[libcxx-dev] Contributions to libc++abi for different ABIs

Petr Hosek via libcxx-dev libcxx-dev at lists.llvm.org
Fri Jan 24 12:02:17 PST 2020

On Fri, Jan 24, 2020 at 11:42 AM Leonard Chan <leonardchan at google.com>

> Hi all,
> We're attempting to implement a PC-relative vtable ABI for C++ in Fuchsia
> <https://reviews.llvm.org/D72959> and I wanted to ask about the etiquette
> for contributions to libc++abi for different ABIs.
> The purpose of this ABI is to replace the 64 bit absolute address of
> virtual functions in a vtable with 32 bit relative offsets between the
> vtable and the virtual function. This would effectively make the vtable
> readonly and help save on memory by reducing copy on write pages. The same
> technique is applied to the typeinfo pointer in the vtable, replacing it
> with an offset to the typeinfo object.
> We ran into an issue though where changing the size and value of this
> typeinfo component breaks dynamic_cast. More specifically, in this snippet
> from __dynamic_cast in libc++abi:
> ```
>     // Get (dynamic_ptr, dynamic_type) from static_ptr
>     void **vtable = *static_cast<void ** const *>(static_ptr);
>     ptrdiff_t offset_to_derived = reinterpret_cast<ptrdiff_t>(vtable[-2]);
>     const void* dynamic_ptr = static_cast<const char*>(static_ptr) +
> offset_to_derived;
>     const __class_type_info* dynamic_type = static_cast<const
> __class_type_info*>(vtable[-1]);
> ```
> there is an assumption that the offset_to_derived is 16 bytes behind the
> first function in the vtable, where for us it would take 12 bytes (8 for
> the offset to top + 4 for the relative rtti component). The same thing
> applies to __class_type_info which instead should be 4 bytes behind the
> vtable pointer and would require an additional dereference and add since
> it's an offset for us.
> Off the top of my head, it seems like a simple solution to this is to
> create another __dynamic_cast function (like
> __dynamic_cast_fuchsia_relative or something) that would calculate these
> variables correctly for our ABI.

Could this be a build configuration (e.g. CMake option that changes the
behavior or __dynamic_cast) that we would set when compiling libc++abi for
Fuchsia? I don't think we want to change the existing code to use a
different function when compiling for Fuchsia, rather we want the new ABI
to be the default on Fuchsia and be completely transparent to the code and

> So, finally to my question, is the proper etiquette for making ABI changes
> this way to contribute them to libc++abi directly, or is there another
> recommended way? Or is there a better way to address this problem that I
> haven't thought of yet? So far, this seems to be the only RTTI breakage I
> could find, but I'm still actively looking.
> Thanks,
> Leonard
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