Re: [PATCH v8 0/1] MR1065: ntdll: Add CFI unwind info to __wine_syscall_dispatcher (x86_64)

[Florian Kübler (＠florian-kuebler)]

On Wed Oct 19 16:21:50 2022 +0000, Rémi Bernon wrote:
> Looking at GDB source I believe my understanding is correct, and so
> `.cfi_same_value` was right (and as we're using it already elsewhere,
> probably better):
>     case DWARF2_FRAME_REG_SAVED_REG:
>       realnum = dwarf_reg_to_regnum_or_error
>    (gdbarch, cache->reg[regnum].loc.reg);
>       return frame_unwind_got_register (this_frame, regnum, realnum);
> /* ... */
>     case DWARF2_FRAME_REG_UNSPECIFIED:
>       /* GCC, in its infinite wisdom decided to not provide unwind
>     information for registers that are "same value".  Since
>     DWARF2 (3 draft 7) doesn't define such behavior, said
>     registers are actually undefined (which is different to CFI
>     "undefined").  Code above issues a complaint about this.
>     Here just fudge the books, assume GCC, and that the value is
>     more inner on the stack.  */
>       return frame_unwind_got_register (this_frame, regnum, regnum);
>     case DWARF2_FRAME_REG_SAME_VALUE:
>       return frame_unwind_got_register (this_frame, regnum, regnum);
>
> As you can see, it implements `.cfi_same_value x` the same as
> `.cfi_register x, x` here. It also assumes unspecified registers have a
> default of `.cfi_same_value` too, so I'd say it's not required to
> specify that for xmm register at the top of the function.

Thanks. You were right. We discussed this internally. When writing this, I somehow confused the semantic of "previous frame". I also think it is save to assume "undefined" being "same value" for most unwinders, as this seem to be also what clang and gcc are producing.