[patch/rfc] Try to get dummy calls working on hpux again

[Andrew Cagney]

>>Can push_dummy_call set these space registers, and the generic pop dummy 
>>> frame code restore them?
> 
> 
> No. Unfortunately it's not possible to use the regular ttrace mechanism
> to set the pcsqh/pcsqt (aka iasq[0] and iasq[1]) registers.... bascially
> you need to execute a bit of code:
> 
> (assuming pcoqh (aka pc) is in r21 and pcsqh is in r22)
> ldsid (%r22), %r1
> mtsp %r1, %sr0
> be 0(%sr0, %r21)
> 
> this branches to (%r22, %r21), thereby setting the space register ...

Sigh (the relevant code will need comments explaining this).  Are you 
absolutely positively certain this is true (for both HP/UX 10.20 / 
11.xx)? :-)

Also that sequence in the call path, the return path, or both?

Having the dummy-code containing a non-trivial sequence of instructions 
opens up the problem of needing to be able to step through them.  Issues 
similar to the grief I've been going through with signal trampolines.

Andrew