Revamp sniffer; Was: [obish?sym;rfa:doc] Wire up vsyscall

[Andrew Cagney <cagney@gnu.org>]

2004-01-25  Daniel Jacobowitz  <drow@mvista.com>

	* i386-tdep.c (i386_gdbarch_init): Check for signal frame first.
	* i386-linux-tdep.c (i386_linux_pc_in_sigtramp): Handle
	__kernel_sigreturn.
[...]
I'm somewhat reluctant to throw the CFI completely overboard.
Yes.

How
about extending the DWARF2 unwinder such that we can set the
[SIGTRAMP_FRAME] attribute based on the function's name?
The frame attributes (tramp et.al.) in many cases are orthogonal to the 
unwind mechanism, but sometimes they are not).

At present the frame sniffer is focused solely on identifying and 
returning an unwinder.  The frame's type being identified as an 
unexpected sid effect.  I think this should be inverted vis:

frame_sniffer () ->
	- attributes
	- unwinder (or NULL, indicating sniff for unwinder)
	- base (or NULL, indicating sniff for base)
	- ...
so while the type is determined up front, other things are optional. 
That way here the tramp code would look like:

  if (name == _kernel_sigreturn)
    if (cfi at address)
      return { attribute=TRAMP; unwinder=cfi_unwinder; base=NULL };
    else
      return { attribute=TRAMP; unwinder=alt_unwinder; base=NULL };
this also opens the way for us doing:

  if (name == solib-trampoline)
    return { attribute=TRAMP|CALLEE; unwinder=NULL; base=NULL }
i.e., it's a trampoline (need to skip it) but let something more generic 
identify the unwinder.

Andrew