[PATCH] D39347: Fix __has_unique_object_representations based on rsmith's input

[Erich Keane via Phabricator via cfe-commits]

erichkeane added inline comments.


================
Comment at: lib/AST/ASTContext.cpp:2213
+        Field->isBitField()
+            ? Field->getBitWidthValue(Context)
+            : Context.toBits(Context.getTypeSizeInChars(Field->getType()));
----------------
erichkeane wrote:
> rsmith wrote:
> > This isn't quite right; you want min(bitfield length, bit size of type) here. Eg, a `bool b : 8` or `int n : 1000` bitfield has padding bits.
> I guess I'm missing something with this comment... why would a bitfield with bool b: 8 have padding?
> 
> Additionally, isn't int n : 1000 illegal, since 1000 > 32 (# bits in an int)?
> Both clang and GCC reject the 2nd case.  Curiously, GCC rejects bool b: 9, but Clang seems to allow 'bool' to have ANY size.  Is that an error itself?
> 
> Finally: Should we consider ANY bool to be not a unique object representation?  It has 1 bit of data, but 8 bits of storage.  GCC's implementation of this trait accepts them, but I'm second guessing that at the moment...
I did a bit more looking into this... In the 'TYPE b: X' case, 'X' is always the size of the field, so :

    struct Bitfield {
      bool b: 16;
    };
    static_assert(sizeof(Bitfield) == 2);

The rest of my padding detection works correctly.


https://reviews.llvm.org/D39347