Re: Removing TYPE_VPTR_FIELDNO uses (was: Re: [patch ping] SetTYPE_VPTR_BASETYPE/TYPE_VPTR_FIELDNO of XL C++ virtual class)

[Wu Zhou <woodzltc at cn dot ibm dot com>]

Hi Daniel,

Thanks for your comments.  Some more questions about these code. 

> > - gdbtypes.c/gdbtype.h: to initialize VPTR_FIELDNO (in alloc_type and
> > create_array_type), fill VPTRs (in fill_in_vptr_fieldno), and dump VPTRs
> > (in recursive_dump_type).  Maybe some change to the type dumping is
> > needed.
> 
> Not if we leave them for older ABIs and stabs.

Maybe we need to add some code to dump VPTRs for gnu-v3 ABI after removing 
TYPE_VPTR_FIELDNO?

> > - eval.c (evaluate_subexp_standard): TYPE_VPTR_BASETYPE is used to iterate
> > the baseclasses to find the real address of the virtual function.
> 
> This code needs to be (A) read and thought about, so that we can figure
> out what it used to do, and (B) replaced with something less broken. 
> It hasn't worked in forever.  Take a look at what METHOD_PTR_IS_VIRTUAL
> expands to!

It seems that the definition for METHOD_PTR_IS_VIRTUAL is at least error 
for 64-bit arch.  Seen from the changelogs I found it was introduced in 
gdb since 1992.  Will this still stands after more than ten years? 

#define METHOD_PTR_IS_VIRTUAL(ADDR)  ((ADDR) & 0x80000000)

Didn't all these different compilers reached an agreement on how to 
predicate a pointer-to-method is virtual?

> > - gnu-v3-abi.c: VPTRs is used for rtti, virtual function and virtual base 
> > class offset.
> 
> In this file, I think we can skip all the rigamarole with debug
> information to find the vptr.  It's very easy: it's the size of a
> pointer, and it's at offset 0.  Always.

If so, we can use this to find the vptr.

> Unless of course there isn't one.  We may need to figure out what the
> field at offset 0 is to see whether it's a vptr or a user variable.  I
> haven't thought about that in a while; maybe we can assume that there
> is one by the time we get into this file.

Do you have any clues on how to determine whether this assumption stands?

Best Regards
- Wu Zhou