[LLVMdev] MCJIT handling of linkonce_odr

[Axel Naumann]

Hi Keno,

The part that scares me a bit is

> and then adjust the other methods to not
> bail out two quickly when encountering a weak symbol.

I would very much appreciate if you could implement this; I don't have
enough knowledge of the MCJIT nor llvm CodeGen internals... I will
happily try it out and provide you with feedback, though! :-)

Thank you *so* much for your fast reaction!

Cheers, Axel 



On 13.01.2015 00:30, Keno Fischer wrote:
> Hi Axel,
> 
> the problem is that weak symbols are not handled in RuntimeDyld at all.
> The proper solution is probably to add a separate case for weak symbols
> (where we're already taking care of common symbols) in
> RuntimeDyld::loadObjectImpl and then adjust the other methods to not
> bail out two quickly when encountering a weak symbol. 
> 
> Please let me know if you're planning to work on a patch for this,
> otherwise I'll have a go tomorrow. 
> 
> Thanks,
> Keno
> 
> On Mon, Jan 12, 2015 at 11:45 AM, Axel Naumann <Axel.Naumann at cern.ch
> <mailto:Axel.Naumann at cern.ch>> wrote:
> 
>     Hi,
> 
>     I'm finally moving cling to MCJIT - and MCJIT is wonderful! So far I
>     only ran into this issue:
> 
>     $ cat linkonceodr.cxx
>     extern "C" int printf(const char*,...);
> 
>     template <class T> struct StaticStuff {
>       static T s_data;
>     };
>     template <class T> T StaticStuff<T>::s_data = 42;
> 
>     int compareAddr(int* mcjit);
> 
>     #ifdef BUILD_SHARED
>     int compareAddr(int* mcjit) {
>       if (mcjit != &StaticStuff<int>::s_data) {
>         printf("Wrong address, %ld in shared lib, %ld in mcjit!\n",
>                (long)&StaticStuff<int>::s_data, (long)mcjit);
>         return 1;
>       }
>       return 0;
>     }
>     #else
>     int main(int, char**) {
>       return compareAddr(&StaticStuff<int>::s_data);
>     }
>     #endif
> 
> 
>     $ clang++ -fPIC -shared -DBUILD_SHARED -o liblinkonceodr.so
>     linkonceodr.cxx
>     $ clang++ -emit-llvm -c linkonceodr.cxx -o - |
>     LD_PRELOAD=./liblinkonceodr.so lli -
>     Wrong address, 140449908087496 in shared lib, 140449908076544 in mcjit!
> 
> 
>     I.e. while compareAddr is resolved from the dylib, this:
> 
>     @_ZN11StaticStuffIiE6s_dataE = linkonce_odr global i32 42, align 4
> 
>     is not: it is re-emitted by the MCJIT. When building a binary and
>     linking against that dylib, _ZN11StaticStuffIiE6s is correctly picked up
>     from the dylib. I would expect MCJIT to behave the same.
> 
>     I'll try to fix that myself, but I'd appreciate a hint where to do that.
>     Should I collect linkonce_odr-s and "replace" their emitted code as part
>     of llvm::RuntimeDyldImpl::resolveRelocations()? Or is this just a
>     missing case somewhere?
> 
>     That's e.g. with
> 
>     $ lli --version
>     LLVM (http://llvm.org/):
>       LLVM version 3.6.0svn
>       Optimized build.
>       Built Jan 12 2015 (10:52:59).
>       Default target: x86_64-unknown-linux-gnu
>       Host CPU: corei7-avx
> 
> 
>     Cheers, Axel.
> 
> 
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