Michael137 wrote:
While fixing the libc++ formatters in preparation for the [compressed_pair
change](https://github.com/llvm/llvm-project/issues/93069), i encountered
another issue which I'm not sure entirely how to best reconcile. There's [this
assumption in
`RecordLayoutBuilder`](https://github.com/llvm/llvm-project/blob/f782ff8fc6426890863be0791a9ace2394da3887/clang/lib/AST/RecordLayoutBuilder.cpp#L2258-L2264)
for external layouts, where, if we encounter overlapping field offsets, we
assume the structure is packed and set the alignment back to `1`:
```
uint64_t
ItaniumRecordLayoutBuilder::updateExternalFieldOffset(const FieldDecl *Field,
uint64_t ComputedOffset) {
uint64_t ExternalFieldOffset = External.getExternalFieldOffset(Field);
if (InferAlignment && ExternalFieldOffset < ComputedOffset) {
// The externally-supplied field offset is before the field offset we
// computed. Assume that the structure is packed.
Alignment = CharUnits::One();
PreferredAlignment = CharUnits::One();
InferAlignment = false;
}
...
```
The assumption in that comment doesn't hold for layouts where the overlap
occurred because of `[[no_unique_address]]`. In those cases, the alignment of
`1` will prevent us from aligning the `FieldOffset` correctly and cause LLDB to
read out the fields incorrectly.
We can't guard this with `Field->isZeroSize()` because we don't have the
attribute in the AST.
Can we infer packedness more accurately here?
Or maybe that's just putting a bandaid on a bigger underlying issue
https://github.com/llvm/llvm-project/pull/93809
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