[llvm-dev] RFC: A new ABI for virtual calls, and a change to the virtual call representation in the IR

[Peter Collingbourne via llvm-dev]

On Fri, Mar 4, 2016 at 2:48 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:

> On Mon, Feb 29, 2016 at 1:53 PM, <> wrote:
>>
>> @A_vtable = {i8*, i8*, i32, i32} {0, @A::rtti, @A::f - (@A_vtable + 16),
>> @A::g - (@A_vtable + 16)}
>>
>
> There's a subtlety about this aspect of the ABI that I should call
> attention to. The virtual function references can only be resolved directly
> by the static linker if they are defined in the same executable/DSO as the
> virtual table. I expect this to be the overwhelmingly common case, as
> classes are normally wholly defined within a single executable or DSO, so
> our implementation should be optimized around that case.
>
> If we expected cross-DSO references to be relatively common, we could make
> vtable entries be relative to GOT entries, but that would introduce an
> additional level of indirection and additional relocations, probably
> costing us more in binary size and memory bandwidth than the current ABI.
>
> However, it is technically possible to split the implementation of a
> class's virtual functions between DSOs, and there are more practical cases
> where we might expect to see cross-DSO references:
>
> - one DSO could derive from a class defined in another DSO, and only
> override some of its virtual functions
> - the vtable could contain a reference to __cxa_pure_virtual which would
> be defined by the standard library
>
> We can handle these cases by having the vtable refer to a PLT entry for
> each function that is not defined within the module. This can be done by
> using a specific type of relative relocation that refers directly to the
> symbol if defined within the current module, or to a PLT entry if not. This
> is the same type of relocation that is needed to implement relative
> branches on x86, so I'd expect it to be generally available on that
> architecture (ELF has R_{386,X86_64}_PLT32, Mach-O has X86_64_RELOC_BRANCH,
> COFF has IMAGE_REL_{AMD64,I386}_REL32, which may resolve to a thunk [1],
> which is essentially the same thing as a PLT entry). It is also present on
> ARM (R_ARM_PREL31, which was apparently added to support unwind tables).
>
> We still need some way to create PLT relocations in the vtable's
> initializer without breaking the semantics of a load from the vtable.
> Rafael and I discussed this and we believe that if the target function is
> unnamed_addr, this indicates that the function's address isn't observable
> (this is true for virtual functions, as it isn't possible to take their
> address), and so it could be substituted with the address of a PLT entry.
>

I've discovered a problem with this idea. Since we are using 32-bit
displacements, the offset from the vtable to the function must fit within
32 bits. This is assumed to be true in the medium code model, so long as
the displacement points to a real function address or a PLT entry. However,
if we combine a vtable load at a virtual call site, the code will evaluate
the function address to the actual address of the function via the GOT, and
that could push the displacement outside of the 32-bit boundary and cause
an error in the evaluation of the function address.

To solve this problem, I reckon that the @llvm.vtable.load.relative
intrinsic I mentioned earlier will be required for correctness, and we
would have to lower it very late, e.g. in the pre-backend passes.

Peter
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