[commit] Remove setting value address for reference entry value target data value

Jan Kratochvil
2014-07-22T20:21:37Z

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On Tue, 22 Jul 2014 21:19:22 +0200, Pedro Alves wrote:
> Thanks Jan. Indeed I'd much prefer removing it.
> It's fine with me to still leave it in 7.8 in case we missed
> something.

Removed on trunk.

Jan

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commit 45326f6fbe28ef5bac22dac447a4181c44cb945a
Author: Jan Kratochvil <jan.kratochvil@redhat.com>
Date: Tue Jul 22 22:15:27 2014 +0200

Remove setting value address for reference entry value target data value.

I cannot reproduce any wrong case having the code removed.

I just do not find it correct to have it disabled. But at the same time I do
like much / I do not find correct the code myself. It is a bit problematic to
have struct value describing a memory content which is no longer present
there.

What happens there:
------------------------------------------------------------------------------
volatile int vv;
static __attribute__((noinline)) int
bar (int &ref) {
ref = 20;
vv++; /* break-here */
return ref;
}
int main (void) {
int var = 10;
return bar (var);
}
------------------------------------------------------------------------------
<4><c7>: Abbrev Number: 13 (DW_TAG_GNU_call_site_parameter)
<c8> DW_AT_location : 1 byte block: 55 (DW_OP_reg5 (rdi))
<ca> DW_AT_GNU_call_site_value: 2 byte block: 91 74 (DW_OP_fbreg: -12)
<cd> DW_AT_GNU_call_site_data_value: 1 byte block: 3a (DW_OP_lit10)
------------------------------------------------------------------------------
gdb -ex 'b value_addr' -ex r --args ../gdb ./1 -ex 'watch vv' -ex r -ex 'p &ref@entry'
->
6 return ref;
bar (ref=@0x7fffffffd944: 20, ref@entry=@0x7fffffffd944: 10) at 1.C:25
------------------------------------------------------------------------------
At /* break-here */ struct value variable 'ref' is TYPE_CODE_REF.

With FSF GDB HEAD:
(gdb) x/gx arg1.contents
0x6004000a4ad0: 0x00007fffffffd944
(gdb) p ((struct value *)arg1.location.computed.closure).lval
$1 = lval_memory
(gdb) p/x ((struct value *)arg1.location.computed.closure).location.address
$3 = 0x7fffffffd944

With your #if0-ed code:
(gdb) x/gx arg1.contents
0x6004000a4ad0: 0x00007fffffffd944
(gdb) p ((struct value *)arg1.location.computed.closure).lval
$8 = not_lval
(gdb) p/x ((struct value *)arg1.location.computed.closure).location.address
$9 = 0x0

I do not see how to access
((struct value *)arg1.location.computed.closure).location.address
from GDB CLI. Trying
(gdb) p &ref@entry
will invoke value_addr()'s:
if (TYPE_CODE (type) == TYPE_CODE_REF)
/* Copy the value, but change the type from (T&) to (T*). We
keep the same location information, which is efficient, and
allows &(&X) to get the location containing the reference. */
and therefore the address gets fetched already from
arg1.contents
and not from
((struct value *)arg1.location.computed.closure).location.address
.

And for any other type than TYPE_CODE_REF this code you removed does not get
executed at all. This DW_AT_GNU_call_site_data_value DWARF was meant
primarily for Fortran but with -O0 entry values do not get produced
and with -Og and higher Fortran always optimizes out the passing by reference.

If you do not like the removed code there I am OK with removing it as I do not
know how to make it's use reproducible for user anyway. In the worst case
- if there really is some way how to exploit it - one should just get
Attempt to take address of value not located in memory.
instead of some wrong value and it may be easy to fix then.

gdb/
2014-07-22 Jan Kratochvil <jan.kratochvil@redhat.com>

* dwarf2loc.c (value_of_dwarf_reg_entry): Remove setting value address
for reference entry value target data value.

Message-ID: <20140720150727.GA18488@host2.jankratochvil.net>

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 73a13d3..687e2fe 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,10 @@
2014-07-22 Jan Kratochvil <jan.kratochvil@redhat.com>

+ * dwarf2loc.c (value_of_dwarf_reg_entry): Remove setting value address
+ for reference entry value target data value.
+
+2014-07-22 Jan Kratochvil <jan.kratochvil@redhat.com>
+
* stack.c (read_frame_arg): Verify value_optimized_out before calling
value_available_contents_eq.

diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
index fcab9b9..b1c7ee1 100644
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -1312,7 +1312,6 @@ value_of_dwarf_reg_entry (struct type *type, struct frame_info *frame,
struct value *outer_val, *target_val, *val;
struct call_site_parameter *parameter;
struct dwarf2_per_cu_data *caller_per_cu;
- CORE_ADDR addr;

parameter = dwarf_expr_reg_to_entry_parameter (frame, kind, kind_u,
&caller_per_cu);
@@ -1335,14 +1334,6 @@ value_of_dwarf_reg_entry (struct type *type, struct frame_info *frame,
target_type, caller_frame,
caller_per_cu);

- /* value_as_address dereferences TYPE_CODE_REF. */
- addr = extract_typed_address (value_contents (outer_val), checked_type);
-
- /* The target entry value has artificial address of the entry value
- reference. */
- VALUE_LVAL (target_val) = lval_memory;
- set_value_address (target_val, addr);
-
release_value (target_val);
val = allocate_computed_value (type, &entry_data_value_funcs,
target_val /* closure */);