[cfe-dev] CGRecordLayoutBuilder for MS ABI
Дмитрий Соколов
r4start at gmail.com
Fri Oct 28 03:18:17 PDT 2011
2011/10/28 John McCall <rjmccall at apple.com>
> On Oct 28, 2011, at 1:13 AM, r4start wrote:
> > On 27/10/2011 23:31, John McCall wrote:
> >> + } else if (
> >> + Types.getContext().getTargetInfo().getCXXABI() == CXXABI_Microsoft)
> {
> >> + // In MS mode this code helps insert padding bytes before fields,
> >> + // like MSVC does.
> >> + CharUnits padding = fieldOffset - NextFieldOffset;
> >> +
> >> + AppendBytes(padding);
> >>
> >> This still shouldn't be necessary. This routine just adds padding bytes
> >> to get us up to fieldOffset, which is determined by the ASTRecordLayout.
> >> If the alignment of the next IR type is high enough, we don't need
> explicit
> >> padding bytes, which is what the 'if' condition above this is testing.
> This
> >> isn't any different in the MS ABI.
> >>
> >> That said, it does look like there's a bug here, because we *always*
> need
> >> to add padding bytes if we're building a packed layout, and that can
> >> happen in MSVC; see above. So I think the condition should be:
> >>
> >> if (alignedNextFieldOffset< fieldOffset ||
> >> (Packed&& fieldOffset != NextFieldOffset)) {
> > This condition doesn't work for class I from test.
> > We have such layout
> > struct I
> > 0 | (I vftable pointer)
> > 8 | (I vbtable pointer)
> > 16 | double q
> > 24 | class D (virtual base)
> > 24 | (D vftable pointer)
> > 32 | double a
> > and layout must be %struct.I = type { i32 (...)**, i32*, [4 x i8],
> double, %class.D }
> > But we will have %struct.I = type { i32 (...)**, i32*, double, %class.D }
> and Packed = false.
>
> Neither of these types is correct; both incorrectly place the
> vbtbl at offset 4. It's just that the first type happens to create
> some extra padding in the wrong place, and that makes the
> total struct size right.
>
> The type should be this:
> type { i32 (...)**, [4 x i8], i32*, double, %class.D }
>
> If you place the vbtbl at the right offset, you should find that
> everything else falls out correctly.
>
> As you can see in my patch I place vbptr on offset from RecordLayout.
So I expect that LLVM add padding between them.
If we want show layout like MSVC then type must be
type { i32 (...)**, [4 x i8], i32*, [4 x i8], double, %class.D } .
But MSVC never shows padding between vfptr and vbptr.
- Dmitry.
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