[PATCH 2/2] PowerPC, fix support for printing the function return value for non-trivial values.

[Carl Love <cel@us.ibm.com>]

GDB maintainers:

On PowerPC, a non-trivial return value for a function cannot be
reliably determined with the current PowerPC ABI.  The issue is the
address of the return buffer for a non-trivial value is passed to the
function in register r3.  However, at the end of the function the ABI
does not guarantee r3 will not be changed.  Hence it cannot be reliably
used to get the address of the return buffer.

This patch adds a new GDB ABI to allow GDB to obtain the input value of
r3 when the function returns.  This is done by using the DW_OP_entry
value for the DWARF entries to obtain the value of r3 on entry to the
function.  The ABI allows GDB to get the correct address for the return
buffer on exit from the function.

This patch fixes the five test failures in gdb.cp/non-trivial-
retval.exp.

The patch has been tested on PowerPC and X86-64 with no regression
failures.

Please let me know if this patch is acceptable for GDB mainline.
Thanks.

                         Carl Love

--------------------------------------
PowerPC, fix support for printing the function return value for non-trivial values.

Currently, a non-trivial return value from a function cannot currently be
reliably determined on PowerPC.  This is due to the fact that the PowerPC
ABI uses register r3 to store the address of the buffer containing the
non-trivial return value when the function is called.  The PowerPC ABI
does not guarantee the value in register r3 is not modified in the
function.  Thus the value in r3 cannot be reliably used to obtain the
return addreses on exit from the function.

This patch adds a new gdb ABI to allow PowerPC to access the value of r3
on entry to a function. The new gdb ABI attempts to use the DW_OP_entry
value for the DWARF entries, when exiting the function, to determine the
value of r3 on entry to the function.  This requires the use of the
-fvar-tracking compiler option to compile the user application thus
generating the DW_OP_entry_value in the binary.  The DW_OP_entries in the
binary file enables GDB can resolve the DW_TAG_call_sites.  This new API
is used to get the return buffer address, in the case of a function
returning a nontrivial data type, on exit from the function.  The GDB
function should_stop checks to see if RETURN_BUF is non-zero.  By default,
RETURN_BUF will be set to zero by the new ABI call for all architectures
but PowerPC.  The get_return_value function will be used to obtain the
return value on these architectures as is currently being done if
RETURN_BUF is zero.  On PowerPC, the new ABI will return a nonzero address
in RETURN_BUF if the value can be determined.  The value_at function uses
the return buffer address to get the return value.

This patch fixes five testcase failures in gdb.cp/non-trivial-retval.exp.
The correct function return values are now reported.

Note this patch is dependent on patch: "PowerPC, function
ppc64_sysv_abi_return_value add missing return value convention".

This patch has been tested on Power 10 and x86-64 with no regressions.
---
 gdb/arch-utils.c                            |  6 +++
 gdb/arch-utils.h                            |  4 ++
 gdb/dwarf2/loc.c                            |  2 +-
 gdb/dwarf2/loc.h                            | 11 ++++++
 gdb/gdbarch-components.py                   | 15 ++++++++
 gdb/gdbarch-gen.h                           | 11 ++++++
 gdb/gdbarch.c                               | 23 ++++++++++++
 gdb/infcmd.c                                | 41 +++++++++++++++++++--
 gdb/ppc-sysv-tdep.c                         | 37 +++++++++++++++++++
 gdb/ppc-tdep.h                              |  2 +
 gdb/rs6000-tdep.c                           |  6 ++-
 gdb/testsuite/gdb.cp/non-trivial-retval.exp |  9 ++++-
 gdb/testsuite/lib/gdb.exp                   |  8 ++++
 13 files changed, 168 insertions(+), 7 deletions(-)

diff --git a/gdb/arch-utils.c b/gdb/arch-utils.c
index 5943965bd75..12d6d1cdd75 100644
--- a/gdb/arch-utils.c
+++ b/gdb/arch-utils.c
@@ -1093,6 +1093,12 @@ default_read_core_file_mappings
 {
 }
 
+CORE_ADDR
+default_get_return_buf_addr (struct type *val_type, frame_info *cur_frame)
+{
+  return 0;
+}
+
 /* Non-zero if we want to trace architecture code.  */
 
 #ifndef GDBARCH_DEBUG
diff --git a/gdb/arch-utils.h b/gdb/arch-utils.h
index f850e5fd6e7..a1944ff9c3c 100644
--- a/gdb/arch-utils.h
+++ b/gdb/arch-utils.h
@@ -300,4 +300,8 @@ extern void default_read_core_file_mappings
    struct bfd *cbfd,
    read_core_file_mappings_pre_loop_ftype pre_loop_cb,
    read_core_file_mappings_loop_ftype loop_cb);
+
+/* Default implementation of gdbarch default_get_return_buf_addr method.  */
+extern CORE_ADDR default_get_return_buf_addr (struct type *val_typegdbarch,
+					      frame_info *cur_frame);
 #endif /* ARCH_UTILS_H */
diff --git a/gdb/dwarf2/loc.c b/gdb/dwarf2/loc.c
index ad45d57a654..25a7499fe9b 100644
--- a/gdb/dwarf2/loc.c
+++ b/gdb/dwarf2/loc.c
@@ -1332,7 +1332,7 @@ static const struct lval_funcs entry_data_value_funcs =
    Function always returns non-NULL value.  It throws NO_ENTRY_VALUE_ERROR if it
    cannot resolve the parameter for any reason.  */
 
-static struct value *
+extern struct value *
 value_of_dwarf_reg_entry (struct type *type, struct frame_info *frame,
 			  enum call_site_parameter_kind kind,
 			  union call_site_parameter_u kind_u)
diff --git a/gdb/dwarf2/loc.h b/gdb/dwarf2/loc.h
index a9834d32066..f2cf93ba28e 100644
--- a/gdb/dwarf2/loc.h
+++ b/gdb/dwarf2/loc.h
@@ -296,4 +296,15 @@ extern struct value *indirect_synthetic_pointer
    dwarf2_per_objfile *per_objfile, struct frame_info *frame,
    struct type *type, bool resolve_abstract_p = false);
 
+/* Read parameter of TYPE at (callee) FRAME's function entry.  KIND and KIND_U
+   are used to match DW_AT_location at the caller's
+   DW_TAG_call_site_parameter.
+
+   Function always returns non-NULL value.  It throws NO_ENTRY_VALUE_ERROR if
+   it cannot resolve the parameter for any reason.  */
+
+extern struct value *value_of_dwarf_reg_entry (struct type *type,
+					       struct frame_info *frame,
+					       enum call_site_parameter_kind kind,
+					       union call_site_parameter_u kind_u);
 #endif /* DWARF2LOC_H */
diff --git a/gdb/gdbarch-components.py b/gdb/gdbarch-components.py
index ad71bf754de..6c623d668c5 100644
--- a/gdb/gdbarch-components.py
+++ b/gdb/gdbarch-components.py
@@ -845,6 +845,21 @@ for instance).
     invalid=True,
 )
 
+Function(
+    comment="""
+Return the address at which the value being returned from
+the current function will be stored.  This routine is only
+called if the current function uses the the "struct return
+convention".
+
+May return 0 when unable to determine that address.""",
+    type="CORE_ADDR",
+    name="get_return_buf_addr",
+    params=[("struct type *", "val_type"), ("frame_info *", "cur_frame")],
+    predefault="default_get_return_buf_addr",
+    invalid=False,
+)
+
 Method(
     comment="""
 Return true if the return value of function is stored in the first hidden
diff --git a/gdb/gdbarch-gen.h b/gdb/gdbarch-gen.h
index c7a24704c7c..3e363b3a069 100644
--- a/gdb/gdbarch-gen.h
+++ b/gdb/gdbarch-gen.h
@@ -435,6 +435,17 @@ typedef enum return_value_convention (gdbarch_return_value_ftype) (struct gdbarc
 extern enum return_value_convention gdbarch_return_value (struct gdbarch *gdbarch, struct value *function, struct type *valtype, struct regcache *regcache, gdb_byte *readbuf, const gdb_byte *writebuf);
 extern void set_gdbarch_return_value (struct gdbarch *gdbarch, gdbarch_return_value_ftype *return_value);
 
+/* Return the address at which the value being returned from
+   the current function will be stored.  This routine is only
+   called if the current function uses the the "struct return
+   convention".
+
+   May return 0 when unable to determine that address. */
+
+typedef CORE_ADDR (gdbarch_get_return_buf_addr_ftype) (struct type *val_type, frame_info *cur_frame);
+extern CORE_ADDR gdbarch_get_return_buf_addr (struct gdbarch *gdbarch, struct type *val_type, frame_info *cur_frame);
+extern void set_gdbarch_get_return_buf_addr (struct gdbarch *gdbarch, gdbarch_get_return_buf_addr_ftype *get_return_buf_addr);
+
 /* Return true if the return value of function is stored in the first hidden
    parameter.  In theory, this feature should be language-dependent, specified
    by language and its ABI, such as C++.  Unfortunately, compiler may
diff --git a/gdb/gdbarch.c b/gdb/gdbarch.c
index 18d46a39d7a..da3e2ad0e36 100644
--- a/gdb/gdbarch.c
+++ b/gdb/gdbarch.c
@@ -116,6 +116,7 @@ struct gdbarch
   gdbarch_address_to_pointer_ftype *address_to_pointer = nullptr;
   gdbarch_integer_to_address_ftype *integer_to_address = nullptr;
   gdbarch_return_value_ftype *return_value = nullptr;
+  gdbarch_get_return_buf_addr_ftype *get_return_buf_addr = nullptr;
   gdbarch_return_in_first_hidden_param_p_ftype *return_in_first_hidden_param_p = nullptr;
   gdbarch_skip_prologue_ftype *skip_prologue = nullptr;
   gdbarch_skip_main_prologue_ftype *skip_main_prologue = nullptr;
@@ -313,6 +314,7 @@ gdbarch_alloc (const struct gdbarch_info *info,
   gdbarch->value_from_register = default_value_from_register;
   gdbarch->pointer_to_address = unsigned_pointer_to_address;
   gdbarch->address_to_pointer = unsigned_address_to_pointer;
+  gdbarch->get_return_buf_addr = default_get_return_buf_addr;
   gdbarch->return_in_first_hidden_param_p = default_return_in_first_hidden_param_p;
   gdbarch->breakpoint_from_pc = default_breakpoint_from_pc;
   gdbarch->sw_breakpoint_from_kind = NULL;
@@ -465,6 +467,7 @@ verify_gdbarch (struct gdbarch *gdbarch)
   /* Skip verify of address_to_pointer, invalid_p == 0 */
   /* Skip verify of integer_to_address, has predicate.  */
   /* Skip verify of return_value, has predicate.  */
+  /* Skip verify of get_return_buf_addr, invalid_p == 0 */
   /* Skip verify of return_in_first_hidden_param_p, invalid_p == 0 */
   if (gdbarch->skip_prologue == 0)
     log.puts ("\n\tskip_prologue");
@@ -877,6 +880,9 @@ gdbarch_dump (struct gdbarch *gdbarch, struct ui_file *file)
   gdb_printf (file,
                       "gdbarch_dump: return_value = <%s>\n",
                       host_address_to_string (gdbarch->return_value));
+  gdb_printf (file,
+                      "gdbarch_dump: get_return_buf_addr = <%s>\n",
+                      host_address_to_string (gdbarch->get_return_buf_addr));
   gdb_printf (file,
                       "gdbarch_dump: return_in_first_hidden_param_p = <%s>\n",
                       host_address_to_string (gdbarch->return_in_first_hidden_param_p));
@@ -2680,6 +2686,23 @@ set_gdbarch_return_value (struct gdbarch *gdbarch,
   gdbarch->return_value = return_value;
 }
 
+CORE_ADDR
+gdbarch_get_return_buf_addr (struct gdbarch *gdbarch, struct type *val_type, frame_info *cur_frame)
+{
+  gdb_assert (gdbarch != NULL);
+  gdb_assert (gdbarch->get_return_buf_addr != NULL);
+  if (gdbarch_debug >= 2)
+    gdb_printf (gdb_stdlog, "gdbarch_get_return_buf_addr called\n");
+  return gdbarch->get_return_buf_addr (val_type, cur_frame);
+}
+
+void
+set_gdbarch_get_return_buf_addr (struct gdbarch *gdbarch,
+                                 gdbarch_get_return_buf_addr_ftype get_return_buf_addr)
+{
+  gdbarch->get_return_buf_addr = get_return_buf_addr;
+}
+
 int
 gdbarch_return_in_first_hidden_param_p (struct gdbarch *gdbarch, struct type *type)
 {
diff --git a/gdb/infcmd.c b/gdb/infcmd.c
index b788f454432..7964baa7f92 100644
--- a/gdb/infcmd.c
+++ b/gdb/infcmd.c
@@ -55,6 +55,7 @@
 #include "gdbsupport/gdb_optional.h"
 #include "source.h"
 #include "cli/cli-style.h"
+#include "dwarf2/loc.h"
 
 /* Local functions: */
 
@@ -1598,6 +1599,12 @@ struct finish_command_fsm : public thread_fsm
      return value.  */
   struct return_value_info return_value_info {};
 
+  /* If the current function uses the "struct return convention",
+     this holds the address at which the value being returned will
+     be stored, or zero if that address could not be determined or
+     the "struct return convention" is not being used.  */
+  CORE_ADDR return_buf;
+
   explicit finish_command_fsm (struct interp *cmd_interp)
     : thread_fsm (cmd_interp)
   {
@@ -1636,7 +1643,13 @@ finish_command_fsm::should_stop (struct thread_info *tp)
 	  struct value *func;
 
 	  func = read_var_value (function, NULL, get_current_frame ());
-	  rv->value = get_return_value (function, func);
+
+	  if (return_buf != 0)
+	    /* Retrieve return value from the buffer where it was saved.  */
+	      rv->value = value_at (rv->type, return_buf);
+	  else
+	      rv->value = get_return_value (function, func);
+
 	  if (rv->value != NULL)
 	    rv->value_history_index = record_latest_value (rv->value);
 	}
@@ -1851,8 +1864,28 @@ finish_command (const char *arg, int from_tty)
     }
 
   /* Find the function we will return from.  */
-
-  sm->function = find_pc_function (get_frame_pc (get_selected_frame (NULL)));
+  frame_info *callee_frame = get_selected_frame (NULL);
+  sm->function = find_pc_function (get_frame_pc (callee_frame));
+
+  /* Determine the return convention.  If it is RETURN_VALUE_STRUCT_CONVENTION,
+     attempt to determine the address of the return buffer.  */
+  enum return_value_convention return_value;
+  struct gdbarch *gdbarch = get_frame_arch (callee_frame);
+
+  struct type * val_type
+    = check_typedef (sm->function->type()->target_type ());
+
+  return_value = gdbarch_return_value (gdbarch,
+				       read_var_value (sm->function, NULL,
+						       callee_frame),
+				       val_type, NULL, NULL, NULL);
+
+  if (return_value == RETURN_VALUE_STRUCT_CONVENTION
+      && val_type->code() != TYPE_CODE_VOID)
+    sm->return_buf = gdbarch_get_return_buf_addr (gdbarch, val_type,
+						  callee_frame);
+  else
+    sm->return_buf = 0;
 
   /* Print info on the selected frame, including level number but not
      source.  */
@@ -1870,7 +1903,7 @@ finish_command (const char *arg, int from_tty)
 	  gdb_printf (_("Run till exit from "));
 	}
 
-      print_stack_frame (get_selected_frame (NULL), 1, LOCATION, 0);
+      print_stack_frame (callee_frame, 1, LOCATION, 0);
     }
 
   if (execution_direction == EXEC_REVERSE)
diff --git a/gdb/ppc-sysv-tdep.c b/gdb/ppc-sysv-tdep.c
index 14effb93210..59145f9f166 100644
--- a/gdb/ppc-sysv-tdep.c
+++ b/gdb/ppc-sysv-tdep.c
@@ -28,6 +28,7 @@
 #include "objfiles.h"
 #include "infcall.h"
 #include "dwarf2.h"
+#include "dwarf2/loc.h"
 #include "target-float.h"
 #include <algorithm>
 
@@ -2156,3 +2157,39 @@ ppc64_sysv_abi_return_value (struct gdbarch *gdbarch, struct value *function,
   return RETURN_VALUE_STRUCT_CONVENTION;
 }
 
+CORE_ADDR ppc64_sysv_get_return_buf_addr (struct type *val_type,
+					  frame_info *cur_frame)
+{
+  /* On PowerPC, the ABI specifies r3 is the address where an argument
+     and the return value is stored.  The PowerPC ABI does not guarantee
+     r3 will be preserved when the function returns.  The function returns
+     the address of the buffer stored in r3.
+
+     Attempt to determine the value of r3 on entry to the function using the
+     DW_OP_entry_value DWARF entries.  This requires compiling the user
+     program with -fvar-tracking to resolve the DW_TAG_call_sites in the
+     binary file.  */
+  union call_site_parameter_u kind_u;
+  enum call_site_parameter_kind kind;
+  CORE_ADDR return_val = 0;
+
+  kind_u.dwarf_reg = 3;  /* First passed arg/return value is in r3.  */
+  kind = CALL_SITE_PARAMETER_DWARF_REG;
+
+  /* val_type is the type of the return value.  Need the pointer type
+     to the return value.  */
+  val_type = lookup_pointer_type (val_type);
+
+  try
+    {
+      return_val = value_as_address (value_of_dwarf_reg_entry (val_type,
+							       cur_frame,
+							       kind, kind_u));
+    }
+  catch (const gdb_exception_error &e)
+    {
+      warning ("Cannot determine the function return value.\n"
+	       "Try compiling with -fvar-tracking.");
+    }
+  return return_val;
+}
diff --git a/gdb/ppc-tdep.h b/gdb/ppc-tdep.h
index 44f63b145c6..299c87a169d 100644
--- a/gdb/ppc-tdep.h
+++ b/gdb/ppc-tdep.h
@@ -175,6 +175,8 @@ extern void ppc_collect_vsxregset (const struct regset *regset,
 				  const struct regcache *regcache,
 				  int regnum, void *vsxregs, size_t len);
 
+extern CORE_ADDR ppc64_sysv_get_return_buf_addr (type*, frame_info*);
+
 /* Private data that this module attaches to struct gdbarch.  */
 
 /* ELF ABI version used by the inferior.  */
diff --git a/gdb/rs6000-tdep.c b/gdb/rs6000-tdep.c
index 87b2faad36a..0a4f4458845 100644
--- a/gdb/rs6000-tdep.c
+++ b/gdb/rs6000-tdep.c
@@ -8235,7 +8235,11 @@ rs6000_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
   set_gdbarch_ps_regnum (gdbarch, tdep->ppc_ps_regnum);
 
   if (wordsize == 8)
-    set_gdbarch_return_value (gdbarch, ppc64_sysv_abi_return_value);
+    {
+      set_gdbarch_return_value (gdbarch, ppc64_sysv_abi_return_value);
+      set_gdbarch_get_return_buf_addr (gdbarch,
+				       ppc64_sysv_get_return_buf_addr);
+    }
   else
     set_gdbarch_return_value (gdbarch, ppc_sysv_abi_return_value);
 
diff --git a/gdb/testsuite/gdb.cp/non-trivial-retval.exp b/gdb/testsuite/gdb.cp/non-trivial-retval.exp
index 21c390bc937..93a3a6832f7 100644
--- a/gdb/testsuite/gdb.cp/non-trivial-retval.exp
+++ b/gdb/testsuite/gdb.cp/non-trivial-retval.exp
@@ -15,11 +15,18 @@
 
 # This file is part of the gdb testsuite
 
+set additional_flags ""
+
 if {[skip_cplus_tests]} { continue }
 
 standard_testfile .cc
 
-if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug c++}]} {
+if {[have_fvar_tracking]} {
+    set additional_flags "additional_flags= -fvar-tracking"
+}
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile [list debug c++ $additional_flags]]} {
+
     return -1
 }
 
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 5ab4df1bcf3..a9ebc11dfea 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -8468,6 +8468,14 @@ gdb_caching_proc have_fuse_ld_gold {
     return [gdb_simple_compile $me $src executable $flags]
 }
 
+# Return 1 if compiler supports fvar-tracking, otherwise return 0.
+gdb_caching_proc have_fvar_tracking {
+    set me "have_fvar_tracking"
+    set flags "additional_flags=-fvar-tracking"
+    set src { int main() { return 0; } }
+    return [gdb_simple_compile $me $src executable $flags]
+}
+
 # Return 1 if linker supports -Ttext-segment, otherwise return 0.
 gdb_caching_proc linker_supports_Ttext_segment_flag {
     set me "linker_supports_Ttext_segment_flag"
-- 
2.31.1