[libunwind] Several function exit code paths

[Johan Walles]

Assume we have a compiler that doesn't do any optimizations at all.  Then, if we 
have a function like this...

{
  int i;

  if (foo) {
    return 42;
  }

  bar();
  return 25;
}

... it will be compiled into something like this:

 1 alloc
 2 save the return address
 3 move the usp by -8

 4 test foo
 5 if foo was 0, goto fooIs0
 6 set r8=42
 7 move the usp by 8 (restore it)
 8 restore the return address
 9 br.ret

fooIs0:
10 call bar()
11 set r8=25
12 move the usp by 8 (restore it)
13 restore the return address
14 br.ret

... the function will (in our case) get a frame entry of three instructions at 
1-3.

Then, it will get two corresponding frame exits at lines 7-9 and 12-14).

Now, let's say I correctly register the frame entry code with libunwind.  I also 
register the two frame exits with libunwind.  In this situation, how does 
libunwind know that on line 10, the user stack pointer has the same value it had 
on line 4?

Should I register with libunwind that the br.ret at line 9 does the same thing 
as both line 2 and line 3?

If this is the way to go, can I use the same "when" parameter to several calls 
to _U_dyn_op()?

  Cheers //Johan