Re: [PATCH 1/4] New gdb arch hook: return_with_first_hidden_param_p

[Mark Kettenis <mark.kettenis@xs4all.nl>]

> Date: Wed, 16 May 2012 09:55:00 +0800
> From: Yao Qi <yao@codesourcery.com>
> 
> On 05/16/2012 02:02 AM, Mark Kettenis wrote:
> >> > No, the extra hidden parameter's type is the reference of the function's
> >> > return type.  My point is if we only examine debug info, we don't know
> >> > whether hidden parameter is passed, because the debug info is the same
> >> > regardless of hidden parameter is passed or not.
> > I'm still very confused.  This "hidden parameter" thing sounds exactly
> > like "struct_return".  So are you saying that on some architectures
> > gdbarch_return_value() return RETUN_VALUE_REGISTER_CONVENTION when if
> > fact for C++ (and not for plain C) it should return
> > RETURN_VALUE_STRUCT_CONVENTION?
> 
> In an inferior call, there are two steps related to ABI, 1) passing
> parameters, 2) fetching return value.  The "hidden parameter" is
> relevant to both steps.  Your description above is correct, and it is
> about step 2.

No, that's not quite correct.  push_dummy_call() gets passed a "struct
return" argument, so it is also about step 1.  Except that this
decision is unconditionally bypassed for GNU v3 C++ ABI because of the
language_pass_by_reference() check.

> My patches are trying to fix the problems in step 1.  So
> far, GDB passes hidden parameter for some language, but on some targets,
> the hidden parameter is not passed by parameter register, it is passed
> by other register specified by its ABI.  GDB is wrong if it still pass
> hidden parameter to parameter register.

But the proper place to take that decision is in
gnu-v3-abi.c:gnuv3_pass_by_reference(), since this problem is rather
specific to the C++ ABI used by GCC.  So I think you need a
gnuv3_pass_by_reference() gdbarch hook, and call that if it is it is
installed.  It's easy to do that, since get_type_arch() on the type
passed as an argument to gnuv3_pass_by_reference() function will give
you the gdbarch.

I think that would be far less confusing.