On 11 Oct 2010, at 14:20, Douglas Gregor wrote:
>
>>>>
>>>> To be more clear, when using clang, the specific executable being linked
>>>> fails with linker error because the VTable symbol is not defined in the
>>>> object files being linked.
>>>> With gcc, the VTable is emitted (as in the test case I posted) and the key
>>>> function is resolved dynamically when the executable is run.
>>>
>>> But where does the body of the key function come from? Some other library
>>> that's loaded dynamically?
>>
>> Yes, exactly.
>>
>>>
>>> More importantly, did the key function get instantiated somewhere and we
>>> failed to emit the vtables + RTTI at that point?
>>
>> No, it gets instantiated at a library outside of the executable.
>
>
> That is absolutely disgusting :(
>
> Alright, we're stuck with this approach, then. Thanks for fixing this.
Some more mind-numbing details..
The VTable symbol cannot be resolved dynamically through the library when using
clang because it's compiled with -fvisibility=hidden, resulting in hidden
linkage for the VTable symbol.
On another note, another difference with gcc I noticed is this:
$ cat t.cpp
template <typename T>
struct S {
virtual void m();
};
template<typename T>
void S<T>::m() { }
template void S<int>::m();
$ llvm-gcc -S -emit-llvm t.cpp -o -
[...]
%"struct.S<int>" = type { i32 (...)** }
define void @_ZN1SIiE1mEv(%"struct.S<int>"* %this) nounwind ssp align 2 {
entry:
%this_addr = alloca %"struct.S<int>"*, align 4
%"alloca point" = bitcast i32 0 to i32
store %"struct.S<int>"* %this, %"struct.S<int>"** %this_addr
br label %return
return: ; preds = %entry
ret void
}
$ clang -S -emit-llvm t.cpp -o -
[...]
%0 = type { i8*, i8* }
%struct.S = type { i32 (...)** }
@_ZTV1SIiE = weak_odr constant [3 x i8*] [i8* null, i8* bitcast (%0* @_ZTI1SIiE
to i8*), i8* bitcast (void (%struct.S*)* @_ZN1SIiE1mEv to i8*)]
@_ZTVN10__cxxabiv117__class_type_infoE = external global i8*
@_ZTS1SIiE = weak_odr constant [6 x i8] c"1SIiE\00"
@_ZTI1SIiE = weak_odr constant %0 { i8* bitcast (i8** getelementptr inbounds
(i8** @_ZTVN10__cxxabiv117__class_type_infoE, i64 2) to i8*), i8* getelementptr
inbounds ([6 x i8]* @_ZTS1SIiE, i32 0, i32 0) }
define weak_odr void @_ZN1SIiE1mEv(%struct.S* %this) nounwind ssp align 2 {
%1 = alloca %struct.S*, align 8
store %struct.S* %this, %struct.S** %1, align 8
%2 = load %struct.S** %1
ret void
}
So we emit the VTable+RTTI symbols while gcc does not.
I think this is because gcc doesn't consider that template instantiations have
key functions (due to C++ ABI 5.2.6 probably).
Do we want to have the same behavior ?
-Argiris
>
> - Doug
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