Re: breakpoints in constructors

[Daniel Jacobowitz <drow at mvista dot com>]

On Wed, Apr 30, 2003 at 11:58:16AM -0700, Michael Eager wrote:
> Daniel Jacobowitz wrote:
> > 
> > On Tue, Apr 29, 2003 at 04:45:06PM -0400, Paul Koning wrote:
> > > >>>>> "Daniel" == Daniel Jacobowitz <drow@mvista.com> writes:
> > >
> > >  Daniel> On Fri, Apr 18, 2003 at 01:04:46PM -0700, David Carlton
> > >  Daniel> wrote:
> > >  >> I might have some time over the next few weeks (/months) to work
> > >  >> on the "breakpoints in constructors" issue.  Daniel: clearly
> > >  >> you've thought about this already, so if you happen to have time
> > >  >> to do a bit of a brain dump on the issue at some point, I'd
> > >  >> appreciate it.
> > >
> > >  Daniel> Sure.  First of all, a rough overview of the problem; might
> > >  Daniel> as well keep everything in one place.
> > >
> > >  Daniel> With the new GCC 3.x multi-vendor C++ ABI, constructors are
> > >  Daniel> implemented as multiple functions: C1, the complete object
> > >  Daniel> constructor [in-charge] C2, the base object constructor
> > >  Daniel> [not-in-charge] C3, the allocating constructor [not currently
> > >  Daniel> used]
> > >
> > >  Daniel> Similarly for destructors - most of the rest of this message
> > >  Daniel> applies to destructors too.  The base constructor is
> > >  Daniel> generally called for the base objects of a derived class,
> > >  Daniel> esp. with virtual inheritance; it's been a while since I
> > >  Daniel> looked at exactly when.
> > >
> > >  Daniel> GCC has chosen to implement this by duplicating the function,
> > >  Daniel> including any user-provided code and any compiler-added code.
> > >  Daniel> A better implementation would have one copy and labels for
> > >  Daniel> multiple entry points, on systems where that is supported;
> > >  Daniel> that's temporarily tabled pending a better description of the
> > >  Daniel> GCC tree structure to describe multiple entry points.
> > >
> > > I looked at a few examples to see how they differ.  Didn't see any
> > > where the two constructors that gcc generates differ at all.  Ditto
> > > for the two (in charge vs. not in charge) destructors.
> > >
> > > The "deleting" constructor does what the name suggests, it frees the
> > > item at the end.  Since the difference is at the end, that doesn't
> > > sound like a case where multiple entry points can help.
> > >
> > > Couldn't one constructor/destructor call another, so that there one
> > > "bottom level" constructor or destructor where all three variants
> > > eventually end up?  Then that would be the one you'd want to match
> > > when you set a breakpoint by name or by line.
> > >
> > > The only drawback I can see is that you'd see an extraneous frame in
> > > the callstack.
> > 
> > Wow, Paul, you're really on top of this one.  Yes, that's what Apple
> > implemented, and I'm looking over their patches right now :)  There are
> > some quirks in the implementation which are throwing me for a loop.
> > 
> > The constructors will differ in some cases involving virtual bases;
> > that's what they're for.
> 
> I think that you can generate a jump rather than a call, so there is no 
> extraneous call frame on the stack.  (I think that's what the Apple patch
> does, actually.)

Ah, yes, you're right.  But you end up with the extra call if you
compile with -O0.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer