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* [Bug mi/29554] New: --simple-values does not take reference types into account
@ 2022-09-07 15:46 grees at undo dot io
  2022-09-08  6:55 ` [Bug mi/29554] " grees at undo dot io
                   ` (5 more replies)
  0 siblings, 6 replies; 7+ messages in thread
From: grees at undo dot io @ 2022-09-07 15:46 UTC (permalink / raw)
  To: gdb-prs

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

            Bug ID: 29554
           Summary: --simple-values does not take reference types into
                    account
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: mi
          Assignee: unassigned at sourceware dot org
          Reporter: grees at undo dot io
  Target Milestone: ---

Created attachment 14321
  --> https://sourceware.org/bugzilla/attachment.cgi?id=14321&action=edit
Patch against 154f2735ad4

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, '--simplest-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 '--simplest-values' argument is
   available for use when refreshing the call stack.


PATCH

The attached patch implements solution (1) as I think the current
behaviour of not printing structures, but printing references to
structures, is surprising. However, if you prefer solution (2) I would
be happy to implement that instead.

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^ permalink raw reply	[flat|nested] 7+ messages in thread
end of thread, other threads:[~2023-05-04 16:32 UTC | newest]

Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-09-07 15:46 [Bug mi/29554] New: --simple-values does not take reference types into account grees at undo dot io
2022-09-08  6:55 ` [Bug mi/29554] " grees at undo dot io
2022-09-09  8:09 ` grees at undo dot io
2022-10-04  9:05 ` grees at undo dot io
2022-10-20 17:49 ` grees at undo dot io
2023-05-04 15:08 ` cvs-commit at gcc dot gnu.org
2023-05-04 16:32 ` tromey at sourceware dot org

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