bug#34589: 26.1.91; GDB-MI Display Complex Data Types

[Gustaf Waldemarson <gustaf.waldemarson@gmail.com>]

Hello,

I've got a minor feature request to the gdb-mi (and possibly a bug to
report). I've recently been coding a lot of C++ where almost every variable is a
class/struct of some kind and up until now I've simply ignored the "locals"
windows since almost every data-type is listed as "<complex data type>".

Eventually though, I looked up why it was doing that instead of doing
some kind of pretty printing, and finally I noticed the
"--simple-values" flag, which causes GDB to print the variable name and
type, but not the value (for "complex" data types).

Since as C++ (and other languages) is moving more and more towards these
kinds non-simple data types, I figured I should send out a feature
request for something like the following:

- Remove "--simple-values" by default.

- Add a customizable variable such as `gdb-simple-values-only`, which
  adds the above flag.

- Add a customizable variable such as `gdb-locals-value-limit` to limit
  the length a value is allowed to be shown.

- Add a simple filter of the retrieved value string to make it fit on a
  single line (e.g., a simple version could just remove all newlines).

Naturally, the having "--simple-values" be default or not is really a
matter of taste, the above merely reflects my personal preference.

There might also be some efficiency concerns for really large
data-structure being sent through the GDB interface, but perhaps that's
already handled elsewhere?

Also, since it is tangential with the above topic: reference and pointer
variables to complex data types *are* being displayed already presumably
unintentionally. And since the output is not filtered, this leads to a
rather distorted local variable window.

A simple example showing the above "bug" can be seen in the program:

#include <iostream>
#include <vector>

using namespace std;

int main(void)
{
    size_t idx = 0;
    vector<int> v{1, 2, 3, 4, 5};
    auto &vr = v;
    for (auto i : v)
    {
        idx++;
        cout << i << endl;
    }
    return 0;
}

(Compile with CXXFLAGS="-std=c++11 -g" and open with gdb-mi as usual).

I guess this is caused by GDB considering references themselves to be
simple values. I believe the proposed feature(s) would fix this issue
and provide a more useful debugging experience.

Best regards,
Gustaf



In GNU Emacs 26.1.91 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.18.9)
 of 2019-01-10 built on E105888
Repository revision: 9845044fb659735251269213e6e4e6debdd08eee
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