[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.

8 files changed, 628 insertions, 16 deletions

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"
+}