llvm dev - Oct 2020 - Associating Clang Variable Names with Arguments to Offloading Runtime Functions in OpenMP CodeGen

Hello,

A commonly requested feature in the OpenMP Target Runtime is to be able to
associate offloaded pointers with their original variable names. We figured the
most straightforward way to accomplish this is to make an additional argument to
the mapper functions that contains an array of structs containing source
information about the variables similar to how we pass in the size and offset
already. This will only happen if debugging symbols are enabled, otherwise
we'll probably just pass a null pointer. So, basically we're planning on
turning this

void __tgt_target_data_begin_mapper(int64_t device_id, int32_t arg_num,
                                    void **args_base, void **args,
                                    int64_t *arg_sizes, int64_t *arg_types,
                                    void **arg_mappers);

Into something like this for better debugging.

void __tgt_target_data_begin_mapper(ident_t *loc, int64_t device_id, int32_t
arg_num,
                                    void **args_base, void **args,
                                    int64_t *arg_sizes, arg_info_struct
**arg_info, int64_t *arg_types,
                                    void **arg_mappers);

I figured we could trace back the original VarDecls used to create the mapper
arguments and just build a struct using the names (if availible). There's a
ValueDecl listed in the BasePointerInfo class at CGOpenMPRuntime.cpp:7048 but
when I tried to access its fields it was just a nullptr when I was passing in a
single variable. The method 'setDevicePtrDecl` is used to set it, but
it's only called in a single location at CGOpenMPRuntime.cpp:8240 so I'm
assuming this is not available in the general case.

What do you think would be the best way to approach this? Basically just getting
the name and line number of the variable declaration that was used to build the
argument to the offload function.

Thanks,
Joseph Huber
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Hi,

Did you think about using something like Debugger API to translate
addresses to the var names?


-------------
Best regards,
Alexey Bataev

08.10.2020 11:38 AM, Huber, Joseph via llvm-dev пишет:
> Hello, 
>
> A commonly requested feature in the OpenMP Target Runtime is to be
> able to associate offloaded pointers with their original variable
> names. We figured the most straightforward way to accomplish this is
> to make an additional argument to the mapper functions that contains
> an array of structs containing source information about the variables
> similar to how we pass in the size and offset already. This will only
> happen if debugging symbols are enabled, otherwise we'll probably just
> pass a null pointer. So, basically we're planning on turning this
>
> void __tgt_target_data_begin_mapper(int64_t device_id, int32_t arg_num,
>                                     void **args_base, void **args,
>                                     int64_t *arg_sizes, int64_t
> *arg_types,
>                                     void **arg_mappers);
>
> Into something like this for better debugging.
>
> void __tgt_target_data_begin_mapper(ident_t *loc, int64_t device_id,
> int32_t arg_num,
>                                     void **args_base, void **args,
>                                     int64_t *arg_sizes,
> arg_info_struct **arg_info, int64_t *arg_types,
>                                     void **arg_mappers);
>
> I figured we could trace back the original VarDecls used to create the
> mapper arguments and just build a struct using the names (if
> availible). There's a ValueDecl listed in the BasePointerInfo class at
> CGOpenMPRuntime.cpp:7048 but when I tried to access its fields it was
> just a nullptr when I was passing in a single variable. The method
> 'setDevicePtrDecl` is used to set it, but it's only called in a
single
> location at CGOpenMPRuntime.cpp:8240 so I'm assuming this is not
> available in the general case.
>
> What do you think would be the best way to approach this? Basically
> just getting the name and line number of the variable declaration that
> was used to build the argument to the offload function.
>
> Thanks,
> Joseph Huber
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