rsmith added inline comments.
================
Comment at: lib/AST/ASTContext.cpp:2183-2184
+ for (const auto *Field : RD->fields()) {
+ if (!Context.hasUniqueObjectRepresentations(Field->getType()))
+ return false;
+ int64_t FieldSizeInBits =
----------------
erichkeane wrote:
> rsmith wrote:
> > erichkeane wrote:
> > > rsmith wrote:
> > > > What should happen for fields of reference type?
> > > References are not trivially copyable, so they will prevent the struct
> > > from having a unique object representation.
> > That sounds like the wrong behavior to me. If two structs have references
> > that bind to the same object, then they have the same object
> > representation, so the struct does have unique object representations.
> I didn't think of it that way... I WILL note that GCC rejects references in
> their implementation, but that could be a bug on their part.
The wording you quote below is defective in this regard (it doesn't discuss
what "same value" means for reference members) -- that's at least partly my
wording, sorry about that! But at least my intent when we were working on the
rules was that "same value" would have the obvious structurally-recursive
meaning, which for references means we're considering whether they have the
same referent.
So I think references, like pointers, should always be considered to have
unique object representations when considered as members of objects of class
type. (But `__has_unique_object_representations(T&)` should still return
`false` because `T&` is not a trivially-copyable type, even though a class
containing a `T&` might be.)
================
Comment at: lib/AST/MicrosoftCXXABI.cpp:253
+
+ MPI.HasPadding = MPI.Width % MPI.Align == 0;
----------------
erichkeane wrote:
> rsmith wrote:
> > This seems to be backwards?
> >
> > Also, I'm not sure whether this is correct. In the strange case where
> > `Width` is not a multiple of `Align` (because we don't round up the width),
> > there is no padding. We should only set `HasPadding` to `true` in the
> > `alignTo` case below.
> I think you're right about it being backwards.
>
> However, doesn't the struct with a single Ptr and either 1 or 3 Ints have
> tail padding as well? I do believe you're right about the alignTo situation
> below, but only if Width changes, right?
I think the idea is that a struct with one pointer and an odd number of ints,
on 32-bit Windows, will have no padding per se, but its size will simply not be
a multiple of its alignment. (So struct layout can put things in the four bytes
after it, but will insert padding before it to place it at an 8 byte boundary.)
https://reviews.llvm.org/D39347
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