[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.

12 files changed, 851 insertions, 93 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 04c1686..7b25bc0 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,18 @@
+2016-05-31  Martin Galvan  <martin.galvan@tallertechnologies.com>
+
+	PR c++/19893
+	* 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.
+
 2016-05-30  Jan Kratochvil  <jan.kratochvil@redhat.com>
 
 	PR c++/15231
diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
index bfe1173..adb0ac2 100644
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -2061,6 +2061,66 @@ get_frame_address_in_block_wrapper (void *baton)
   return get_frame_address_in_block ((struct frame_info *) baton);
 }
 
+/* Fetch a DW_AT_const_value through a synthetic pointer.  */
+
+static struct value *
+fetch_const_value_from_synthetic_pointer (sect_offset die, LONGEST byte_offset,
+					  struct dwarf2_per_cu_data *per_cu,
+					  struct type *type)
+{
+  struct value *result = NULL;
+  struct obstack temp_obstack;
+  struct cleanup *cleanup;
+  const gdb_byte *bytes;
+  LONGEST len;
+
+  obstack_init (&temp_obstack);
+  cleanup = make_cleanup_obstack_free (&temp_obstack);
+  bytes = dwarf2_fetch_constant_bytes (die, per_cu, &temp_obstack, &len);
+
+  if (bytes != NULL)
+    {
+      if (byte_offset >= 0
+	  && byte_offset + TYPE_LENGTH (TYPE_TARGET_TYPE (type)) <= len)
+	{
+	  bytes += byte_offset;
+	  result = value_from_contents (TYPE_TARGET_TYPE (type), bytes);
+	}
+      else
+	invalid_synthetic_pointer ();
+    }
+  else
+    result = allocate_optimized_out_value (TYPE_TARGET_TYPE (type));
+
+  do_cleanups (cleanup);
+
+  return result;
+}
+
+/* Fetch the value pointed to by a synthetic pointer.  */
+
+static struct value *
+indirect_synthetic_pointer (sect_offset die, LONGEST byte_offset,
+			    struct dwarf2_per_cu_data *per_cu,
+			    struct frame_info *frame, struct type *type)
+{
+  /* Fetch the location expression of the DIE we're pointing to.  */
+  struct dwarf2_locexpr_baton baton
+    = dwarf2_fetch_die_loc_sect_off (die, per_cu,
+				     get_frame_address_in_block_wrapper, frame);
+
+  /* If pointed-to DIE has a DW_AT_location, evaluate it and return the
+     resulting value.  Otherwise, it may have a DW_AT_const_value instead,
+     or it may've been optimized out.  */
+  if (baton.data != NULL)
+    return dwarf2_evaluate_loc_desc_full (TYPE_TARGET_TYPE (type), frame,
+					  baton.data, baton.size, baton.per_cu,
+					  byte_offset);
+  else
+    return fetch_const_value_from_synthetic_pointer (die, byte_offset, per_cu,
+						     type);
+}
+
 /* An implementation of an lval_funcs method to indirect through a
    pointer.  This handles the synthetic pointer case when needed.  */
 
@@ -2115,6 +2175,7 @@ indirect_pieced_value (struct value *value)
       break;
     }
 
+  gdb_assert (piece != NULL);
   frame = get_selected_frame (_("No frame selected."));
 
   /* This is an offset requested by GDB, such as value subscripts.
@@ -2132,43 +2193,40 @@ indirect_pieced_value (struct value *value)
 					TYPE_LENGTH (type), byte_order);
   byte_offset += piece->v.ptr.offset;
 
-  gdb_assert (piece);
-  baton
-    = dwarf2_fetch_die_loc_sect_off (piece->v.ptr.die, c->per_cu,
-				     get_frame_address_in_block_wrapper,
-				     frame);
+  return indirect_synthetic_pointer (piece->v.ptr.die, byte_offset, c->per_cu,
+				     frame, type);
+}
 
-  if (baton.data != NULL)
-    return dwarf2_evaluate_loc_desc_full (TYPE_TARGET_TYPE (type), frame,
-					  baton.data, baton.size, baton.per_cu,
-					  byte_offset);
+/* Implementation of the coerce_ref method of lval_funcs for synthetic C++
+   references.  */
 
-  {
-    struct obstack temp_obstack;
-    struct cleanup *cleanup;
-    const gdb_byte *bytes;
-    LONGEST len;
-    struct value *result;
-
-    obstack_init (&temp_obstack);
-    cleanup = make_cleanup_obstack_free (&temp_obstack);
-
-    bytes = dwarf2_fetch_constant_bytes (piece->v.ptr.die, c->per_cu,
-					 &temp_obstack, &len);
-    if (bytes == NULL)
-      result = allocate_optimized_out_value (TYPE_TARGET_TYPE (type));
-    else
-      {
-	if (byte_offset < 0
-	    || byte_offset + TYPE_LENGTH (TYPE_TARGET_TYPE (type)) > len)
-	  invalid_synthetic_pointer ();
-	bytes += byte_offset;
-	result = value_from_contents (TYPE_TARGET_TYPE (type), bytes);
-      }
+static struct value *
+coerce_pieced_ref (const struct value *value)
+{
+  struct type *type = check_typedef (value_type (value));
 
-    do_cleanups (cleanup);
-    return result;
-  }
+  if (value_bits_synthetic_pointer (value, value_embedded_offset (value),
+				    TARGET_CHAR_BIT * TYPE_LENGTH (type)))
+    {
+      const struct piece_closure *closure
+	= (struct piece_closure *) value_computed_closure (value);
+      struct frame_info *frame
+	= get_selected_frame (_("No frame selected."));
+
+      /* gdb represents synthetic pointers as pieced values with a single
+	 piece.  */
+      gdb_assert (closure != NULL);
+      gdb_assert (closure->n_pieces == 1);
+
+      return indirect_synthetic_pointer (closure->pieces->v.ptr.die,
+					 closure->pieces->v.ptr.offset,
+					 closure->per_cu, frame, type);
+    }
+  else
+    {
+      /* Else: not a synthetic reference; do nothing.  */
+      return NULL;
+    }
 }
 
 static void *
@@ -2206,7 +2264,7 @@ static const struct lval_funcs pieced_value_funcs = {
   read_pieced_value,
   write_pieced_value,
   indirect_pieced_value,
-  NULL,	/* coerce_ref */
+  coerce_pieced_ref,
   check_pieced_synthetic_pointer,
   copy_pieced_value_closure,
   free_pieced_value_closure
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index b91ddce..da4dd56 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,15 @@
+2016-05-31  Martin Galvan  <martin.galvan@tallertechnologies.com>
+
+	PR c++/19893
+	* 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.
+
 2016-05-30  Antoine Tremblay  <antoine.tremblay@ericsson.com>
 
 	* gdb.trace/trace-condition.exp: Add 64bit tests.
diff --git a/gdb/testsuite/gdb.dwarf2/implref-array.c b/gdb/testsuite/gdb.dwarf2/implref-array.c
new file mode 100644
index 0000000..2470412
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/implref-array.c
@@ -0,0 +1,27 @@
+/* Copyright (C) 2016 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Test program for synthetic C++ references to arrays.  */
+
+int array[5] = {0, 1, 2, 3, 4};
+
+int
+main (void)
+{
+  asm ("main_label: .globl main_label");
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.dwarf2/implref-array.exp b/gdb/testsuite/gdb.dwarf2/implref-array.exp
new file mode 100644
index 0000000..b612763
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/implref-array.exp
@@ -0,0 +1,171 @@
+# Copyright 2016 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test a C++ reference marked with DW_OP_GNU_implicit_pointer.
+# The referenced value is a global array whose location is a DW_OP_addr.
+
+if [skip_cplus_tests] {
+    continue
+}
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if ![dwarf2_support] {
+    return 0
+}
+
+# We'll place the output of Dwarf::assemble in implref-array.S.
+standard_testfile .c .S
+
+# ${testfile} is now "implref-array".  srcfile2 is "implref-array.S".
+set executable ${testfile}
+set asm_file [standard_output_file ${srcfile2}]
+
+# We need to know the size of integer and address types in order
+# to write some of the debugging info we'd like to generate.
+#
+# For that, we ask GDB by debugging our implref-array program.
+# Any program would do, but since we already have implref-array
+# specifically for this testcase, might as well use that.
+if { [prepare_for_testing ${testfile}.exp ${testfile} ${srcfile}] } {
+    untested ${testfile}.exp
+    return -1
+}
+
+set array_length [get_valueof "/u" "sizeof(array) / sizeof(array\[0\])" -1]
+
+# Create the DWARF.  We need a regular variable which represents the array, and
+# a reference to it that'll be marked with DW_OP_GNU_implicit_pointer.
+# The variable must be global so that its name is an exported symbol that we
+# can reference from the DWARF using gdb_target_symbol.
+Dwarf::assemble ${asm_file} {
+    global srcdir subdir srcfile array_length
+
+    cu {} {
+	DW_TAG_compile_unit {
+	    {DW_AT_language @DW_LANG_C_plus_plus}
+	} {
+	    declare_labels int_label sizetype_label array_label variable_label ref_label
+	    set int_size [get_sizeof "int" -1]
+	    set upper_bound [expr ${array_length} - 1]
+
+	    # gdb always assumes references are implemented as pointers.
+	    set addr_size [get_sizeof "void *" -1]
+
+	    int_label: DW_TAG_base_type {
+		{DW_AT_byte_size ${int_size} DW_FORM_udata}
+		{DW_AT_encoding @DW_ATE_signed}
+		{DW_AT_name "int"}
+	    }
+
+	    sizetype_label: DW_TAG_base_type {
+		{DW_AT_byte_size ${int_size} DW_FORM_udata}
+		{DW_AT_encoding @DW_ATE_unsigned}
+		{DW_AT_name "sizetype"}
+	    }
+
+	    array_label: DW_TAG_array_type {
+		    {DW_AT_type :${int_label}}
+	    } {
+		DW_TAG_subrange_type {
+		    {DW_AT_type :${sizetype_label}}
+		    {DW_AT_lower_bound 0 DW_FORM_udata}
+		    {DW_AT_upper_bound ${upper_bound} DW_FORM_udata}
+		}
+	    }
+
+	    ref_label: DW_TAG_reference_type {
+		{DW_AT_byte_size ${addr_size} DW_FORM_udata}
+		{DW_AT_type :${array_label}}
+	    }
+
+	    variable_label: DW_TAG_variable {
+		{DW_AT_name "array"}
+		{DW_AT_type :${array_label}}
+		{DW_AT_external 1 DW_FORM_flag}
+		{DW_AT_location {DW_OP_addr [gdb_target_symbol "array"]} SPECIAL_expr}
+	    }
+
+	    DW_TAG_subprogram {
+		{MACRO_AT_func { "main" "${srcdir}/${subdir}/${srcfile}" }}
+		{DW_AT_type :${int_label}}
+		{DW_AT_external 1 DW_FORM_flag}
+	    } {
+		DW_TAG_variable {
+		    {DW_AT_name "ref"}
+		    {DW_AT_type :${ref_label}}
+		    {DW_AT_location {DW_OP_GNU_implicit_pointer ${variable_label} 0} SPECIAL_expr}
+		}
+	    }
+	}
+    }
+}
+
+if [prepare_for_testing ${testfile}.exp ${executable} [list ${asm_file} ${srcfile}] {}] {
+    return -1
+}
+
+# DW_OP_GNU_implicit_pointer implementation requires a valid frame.
+if ![runto_main] {
+    return -1
+}
+
+# This matches e.g. '(int (&)[5])'
+set ref_type [format {\(int \(&\)\[%d\]\)} ${array_length}]
+
+# This matches e.g. '(int (*)[5])'
+set ptr_type [format {\(int \(\*\)\[%d\]\)} ${array_length}]
+
+# Contents of the array.  Trim leading/trailing whitespace, '{' and '}'
+# since they confuse TCL to no end.
+set contents [get_valueof "" "array" ""]
+set contents [string trim ${contents}]
+set contents [string trim ${contents} "{}"]
+
+# Address of the referenced value.
+set address [get_hexadecimal_valueof "&array" ""]
+
+# Doing 'print ref' should show us e.g. '(int (&)[5]) 0xdeadbeef: {0, 1, 2, 3, 4}'.
+gdb_test "print ref" " = ${ref_type} @${address}: \\{${contents}\\}"
+
+# Doing 'print &ref' should show us e.g. '(int (*)[5]) 0xdeadbeef <array>'.
+gdb_test "print &ref" " = ${ptr_type} ${address} <array>"
+
+# gdb assumes C++ references are implemented as pointers, and print &(&ref)
+# shows us the underlying pointer's address.  Since in this case there's no
+# physical pointer, gdb should tell us so.
+gdb_test "print &(&ref)" "Attempt to take address of value not located in memory."
+
+# Test assignment through the synthetic reference.
+set first_value 10
+gdb_test_no_output "set (ref\[0\] = ${first_value})"
+
+# This matches '{10, 1, 2, 3, 4}'.
+set new_contents [format {\{%d, 1, 2, 3, 4\}} ${first_value}]
+
+# Doing 'print ref' should now show us e.g.
+# '(int (&)[5]) <synthetic pointer>: {10, 1, 2, 3, 4}'.
+gdb_test "print ref" " = ${ref_type} @${address}: ${new_contents}" "print ref after assignment"
+gdb_test "print array" " = ${new_contents}" "print array after assignment"
+
+# Test treating the array as a pointer.
+set second_value 20
+set new_contents [format {\{%d, %d, 2, 3, 4\}} ${first_value} ${second_value}]
+
+gdb_test "print *ref" " = ${first_value}"
+gdb_test_no_output "set (*(ref + 1) = ${second_value})"
+gdb_test "print ref\[1\]" " = ${second_value}"
+gdb_test "print array" " = ${new_contents}" "print array after second assignment"
diff --git a/gdb/testsuite/gdb.dwarf2/implref.exp b/gdb/testsuite/gdb.dwarf2/implref-const.exp
index ca4766e..9b6d5fe 100644
--- a/gdb/testsuite/gdb.dwarf2/implref.exp
+++ b/gdb/testsuite/gdb.dwarf2/implref-const.exp
@@ -13,45 +13,62 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-# Test C++ references marked with DW_OP_GNU_implicit_pointer.
+# Test a C++ reference marked with DW_OP_GNU_implicit_pointer.
+# The referenced value is a DW_AT_const_value.
 
-# TODO: Add more test statements after fixing bug #19893:
-# https://sourceware.org/bugzilla/show_bug.cgi?id=19893.
+if [skip_cplus_tests] {
+    continue
+}
 
 load_lib dwarf.exp
 
 # This test can only be run on targets which support DWARF-2 and use gas.
-if {![dwarf2_support]} {
+if ![dwarf2_support] {
     return 0
 }
 
-# We'll place the output of Dwarf::assemble in implref.S.
+# We'll place the output of Dwarf::assemble in implref-const.S.
 standard_testfile main.c .S
 
-# ${testfile} is now "implref".  srcfile2 is "implref.S".
+# ${testfile} is now "implref-const".  srcfile2 is "implref-const.S".
 set executable ${testfile}
 set asm_file [standard_output_file ${srcfile2}]
 
+# We need to know the size of integer and address types in order
+# to write some of the debugging info we'd like to generate.
+#
+# For that, we ask GDB by debugging our implref-const program.
+# Any program would do, but since we already have implref-const
+# specifically for this testcase, might as well use that.
+if [prepare_for_testing ${testfile}.exp ${testfile} ${srcfile}] {
+    untested ${testfile}.exp
+    return -1
+}
+
 # Create the DWARF.  We need a regular variable and a reference to it that'll
 # be marked with DW_OP_GNU_implicit_pointer.
-Dwarf::assemble $asm_file {
+Dwarf::assemble ${asm_file} {
     global srcdir subdir srcfile
 
-    cu { version 3 addr_size 4 } {
+    cu {} {
 	DW_TAG_compile_unit {
-	    {DW_AT_producer "GNU C++ 4.8.4"}
 	    {DW_AT_language @DW_LANG_C_plus_plus}
 	} {
 	    declare_labels int_label const_label variable_label ref_label
+	    set int_size [get_sizeof "int" -1]
+
+	    # gdb always assumes references are implemented as pointers.
+	    set addr_size [get_sizeof "void *" -1]
+	    set var_value 42
 
 	    int_label: DW_TAG_base_type {
-		{DW_AT_byte_size 4 DW_FORM_udata}
+		{DW_AT_byte_size ${int_size} DW_FORM_udata}
 		{DW_AT_encoding @DW_ATE_signed}
 		{DW_AT_name "int"}
 	    }
 
 	    ref_label: DW_TAG_reference_type {
-		{DW_AT_byte_size 4 DW_FORM_udata}
+		{DW_AT_byte_size ${addr_size} DW_FORM_udata}
 		{DW_AT_type :${int_label}}
 	    }
 
@@ -67,7 +84,7 @@ Dwarf::assemble $asm_file {
 		variable_label: DW_TAG_variable {
 		    {DW_AT_name "var"}
 		    {DW_AT_type :${int_label}}
-		    {DW_AT_const_value 42 DW_FORM_udata}
+		    {DW_AT_const_value ${var_value} DW_FORM_udata}
 		}
 
 		DW_TAG_variable {
@@ -80,7 +97,7 @@ Dwarf::assemble $asm_file {
     }
 }
 
-if [prepare_for_testing ${testfile}.exp ${executable} "${asm_file} ${srcfile}" {}] {
+if [prepare_for_testing ${testfile}.exp ${executable} [list ${asm_file} ${srcfile}] {}] {
     return -1
 }
 
@@ -89,10 +106,14 @@ if ![runto_main] {
     return -1
 }
 
-gdb_test "print ref" " = \\(int &\\) <synthetic pointer>" "print ref"
-gdb_test "print &ref" " = \\(int \\*\\) <synthetic pointer>" "print &ref"
+# Doing 'print ref' should show us e.g. '(int &) <synthetic pointer>: 42'.
+gdb_test "print ref" " = \\(int &\\) <synthetic pointer>: \\\d+"
+
+# The variable isn't located in memory, thus we can't take its address.
+gdb_test "print &var" "Can't take address of \"var\" which isn't an lvalue."
+gdb_test "print &ref" "Attempt to take address of value not located in memory."
 
 # gdb assumes C++ references are implemented as pointers, and print &(&ref)
 # shows us the underlying pointer's address.
 # Since in this case there's no physical pointer, gdb should tell us so.
-gdb_test "print &\(&ref\)" "Attempt to take address of value not located in memory." "print &(&ref)"
+gdb_test "print &(&ref)" "Attempt to take address of value not located in memory."
diff --git a/gdb/testsuite/gdb.dwarf2/implref-global.c b/gdb/testsuite/gdb.dwarf2/implref-global.c
new file mode 100644
index 0000000..7814a30
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/implref-global.c
@@ -0,0 +1,27 @@
+/* Copyright (C) 2016 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Test program for synthetic C++ references to global variables.  */
+
+int global_var = 42;
+
+int
+main (void)
+{
+  asm ("main_label: .globl main_label");
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.dwarf2/implref-global.exp b/gdb/testsuite/gdb.dwarf2/implref-global.exp
new file mode 100644
index 0000000..1f4cc05
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/implref-global.exp
@@ -0,0 +1,125 @@
+# Copyright 2016 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test a C++ reference marked with DW_OP_GNU_implicit_pointer.
+# The referenced value is a global variable whose location is a DW_OP_addr.
+
+if [skip_cplus_tests] {
+    continue
+}
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if ![dwarf2_support] {
+    return 0
+}
+
+# We'll place the output of Dwarf::assemble in implref-global.S.
+standard_testfile .c .S
+
+# ${testfile} is now "implref-global".  srcfile2 is "implref-global.S".
+set executable ${testfile}
+set asm_file [standard_output_file ${srcfile2}]
+
+# We need to know the size of integer and address types in order
+# to write some of the debugging info we'd like to generate.
+#
+# For that, we ask GDB by debugging our implref-global program.
+# Any program would do, but since we already have implref-global
+# specifically for this testcase, might as well use that.
+if [prepare_for_testing ${testfile}.exp ${testfile} ${srcfile}] {
+    untested ${testfile}.exp
+    return -1
+}
+
+# Create the DWARF.  We need a regular variable and a reference to it that'll
+# be marked with DW_OP_GNU_implicit_pointer.  The variable must be global so
+# that its name is an exported symbol that we can reference from the DWARF
+# using gdb_target_symbol.
+Dwarf::assemble ${asm_file} {
+    global srcdir subdir srcfile
+
+    cu {} {
+	DW_TAG_compile_unit {
+	    {DW_AT_language @DW_LANG_C_plus_plus}
+	} {
+	    declare_labels int_label variable_label ref_label
+	    set int_size [get_sizeof "int" -1]
+
+	    # gdb always assumes references are implemented as pointers.
+	    set addr_size [get_sizeof "void *" -1]
+
+	    int_label: DW_TAG_base_type {
+		{DW_AT_byte_size ${int_size} DW_FORM_udata}
+		{DW_AT_encoding @DW_ATE_signed}
+		{DW_AT_name "int"}
+	    }
+
+	    ref_label: DW_TAG_reference_type {
+		{DW_AT_byte_size ${addr_size} DW_FORM_udata}
+		{DW_AT_type :${int_label}}
+	    }
+
+	    variable_label: DW_TAG_variable {
+		{DW_AT_name "global_var"}
+		{DW_AT_type :${int_label}}
+		{DW_AT_external 1 DW_FORM_flag}
+		{DW_AT_location {DW_OP_addr [gdb_target_symbol "global_var"]} SPECIAL_expr}
+	    }
+
+	    DW_TAG_subprogram {
+		{MACRO_AT_func { "main" "${srcdir}/${subdir}/${srcfile}" }}
+		{DW_AT_type :${int_label}}
+		{DW_AT_external 1 DW_FORM_flag}
+	    } {
+		DW_TAG_variable {
+		    {DW_AT_name "ref"}
+		    {DW_AT_type :${ref_label}}
+		    {DW_AT_location {DW_OP_GNU_implicit_pointer ${variable_label} 0} SPECIAL_expr}
+		}
+	    }
+	}
+    }
+}
+
+if [prepare_for_testing ${testfile}.exp ${executable} [list ${asm_file} ${srcfile}] {}] {
+    return -1
+}
+
+# DW_OP_GNU_implicit_pointer implementation requires a valid frame.
+if ![runto_main] {
+    return -1
+}
+
+# Address of the referenced value.
+set address [get_hexadecimal_valueof "&global_var" ""]
+
+# Doing 'print ref' should show us e.g. '(int &) @0xdeadbeef: 42'.
+gdb_test "print ref" " = \\(int &\\) @${address}: \\\d+"
+
+# Doing 'print &ref' should show us e.g. '(int *) 0xdeadbeef <global_var>'.
+gdb_test "print &ref" " = \\(int \\*\\) ${address} <global_var>"
+
+# gdb assumes C++ references are implemented as pointers, and print &(&ref)
+# shows us the underlying pointer's address.  Since in this case there's no
+# physical pointer, gdb should tell us so.
+gdb_test "print &(&ref)" "Attempt to take address of value not located in memory."
+
+# Test assignment through the synthetic reference.
+set new_value 10
+gdb_test_no_output "set (ref = ${new_value})"
+gdb_test "print ref" " = \\(int &\\) @${address}: ${new_value}" "print ref after assignment"
+gdb_test "print global_var" " = ${new_value}" "print global_var after assignment"
diff --git a/gdb/testsuite/gdb.dwarf2/implref-struct.c b/gdb/testsuite/gdb.dwarf2/implref-struct.c
new file mode 100644
index 0000000..cc4f730
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/implref-struct.c
@@ -0,0 +1,43 @@
+/* Copyright (C) 2016 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Test program for synthetic C++ references to structs.  */
+
+struct S {
+  int a;
+  int b;
+  int c;
+};
+
+struct S s1 = {
+  0,
+  1,
+  2
+};
+
+struct S s2 = {
+  10,
+  11,
+  12
+};
+
+int
+main (void)
+{
+  asm ("main_label: .globl main_label");
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.dwarf2/implref-struct.exp b/gdb/testsuite/gdb.dwarf2/implref-struct.exp
new file mode 100644
index 0000000..49967e3
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/implref-struct.exp
@@ -0,0 +1,186 @@
+# Copyright 2016 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test a C++ reference marked with DW_OP_GNU_implicit_pointer.
+# The referenced value is a global struct whose location is a DW_OP_addr.
+
+if [skip_cplus_tests] {
+    continue
+}
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if ![dwarf2_support] {
+    return 0
+}
+
+# We'll place the output of Dwarf::assemble in implref-struct.S.
+standard_testfile .c .S
+
+# ${testfile} is now "implref-struct".  srcfile2 is "implref-struct.S".
+set executable ${testfile}
+set asm_file [standard_output_file ${srcfile2}]
+
+# We need to know the size of integer and address types in order
+# to write some of the debugging info we'd like to generate.
+#
+# For that, we ask GDB by debugging our implref-struct program.
+# Any program would do, but since we already have implref-struct
+# specifically for this testcase, might as well use that.
+if [prepare_for_testing ${testfile}.exp ${testfile} ${srcfile} {debug c++}] {
+    untested ${testfile}.exp
+    return -1
+}
+
+# Create the DWARF.  We need a regular variable for the struct and a reference
+# to it that'll be marked with DW_OP_GNU_implicit_pointer. The variable must be
+# global so that its name is an exported symbol that can we can reference from
+# the DWARF using gdb_target_symbol.
+Dwarf::assemble ${asm_file} {
+    global srcdir subdir srcfile
+
+    cu {} {
+	DW_TAG_compile_unit {
+	    {DW_AT_language @DW_LANG_C_plus_plus}
+	} {
+	    declare_labels int_label struct_label variable_label ref_label
+	    set int_size [get_sizeof "int" -1]
+
+	    # gdb always assumes references are implemented as pointers.
+	    set addr_size [get_sizeof "void *" -1]
+	    set S_size [get_sizeof "S" -1]
+
+	    # The compiler shouldn't introduce structure padding here.
+	    set b_offset 4
+	    set c_offset 8
+
+	    int_label: DW_TAG_base_type {
+		{DW_AT_byte_size ${int_size} DW_FORM_udata}
+		{DW_AT_encoding @DW_ATE_signed}
+		{DW_AT_name "int"}
+	    }
+
+	    struct_label: DW_TAG_structure_type {
+		{DW_AT_name "S"}
+		{DW_AT_byte_size ${S_size} DW_FORM_udata}
+	    } {
+		DW_TAG_member {
+		    {DW_AT_name "a"}
+		    {DW_AT_type :${int_label}}
+		    {DW_AT_data_member_location 0 DW_FORM_udata}
+		}
+
+		DW_TAG_member {
+		    {DW_AT_name "b"}
+		    {DW_AT_type :${int_label}}
+		    {DW_AT_data_member_location ${b_offset} DW_FORM_udata}
+		}
+
+		DW_TAG_member {
+		    {DW_AT_name "c"}
+		    {DW_AT_type :${int_label}}
+		    {DW_AT_data_member_location ${c_offset} DW_FORM_udata}
+		}
+	    }
+
+	    ref_label: DW_TAG_reference_type {
+		{DW_AT_byte_size ${addr_size} DW_FORM_udata}
+		{DW_AT_type :${struct_label}}
+	    }
+
+	    variable_label: DW_TAG_variable {
+		{DW_AT_name "s1"}
+		{DW_AT_type :${struct_label}}
+		{DW_AT_external 1 DW_FORM_flag}
+		{DW_AT_location {DW_OP_addr [gdb_target_symbol "s1"]} SPECIAL_expr}
+	    }
+
+	    DW_TAG_variable {
+		{DW_AT_name "s2"}
+		{DW_AT_type :${struct_label}}
+		{DW_AT_external 1 DW_FORM_flag}
+		{DW_AT_location {DW_OP_addr [gdb_target_symbol "s2"]} SPECIAL_expr}
+	    }
+
+	    DW_TAG_subprogram {
+		{MACRO_AT_func { "main" "${srcdir}/${subdir}/${srcfile}" }}
+		{DW_AT_type :${int_label}}
+		{DW_AT_external 1 DW_FORM_flag}
+	    } {
+	        DW_TAG_variable {
+		    {DW_AT_name "ref"}
+		    {DW_AT_type :${ref_label}}
+		    {DW_AT_location {DW_OP_GNU_implicit_pointer ${variable_label} 0} SPECIAL_expr}
+		}
+	    }
+	}
+    }
+}
+
+if [prepare_for_testing ${testfile}.exp ${executable} [list ${asm_file} ${srcfile}] {}] {
+    return -1
+}
+
+# DW_OP_GNU_implicit_pointer implementation requires a valid frame.
+if ![runto_main] {
+    return -1
+}
+
+# Returns the struct members, e.g. '{a = 0, b = 1, c = 2}'.
+proc get_members {var} {
+    set members [get_valueof "" ${var} ""]
+
+    # Trim leading/trailing whitespace, '{' and '}' since they confuse TCL to no end.
+    set members [string trim ${members}]
+    set members [string trim ${members} "{}"]
+
+    return ${members}
+}
+
+# Values of the struct members.
+set s1_members [get_members "s1"]
+set s2_members [get_members "s2"]
+
+# Address of the referenced value.
+set address [get_hexadecimal_valueof "&s1" ""]
+
+# Test printing with both 'set print object off' and 'on', just to make sure
+# the output doesn't change.
+foreach_with_prefix print-object {"off" "on"} {
+    gdb_test_no_output "set print object ${print-object}"
+
+    # Doing 'print ref' should show us e.g.
+    # '(S &) @0xdeadbeef: {a = 0, b = 1, c = 2}'.
+    gdb_test "print ref" " = \\(S &\\) @${address}: \\{${s1_members}\\}"
+
+    # Doing 'print &ref' should show us e.g. '(S *) 0xdeadbeef <s1>'.
+    gdb_test "print &ref" " = \\(S \\*\\) ${address} <s1>"
+
+    # gdb assumes C++ references are implemented as pointers, and
+    # print &(&ref) shows us the underlying pointer's address.
+    # Since in this case there's no physical pointer, gdb should tell us so.
+    gdb_test "print &(&ref)" "Attempt to take address of value not located in memory."
+}
+
+# Test assignment through the synthetic reference.
+gdb_test_no_output "set (ref = s2)"
+
+foreach_with_prefix print-object {"off" "on"} {
+    gdb_test_no_output "set print object ${print-object}"
+
+    gdb_test "print ref" " = \\(S &\\) @${address}: \\{${s2_members}\\}" "print ref after assignment"
+    gdb_test "print s1" " = \\{${s2_members}\\}" "print s1 after assignment"
+}
diff --git a/gdb/valops.c b/gdb/valops.c
index 71fb1b3..7f9cb93 100644
--- a/gdb/valops.c
+++ b/gdb/valops.c
@@ -1465,22 +1465,28 @@ value_addr (struct value *arg1)
 
   if (TYPE_CODE (type) == TYPE_CODE_REF)
     {
-      /* Copy the value, but change the type from (T&) to (T*).  We
-	 keep the same location information, which is efficient, and
-	 allows &(&X) to get the location containing the reference.
-	 Do the same to its enclosing type for consistency.  */
-      struct type *type_ptr
-        = lookup_pointer_type (TYPE_TARGET_TYPE (type));
-      struct type *enclosing_type
-        = check_typedef (value_enclosing_type (arg1));
-      struct type *enclosing_type_ptr
-        = lookup_pointer_type (TYPE_TARGET_TYPE (enclosing_type));
-
-      arg2 = value_copy (arg1);
-      deprecated_set_value_type (arg2, type_ptr);
-      set_value_enclosing_type (arg2, enclosing_type_ptr);
+      if (value_bits_synthetic_pointer (arg1, value_embedded_offset (arg1),
+	  TARGET_CHAR_BIT * TYPE_LENGTH (type)))
+	arg1 = coerce_ref (arg1);
+      else
+	{
+	  /* Copy the value, but change the type from (T&) to (T*).  We
+	     keep the same location information, which is efficient, and
+	     allows &(&X) to get the location containing the reference.
+	     Do the same to its enclosing type for consistency.  */
+	  struct type *type_ptr
+	    = lookup_pointer_type (TYPE_TARGET_TYPE (type));
+	  struct type *enclosing_type
+	    = check_typedef (value_enclosing_type (arg1));
+	  struct type *enclosing_type_ptr
+	    = lookup_pointer_type (TYPE_TARGET_TYPE (enclosing_type));
+
+	  arg2 = value_copy (arg1);
+	  deprecated_set_value_type (arg2, type_ptr);
+	  set_value_enclosing_type (arg2, enclosing_type_ptr);
 
-      return arg2;
+	  return arg2;
+	}
     }
   if (TYPE_CODE (type) == TYPE_CODE_FUNC)
     return value_coerce_function (arg1);
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);
     }