[PR gdb/19893] Fix handling of synthetic C++ references

https://sourceware.org/bugzilla/show_bug.cgi?id=19893

I've traced the main source of the problem to pieced_value_funcs.coerce_ref not being
implemented. Since gdb always assumes references are implemented as pointers, this
causes it to think that it's dealing with a NULL pointer, thus breaking any operations
involving synthetic references.

What I did here was implementing pieced_value_funcs.coerce_ref using some of the synthetic
pointer handling code from indirect_pieced_value, as Pedro suggested. I also made a few
adjustments to the reference printing code so that it correctly shows either the address
of the referenced value or (if it's non-addressable) the "<synthetic pointer>" string.

I also wrote some unit tests based on Dwarf::assemble; these took a while to make
because in most cases I needed a synthetic reference to a physical variable. Additionally,
I started working on a unit test for classes that have a vtable, but ran into a few issues
so that'll probably go in a future patch. One thing that should definitely be fixed is that
proc function_range (called for MACRO_AT_func) will always try to compile/link using gcc
with the default options instead of g++, thus breaking C++ compilations that require e.g. libstdc++.

gdb/ChangeLog:

	* dwarf2loc.c (coerce_pieced_ref, indirect_synthetic_pointer,
	fetch_const_value_from_synthetic_pointer): New functions.
	(indirect_pieced_value): Move lower half to indirect_synthetic_pointer.
	(pieced_value_funcs): Implement coerce_ref.
	* valops.c (value_addr): Call coerce_ref for synthetic references.
	* valprint.c (valprint_check_validity): Return true for synthetic
	references.  Also, don't show "<synthetic pointer>" if they reference
	addressable values.
	(generic_val_print_ref): Handle synthetic references.  Also move some
	code to print_ref_address.
	(print_ref_address, get_value_addr_contents): New functions.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/implref.exp: Rename to...
	* gdb.dwarf2/implref-const.exp: ...this.  Also add more test statements.
	* gdb.dwarf2/implref-array.c: New file.
	* gdb.dwarf2/implref-array.exp: Likewise.
	* gdb.dwarf2/implref-global.c: Likewise.
	* gdb.dwarf2/implref-global.exp: Likewise.
	* gdb.dwarf2/implref-struct.c: Likewise.
	* gdb.dwarf2/implref-struct.exp: Likewise.

1 files changed, 94 insertions, 27 deletions

diff --git a/gdb/valprint.c b/gdb/valprint.c
index cea69f3..64407e8 100644
--- a/gdb/valprint.c
+++ b/gdb/valprint.c
@@ -335,8 +335,22 @@ valprint_check_validity (struct ui_file *stream,
       if (value_bits_synthetic_pointer (val, TARGET_CHAR_BIT * embedded_offset,
 					TARGET_CHAR_BIT * TYPE_LENGTH (type)))
 	{
-	  fputs_filtered (_("<synthetic pointer>"), stream);
-	  return 0;
+	  const int is_ref = TYPE_CODE (type) == TYPE_CODE_REF;
+	  int ref_is_addressable = 0;
+
+	  if (is_ref)
+	    {
+	      const struct value *deref_val = coerce_ref_if_computed (val);
+
+	      if (deref_val != NULL)
+		ref_is_addressable = value_lval_const (deref_val) == lval_memory;
+	    }
+
+	  if (!is_ref || !ref_is_addressable)
+	    fputs_filtered (_("<synthetic pointer>"), stream);
+
+	  /* C++ references should be valid even if they're synthetic.  */
+	  return is_ref;
 	}
 
       if (!value_bytes_available (val, embedded_offset, TYPE_LENGTH (type)))
@@ -484,6 +498,42 @@ generic_val_print_memberptr (struct type *type, const gdb_byte *valaddr,
 			      original_value, options, 0, stream);
 }
 
+/* Print '@' followed by the address contained in ADDRESS_BUFFER.  */
+
+static void
+print_ref_address (struct type *type, const gdb_byte *address_buffer,
+		  int embedded_offset, struct ui_file *stream)
+{
+  struct gdbarch *gdbarch = get_type_arch (type);
+
+  if (address_buffer != NULL)
+    {
+      CORE_ADDR address
+	= extract_typed_address (address_buffer + embedded_offset, type);
+
+      fprintf_filtered (stream, "@");
+      fputs_filtered (paddress (gdbarch, address), stream);
+    }
+  /* Else: we have a non-addressable value, such as a DW_AT_const_value.  */
+}
+
+/* If VAL is addressable, return the value contents buffer of a value that
+   represents a pointer to VAL.  Otherwise return NULL.  */
+
+static const gdb_byte *
+get_value_addr_contents (struct value *deref_val)
+{
+  gdb_assert (deref_val != NULL);
+
+  if (value_lval_const (deref_val) == lval_memory)
+    return value_contents_for_printing_const (value_addr (deref_val));
+  else
+    {
+      /* We have a non-addressable value, such as a DW_AT_const_value.  */
+      return NULL;
+    }
+}
+
 /* generic_val_print helper for TYPE_CODE_REF.  */
 
 static void
@@ -492,41 +542,58 @@ generic_val_print_ref (struct type *type, const gdb_byte *valaddr,
 		       const struct value *original_value,
 		       const struct value_print_options *options)
 {
-  struct gdbarch *gdbarch = get_type_arch (type);
   struct type *elttype = check_typedef (TYPE_TARGET_TYPE (type));
+  struct value *deref_val = NULL;
+  const int value_is_synthetic
+    = value_bits_synthetic_pointer (original_value,
+				    TARGET_CHAR_BIT * embedded_offset,
+				    TARGET_CHAR_BIT * TYPE_LENGTH (type));
+  const int must_coerce_ref = ((options->addressprint && value_is_synthetic)
+			       || options->deref_ref);
+  const int type_is_defined = TYPE_CODE (elttype) != TYPE_CODE_UNDEF;
+
+  if (must_coerce_ref && type_is_defined)
+    {
+      deref_val = coerce_ref_if_computed (original_value);
+
+      if (deref_val != NULL)
+	{
+	  /* More complicated computed references are not supported.  */
+	  gdb_assert (embedded_offset == 0);
+	}
+      else
+	deref_val = value_at (TYPE_TARGET_TYPE (type),
+			      unpack_pointer (type, valaddr + embedded_offset));
+    }
+  /* Else, original_value isn't a synthetic reference or we don't have to print
+     the reference's contents.
+
+     Notice that for references to TYPE_CODE_STRUCT, 'set print object on' will
+     cause original_value to be a not_lval instead of an lval_computed,
+     which will make value_bits_synthetic_pointer return false.
+     This happens because if options->objectprint is true, c_value_print will
+     overwrite original_value's contents with the result of coercing
+     the reference through value_addr, and then set its type back to
+     TYPE_CODE_REF.  In that case we don't have to coerce the reference again;
+     we can simply treat it as non-synthetic and move on.  */
 
   if (options->addressprint)
     {
-      CORE_ADDR addr
-	= extract_typed_address (valaddr + embedded_offset, type);
+      const gdb_byte *address = (value_is_synthetic && type_is_defined
+				 ? get_value_addr_contents (deref_val)
+				 : valaddr);
+
+      print_ref_address (type, address, embedded_offset, stream);
 
-      fprintf_filtered (stream, "@");
-      fputs_filtered (paddress (gdbarch, addr), stream);
       if (options->deref_ref)
 	fputs_filtered (": ", stream);
     }
-  /* De-reference the reference.  */
+
   if (options->deref_ref)
     {
-      if (TYPE_CODE (elttype) != TYPE_CODE_UNDEF)
-	{
-	  struct value *deref_val;
-
-	  deref_val = coerce_ref_if_computed (original_value);
-	  if (deref_val != NULL)
-	    {
-	      /* More complicated computed references are not supported.  */
-	      gdb_assert (embedded_offset == 0);
-	    }
-	  else
-	    deref_val = value_at (TYPE_TARGET_TYPE (type),
-				  unpack_pointer (type,
-						  (valaddr
-						   + embedded_offset)));
-
-	  common_val_print (deref_val, stream, recurse, options,
-			    current_language);
-	}
+      if (type_is_defined)
+	common_val_print (deref_val, stream, recurse, options,
+			  current_language);
       else
 	fputs_filtered ("???", stream);
     }