[Bug mi/29554] --simple-values does not take reference types into account

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29554

--- Comment #5 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Tom Tromey <tromey@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=51f8dafba8175a8d59716f220a46de4e626e5073

commit 51f8dafba8175a8d59716f220a46de4e626e5073
Author: Gareth Rees <grees@undo.io>
Date:   Sat Mar 11 11:49:34 2023 +0000

    Don't treat references to compound values as "simple".

    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

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