PATCH: PR other/48007: Unwind library doesn't work with UNITS_PER_WORD > sizeof (void *)

[H.J. Lu]

On Wed, Mar 23, 2011 at 12:22 PM, Ulrich Weigand <uweigand@de.ibm.com> wrote:
> Richard Henderson wrote:
>> Because, really, if we consider the structure truly public, we can't even
>> change the number of registers for a given port to support new features of
>> the cpu.
>
> Indeed, and I remember we got bitten by that a long time ago, which is why
> s390.h now has this comment:
>
> /* Number of hardware registers that go into the DWARF-2 unwind info.
>   To avoid ABI incompatibility, this number must not change even as
>   'fake' hard registers are added or removed.  */
> #define DWARF_FRAME_REGISTERS 34
>
>> I don't suppose there's any way that we can declare these old
>> programs Just Broken, and drop this compatibility stuff?
>
> I wouldn't like that ... we did run into this problem in the wild, and
> some s390 users really run very old programs for some reason.
>
> However, I'm wondering: this bug that leaked the implementation of
> _Unwind_Context only ever affected the *original* version of the
> structure -- it was fixed before the extended context was ever
> added, right?
>
> If this is true, we'd still need to keep the original context format
> unchanged, but we'd be free to modify the *extended* format at any
> time, without ABI considerations and need for further versioning ...
>

From what I can tell, the issues are:

1. _Unwind_Context is supposed to be opaque and we are free to
change it.  We should be able to extend DWARF_FRAME_REGISTERS
to support the new hard registers if needed, without breaking binary
compatibility.
2.  _Unwind_Context was leaked and wasn't really opaque.  To
provide backward binary compatibility, we are stuck with what we
had.

Is that possible to implement something along the line:

1. Add some bits to _Unwind_Context so that we can detect
the leaked _Unwind_Context.
2. When a leaked _Unwind_Context is detected at run-time,
as a compile time option, a target can either provide binary
compatibility or issue a run-time error.


-- 
H.J.