Re: [RFC/RFA?] Should break FILE:LINENO skip prologue?

Jim Blandy
2008-01-09T19:11:26Z

Joel Brobecker <brobecker at adacore.com> writes:
> I would like to revive a discussion that started sometime in 2002.
> The idea is the following: When breaking on a given source line, if
> that line is inside a function prologue, skip the prologue (using
> the linetable to do so).
>
> In our experience, we have found that most users are not aware of
> the existence of function prologues. When they break on the line
> where a function is defined, they think the debugger is doing the
> same thing than it would do if it inserted the breakpoint using
> that function name. Unfortunately, it doesn't and they end up
> having problems trying to print parameter values [1].
>
> When we discussed this back in 2002, there wasn't an overwhelming
> support for this proposal, but I think that we had mild support.
> Both Apple and AdaCore have decided to implement this, and JimB
> was also supporting the idea. I searched the archives, and I didn't
> really find any negative support. I personally think that it's
> friendlier and is usually what the typical user expects.
>
> The only counter argument, IMO, is the fact that we're changing a
> behavior that has been there for a long time... But the current
> behavior is currently undocumented (or at least it wasn't in 2002
> :-), and users can use the "break *FUNC_NAME" syntax if they don't
> want the prologue to be skipped. Like Jim, I think this syntax
> makes better sense - in fact, I have always naturally used the
> *FUNC_NAME syntax in these cases, never really used line numbers.
>
> So I am making this proposal again, with the hope of reaching
> a decision, either yes or no. Or, as a compromise, we can control
> the behavior through as setting. I would argue for the default value
> to change the behavior, since: Deliberately breaking inside the
> prologue is a relatively uncommon operation; and the users who expect
> to break inside the prologue know what they are doing and will quickly
> find a way around.

I don't remember the original conversation, but the suggestion still
sounds like a good idea to me. (Whew!)

> [1]: I think we have made some progress with parameter/variable
> tracking with DWARF, but I think it's not activated by default
> and we also still support platforms where DWARF is not available.

What is "not activated by default", exactly? Is it in GCC? In GDB?
Where in GDB?

I've long had this dream that eventually we would just stop skipping
prologues altogether, because everything would actually work at every
instruction in the function.