dwarf2loc.c: Perform a pointer to address conversion for DWARF_VALUE_MEMORY

  This patch fixes the following failures for rl78-elf:

FAIL: gdb.base/vla-datatypes.exp: print int_vla
FAIL: gdb.base/vla-datatypes.exp: print unsigned_int_vla
FAIL: gdb.base/vla-datatypes.exp: print double_vla
FAIL: gdb.base/vla-datatypes.exp: print float_vla
FAIL: gdb.base/vla-datatypes.exp: print long_vla
FAIL: gdb.base/vla-datatypes.exp: print unsigned_long_vla
FAIL: gdb.base/vla-datatypes.exp: print char_vla
FAIL: gdb.base/vla-datatypes.exp: print short_vla
FAIL: gdb.base/vla-datatypes.exp: print unsigned_short_vla
FAIL: gdb.base/vla-datatypes.exp: print unsigned_char_vla
FAIL: gdb.base/vla-datatypes.exp: print foo_vla
FAIL: gdb.base/vla-datatypes.exp: print bar_vla
FAIL: gdb.base/vla-datatypes.exp: print vla_struct_object
FAIL: gdb.base/vla-datatypes.exp: print vla_union_object
FAIL: gdb.base/vla-ptr.exp: print td_vla
FAIL: gdb.mi/mi-vla-c99.exp: evaluate complete vla

The first failure in this bunch occurs due to printing an incorrect
result for a variable length array:

    print int_vla
    $1 = {-1, -1, -1, -1, -1}

The result should actually be this:

    $1 = {0, 2, 4, 6, 8}

When I started examining this bug, I found that printing an
individual array element worked correctly.  E.g. "print int_vla[2]"
resulted in 4 being printed.  I have not looked closely to see why
this is the case.

I found that evaluation of the location expression for int_vla was
causing problems.  This is the relevant DWARF entry for int_vla:

<2><15a>: Abbrev Number: 10 (DW_TAG_variable)
    <15b>   DW_AT_name        : (indirect string, offset: 0xbf): int_vla
    <15f>   DW_AT_decl_file   : 1
    <160>   DW_AT_decl_line   : 35
    <161>   DW_AT_type        : <0x393>
    <165>   DW_AT_location    : 4 byte block: 86 7a 94 2  (DW_OP_breg22 (r22): -6; DW_OP_deref_size: 2)

I found that DW_OP_breg22 was providing a correct result.
DW_OP_deref_size was fetching the correct value from memory.  However,
the value being fetched should be considered a pointer.
DW_OP_deref_size zero extends the fetched value prior to pushing
it onto the evaluation stack.  (The DWARF-4 document specifies this
action; so GDB is faithfully implementing the DWARF-4 specification.)

However, zero extending the pointer is not sufficient for converting
that value to an address for rl78 and (perhaps) other architectures
which define a `pointer_to_address' method.  (I suspect that m32c
would have the same problem.)

Ideally, we would perform the pointer to address conversion in
DW_OP_deref_size.  We don't, however, know the type of the object
that the address refers to in DW_OP_deref_size.  I can't think
of a way to infer the type at that point in the code.

Before proceeding, I should note that there are two other DWARF
operations that could be used in place of DW_OP_deref_size.  One of
these is DW_OP_GNU_deref_type.  Current GDB implements this operation,
but as is obvious from the name, it is non-standard DWARF.  The other
operation is DW_OP_xderef_size.  Even though it's part of DWARF-2
through DWARF-4 specifications, it's not presently implemented in GDB.
Present day GCC does not output dwarf expressions containing this
operation either.  [Of the two, I like DW_OP_GNU_deref_type better.
Using it avoids the need to specify an "address space identifier".
(GCC, GDB, and other non-free tools all need to agree on the meanings
of these identifiers.)]

Back to the bug analysis...

The closest consumer of the DW_OP_deref_size result is the
DWARF_VALUE_MEMORY case in dwarf2_evaluate_loc_desc_full.  At that
location, we do know the object type to which the address is intended
to refer.  I added code to perform a pointer to address conversion at
this location.  (See the patch.)

I do have some misgivings regarding this patch.  As noted earlier, it
would really be better to perform the pointer to address conversion in
DW_OP_deref_size.  I can't, however, think of a way to make this work.
Changing GCC to output one of the other aforementioned operations might
be preferable but, as noted earlier, these solutions have problems as
well.  Long term, I think it'd be good to have something like
DW_OP_GNU_deref_type become part of the standard.  If that can't or
won't happen, we'll need to implement DW_OP_xderef_size.

But until that happens, this patch will work for expressions in which
DW_OP_deref_size occurs last.  It should even work for dereferences
followed by adding an offset.  I don't think it'll work for more than
one dereference in the same expression.

gdb/ChangeLog:

    	* dwarf2loc.c (dwarf2_evaluate_loc_desc_full): Perform a pointer
    	to address conversion for DWARF_VALUE_MEMORY.
---
 gdb/dwarf2loc.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

dwarf2loc.c: Perform a pointer to address conversion for DWARF_VALUE_MEMORY

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