[PING] [PATCH v6] [gdb/mi] Don't treat references to compound values as "simple".

[Gareth Rees <grees@undo.io>]

This is a patch modifying the behaviour of the --simple-values option
but as discussed previously, if GDB maintainers agree that adding a
new option is the best approach, I would be happy to submit a patch
for that solution instead.

On Sat, 11 Mar 2023 at 11:49, Gareth Rees <grees@undo.io> wrote:
>
> SUMMARY
>
> The '--simple-values' argument to '-stack-list-arguments' and similar
> GDB/MI commands does not take reference types into account, so that
> references to arbitrarily large structures are considered "simple" and
> printed. This means that the '--simple-values' argument cannot be used
> by IDEs when tracing the stack due to the time taken to print large
> structures passed by reference.
>
> DETAILS
>
> Various GDB/MI commands ('-stack-list-arguments', '-stack-list-locals',
> '-stack-list-variables' and so on) take a PRINT-VALUES argument which
> may be '--no-values' (0), '--all-values' (1) or '--simple-values' (2).
> In the '--simple-values' case, the command is supposed to print the
> name, type, and value of variables with simple types, and print only the
> name and type of variables with compound types.
>
> The '--simple-values' argument ought to be suitable for IDEs that need
> to update their user interface with the program's call stack every time
> the program stops. However, it does not take C++ reference types into
> account, and this makes the argument unsuitable for this purpose.
>
> For example, consider the following C++ program:
>
>     struct s {
>         int v[10];
>     };
>
>     int
>     sum(const struct s &s)
>     {
>         int total = 0;
>         for (int i = 0; i < 10; ++i) total += s.v[i];
>         return total;
>     }
>
>     int
>     main(void)
>     {
>         struct s s = { { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 } };
>         return sum(s);
>     }
>
> If we start GDB in MI mode and continue to 'sum', the behaviour of
> '-stack-list-arguments' is as follows:
>
>     (gdb)
>     -stack-list-arguments --simple-values
>     ^done,stack-args=[frame={level="0",args=[{name="s",type="const s &",value="@0x7fffffffe310: {v = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}}"}]},frame={level="1",args=[]}]
>
> Note that the value of the argument 's' was printed, even though 's' is
> a reference to a structure, which is not a simple value.
>
> See https://github.com/microsoft/MIEngine/pull/673 for a case where this
> behaviour caused Microsoft to avoid the use of '--simple-values' in
> their MIEngine debug adapter, because it caused Visual Studio Code to
> take too long to refresh the call stack in the user interface.
>
> SOLUTIONS
>
> There are two ways we could fix this problem, depending on whether we
> consider the current behaviour to be a bug.
>
> 1. If the current behaviour is a bug, then we can update the behaviour
>    of '--simple-values' so that it takes reference types into account:
>    that is, a value is simple if it is neither an array, struct, or
>    union, nor a reference to an array, struct or union.
>
>    In this case we must add a feature to the '-list-features' command so
>    that IDEs can detect that it is safe to use the '--simple-values'
>    argument when refreshing the call stack.
>
> 2. If the current behaviour is not a bug, then we can add a new option
>    for the PRINT-VALUES argument, for example, '--scalar-values' (3),
>    that would be suitable for use by IDEs.
>
>    In this case we must add a feature to the '-list-features' command
>    so that IDEs can detect that the '--scalar-values' argument is
>    available for use when refreshing the call stack.
>
> PATCH
>
> This patch implements solution (1) as I think the current behaviour of
> not printing structures, but printing references to structures, is
> contrary to reasonable expectation.
>
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29554
> ---
>  gdb/NEWS                                     |  8 +++
>  gdb/doc/gdb.texinfo                          |  7 +++
>  gdb/mi/mi-cmd-stack.c                        |  9 +--
>  gdb/mi/mi-cmd-var.c                          | 27 ++++++---
>  gdb/mi/mi-cmds.h                             |  5 ++
>  gdb/mi/mi-main.c                             |  7 +--
>  gdb/python/py-framefilter.c                  |  6 +-
>  gdb/testsuite/gdb.mi/mi-stack.exp            | 16 ++++-
>  gdb/testsuite/gdb.mi/print-simple-values.cc  | 62 ++++++++++++++++++++
>  gdb/testsuite/gdb.mi/print-simple-values.exp | 53 +++++++++++++++++
>  10 files changed, 175 insertions(+), 25 deletions(-)
>  create mode 100644 gdb/testsuite/gdb.mi/print-simple-values.cc
>  create mode 100644 gdb/testsuite/gdb.mi/print-simple-values.exp
>
> diff --git a/gdb/NEWS b/gdb/NEWS
> index cc262f1f8a6..1971b76a736 100644
> --- a/gdb/NEWS
> +++ b/gdb/NEWS
> @@ -104,6 +104,14 @@ show always-read-ctf
>     without a thread restriction.  The same is also true for the 'task'
>     field of an Ada task-specific breakpoint.
>
> +** The '--simple-values' argument to the '-stack-list-arguments',
> +   '-stack-list-locals', '-stack-list-variables', and '-var-list-children'
> +   commands now takes reference types into account: that is, a value is now
> +   considered simple if it is neither an array, structure, or union, nor a
> +   reference to an array, structure, or union.  (Previously all references were
> +   considered simple.)  Support for this feature can be verified by using the
> +   '-list-features' command, which should contain "simple-values-ref-types".
> +
>  *** Changes in GDB 13
>
>  * MI version 1 is deprecated, and will be removed in GDB 14.
> diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
> index 954f1481dae..dff7ba374bd 100644
> --- a/gdb/doc/gdb.texinfo
> +++ b/gdb/doc/gdb.texinfo
> @@ -37948,6 +37948,13 @@ option (@pxref{GDB/MI Program Execution}).
>  @item data-disassemble-a-option
>  Indicates that the @code{-data-disassemble} command supports the @option{-a}
>  option (@pxref{GDB/MI Data Manipulation}).
> +@item simple-values-ref-types
> +Indicates that the @code{--simple-values} argument to the
> +@code{-stack-list-arguments}, @code{-stack-list-locals},
> +@code{-stack-list-variables}, and @code{-var-list-children} commands
> +takes reference types into account: that is, a value is considered
> +simple if it is neither an array, structure, or union, nor a reference
> +to an array, structure, or union.
>  @end ftable
>
>  @findex -list-target-features
> diff --git a/gdb/mi/mi-cmd-stack.c b/gdb/mi/mi-cmd-stack.c
> index 4c4662ab5d7..406bf85ceec 100644
> --- a/gdb/mi/mi-cmd-stack.c
> +++ b/gdb/mi/mi-cmd-stack.c
> @@ -645,13 +645,8 @@ list_args_or_locals (const frame_print_options &fp_opts,
>               switch (values)
>                 {
>                 case PRINT_SIMPLE_VALUES:
> -                 {
> -                   struct type *type = check_typedef (sym2->type ());
> -                   if (type->code () == TYPE_CODE_ARRAY
> -                       || type->code () == TYPE_CODE_STRUCT
> -                       || type->code () == TYPE_CODE_UNION)
> -                     break;
> -                 }
> +                  if (!mi_simple_type_p (sym2->type ()))
> +                   break;
>                   /* FALLTHROUGH */
>
>                 case PRINT_ALL_VALUES:
> diff --git a/gdb/mi/mi-cmd-var.c b/gdb/mi/mi-cmd-var.c
> index e026c239b87..36eea4619a7 100644
> --- a/gdb/mi/mi-cmd-var.c
> +++ b/gdb/mi/mi-cmd-var.c
> @@ -335,14 +335,27 @@ mi_print_value_p (struct varobj *var, enum print_values print_values)
>    if (type == NULL)
>      return 1;
>    else
> -    {
> -      type = check_typedef (type);
> +    return mi_simple_type_p (type);
> +}
>
> -      /* For PRINT_SIMPLE_VALUES, only print the value if it has a type
> -        and that type is not a compound type.  */
> -      return (type->code () != TYPE_CODE_ARRAY
> -             && type->code () != TYPE_CODE_STRUCT
> -             && type->code () != TYPE_CODE_UNION);
> +/* See mi-cmds.h.  */
> +
> +bool
> +mi_simple_type_p (struct type *type)
> +{
> +  type = check_typedef (type);
> +
> +  if (TYPE_IS_REFERENCE (type))
> +    type = check_typedef (type->target_type ());
> +
> +  switch (type->code ())
> +    {
> +    case TYPE_CODE_ARRAY:
> +    case TYPE_CODE_STRUCT:
> +    case TYPE_CODE_UNION:
> +      return false;
> +    default:
> +      return true;
>      }
>  }
>
> diff --git a/gdb/mi/mi-cmds.h b/gdb/mi/mi-cmds.h
> index 490f50484d9..d867c298df9 100644
> --- a/gdb/mi/mi-cmds.h
> +++ b/gdb/mi/mi-cmds.h
> @@ -226,4 +226,9 @@ using remove_mi_cmd_entries_ftype
>    = gdb::function_view<bool (mi_command *)>;
>  extern void remove_mi_cmd_entries (remove_mi_cmd_entries_ftype callback);
>
> +/* Return true if type is a simple type: that is, neither an array, structure,
> +   or union, nor a reference to an array, structure, or union.  */
> +
> +extern bool mi_simple_type_p (struct type *type);
> +
>  #endif /* MI_MI_CMDS_H */
> diff --git a/gdb/mi/mi-main.c b/gdb/mi/mi-main.c
> index 0013e5dfafd..cedad60d0b8 100644
> --- a/gdb/mi/mi-main.c
> +++ b/gdb/mi/mi-main.c
> @@ -1656,6 +1656,7 @@ mi_cmd_list_features (const char *command, char **argv, int argc)
>        uiout->field_string (NULL, "undefined-command-error-code");
>        uiout->field_string (NULL, "exec-run-start-option");
>        uiout->field_string (NULL, "data-disassemble-a-option");
> +      uiout->field_string (NULL, "simple-values-ref-types");
>
>        if (ext_lang_initialized_p (get_ext_lang_defn (EXT_LANG_PYTHON)))
>         uiout->field_string (NULL, "python");
> @@ -2458,7 +2459,6 @@ static void
>  print_variable_or_computed (const char *expression, enum print_values values)
>  {
>    struct value *val;
> -  struct type *type;
>    struct ui_out *uiout = current_uiout;
>
>    string_file stb;
> @@ -2478,12 +2478,9 @@ print_variable_or_computed (const char *expression, enum print_values values)
>    switch (values)
>      {
>      case PRINT_SIMPLE_VALUES:
> -      type = check_typedef (val->type ());
>        type_print (val->type (), "", &stb, -1);
>        uiout->field_stream ("type", stb);
> -      if (type->code () != TYPE_CODE_ARRAY
> -         && type->code () != TYPE_CODE_STRUCT
> -         && type->code () != TYPE_CODE_UNION)
> +      if (mi_simple_type_p (val->type ()))
>         {
>           struct value_print_options opts;
>
> diff --git a/gdb/python/py-framefilter.c b/gdb/python/py-framefilter.c
> index 0e8b2409636..e555dc3d879 100644
> --- a/gdb/python/py-framefilter.c
> +++ b/gdb/python/py-framefilter.c
> @@ -235,14 +235,10 @@ py_print_value (struct ui_out *out, struct value *val,
>    if (args_type == MI_PRINT_SIMPLE_VALUES
>        || args_type == MI_PRINT_ALL_VALUES)
>      {
> -      struct type *type = check_typedef (val->type ());
> -
>        if (args_type == MI_PRINT_ALL_VALUES)
>         should_print = 1;
>        else if (args_type == MI_PRINT_SIMPLE_VALUES
> -              && type->code () != TYPE_CODE_ARRAY
> -              && type->code () != TYPE_CODE_STRUCT
> -              && type->code () != TYPE_CODE_UNION)
> +              && mi_simple_type_p (val->type ()))
>         should_print = 1;
>      }
>    else if (args_type != NO_VALUES)
> diff --git a/gdb/testsuite/gdb.mi/mi-stack.exp b/gdb/testsuite/gdb.mi/mi-stack.exp
> index 633e090c13a..777a425894f 100644
> --- a/gdb/testsuite/gdb.mi/mi-stack.exp
> +++ b/gdb/testsuite/gdb.mi/mi-stack.exp
> @@ -87,6 +87,9 @@ proc test_stack_args_listing {} {
>      # -stack-list-arguments 1 1 1
>      # -stack-list-arguments 1 1 3
>      # -stack-list-arguments
> +    # -stack-list-arguments 1 1 300
> +    # -stack-list-arguments 2 1 1
> +    # -stack-list-arguments --simple-values 1 1
>
>      mi_gdb_test "231-stack-list-arguments 0" \
>         "231\\^done,stack-args=\\\[frame=\{level=\"0\",args=\\\[\\\]\},frame=\{level=\"1\",args=\\\[name=\"strarg\"\\\]\},frame=\{level=\"2\",args=\\\[name=\"intarg\",name=\"strarg\"\\\]\},frame=\{level=\"3\",args=\\\[name=\"intarg\",name=\"strarg\",name=\"fltarg\"\\\]\},frame=\{level=\"4\",args=\\\[\\\]\}\\\]" \
> @@ -119,6 +122,14 @@ proc test_stack_args_listing {} {
>      mi_gdb_test "235-stack-list-arguments 1 1 300" \
>         "235\\^done,stack-args=\\\[frame=\{level=\"1\",args=\\\[\{name=\"strarg\",value=\"$hex \\\\\"A string argument.\\\\\"\"\}\\\]\},frame=\{level=\"2\",args=\\\[\{name=\"intarg\",value=\"2\"\},\{name=\"strarg\",value=\"$hex \\\\\"A string argument.\\\\\"\"\}\\\]\},frame=\{level=\"3\",args=\\\[\{name=\"intarg\",value=\"2\"\},\{name=\"strarg\",value=\"$hex \\\\\"A string argument.\\\\\"\"\},\{name=\"fltarg\",value=\"3.5\"\}\\\]\},frame=\{level=\"4\",args=\\\[\\\]\}\\\]" \
>         "stack args listing 1 1 300"
> +
> +    mi_gdb_test "236-stack-list-arguments 2 1 1" \
> +       "236\\^done,stack-args=\\\[frame=\\{level=\"1\",args=\\\[\\{name=\"strarg\",type=\"char \\*\",value=\"$hex \\\\\"A string argument.\\\\\"\"\\}\\\]\\}\\\]" \
> +       "stack args listing 2 1 1"
> +
> +    mi_gdb_test "237-stack-list-arguments --simple-values 1 1" \
> +       "237\\^done,stack-args=\\\[frame=\\{level=\"1\",args=\\\[\\{name=\"strarg\",type=\"char \\*\",value=\"$hex \\\\\"A string argument.\\\\\"\"\\}\\\]\\}\\\]" \
> +       "stack args listing --simple-values 1 1"
>  }
>
>  proc test_stack_info_depth {} {
> @@ -157,7 +168,6 @@ proc test_stack_locals_listing {} {
>      # -stack-list-locals 0 (--no-values)
>      # -stack-list-locals 1 (--all-values)
>      # -stack-list-locals 2 (--simple-values)
> -    # -stack-list-arguments
>
>      mi_gdb_test "232-stack-list-locals 0" \
>         "232\\^done,locals=\\\[name=\"A\",name=\"B\",name=\"C\",name=\"D\"\\\]" \
> @@ -173,6 +183,10 @@ proc test_stack_locals_listing {} {
>         "232\\^done,locals=\\\[\{name=\"A\",value=\"1\"\},\{name=\"B\",value=\"2\"\},\{name=\"C\",value=\"3\"\},\{name=\"D\",value=\"\\{0, 1, 2\\}\"\}\\\]" \
>         "stack locals listing of names and values"
>
> +    mi_gdb_test "232-stack-list-locals 2" \
> +       "232\\^done,locals=\\\[\{name=\"A\",type=\"int\",value=\"1\"\},\{name=\"B\",type=\"int\",value=\"2\"\},\{name=\"C\",type=\"int\",value=\"3\"\},\{name=\"D\",type=\"int \\\[3\\\]\"\}\\\]" \
> +       "stack locals listing 2"
> +
>      mi_gdb_test "232-stack-list-locals --simple-values" \
>         "232\\^done,locals=\\\[\{name=\"A\",type=\"int\",value=\"1\"\},\{name=\"B\",type=\"int\",value=\"2\"\},\{name=\"C\",type=\"int\",value=\"3\"\},\{name=\"D\",type=\"int \\\[3\\\]\"\}\\\]" \
>         "stack locals listing, simple types: names and values, complex type: names and types"
> diff --git a/gdb/testsuite/gdb.mi/print-simple-values.cc b/gdb/testsuite/gdb.mi/print-simple-values.cc
> new file mode 100644
> index 00000000000..1aad724efb9
> --- /dev/null
> +++ b/gdb/testsuite/gdb.mi/print-simple-values.cc
> @@ -0,0 +1,62 @@
> +/* This test case is part of GDB, the GNU debugger.
> +
> +   Copyright 2022-2023 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 program for PRINT_SIMPLE_VALUES.
> +
> +   In the function f:
> +
> +   * The arguments i, ir, and irr are ints or references to ints, which
> +     must be printed by PRINT_SIMPLE_VALUES.
> +
> +   * The arguments a, s, and u are non-scalar values, which must not be
> +     printed by PRINT_SIMPLE_VALUES.
> +
> +   * The arguments ar, arr, sr, srr, ur, and urr are references to
> +     non-scalar values, which must not be printed by
> +     PRINT_SIMPLE_VALUES.  */
> +
> +struct s
> +{
> +  int v;
> +};
> +
> +union u
> +{
> +  int v;
> +};
> +
> +int
> +f (int i, int &ir, int &&irr,
> +   int a[1], int (&ar)[1], int (&&arr)[1],
> +   struct s s, struct s &sr, struct s &&srr,
> +   union u u, union u &ur, union u &&urr)
> +{
> +  return (i + ir + irr
> +          + a[0] + ar[0] + arr[0]
> +          + s.v + sr.v + srr.v
> +          + u.v + ur.v + urr.v);
> +}
> +
> +int
> +main (void)
> +{
> +  int i = 1, j = 2;
> +  int a[1] = { 4 }, b[1] = { 5 };
> +  struct s s = { 7 }, t = { 8 };
> +  union u u = { 10 }, v = { 11 };
> +  return f (i, j, 3, a, b, { 6 }, s, t, { 9 }, u, v, { 12 });
> +}
> diff --git a/gdb/testsuite/gdb.mi/print-simple-values.exp b/gdb/testsuite/gdb.mi/print-simple-values.exp
> new file mode 100644
> index 00000000000..9436645df84
> --- /dev/null
> +++ b/gdb/testsuite/gdb.mi/print-simple-values.exp
> @@ -0,0 +1,53 @@
> +# Copyright 2022-2023 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 of PRINT_SIMPLE_VALUES.
> +#
> +# Test that PRINT_SIMPLE_VALUES distinguishes simple and compound types,
> +# including C++ reference and rvalue reference types.
> +
> +require allow_cplus_tests
> +
> +load_lib mi-support.exp
> +set MIFLAGS "-i=mi"
> +
> +standard_testfile .cc
> +
> +if [build_executable "failed to prepare" $testfile $srcfile {debug c++}] {
> +    return -1
> +}
> +
> +if [mi_clean_restart $binfile] {
> +    return
> +}
> +
> +mi_runto_main
> +
> +mi_gdb_test "-break-insert f" "\\^done.*" "set breakpoint on f"
> +
> +mi_send_resuming_command "exec-continue" "exec-continue to breakpoint on f"
> +
> +mi_expect_stop "breakpoint-hit" "f" ".*" ".*" ".*" {.* disp="keep"} \
> +    "run until breakpoint on f"
> +
> +mi_gdb_test "-stack-list-arguments 2" \
> +    "\\^done,stack-args=\\\[frame=\\{level=\"0\",args=\\\[\\{name=\"i\",type=\"int\",value=\"1\"\\},\\{name=\"ir\",type=\"int &\",value=\"@$hex: 2\"\\},\\{name=\"irr\",type=\"int &&\",value=\"@$hex: 3\"\\},\\{name=\"a\",type=\"int \\*\",value=\"$hex\"\\},\\{name=\"ar\",type=\"int \\(&\\)\\\[1\\\]\"\\},\\{name=\"arr\",type=\"int \\(&&\\)\\\[1\\\]\"\\},\\{name=\"s\",type=\"s\"\\},\\{name=\"sr\",type=\"s &\"\\},\\{name=\"srr\",type=\"s &&\"\\},\\{name=\"u\",type=\"u\"\\},\\{name=\"ur\",type=\"u &\"\\},\\{name=\"urr\",type=\"u &&\"\\}\\\]\\},frame=\\{level=\"1\",args=\\\[\\\]\\}\\\]" \
> +    "stack arguments listing 2"
> +
> +mi_gdb_test "-stack-list-arguments --simple-values" \
> +    "\\^done,stack-args=\\\[frame=\\{level=\"0\",args=\\\[\\{name=\"i\",type=\"int\",value=\"1\"\\},\\{name=\"ir\",type=\"int &\",value=\"@$hex: 2\"\\},\\{name=\"irr\",type=\"int &&\",value=\"@$hex: 3\"\\},\\{name=\"a\",type=\"int \\*\",value=\"$hex\"\\},\\{name=\"ar\",type=\"int \\(&\\)\\\[1\\\]\"\\},\\{name=\"arr\",type=\"int \\(&&\\)\\\[1\\\]\"\\},\\{name=\"s\",type=\"s\"\\},\\{name=\"sr\",type=\"s &\"\\},\\{name=\"srr\",type=\"s &&\"\\},\\{name=\"u\",type=\"u\"\\},\\{name=\"ur\",type=\"u &\"\\},\\{name=\"urr\",type=\"u &&\"\\}\\\]\\},frame=\\{level=\"1\",args=\\\[\\\]\\}\\\]" \
> +    "stack arguments listing --simple-values"
> +
> +mi_gdb_exit
> --
> 2.26.0
>