Re: [RFC] expected behavior for "bt" command used with "set language ..." ?

[Pedro Alves <palves at redhat dot com>]

On 01/23/2018 11:51 AM, Joel Brobecker wrote:
>> Xavier> When printing one frame arguments, should we do it using the language
>> Xavier> of the frame, and it may be different for each frame in a single "bt"
>> Xavier> command or should we leave things as they are, and possibly allow the
>> Xavier> "bt" command to display weird values for frame arguments or even
>> Xavier> worse, crash GDB because the user set language manually so he has to
>> Xavier> know what he's doing ?
>>
>> I tend to think the answer should be:
>>
>> * If the language is "auto", then use each frame's language; otherwise
>> * If the user specified a particular language, use that language for
>>   everything.

I think that this is what makes sense.

The way I see it, "bt" should print frames as-if the user had manually
done a sequence of "up", "frame", "up", frame", ...
And in that case, we print each frame's arguments using the
frame's language (if language is "auto"), because "up" selects a
new frame, and that switches the language accordingly.  Right?

> 
> I don't really have a strong opinion on this. But I thought I'd mention
> that using a language to dump the value of a variable described using
> another language can be a bit iffy, and lead to fairly mysterious
> errors. If I was a fan of FUD, I might even say it can lead to crashes,
> if the code is not careful enough. For instance, who knows what it's
> going to look like asking Ada to print come C++ stuff, or vice-versa...

IMO, the uncertainty comes from lack of tests.  So instead of FUDing, we
should add some tests to raise the confidence level.

For example, we could have a testcase that picks a couple Ada constructs
that are quite unique to Ada, and then cycle through all languages,
printing the objects.  Same for C++.

And another testcase that calls C++ from Ada, and then does something
like:

while !$outermost_frame
  foreach lang $supported_languages
    gdb_test "set language $lang"
    gdb_test "frame"
  gdb_test "up"

Thanks,
Pedro Alves