OpenMP failing offload of static object if bitfields

[KaruroChori]

Tested in one of the latest commits. This works as intended:

    struct hello{
        struct sub{
            uint32_t uid;
            uint32_t gid ;
            uint32_t idx;
            uint32_t weak ;
        };
        
        #pragma omp declare target
        constexpr static sub A={1,1,1,false};
        #pragma omp end declare target
    };

while this does not:

    struct hello{
        struct sub{
            uint32_t uid:32;
            uint32_t gid:32;
            uint32_t idx:32;
            uint32_t weak:32;
        };
        
        #pragma omp declare target
        constexpr static sub A={1,1,1,false};
        #pragma omp end declare target
    };

Bitfields are intrinsically not working, and the error message is a generic sounding Offloading entry for declare target variable _ZN3sdf5hello1AE is incorrect: the address is invalid.
At the very least, a more explicit description would help but still, I see no reason why bitfields should not be allowed.

Potential workarounds like wrapping it in an union and setting it up through a different field will not work.

[llvmbot]

@llvm/issue-subscribers-openmp

Author: None (KaruroChori)

Tested in one of the latest commits. This works as intended: ``` struct hello{ struct sub{ uint32_t uid; uint32_t gid ; uint32_t idx; uint32_t weak ; };

    #pragma omp declare target
    constexpr static sub A={1,1,1,false};
    #pragma omp end declare target
};

while this does not:

struct hello{
    struct sub{
        uint32_t uid:32;
        uint32_t gid:32;
        uint32_t idx:32;
        uint32_t weak:32;
    };
    
    #pragma omp declare target
    constexpr static sub A={1,1,1,false};
    #pragma omp end declare target
};

Bitfields are intrinsically not working, and the error message is a generic sounding `Offloading entry for declare target variable _ZN3sdf5hello1AE is incorrect: the address is invalid.`
At the very least, a more explicit description would help but still, I see no reason why bitfields should not be allowed.

Potential workarounds like wrapping it in an union and setting it up through a different field will not work.
</details>

[shiltian]

Hmm, wonder what the difference is in terms of IR representation of the two structs.

[KaruroChori]

Honestly, no idea and I have no clue where to start to check that. But I would be mildly surprised if that was the case.
At the very least, I am pretty positive the layout of the final structs is equivalent, or my application would be broken (more broken).
I guess there could be a potential problem with endianess on some combinations of architectures maybe?

[KaruroChori]

More info as I tested this further:

• The issue is not specific for one single target. It will fail for both amd64 and nvptx.
• Bitfields are perfectly usable within target regions. Also copying in and out of target regions seems fine for all scenarios I tested. The problem is specifically when using the declare target clause.

I also tried to compare the llvm bytecode, but I cannot spot any meaningful difference
https://godbolt.org/z/odKE5df1e

[jhuber6]

The error is emitted here https://github.com/llvm/llvm-project/blob/main/llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp#L9499. We need an address to be able to emit an offloading entry, because an offloading entry fundamentally is a named symbol in the ELF that the device runtime can look up and copy / read to. A bitfield should have an address, but I'm going to guess that something inside of OpenMP gets confused when it's used. I don't know anything more than that, maybe @alexey-bataev has a guess.

[alexey-bataev]

Bitfields are not addressable by the standard

[KaruroChori]

The bit-aligned fields inside a structure surely are not, but I would assume there should be no problem addressing full instances of structs, even if they have bitfields inside.
Is it failing because it wants each field to be addressable?