Fix AMD64 return value ABI in expression evaluation

The AMD64 System V ABI specifies that when a function has a return type
classified as MEMORY, the caller provides space for the value and passes
the address to this space as the first argument to the function (before
even the "this" pointer). The classification of MEMORY is applied to
struct that are sufficiently large, or ones with unaligned fields.

The expression evaluator uses call_function_by_hand to call functions,
and the hand-built frame has to push arguments in a way that matches the
ABI of the called function. call_function_by_hand supports ABI-based
struct returns, based on the value of gdbarch_return_value, however on
AMD64 the implementation of the classifier incorrectly assumed that all
non-POD types (implemented as "all types with a base class") should be
classified as MEMORY and use the struct return.

This ABI mismatch resulted in issues when calling a function that returns
a class of size <16 bytes which has a base class, including issues such
as the "this" pointer being incorrect (as it was passed as the second
argument rather than the first).

This is now fixed by checking for field alignment rather than POD-ness,
and a testsuite is added to test expression evaluation for AMD64.

gdb/ChangeLog:

	* amd64-tdep.c (amd64_classify_aggregate): Use cp_pass_by_reference
	rather than a hand-rolled POD check when checking for forced MEMORY
	classification.

gdb/testsuite/ChangeLog:

	* gdb.arch/amd64-eval.cc: New file.
	* gdb.arch/amd64-eval.exp: New file.

2 files changed, 163 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.arch/amd64-eval.cc b/gdb/testsuite/gdb.arch/amd64-eval.cc
new file mode 100644
index 00000000000..a986a49db34
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/amd64-eval.cc
@@ -0,0 +1,120 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2019 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/>.  */
+
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cassert>
+
+/* A simple structure with a single integer field. Should be returned in
+   a register.  */
+struct SimpleBase
+{
+  SimpleBase (int32_t x) : x (x) {}
+
+  int32_t x;
+};
+
+/* A simple structure derived from the simple base. Should be returned in
+   a register.  */
+struct SimpleDerived : public SimpleBase
+{
+  SimpleDerived (int32_t x) : SimpleBase (x) {}
+};
+
+/* A structure derived from the simple base with a non-trivial destructor.
+   Should be returned on the stack.  */
+struct NonTrivialDestructorDerived : public SimpleBase
+{
+  NonTrivialDestructorDerived (int32_t x) : SimpleBase (x) {}
+  ~NonTrivialDestructorDerived() { x = 1; }
+};
+
+/* A structure with unaligned fields. Should be returned on the stack.  */
+struct UnalignedFields
+{
+  UnalignedFields (int32_t x, double y) : x (x), y (y) {}
+
+  int32_t x;
+  double y;
+} __attribute__((packed));
+
+/* A structure with unaligned fields in its base class. Should be
+   returned on the stack.  */
+struct UnalignedFieldsInBase : public UnalignedFields
+{
+  UnalignedFieldsInBase (int32_t x, double y, int32_t x2)
+  : UnalignedFields (x, y), x2 (x2) {}
+
+  int32_t x2;
+};
+
+class Foo
+{
+public:
+  SimpleBase
+  return_simple_base (int32_t x)
+  {
+    assert (this->tag == EXPECTED_TAG);
+    return SimpleBase (x);
+  }
+
+  SimpleDerived
+  return_simple_derived (int32_t x)
+  {
+    assert (this->tag == EXPECTED_TAG);
+    return SimpleDerived (x);
+  }
+
+  NonTrivialDestructorDerived
+  return_non_trivial_destructor (int32_t x)
+  {
+    assert (this->tag == EXPECTED_TAG);
+    return NonTrivialDestructorDerived (x);
+  }
+
+  UnalignedFields
+  return_unaligned (int32_t x, double y)
+  {
+    assert (this->tag == EXPECTED_TAG);
+    return UnalignedFields (x, y);
+  }
+
+  UnalignedFieldsInBase
+  return_unaligned_in_base (int32_t x, double y, int32_t x2)
+  {
+    assert (this->tag == EXPECTED_TAG);
+    return UnalignedFieldsInBase (x, y, x2);
+  }
+
+private:
+  /* Use a tag to detect if the "this" value is correct.  */
+  static const int EXPECTED_TAG = 0xF00F00F0;
+  int tag = EXPECTED_TAG;
+};
+
+int
+main (int argc, char *argv[])
+{
+  Foo foo;
+  foo.return_simple_base(1);
+  foo.return_simple_derived(2);
+  foo.return_non_trivial_destructor(3);
+  foo.return_unaligned(4, 5);
+  foo.return_unaligned_in_base(6, 7, 8);
+  return 0;  // break-here
+}
diff --git a/gdb/testsuite/gdb.arch/amd64-eval.exp b/gdb/testsuite/gdb.arch/amd64-eval.exp
new file mode 100644
index 00000000000..c33777d2b41
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/amd64-eval.exp
@@ -0,0 +1,43 @@
+# This testcase is part of GDB, the GNU debugger.
+
+# Copyright 2019 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/>.
+
+# This testcase exercises evaluation with amd64 calling conventions.
+
+standard_testfile .cc
+
+if { [prepare_for_testing "failed to prepare" $testfile $srcfile \
+	  { debug c++ }] } {
+    return -1
+}
+
+if ![runto_main] {
+    return -1
+}
+
+gdb_breakpoint [gdb_get_line_number "break-here"]
+gdb_continue_to_breakpoint "break-here"
+
+gdb_test "call foo.return_simple_base(12)" \
+    " = {x = 12}"
+gdb_test "call foo.return_simple_derived(34)" \
+    " = {<SimpleBase> = {x = 34}, <No data fields>}"
+gdb_test "call foo.return_non_trivial_destructor(56)" \
+    " = {<SimpleBase> = {x = 56}, <No data fields>}"
+gdb_test "call foo.return_unaligned(78, 9.25)" \
+    " = {x = 78, y = 9.25}"
+gdb_test "call foo.return_unaligned_in_base(23, 4.5, 67)" \
+    " = {<UnalignedFields> = {x = 23, y = 4.5}, x2 = 67}"