[LLVMdev] Offset to C++ structure members

[Eli Friedman]

On Tue, Oct 2, 2012 at 3:28 PM, Paul J. Lucas <paul at lucasmail.org> wrote:
> On Oct 2, 2012, at 2:34 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>
>> On Tue, Oct 2, 2012 at 11:33 AM, Paul J. Lucas <paul at lucasmail.org> wrote:
>>
>>> My understanding is that, in order to use GEP, you have to provide the LLVM code with the struct layout, i.e., build a StructType object.  In my case, that struct is declared in C++ code already and, in order to use GEP, I'd have to replicate the struct layout (exactly as the C++ compiler would) in LLVM code -- something that I'd rather not do, not to mention that it's fairly "brittle" even if I could manage to get it right.  (Simple structs would probably be easy, but struct that have virtual functions or multiple base classes would be much harder.)
>>
>> No, you don't have to... you can just use GEP on i8*'s.  The LLVM type
>> system doesn't have any semantic significance.
>
> Oh!  I just tried it and it works.  :-)
>
>>>> I'm not entirely sure how you're using mbr_offset_of
>>>
>>> Given 't', an instance of some class T, and some member T::m, find the integer offset in bytes from &t to &t.m.  This offset, when added to &t, should be &t.m.
>>>
>>> I'm using mbr_offset_of to get the C++ compiler to do the work of telling me what the correct offset is for the already existing struct.
>>
>> If you can do that, why not just generate a thunk to perform the addressing?
>
> Because if I can create a thunk to do that, I can just as easily create a thunk to provide a "setter" for the struct member (something I'd prefer not to do).
>
> I'm trying to compute the offset "inline" in the LLVM code rather than (a) have to create yet another C thunk and (b) call it.
>
>>>> but it's broken if there are any classes with virtual bases involved.
>>>
>>> Really?  This simple code works just fine:
>>>
>>>        struct A             { int ai; };
>>>        struct X : virtual A { int xi; };
>>>        struct Y : virtual A { int yi; };
>>>
>>>        struct S : X, Y {
>>>          string a;
>>>          string b;
>>>        };
>>>
>>>        template<class ClassType,class MbrType> inline
>>>        ptrdiff_t mbr_offset_of( MbrType ClassType::*p ) {
>>>          ClassType const *const c = static_cast<ClassType*>( nullptr );
>>>          return reinterpret_cast<ptrdiff_t>( &(c->*p) );
>>>        }
>>>
>>>        int main() {
>>>          ptrdiff_t offset = mbr_offset_of( &S::b );
>>>          S s;
>>>          string *p = (string*)((char*)&s + offset);
>>>          p->assign( "Hello, world!" );
>>>          cout << *p << endl;
>>>         return 0;
>>>        }
>>
>> It starts to become an issue when you try to compute the offset to
>> e.g. A::ai in your example.
>
> Hmmmm.... I just changed:
>
>         s/int ai/string as/
>         s/&S::b/&S::as/
>
> and the code still works.  The offset is 0 which is what you'd expect with class 'A' being a public virtual (shared) base class.

The offset of as is 0 given an A*, but not given an S*.  If you handle
that already, it's okay, I guess.

-Eli