Dataflow issues in the MIPS backend.

[David Daney]

As noted in http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32437#c5

 > I'm not surprised that converting it to an unspec_volatile stops
 > us from deleting the instruction, but that wasn't really my concern.
 >
 > As I said earlier, several other ports use top-level unspecs (rather than
 > unspec_volatiles), and it appears that after the dataflow merge, that 
construct
 > is no longer allowed.  Thus I think we should first nail down whether 
this
 > is a deliberate change.  If it is (and I can see that it might be),
 > then I think we should change the RTL generators so that top-level
 > unspecs are not allowed in define_insns.  I do not think we should the
 > MIPS port until we have established what the new rules are.
 >
 > (This isn't an issue about dataflow being more accurate per se;
 > the old code could also have deleted top-level unspecs with no
 > register dependencies, if it had thought that doing so was allowed.)

I would appreciate it if one or more dataflow maintainers were to opine 
on the correctness of the patch in the bug report and to address 
Richard's concerns.

Thanks,
David Daney