[llvm-dev] RFC: Refactor SubclassData

[Reid Kleckner via llvm-dev]

First, please test your technique with MSVC on godbolt. If you change the
type (bool a : 1; int b : 2;), it will not pack them into the same bitfield.

Second, do we really need macros to do this? See the techniques used in
clang::VarDecl:
https://github.com/llvm/llvm-project/blob/f20fc65887e2085332d111ee0b05736794423c72/clang/include/clang/AST/Decl.h#L878
As written with a union between all the bitfields, the base class ends up
knowing everything about the derived classes, which is not ideal. But, you
could imagine a version of this that uses aligned char storage so that
subclasses could reinterpret_cast the subclass data memory and use it for
their version of subclass data.

On Mon, Dec 23, 2019 at 11:16 PM Ehud Katz via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello devs,
>
> Recently I've been working on a bug that can probably be fixed with the
> addition of a simple flag to a class descendant of `llvm::Value`.
> Without increasing the size of the class, I can just add this flag to
> `llvm::Value::SubclassData`. But this is not an easy task!
>
> This is because the offsetes/sizes of the data stored in the
> `SubclassData`, are hardcoded literals/enums.
> If you change but a single bit in one of those offsets/sizes in a base
> class, then all the classes derived, will have to be adjusted accordingly -
> which can be very tedious and error-prone.
>
> I offer a small set of macros which will take care of the whole
> Subclass-Data.
> It will work as follows:
>
> struct A {
>   int SubclassData : 14;
>
>   DEFINE_SUBCLASS_DATA()
> };
>
> struct B : A {
>   BEGIN_SUBCLASS_DATA()
>     ADD_SUBCLASS_BITFIELD(int,   5, B1) // A::SubclassData bits [0,5)
> ADD_SUBCLASS_BITFIELD(bool,  1, B2) // A::SubclassData bits [5,6)
> ADD_SUBCLASS_BITFIELD(short, 6, B3) // A::SubclassData bits [6,12)
>  // ADD_SUBCLASS_BITFIELD(int,   6, B4) // A::SubclassData bits [12,18) -
> triggers a static_assert, as it exceeds the 14 bits in A::SubclassData
>     END_SUBCLASS_DATA()
> };
>
> struct C : B {
>   BEGIN_SUBCLASS_DATA()
>     ADD_SUBCLASS_BITFIELD(bool, 1, C1) // A::SubclassData bits [12,13)
>     END_SUBCLASS_DATA()
> };
>
>
> I would appreciate your thoughts on the matter, before I submit a patch
> for review.
>
> Cheers,
> Ehud.
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