[PATCH 40/43] Add support for any location description in CFI

Zoran Zaric via Gdb-patches
2021-03-01T14:48:10Z

One of the main benefits of allowing location description to be on the
DWARF stack is that now CFI expression based register rules can be
defined using a location description operations. This allows a register
of one frame to be saved in any location, including any composite
location.

To fully support this feature, the execute_stack_op function in
dwarf2/frame.c needs to return a single struct value object instead of
just an address.

Function put_frame_register_bytes also needs to change to support any
location description.

This support is a one of the key features to truly support optimized
code.

gdb/ChangeLog:

* dwarf2/frame.c (execute_stack_op): Change to return a struct
value object.
(dwarf2_frame_cache): Change to call new execute_stack_op
definition.
(dwarf2_frame_prev_register): Change to call new execute_stack_op
definition.
* frame.c (put_frame_register_bytes): Add support for writing to
composite location description.
---
gdb/dwarf2/frame.c | 54 ++++++++++++++++++++++++++--------------------
gdb/frame.c | 37 +++++++++++++++++++++++++------
2 files changed, 62 insertions(+), 29 deletions(-)

diff --git a/gdb/dwarf2/frame.c b/gdb/dwarf2/frame.c
index 46cbf7016c2..88f118d6374 100644
--- a/gdb/dwarf2/frame.c
+++ b/gdb/dwarf2/frame.c
@@ -235,16 +235,17 @@ register %s (#%d) at %s"),
}
}

-static CORE_ADDR
+static value *
execute_stack_op (const gdb_byte *exp, ULONGEST len, int addr_size,
struct frame_info *this_frame, CORE_ADDR initial,
- int initial_in_stack_memory, dwarf2_per_objfile *per_objfile)
+ int initial_in_stack_memory, dwarf2_per_objfile *per_objfile,
+ struct type* type = nullptr, bool as_lval = true)
{
scoped_value_mark free_values;
- struct type *type = address_type (per_objfile->objfile->arch (),
- addr_size);
+ struct type *init_type = address_type (per_objfile->objfile->arch (),
+ addr_size);

- struct value *init_value = value_at_lazy (type, initial);
+ struct value *init_value = value_at_lazy (init_type, initial);
std::vector<struct value *> init_values;

set_value_stack (init_value, initial_in_stack_memory);
@@ -254,10 +255,15 @@ execute_stack_op (const gdb_byte *exp, ULONGEST len, int addr_size,
= dwarf2_evaluate (exp, len, true, per_objfile, nullptr,
this_frame, addr_size, &init_values, nullptr);

- if (VALUE_LVAL (result_val) == lval_memory)
- return value_address (result_val);
- else
- return value_as_address (result_val);
+ /* We need to clean up all the values that are not needed any more.
+ The problem with a value_ref_ptr class is that it disconnects the
+ RETVAL from the value garbage collection, so we need to make
+ a copy of that value on the stack to keep everything consistent.
+ The value_ref_ptr will clean up after itself at the end of this block. */
+ value_ref_ptr value_holder = value_ref_ptr::new_reference (result_val);
+ free_values.free_to_mark ();
+
+ return value_copy(result_val);
}
\f

@@ -988,10 +994,14 @@ dwarf2_frame_cache (struct frame_info *this_frame, void **this_cache)
break;

case CFA_EXP:
- cache->cfa =
- execute_stack_op (fs.regs.cfa_exp, fs.regs.cfa_exp_len,
- cache->addr_size, this_frame, 0, 0,
- cache->per_objfile);
+ {
+ struct value *value
+ = execute_stack_op (fs.regs.cfa_exp, fs.regs.cfa_exp_len,
+ cache->addr_size, this_frame, 0, 0,
+ cache->per_objfile);
+ cache->cfa = value_address (value);
+ }
+
break;

default:
@@ -1189,24 +1199,22 @@ dwarf2_frame_prev_register (struct frame_info *this_frame, void **this_cache,
return frame_unwind_got_register (this_frame, regnum, realnum);

case DWARF2_FRAME_REG_SAVED_EXP:
- addr = execute_stack_op (cache->reg[regnum].loc.exp.start,
+ return execute_stack_op (cache->reg[regnum].loc.exp.start,
cache->reg[regnum].loc.exp.len,
- cache->addr_size,
- this_frame, cache->cfa, 1,
- cache->per_objfile);
- return frame_unwind_got_memory (this_frame, regnum, addr);
+ cache->addr_size, this_frame,
+ cache->cfa, 1, cache->per_objfile,
+ register_type (gdbarch, regnum));

case DWARF2_FRAME_REG_SAVED_VAL_OFFSET:
addr = cache->cfa + cache->reg[regnum].loc.offset;
return frame_unwind_got_constant (this_frame, regnum, addr);

case DWARF2_FRAME_REG_SAVED_VAL_EXP:
- addr = execute_stack_op (cache->reg[regnum].loc.exp.start,
+ return execute_stack_op (cache->reg[regnum].loc.exp.start,
cache->reg[regnum].loc.exp.len,
- cache->addr_size,
- this_frame, cache->cfa, 1,
- cache->per_objfile);
- return frame_unwind_got_constant (this_frame, regnum, addr);
+ cache->addr_size, this_frame,
+ cache->cfa, 1, cache->per_objfile,
+ register_type (gdbarch, regnum), false);

case DWARF2_FRAME_REG_UNSPECIFIED:
/* GCC, in its infinite wisdom decided to not provide unwind
diff --git a/gdb/frame.c b/gdb/frame.c
index 4578b1acab3..c0d749094ac 100644
--- a/gdb/frame.c
+++ b/gdb/frame.c
@@ -1574,26 +1574,51 @@ put_frame_register_bytes (struct frame_info *frame, int regnum,
{
int curr_len = register_size (gdbarch, regnum) - offset;

+ struct value *value = frame_unwind_register_value (frame->next,
+ regnum);
+
if (curr_len > len)
curr_len = len;

const gdb_byte *myaddr = buffer.data ();
- if (curr_len == register_size (gdbarch, regnum))
+
+ /* Compute value is a special new case. The problem is that
+ the computed callback mechanism only supports a struct
+ value arguments, so we need to make one. */
+ if (value != NULL && VALUE_LVAL (value) == lval_computed)
+ {
+ struct value *from_value;
+ const struct lval_funcs *funcs = value_computed_funcs (value);
+ struct type * reg_type = register_type (gdbarch, regnum);
+
+ if (funcs->write == NULL)
+ error (_("Attempt to assign to an unmodifiable value."));
+
+ from_value = allocate_value (reg_type);
+ memcpy (value_contents_raw (from_value), myaddr,
+ TYPE_LENGTH (reg_type));
+
+ set_value_offset (value, offset);
+
+ funcs->write (value, from_value);
+ release_value (from_value);
+ }
+ else if (curr_len == register_size (gdbarch, regnum))
{
put_frame_register (frame, regnum, myaddr);
}
else
{
- struct value *value = frame_unwind_register_value (frame->next,
- regnum);
gdb_assert (value != NULL);

- memcpy ((char *) value_contents_writeable (value) + offset, myaddr,
- curr_len);
+ memcpy ((char *) value_contents_writeable (value) + offset,
+ myaddr, curr_len);
put_frame_register (frame, regnum, value_contents_raw (value));
- release_value (value);
}

+ if (value != NULL)
+ release_value (value);
+
myaddr += curr_len;
len -= curr_len;
offset = 0;
--
2.17.1