Re: [PATCH v3 14/28] Add new computed struct value callback interface

[Lancelot SIX <lsix@lancelotsix.com>]

Hi,

As for previous patches, I have included (minor) comments below.

On Thu, Oct 14, 2021 at 10:32:21AM +0100, Zoran Zaric via Gdb-patches wrote:
> From: Zoran Zaric <Zoran.Zaric@amd.com>
> 
> At this point all support is there to add a new callback interface
> for the computed struct value infrastructure.
> 
> Original callback interface (piece closure) is going to be removed as
> soon as the switch to the new DWARF entry classes is done in the next
> few patches.
> 
> gdb/ChangeLog:
> 
>         * dwarf2/expr.c (class computed_closure): New class.
>         (closure_value_funcs): New closure callback structure.
>         (copy_value_closure): New function.
>         (free_value_closure): New function.
>         (rw_closure_value): New function.
>         (check_synthetic_pointer): New function.
>         (write_closure_value): New function.
>         (read_closure_value): New function.
>         (is_optimized_out_closure_value): New function.
>         (indirect_closure_value): New function.
>         (coerce_closure_ref): New function.
> ---
>  gdb/dwarf2/expr.c | 307 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 307 insertions(+)
> 
> diff --git a/gdb/dwarf2/expr.c b/gdb/dwarf2/expr.c
> index 1b6622d5f59..e33be7e7c1c 100644
> --- a/gdb/dwarf2/expr.c
> +++ b/gdb/dwarf2/expr.c
> @@ -323,6 +323,92 @@ class dwarf_location;
>  class dwarf_memory;
>  class dwarf_value;
>  
> +/* Closure callback functions.  */
> +
> +static void *
> +copy_value_closure (const value *v);
> +
> +static void
> +free_value_closure (value *v);
> +
> +static void
> +rw_closure_value (value *v, value *from);
> +
> +static int
> +check_synthetic_pointer (const value *value, LONGEST bit_offset,
> +			 int bit_length);
> +
> +static void
> +write_closure_value (value *to, value *from);
> +
> +static void
> +read_closure_value (value *v);
> +
> +static bool
> +is_optimized_out_closure_value (value *v);
> +
> +static value *
> +indirect_closure_value (value *value);
> +
> +static value *
> +coerce_closure_ref (const value *value);
> +
> +/* Functions for accessing a variable described by DW_OP_piece,
> +   DW_OP_bit_piece or DW_OP_implicit_pointer.  */
> +
> +static const lval_funcs closure_value_funcs = {
> +  read_closure_value,
> +  write_closure_value,
> +  is_optimized_out_closure_value,
> +  indirect_closure_value,
> +  coerce_closure_ref,
> +  check_synthetic_pointer,
> +  copy_value_closure,
> +  free_value_closure
> +};
> +
> +/* Closure class that encapsulates a DWARF location description and a
> +   frame information used when that location description was created.
> +   Used for lval_computed value abstraction.  */
> +
> +class computed_closure : public refcounted_object
> +{
> +public:
> +  computed_closure (std::unique_ptr<dwarf_location> location,
> +		    struct frame_id frame_id)
> +    : m_location (std::move (location)), m_frame_id (frame_id)
> +  {}
> +
> +  computed_closure (std::unique_ptr<dwarf_location> location,
> +		    struct frame_info *frame)
> +    : m_location (std::move (location)), m_frame (frame)
> +  {}
> +
> +  const dwarf_location &get_location () const;

Is there a reason not to inline this function definition as well?  It
seems as trivial as get_frame_id and get_frame.

> +
> +  frame_id get_frame_id () const
> +  {

If I understand correctly, m_frame_id and m_frame are mutually
exclusive.  To be on the safe side, I would be tempted to add a check
here:

	gdb_assert (m_frame == nullptr);

> +    return m_frame_id;
> +  }
> +
> +  frame_info *get_frame () const
> +  {
> +    return m_frame;
> +  }
> +
> +private:
> +  /* Entry that this class encloses.  */
> +  const std::unique_ptr<const dwarf_location> m_location;
> +
> +  /* Frame ID context of the closure.  */
> +  frame_id m_frame_id;
> +
> +  /* In the case of frame expression evaluator the frame_id
> +     is not safe to use because the frame itself is being built.
> +     Only in these cases we set and use frame info directly.  */
> +  frame_info *m_frame = NULL;

I guess nullptr is prefered over NULL (across the patch).

> +};
> +
>  /* Base class that describes entries found on a DWARF expression
>     evaluation stack.  */
>  
> @@ -524,6 +610,11 @@ class dwarf_location : public dwarf_entry
>  
>  using dwarf_location_up = std::unique_ptr<dwarf_location>;
>  
> +const dwarf_location &computed_closure::get_location () const
> +{
> +  return *m_location;
> +}
> +
>  void
>  dwarf_location::read_from_gdb_value (frame_info *frame, struct value *value,
>  				     int value_bit_offset,
> @@ -1594,6 +1685,222 @@ dwarf_composite::is_optimized_out (frame_info *frame, bool big_endian,
>    return false;
>  }
>  
> +static void *
> +copy_value_closure (const value *v)
> +{
> +  computed_closure *closure = ((computed_closure*) value_computed_closure (v));
> +
> +  if (closure == nullptr)
> +    internal_error (__FILE__, __LINE__, _("invalid closure type"));
> +
> +  closure->incref ();
> +  return closure;
> +}
> +
> +static void
> +free_value_closure (value *v)
> +{
> +  computed_closure *closure = ((computed_closure*) value_computed_closure (v));
> +
> +  if (closure == nullptr)
> +    internal_error (__FILE__, __LINE__, _("invalid closure type"));
> +
> +  closure->decref ();
> +
> +  if (closure->refcount () == 0)
> +    delete closure;
> +}
> +
> +/* Read or write a closure value V.  If FROM != NULL, operate in "write
> +   mode": copy FROM into the closure comprising V.  If FROM == NULL,
> +   operate in "read mode": fetch the contents of the (lazy) value V by
> +   composing it from its closure.  */
> +
> +static void
> +rw_closure_value (value *v, value *from)
> +{
> +  LONGEST bit_offset = 0, max_bit_size;
> +  computed_closure *closure = (computed_closure*) value_computed_closure (v);
> +  bool big_endian = type_byte_order (value_type (v)) == BFD_ENDIAN_BIG;
> +  const dwarf_location &location = closure->get_location ();
> +
> +  if (from == NULL)
> +    {
> +      if (value_type (v) != value_enclosing_type (v))
> +        internal_error (__FILE__, __LINE__,
> +			_("Should not be able to create a lazy value with "
> +			  "an enclosing type"));
> +    }
> +
> +  ULONGEST bits_to_skip = HOST_CHAR_BIT * value_offset (v);
> +
> +  /* If there are bits that don't complete a byte, count them in.  */
> +  if (value_bitsize (v))
> +    {
> +      bits_to_skip += HOST_CHAR_BIT * value_offset (value_parent (v))
> +		       + value_bitpos (v);
> +      if (from != NULL && big_endian)
> +	{
> +	  /* Use the least significant bits of FROM.  */
> +	  max_bit_size = HOST_CHAR_BIT * TYPE_LENGTH (value_type (from));
> +	  bit_offset = max_bit_size - value_bitsize (v);
> +	}
> +      else
> +	max_bit_size = value_bitsize (v);
> +    }
> +  else
> +    max_bit_size = HOST_CHAR_BIT * TYPE_LENGTH (value_type (v));
> +
> +  frame_info *frame = closure->get_frame ();
> +
> +  if (frame == NULL)
> +    frame = frame_find_by_id (closure->get_frame_id ());
> +
> +  if (from == NULL)
> +    {
> +      location.write_to_gdb_value (frame, v, bit_offset, bits_to_skip,
> +				   max_bit_size - bit_offset, 0);
> +    }
> +  else
> +    {
> +      location.read_from_gdb_value (frame, from, bit_offset, bits_to_skip,
> +				    max_bit_size - bit_offset, 0);
> +    }
> +}
> +
> +static void
> +read_closure_value (value *v)
> +{
> +  rw_closure_value (v, NULL);
> +}
> +
> +static void
> +write_closure_value (value *to, value *from)
> +{
> +  rw_closure_value (to, from);
> +}
> +
> +/* Check if a closure value V contains describes any piece

One of 'contains' and 'describes' might be too much here I guess.

Best,
Lancelot.

> +   of the underlying location description as optimized out.  */
> +
> +static bool
> +is_optimized_out_closure_value (value *v)
> +{
> +  LONGEST max_bit_size;
> +  computed_closure *closure = (computed_closure*) value_computed_closure (v);
> +  bool big_endian = type_byte_order (value_type (v)) == BFD_ENDIAN_BIG;
> +  const dwarf_location &location = closure->get_location ();
> +
> +  if (value_type (v) != value_enclosing_type (v))
> +    internal_error (__FILE__, __LINE__,
> +		    _("Should not be able to create a lazy value with "
> +		      "an enclosing type"));
> +
> +  ULONGEST bits_to_skip = HOST_CHAR_BIT * value_offset (v);
> +
> +  /* If there are bits that don't complete a byte, count them in.  */
> +  if (value_bitsize (v))
> +    {
> +      bits_to_skip += HOST_CHAR_BIT * value_offset (value_parent (v))
> +		      + value_bitpos (v);
> +      max_bit_size = value_bitsize (v);
> +    }
> +  else
> +    max_bit_size = HOST_CHAR_BIT * TYPE_LENGTH (value_type (v));
> +
> +  frame_info *frame = closure->get_frame ();
> +
> +  if (frame == NULL)
> +    frame = frame_find_by_id (closure->get_frame_id ());
> +
> +  return location.is_optimized_out (frame, big_endian, bits_to_skip,
> +				    max_bit_size, 0);
> +}
> +
> +/* An implementation of an lval_funcs method to see whether a value is
> +   a synthetic pointer.  */
> +
> +static int
> +check_synthetic_pointer (const value *value, LONGEST bit_offset,
> +			 int bit_length)
> +{
> +  LONGEST total_bit_offset = bit_offset + HOST_CHAR_BIT * value_offset (value);
> +
> +  if (value_bitsize (value))
> +    total_bit_offset += value_bitpos (value);
> +
> +  computed_closure *closure
> +    = (computed_closure *) value_computed_closure (value);
> +
> +  return closure->get_location ().is_implicit_ptr_at (total_bit_offset,
> +						      bit_length);
> +}
> +
> +/* An implementation of an lval_funcs method to indirect through a
> +   pointer.  This handles the synthetic pointer case when needed.  */
> +
> +static value *
> +indirect_closure_value (value *value)
> +{
> +  computed_closure *closure
> +    = (computed_closure *) value_computed_closure (value);
> +
> +  struct type *type = check_typedef (value_type (value));
> +  if (type->code () != TYPE_CODE_PTR)
> +    return NULL;
> +
> +  LONGEST bit_length = HOST_CHAR_BIT * TYPE_LENGTH (type);
> +  LONGEST bit_offset = HOST_CHAR_BIT * value_offset (value);
> +
> +  if (value_bitsize (value))
> +    bit_offset += value_bitpos (value);
> +
> +  frame_info *frame = get_selected_frame (_("No frame selected."));
> +
> +  /* This is an offset requested by GDB, such as value subscripts.
> +     However, due to how synthetic pointers are implemented, this is
> +     always presented to us as a pointer type.  This means we have to
> +     sign-extend it manually as appropriate.  Use raw
> +     extract_signed_integer directly rather than value_as_address and
> +     sign extend afterwards on architectures that would need it
> +     (mostly everywhere except MIPS, which has signed addresses) as
> +     the later would go through gdbarch_pointer_to_address and thus
> +     return a CORE_ADDR with high bits set on architectures that
> +     encode address spaces and other things in CORE_ADDR.  */
> +  bfd_endian byte_order = gdbarch_byte_order (get_frame_arch (frame));
> +  LONGEST pointer_offset
> +    = extract_signed_integer (value_contents (value),
> +			      TYPE_LENGTH (type), byte_order);
> +
> +  return closure->get_location ().indirect_implicit_ptr (frame, type,
> +							 pointer_offset,
> +							 bit_offset, bit_length);
> +}
> +
> +/* Implementation of the coerce_ref method of lval_funcs for synthetic C++
> +   references.  */
> +
> +static value *
> +coerce_closure_ref (const value *value)
> +{
> +  struct type *type = check_typedef (value_type (value));
> +
> +  if (value_bits_synthetic_pointer (value, value_embedded_offset (value),
> +				    TARGET_CHAR_BIT * TYPE_LENGTH (type)))
> +    {
> +      computed_closure *closure
> +	= (computed_closure *) value_computed_closure (value);
> +      frame_info *frame = get_selected_frame (_("No frame selected."));
> +
> +      return closure->get_location ().indirect_implicit_ptr (frame, type);
> +    }
> +  else
> +    {
> +      /* Else: not a synthetic reference; do nothing.  */
> +      return NULL;
> +    }
> +}
> +
>  struct piece_closure
>  {
>    /* Reference count.  */
> -- 
> 2.17.1
>