rnk added a subscriber: majnemer.
rnk added a comment.
In https://reviews.llvm.org/D45174#1054820, @rsmith wrote:
> +rnk This might also affect the MS ABI, but it does not result in any test
> case failures at least (and MSVC's type trait matches our state after this
> patch rather than before).
I've convinced myself that this is NFC for MS record layout because this is the
only place we use RD->isEmpty() that matters:
if (!FoundBase) {
if (MDCUsesEBO && BaseDecl->isEmpty() &&
BaseLayout.getNonVirtualSize() == CharUnits::Zero()) {
BaseOffset = CharUnits::Zero();
} else {
// Otherwise, lay the base out at the end of the MDC.
BaseOffset = Size = Size.alignTo(Info.Alignment);
}
}
In particular, that `getNonVirtualSize()` check returns false when unnamed
bitfields get involved. Here's a layout:
struct __declspec(empty_bases) A { char : 3; };
struct __declspec(empty_bases) B { char : 3; };
struct UseEmpty : A, B {
char space;
} o[1];
static_assert(sizeof(o) == 3, "incorrect layout");
*** Dumping AST Record Layout
0 | struct A (empty)
0:0-2 | char
| [sizeof=1, align=1,
| nvsize=1, nvalign=1]
*** Dumping AST Record Layout
0 | struct B (empty)
0:0-2 | char
| [sizeof=1, align=1,
| nvsize=1, nvalign=1]
*** Dumping AST Record Layout
0 | struct UseEmpty
0 | struct A (base) (empty)
0:0-2 | char
1 | struct B (base) (empty)
1:0-2 | char
2 | char space
| [sizeof=3, align=1,
| nvsize=3, nvalign=1]
So even though these classes went from "empty" to "non-empty", we previously
allocated space for them. I think @majnemer spent a while on that.
Repository:
rC Clang
https://reviews.llvm.org/D45174
_______________________________________________
cfe-commits mailing list
cfe-commits@lists.llvm.org
http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
