frameless_look_for_prologue

[Michael Snyder]

Andrew Cagney wrote:
> 
> David Taylor wrote:
> >
> > I believe that every target that does:
> >
> >   set_gdbarch_frameless_function_invocation (gdbarch,
> >                                              frameless_look_for_prologue);
> >
> > has a bug.
> >
> > The function frameless_look_for_prologue invokes PROLOGUE_FRAMELESS_P
> > with one argument -- the pc of the *START* of the function.
> >
> > For backtraces, get_prev_frame wants to know not "does this function
> > eventually set up a frame if I execute far enough into it", but rather
> > "does this function have a frame at the point where the program has
> > currently stopped".
> 
> I don't think this is right.  As far as I know, the behavour is:
> 
>         o       gdb sets a breakpoint at the end
>                 of the function prologue
> 
>                 i.e.    break foo
>                 not     break *foo
> 
>         o       the target runs through to the end of
>                 the prologue so that the stack frame's
>                 construction is complete.
> 
> GDB can only do correct backtraces after the frame has been
> constructed.  GDB doesn't handle backtraces part way through a stack
> frame.
> 
> As far as I know, to make things so that GDB could re-construct a
> partially built frame, GDB would need to understand things like dwarf2's
> live range splitting stuff (correct name?) along with a few other dwarf2
> (?) features which would, together, let GDB construct its frame frame
> based on an aribtrary function address.

It varies from ABI to ABI, and base-port to base-port.
For some targets, backtrace can succeed even if you're
stopped at an arbitrary point in the prologue.  Of course, 
evaluation of the function's parameters probably won't work...