[PATCH 20/43] Add new classes that model DWARF stack element

Zoran Zaric via Gdb-patches
2021-03-01T14:47:22Z

The rest of the patch series addresses the issues described in a
"Motivation" section of the AMD’s DWARF standard extensions that
can be found at:

https://llvm.org/docs/AMDGPUDwarfExtensionsForHeterogeneousDebugging.html

The document describes a couple of main issues found when using the
current DWARF 5 version to describe optimized code for SIMD and SIMT
architectures.

Without going much into details described in the document, the main
point is that DWARF version 5 does not allow a proper support for
address spaces and it does not allow a location description to be part
of a DWARF expression, unless it is a leaf node of that expression.

Both issues can be solved in a clean way by introducing a new set of
classes that describe all entry types which can be placed on a DWARF
stack, while keeping a full backward compatibility with the previous
standard version. These entry types can now be either a typed value
or any location description.

Currently, the result of an expression evaluation is kept in a separate
data structure, while with the new approach, it will be always found as
a top element of the DWARF stack. The reason why this approach is
backward compatible is because in version 5, a location description
is only allowed to be a leaf node of the expression or a composite
piece.

Question here is, why do we need a new set of classes and why not just
use the struct value instead?

As it stands, there are couple of issues with using the struct value to
describe a DWARF stack element:

- It is not designed to represent a DWARF location description
specifically, instead it behaves more like unified debug information
format that represents an actual target resource. One example of this
is accessing data of a struct value register location description,
where if the amount of data accessed is larger then the register,
results in accessing more then one register. In DWARF this is not a
valid behavior and locations that span more then one register should be
described as a composite location description.

- There is a tight coupling between struct value and it’s type
information, regardless if the data represented is describing a value
(not_lval) or a location description. While the type information
dictates how the data is accessed for a struct value case, in DWARF,
location description doesn’t have a type so data access is not bound by
it.

- DWARF values only support much simpler base types, while struct value
can be linked to any type. Admittedly, new classes are still using the
same struct value infrastructure for a value based operations at the
moment, but that is planned to change in the near future.

- struct value register location description requires a frame id
information which makes them unsuitable for CFA expression evaluation.

So, there seems to be a lack of separation of concerns in the design
of a struct value infrastructure, while the new classes are handling
one specific purpose and are completely encapsulated in the expr.c.

Additional benefit of this design is that it makes a step in a
right direction for being able to evaluate DWARF expressions on a
gdbserver side in the near future, which sounds like a desirable thing.

It is also worth mentioning that this new location description
representation is based on a bit granularity (the bit_suboffset class
member) even though the DWARF standard has a very limited support for
it (mostly used for DW_OP_bit_piece operation).

By allowing any location description to define a bit sub-offset of the
location, we are able to give more options for supporting of new
concepts (like the existing packed arrays in Ada language).

In this patch, a new set of classes that describe a DWARF stack element
are added. The new classes are:

- Value - describes a numerical value with a DWARF base type.
- Location description - describes a DWARF location description.
- Undefined location - describes a location that is not defined.
- Memory location - describes a location in memory.
- Register location - describes a register location in a frame
context.
- Implicit location - describes a location that implicitly holds a
value that it describes.
- Implicit pointer - describes a concept of an implicit pointer to
a source variable.
- Composite location - describes a location that is composed from
pieces of other location descriptions.

For now, these classes are just defined, and they are planned to be
used by the following patches.

gdb/ChangeLog:

* dwarf2/expr.c (class dwarf_entry): New class.
(class dwarf_value): New class.
(class dwarf_location): New class.
(class dwarf_undefined): New class.
(class dwarf_memory): New class.
(class dwarf_register): New class.
(class dwarf_implicit): New class.
(class dwarf_implicit_pointer): New class.
(class dwarf_composite): New class.
* value.c (pack_unsigned_long): Expose function.
* value.h (pack_unsigned_long): Expose function.
---
gdb/dwarf2/expr.c | 264 ++++++++++++++++++++++++++++++++++++++++++++++
gdb/value.c | 2 +-
gdb/value.h | 2 +
3 files changed, 267 insertions(+), 1 deletion(-)

diff --git a/gdb/dwarf2/expr.c b/gdb/dwarf2/expr.c
index 1abece7d173..5d47dc83aaa 100644
--- a/gdb/dwarf2/expr.c
+++ b/gdb/dwarf2/expr.c
@@ -277,6 +277,270 @@ write_to_memory (CORE_ADDR address, const gdb_byte *buffer,
length, buffer);
}

+/* Base class that describes entries found on a DWARF expression
+ evaluation stack. */
+
+class dwarf_entry : public std::enable_shared_from_this<dwarf_entry>
+{
+public:
+ /* Not expected to be called on it's own. */
+ dwarf_entry () = default;
+
+ virtual ~dwarf_entry () = 0;
+};
+
+dwarf_entry::~dwarf_entry () = default;
+
+/* Location description entry found on a DWARF expression evaluation
+ stack.
+
+ Types of locations descirbed can be: register location, memory
+ location, implicit location, implicit pointer location, undefined
+ location and composite location (composed out of any of the location
+ types including another composite location). */
+
+class dwarf_location : public dwarf_entry
+{
+public:
+ /* Not expected to be called on it's own. */
+ dwarf_location (struct gdbarch *arch, LONGEST offset = 0,
+ LONGEST bit_suboffset = 0)
+ : m_arch (arch), m_initialised (true)
+ {
+ m_offset = offset;
+ m_offset += bit_suboffset / HOST_CHAR_BIT;
+ m_bit_suboffset = bit_suboffset % HOST_CHAR_BIT;
+ }
+
+ virtual ~dwarf_location () = default;
+
+ /* Add bit offset to the location description. */
+ void add_bit_offset (LONGEST bit_offset)
+ {
+ LONGEST bit_total_offset = m_bit_suboffset + bit_offset;
+
+ m_offset += bit_total_offset / HOST_CHAR_BIT;
+ m_bit_suboffset = bit_total_offset % HOST_CHAR_BIT;
+ };
+
+ void set_initialised (bool initialised)
+ {
+ m_initialised = initialised;
+ };
+
+protected:
+ /* Architecture of the location. */
+ struct gdbarch *m_arch;
+
+ /* Byte offset into the location. */
+ LONGEST m_offset;
+
+ /* Bit suboffset of the last byte. */
+ LONGEST m_bit_suboffset;
+
+ /* Whether the location is initialized. Used for non-standard
+ DW_OP_GNU_uninit operation. */
+ bool m_initialised;
+};
+
+/* Value entry found on a DWARF expression evaluation stack. */
+
+class dwarf_value : public dwarf_entry
+{
+public:
+ dwarf_value (const gdb_byte *contents, struct type *type)
+ {
+ size_t type_len = TYPE_LENGTH (type);
+ m_contents.reset ((gdb_byte *) xzalloc (type_len));
+
+ memcpy (m_contents.get (), contents, type_len);
+ m_type = type;
+ }
+
+ dwarf_value (ULONGEST value, struct type *type)
+ {
+ m_contents.reset ((gdb_byte *) xzalloc (TYPE_LENGTH (type)));
+
+ pack_unsigned_long (m_contents.get (), type, value);
+ m_type = type;
+ }
+
+ dwarf_value (LONGEST value, struct type *type)
+ {
+ m_contents.reset ((gdb_byte *) xzalloc (TYPE_LENGTH (type)));
+
+ pack_long (m_contents.get (), type, value);
+ m_type = type;
+ }
+
+ const gdb_byte* get_contents () const
+ {
+ return m_contents.get ();
+ }
+
+ struct type* get_type () const
+ {
+ return m_type;
+ }
+
+ LONGEST to_long () const
+ {
+ return unpack_long (m_type, m_contents.get ());
+ }
+
+private:
+ /* Value contents as a stream of bytes in target byte order. */
+ gdb::unique_xmalloc_ptr<gdb_byte> m_contents;
+
+ /* Type of the value held by the entry. */
+ struct type *m_type;
+};
+
+/* Undefined location description entry. This is a special location
+ description type that describes the location description that is
+ not known. */
+
+class dwarf_undefined : public dwarf_location
+{
+public:
+ dwarf_undefined (struct gdbarch *arch, LONGEST offset = 0,
+ LONGEST bit_suboffset = 0)
+ : dwarf_location (arch, offset, bit_suboffset)
+ {}
+};
+
+class dwarf_memory : public dwarf_location
+{
+public:
+ dwarf_memory (struct gdbarch *arch, LONGEST offset,
+ LONGEST bit_suboffset = 0, bool stack = false)
+ : dwarf_location (arch, offset, bit_suboffset),
+ m_stack (stack)
+ {}
+
+ void set_stack (bool stack)
+ {
+ m_stack = stack;
+ };
+
+private:
+ /* True if the location belongs to a stack memory region. */
+ bool m_stack;
+};
+
+/* Register location description entry. */
+
+class dwarf_register : public dwarf_location
+{
+public:
+ dwarf_register (struct gdbarch *arch, unsigned int regnum,
+ LONGEST offset = 0, LONGEST bit_suboffset = 0)
+ : dwarf_location (arch, offset, bit_suboffset),
+ m_regnum (regnum)
+ {}
+
+private:
+ /* DWARF register number. */
+ unsigned int m_regnum;
+};
+
+/* Implicit location description entry. Describes a location
+ description not found on the target but instead saved in a
+ gdb-allocated buffer. */
+
+class dwarf_implicit : public dwarf_location
+{
+public:
+
+ dwarf_implicit (struct gdbarch *arch, const gdb_byte *contents,
+ size_t size, enum bfd_endian byte_order)
+ : dwarf_location (arch)
+ {
+ m_contents.reset ((gdb_byte *) xzalloc (size));
+
+ memcpy (m_contents.get (), contents, size);
+ m_size = size;
+ m_byte_order = byte_order;
+ }
+
+private:
+ /* Implicit location contents as a stream of bytes in target byte-order. */
+ gdb::unique_xmalloc_ptr<gdb_byte> m_contents;
+
+ /* Contents byte stream size. */
+ size_t m_size;
+
+ /* Contents original byte order. */
+ enum bfd_endian m_byte_order;
+};
+
+/* Implicit pointer location description entry. */
+
+class dwarf_implicit_pointer : public dwarf_location
+{
+public:
+ dwarf_implicit_pointer (struct gdbarch *arch,
+ dwarf2_per_objfile *per_objfile,
+ struct dwarf2_per_cu_data *per_cu,
+ int addr_size, sect_offset die_offset,
+ LONGEST offset, LONGEST bit_suboffset = 0)
+ : dwarf_location (arch, offset, bit_suboffset),
+ m_per_objfile (per_objfile), m_per_cu (per_cu),
+ m_addr_size (addr_size), m_die_offset (die_offset)
+ {}
+
+private:
+ /* Per object file data of the implicit pointer. */
+ dwarf2_per_objfile *m_per_objfile;
+
+ /* Compilation unit context of the implicit pointer. */
+ struct dwarf2_per_cu_data *m_per_cu;
+
+ /* Address size for the evaluation. */
+ int m_addr_size;
+
+ /* DWARF die offset pointed by the implicit pointer. */
+ sect_offset m_die_offset;
+};
+
+/* Composite location description entry. */
+
+class dwarf_composite : public dwarf_location
+{
+public:
+ dwarf_composite (struct gdbarch *arch, struct dwarf2_per_cu_data *per_cu,
+ LONGEST offset = 0, LONGEST bit_suboffset = 0)
+ : dwarf_location (arch, offset, bit_suboffset), m_per_cu (per_cu)
+ {}
+
+ void add_piece (std::shared_ptr<dwarf_location> location, ULONGEST bit_size)
+ {
+ gdb_assert (location != nullptr);
+ m_pieces.emplace_back (location, bit_size);
+ }
+
+private:
+ /* Composite piece that contains a piece location
+ description and it's size. */
+ class piece
+ {
+ public:
+ piece (std::shared_ptr<dwarf_location> location, ULONGEST size)
+ : m_location (location),
+ m_size (size)
+ {}
+
+ std::shared_ptr<dwarf_location> m_location;
+ ULONGEST m_size;
+ };
+
+ /* Compilation unit context of the pointer. */
+ struct dwarf2_per_cu_data *m_per_cu;
+
+ /* Vector of composite pieces. */
+ std::vector<struct piece> m_pieces;
+};
+
struct piece_closure
{
/* Reference count. */
diff --git a/gdb/value.c b/gdb/value.c
index 9f3914a5bc5..52f55f0bb97 100644
--- a/gdb/value.c
+++ b/gdb/value.c
@@ -3436,7 +3436,7 @@ pack_long (gdb_byte *buf, struct type *type, LONGEST num)

/* Pack NUM into BUF using a target format of TYPE. */

-static void
+void
pack_unsigned_long (gdb_byte *buf, struct type *type, ULONGEST num)
{
LONGEST len;
diff --git a/gdb/value.h b/gdb/value.h
index 60a831c38c4..cbb60e2f073 100644
--- a/gdb/value.h
+++ b/gdb/value.h
@@ -678,6 +678,8 @@ extern struct value *value_field_bitfield (struct type *type, int fieldno,
const struct value *val);

extern void pack_long (gdb_byte *buf, struct type *type, LONGEST num);
+extern void pack_unsigned_long (gdb_byte *buf, struct type *type,
+ ULONGEST num);

extern struct value *value_from_longest (struct type *type, LONGEST num);
extern struct value *value_from_ulongest (struct type *type, ULONGEST num);
--
2.17.1