[PATCH] D21489: [codeview] Added support for bitfield type

[David Majnemer via llvm-commits]

majnemer added a comment.

In http://reviews.llvm.org/D21489#467653, @aaboud wrote:

> In http://reviews.llvm.org/D21489#467609, @majnemer wrote:
>
> > Consider this input:
> >
> >   #pragma pack(1)
> >   struct S {
> >     char    : 8;
> >     short   : 8;
> >     short x : 8;
> >   } s;
> >
> >
> > Your patch appears to give:
> >
> >   BitField (0x1001) {
> >     TypeLeafKind: LF_BITFIELD (0x1205)
> >     Type: short (0x11)
> >     BitSize: 8
> >     BitOffset: 0
> >   }
> >   FieldList (0x1002) {
> >     TypeLeafKind: LF_FIELDLIST (0x1203)
> >     DataMember {
> >       AccessSpecifier: Public (0x3)
> >       Type: 0x1001
> >       FieldOffset: 0x2
> >       Name: x
> >     }
> >   }
> >   
> >
> > MSVC gives:
> >
> >   BitField (0x1001) {
> >     TypeLeafKind: LF_BITFIELD (0x1205)
> >     Type: short (0x11)
> >     BitSize: 8
> >     BitOffset: 8
> >   }
> >   FieldList (0x1002) {
> >     TypeLeafKind: LF_FIELDLIST (0x1203)
> >     DataMember {
> >       AccessSpecifier: Public (0x3)
> >       Type: 0x1001
> >       FieldOffset: 0x1
> >       Name: x
> >     }
> >   }
>
>
> You are right, this case is not handled correctly yet.
>  However, this is because Clang (FE) does not provide the information, this is how LLVM IR looks like:
>
>   !5 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "S", file: !1, line: 2, size: 24, align: 8, elements: !6, identifier: ".?AUS@@")
>   !6 = !{!7}
>   !7 = !DIDerivedType(tag: DW_TAG_member, name: "x", scope: !5, file: !1, line: 5, baseType: !8, size: 8, align: 16, offset: 16)
>   !8 = !DIBasicType(name: "short", size: 16, align: 16, encoding: DW_ATE_signed)
>
>
> and this how it should look like to fix the problem, once we make Clang emit the following the above code will work:
>
>   !5 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "S", file: !1, line: 2, size: 24, align: 8, elements: !6, identifier: ".?AUS@@")
>   !6 = !{!7, !9, 11}
>   !7 = !DIDerivedType(tag: DW_TAG_member, scope: !5, file: !1, line: 3, baseType: !8, size: 8, align: 8, offset: 0)
>   !8 = !DIBasicType(name: "char", size: 8, align: 8, encoding: DW_ATE_signed)
>   !9 = !DIDerivedType(tag: DW_TAG_member, scope: !5, file: !1, line: 4, baseType: !10, size: 8, align: 16, offset: 8)
>   !10 = !DIBasicType(name: "short", size: 16, align: 16, encoding: DW_ATE_signed)
>   !11 = !DIDerivedType(tag: DW_TAG_member, name: "x", scope: !5, file: !1, line: 5, baseType: !11, size: 8, align: 16, offset: 16)
>
>
> Then all we need to do, is not to emit members with no name, but to consider the size and alignment they consume.


I don't think this is a good idea.  It would be far cheaper to encode the offset of the underlying allocation in the extraData.


http://reviews.llvm.org/D21489