.e. bits 29–30, not 30–31. It overlaps paddingMask (bits 28–29) at bit 29, meaning that alignment and padding unintentionally share a bit. With `alignmentShift = 30`, storing `shift = 2 << 30` places 1 at bit 31, which the mask doesn't cover.
Correct value is `0xC000_0000` covers bits 30–31, which matches `alignmentShift = 30` so both `shift = 1` and `shift = 2` round-trip, and it does not overlap with `paddingMask = 0x3000_0000` (bits 28–29). It also mirrors the 64-bit layout (top bits of the word reserved for alignment, just 2 of them instead of 1).
- **Scope**: Limited to 32-bit platforms.
- **Issues**: rdar://175799967
- **Risk**: Low due to increased test coverage.
- **Testing**: Previously crashing on 32-bit platforms sample code is now added to the test suite.
Max Desiatov