Add deref method to location description classes

Concept of reading from a location seems to be too low level for the
DWARF standard. What the standard actually describes is a concept of
dereferencing, where the type of the operation result can be
specified in advance.

This can be seen in the definition of the DW_OP_derefX family of
expression operations, but it is also happening implicitly in the case
of DW_OP_fbreg, DW_OP_regval_type and DW_OP_bregX family of operations.

Currently, the DW_OP_derefX operations will take the value from the
DWARF expression stack and implicitly convert it to a memory location
description (in reality treat it as a memory address for a given
target) and apply the dereference operation to it. When we allow any
location description on a DWARF expression stack, these operations need
to work in the same way.

The conclusion here is that we need a universal method that models the
dereference operation for any class derived from a location description
class.

It is worth mentioning that because of how the passed in buffers are
currently being implemented, we needed a specialisation for the deref
method of the dwarf_memory class to support them.

gdb/ChangeLog:

        * dwarf2/expr.c (dwarf_location::deref): New method.
        (dwarf_memory::deref): New method.

diff --git a/gdb/dwarf2/expr.c b/gdb/dwarf2/expr.c
index 6129f4e8ec9484c09c526124190c2c4bed816b55..87a8b87eee7f4dc726146a13c61d97a281a172ae 100644 (file)
--- a/gdb/dwarf2/expr.c
+++ b/gdb/dwarf2/expr.c
@@ -413,6 +413,18 @@ public:
                      bool big_endian, int *optimized,
                      int *unavailable) const = 0;
 
+  /* Apply dereference operation on the DWARF location description.
+     Operation returns a DWARF value of a given TYPE type while FRAME
+     contains a frame context information of the location.  ADDR_INFO
+     (if present) describes a passed in memory buffer if a regular
+     memory read is not desired for certain address range.  If the SIZE
+     is specified, it must be equal or smaller than the TYPE type size.
+     If SIZE is smaller than the type size, the value will be zero
+     extended to the difference.  */
+  virtual std::unique_ptr<dwarf_value> deref
+    (frame_info *frame, const property_addr_info *addr_info,
+     struct type *type, size_t size = 0) const;
+
 protected:
   /* Architecture of the location.  */
   gdbarch *m_arch;
@@ -485,6 +497,43 @@ private:
 
 using dwarf_value_up = std::unique_ptr<dwarf_value>;
 
+std::unique_ptr<dwarf_value>
+dwarf_location::deref (frame_info *frame, const property_addr_info *addr_info,
+                      struct type *type, size_t size) const
+{
+  bool big_endian = type_byte_order (type) == BFD_ENDIAN_BIG;
+  size_t actual_size = size != 0 ? size : TYPE_LENGTH (type);
+
+  if (actual_size > TYPE_LENGTH (type))
+    ill_formed_expression ();
+
+  /* If the size of the object read from memory is different
+     from the type length, we need to zero-extend it.  */
+  gdb::byte_vector read_buf (TYPE_LENGTH (type), 0);
+  gdb_byte *buf_ptr = read_buf.data ();
+  int optimized, unavailable;
+
+  if (big_endian)
+    buf_ptr += TYPE_LENGTH (type) - actual_size;
+
+  this->read (frame, buf_ptr, 0, actual_size * HOST_CHAR_BIT,
+             0, 0, big_endian, &optimized, &unavailable);
+
+  if (optimized)
+    throw_error (OPTIMIZED_OUT_ERROR,
+                _("Can't dereference "
+                  "update bitfield; containing word "
+                  "has been optimized out"));
+  if (unavailable)
+    throw_error (NOT_AVAILABLE_ERROR,
+                _("Can't dereference "
+                  "update bitfield; containing word "
+                  "is unavailable"));
+
+  return make_unique<dwarf_value>
+    (gdb::array_view<const gdb_byte> (read_buf), type);
+}
+
 /* Undefined location description entry.  This is a special location
    description type that describes the location description that is
    not known.  */
@@ -537,6 +586,11 @@ public:
              size_t location_bit_limit, bool big_endian,
              int *optimized, int *unavailable) const override;
 
+  std::unique_ptr<dwarf_value> deref (frame_info *frame,
+                                     const property_addr_info *addr_info,
+                                     struct type *type,
+                                     size_t size = 0) const override;
+
 private:
   /* True if the location belongs to a stack memory region.  */
   bool m_stack;
@@ -659,6 +713,75 @@ dwarf_memory::write (frame_info *frame, const gdb_byte *buf,
     }
 }
 
+std::unique_ptr<dwarf_value>
+dwarf_memory::deref (frame_info *frame, const property_addr_info *addr_info,
+                    struct type *type, size_t size) const
+{
+  bool big_endian = type_byte_order (type) == BFD_ENDIAN_BIG;
+  size_t actual_size = size != 0 ? size : TYPE_LENGTH (type);
+
+  if (actual_size > TYPE_LENGTH (type))
+    ill_formed_expression ();
+
+  gdb::byte_vector read_buf (TYPE_LENGTH (type), 0);
+  size_t size_in_bits = actual_size * HOST_CHAR_BIT;
+  gdb_byte *buf_ptr = read_buf.data ();
+  bool passed_in_buf = false;
+
+  if (big_endian)
+    buf_ptr += TYPE_LENGTH (type) - actual_size;
+
+  /* Covers the case where we have a passed in memory that is not
+     part of the target and requires for the location description
+     to address it instead of addressing the actual target
+     memory.  */
+  LONGEST this_size = bits_to_bytes (m_bit_suboffset, size_in_bits);
+
+  /* We shouldn't have a case where we read from a passed in
+     memory and the same memory being marked as stack. */
+  if (!m_stack && this_size && addr_info != nullptr
+      && addr_info->valaddr.data () != nullptr)
+    {
+      CORE_ADDR offset = (CORE_ADDR) m_offset - addr_info->addr;
+
+      if (offset < addr_info->valaddr.size ()
+         && offset + this_size <= addr_info->valaddr.size ())
+       {
+         /* Using second buffer here because the copy_bitwise
+            doesn't support in place copy.  */
+         gdb::byte_vector temp_buf (this_size);
+
+         memcpy (temp_buf.data (), addr_info->valaddr.data () + offset,
+                 this_size);
+         copy_bitwise (buf_ptr, 0, temp_buf.data (),
+                       m_bit_suboffset, size_in_bits, big_endian);
+         passed_in_buf = true;
+       }
+    }
+
+  if (!passed_in_buf)
+    {
+      int optimized, unavailable;
+
+      this->read (frame, buf_ptr, 0, size_in_bits, 0, 0,
+                 big_endian, &optimized, &unavailable);
+
+      if (optimized)
+       throw_error (OPTIMIZED_OUT_ERROR,
+                    _("Can't dereference "
+                    "update bitfield; containing word "
+                    "has been optimized out"));
+      if (unavailable)
+       throw_error (NOT_AVAILABLE_ERROR,
+                    _("Can't dereference "
+                    "update bitfield; containing word "
+                    "is unavailable"));
+    }
+
+  return make_unique<dwarf_value>
+    (gdb::array_view<const gdb_byte> (read_buf), type);
+}
+
 /* Register location description entry.  */
 
 class dwarf_register final : public dwarf_location