Re: Dwarf unwinder problems with store.exp and preserved regs

[Andreas Jaeger <aj at suse dot de>]

Daniel Jacobowitz <drow@mvista.com> writes:

> I see two kinds of failures in store.exp right now, on the two targets I'm
> testing.  On i386, with DWARF 2 unwinding, I see spurious <value optimized
> out> messages; and on ARM (without DWARF 2 unwinding) I see legitimately
> corrupted values.  The latter are easy, so I'll send a patch momentarily. 
> The former are more interesting.

I see the same problem on x86-64 with the <value optimized out> messages.

> Here's the problem: the unwinder does not know what registers are considered
> call saved.  GDB reads some initial CIE unwind information from the debug
> file:
>
>   /* Instruction sequence to initialize a register set.  */
>   unsigned char *initial_instructions;
>
>   /* True if a 'z' augmentation existed.  */
>   unsigned char saw_z_augmentation;
>
>       cie->saw_z_augmentation = (*augmentation == 'z');
>       if (cie->saw_z_augmentation)
>         {
>           ULONGEST length;
>
>           length = read_unsigned_leb128 (unit->abfd, buf, &bytes_read);
>           buf += bytes_read;
>           cie->initial_instructions = buf + length;
>           augmentation++;
>         }
>
> However, GCC only emits information about the CFA, not about the default
> saved-ness of registers.  So we get:
>
> 168       /* Initialize newly allocated registers.  */
> 169       memset (rs->reg + rs->num_regs, 0, (num_regs - rs->num_regs) * size);
>
> And 0 is UNDEFINED.  So $ebx - a call-saved register on i386 - shows up as
> undefined.
>
> There are two things we can do about this, I think, and perhaps we should
> do both:
>   - Fix GCC.  I -believe-, from reading the spec, that GCC is to blame for
>     not emiting this information.
>   - Provide an architecture method to pre-initialize the register table
>     before initial_instructions is evaluated.

I think the later is the way to go - but didn't read the spec fully to
give an authoritative answer,

Andreas
-- 
 Andreas Jaeger, aj@suse.de, http://www.suse.de/~aj
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