Re: [PATCH 1/9] Gccgo port to s390[x] -- part I

["Ulrich Weigand" <uweigand at de dot ibm dot com>]

Lynn Boger wrote:

> I modified the patch for statements.cc and rebuilt and that eliminates 
> the regressions and fixes the original problem it was intended to fix 
> for both ppc64 BE & LE.  The ABIs are different between BE & LE, so that 
> make_func_code_reference on ppc64 BE is not returning the function's 
> code address but the function pointer from the .opd.  The first 8 bytes 
> of the entry in the .opd is the function's code address.  Here is the 
> change to statements.cc that made it work:

> +#if defined(__powerpc64__) && _CALL_ELF != 2
> +      Expression* pfn =
> +        Expression::make_func_code_reference(function, location);
> +      Type* pfntype =
> +          Type::make_pointer_type(
> +              Type::make_pointer_type(Type::make_void_type()));
> +      Expression* fn = Expression::make_unsafe_cast(pfntype, pfn, 
> location);
> +      Expression* fn_code_addr = Expression::make_unary(OPERATOR_MULT, fn,
> +                                                        location);
> +#else
> +      Expression* fn_code_addr =
> +        Expression::make_func_code_reference(function, location);
> +#endif
> +      Expression* call = Runtime::make_call(Runtime::SET_DEFERING_FN,
> +                                            location, 1, fn_code_addr);
> +      Statement* s = Statement::make_statement(call, true);

This looks wrong when using gcc-go as a cross-compiler.  The #if is
evaluated in the context of the *host*, but you'd need to check the
processor architecture and ABI of the *target*.  This seems difficult
since you'd have to take into account -mabi= options, which are not
readily available to the front end.

It seems more straightforward to keep the front end as is, i.e. generate
code to pass a plain function pointer (as defined by the target ABI) to
the runtime, and have the *runtime* do whatever target-specific fiddling
is required to get from a function pointer to a code address.

For example, you could add something like:

#if defined(__powerpc64__) && _CALL_ELF != 2
  defering_fn = *(void **)defering_fn;
#endif

to __go_set_defering_fn (or possibly __go_can_recover).

[ Since the runtime is compiled for the target with the appropriate
ABI setting, the #if works as intended when in runtime code.  ]

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  GNU/Linux compilers and toolchain
  Ulrich.Weigand@de.ibm.com