Dwarf unwinder problems with store.exp and preserved regs

Daniel Jacobowitz
2003-07-01T21:44:34Z

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.

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.

Thoughts?