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

[Bruno Larsen <blarsen@redhat.com>]

Cheers,
Bruno

On 18/10/2022 20:55, Carl Love wrote:
> GDB maintainers:
>
> Version 2, updated comments per Kevin's comments.  Changed "frame_info
> *" to frame_info_ptr per Bruno's comments.  Discussed supressing the
> new warning after printing it with Kevin.  In the end we decided to not
> suppress the warnings.  Suppression can be added later if deemed
> necessary.  The white space introduced by the gdb/gdbarch-components.py
> needs to be addressed by a future patch to fix the script and the
> generated files.
>
> 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 re-tested on PowerPC and X86-64 with no regression
> failures.
>
> Please let me know if this version of the patch is acceptable for GDB
> mainline.  Thanks.
>
>                           Carl Love

This whole area of code is pure sorcery for me, but I do see that the 
patch fails to apply because of conflicts on gdbarch.c, due to Tom 
Tromey's "Inline initialization of gdbarch members" patch, and some 
style nits inlined.

Sorry for taking so long to reply to this that it is not applying again, 
but in the future I wouldn't wait for my review if someone approves the 
patch.

>
> -----------------------------------
> 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 gdbarch method to allow PowerPC to access the value
> of r3 on entry to a function. On PowerPC, the new gdbarch method 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_entry_value entries in the binary file allows GDB to resolve the
> DW_TAG_call_site entries.  This new gdbarch method 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 gdbarch method call for all architectures except PowerPC.
> The get_return_value function will be used to obtain the return value on
> all other architectures as is currently being done if RETURN_BUF is zero.
> On PowerPC, the new gdbarch method 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.
>
> Signed-off-by: Carl Love <cel@us.ibm.com>
> ---
>   gdb/arch-utils.c                            |  6 +++
>   gdb/arch-utils.h                            |  4 ++
>   gdb/dwarf2/loc.c                            | 10 +----
>   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                         | 41 +++++++++++++++++++++
>   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, 173 insertions(+), 14 deletions(-)
>
> diff --git a/gdb/arch-utils.c b/gdb/arch-utils.c
> index 6a72b3f5efd..1358987c02f 100644
> --- a/gdb/arch-utils.c
> +++ b/gdb/arch-utils.c
> @@ -1092,6 +1092,12 @@ default_read_core_file_mappings
>   {
>   }
>   
> +CORE_ADDR
> +default_get_return_buf_addr (struct type *val_type, frame_info_ptr 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 f6229f434fd..46018a6fbbb 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_ptr cur_frame);
>   #endif /* ARCH_UTILS_H */
> diff --git a/gdb/dwarf2/loc.c b/gdb/dwarf2/loc.c
> index 791648d6e7e..b88226db610 100644
> --- a/gdb/dwarf2/loc.c
> +++ b/gdb/dwarf2/loc.c
> @@ -1325,14 +1325,8 @@ static const struct lval_funcs entry_data_value_funcs =
>     entry_data_value_free_closure
>   };
>   
> -/* 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.  */
> -
> -static struct value *
> +/* See dwarf2/loc.h.  */
> +struct value *
>   value_of_dwarf_reg_entry (struct type *type, frame_info_ptr 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 d6709f2e342..9156e1ee533 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, frame_info_ptr 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_ptr 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 46e7565f293..325801256a2 100644
> --- a/gdb/gdbarch-components.py
> +++ b/gdb/gdbarch-components.py
> @@ -868,6 +868,21 @@ 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_ptr", "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 840de585869..66e2e761385 100644
> --- a/gdb/gdbarch-gen.h
> +++ b/gdb/gdbarch-gen.h
> @@ -441,6 +441,17 @@ 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_ptr cur_frame);
> +extern CORE_ADDR gdbarch_get_return_buf_addr (struct gdbarch *gdbarch, struct type *val_type, frame_info_ptr 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 559e92dee58..3bbb70e7be9 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));
> @@ -2684,6 +2690,23 @@ 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_ptr 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 d729732c81c..2e7cca1a1aa 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);
>   	}
> @@ -1852,8 +1865,28 @@ 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_ptr 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 ());
missing space before (.
> +
> +  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)
same here.
> +    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.  */
> @@ -1871,7 +1904,7 @@ 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);
>       }
>     frame.reinflate ();
>   
> diff --git a/gdb/ppc-sysv-tdep.c b/gdb/ppc-sysv-tdep.c
> index 14effb93210..f720e87e63b 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 +2157,43 @@ 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_ptr cur_frame)
> +{
> +  /* The PowerPC ABI specifies aggregates that are not returned by value
> +     are returned in a storage buffer provided by the caller.  The
> +     address of the storage buffer is provided as a hidden first input
> +     arguement in register r3.  The PowerPC ABI does not guarantee that
> +     register r3 will not be changed while executing the function.  Hence, it
> +     cannot be assumed that r3 will still contain the address of the storage
> +     buffer when execution reaches the end of the function.
> +
> +     This function attempts 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 34b250849b9..cbccd87820d 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_ptr);
> +
>   /* 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 8b6d666bbe7..637cbb41651 100644
> --- a/gdb/rs6000-tdep.c
> +++ b/gdb/rs6000-tdep.c
> @@ -8234,7 +8234,11 @@ 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 bfa9fec628e..d3b77b4869b 100644
> --- a/gdb/testsuite/lib/gdb.exp
> +++ b/gdb/testsuite/lib/gdb.exp
> @@ -8582,6 +8582,14 @@ 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"