Don't delete REG_UNUSED

[Jeffrey A Law]

  In message < 199811281758.RAA15326@phal.cygnus.co.uk >you write:
  > I think the actual problem is that flow didn't handle the PARALLEL
  > correctly.  If a register is set in an instruction but the value is not
  > used, a REG_UNUSED note should be added, no matter if the previous value
  > of the register is used in the insn or not.
I understand your point.  But I do not think it is safe or correct in the
case that Andreas showed.  Though I'm willing to reconsider.

  > Even if you haven't seen such a case in the past, that doesn't mean that
  > it can't happen.
Correct.  Maybe I should have been clearer.  I'm not aware of a case where a
a REG_UNUSED note could be inaccurate after reload and any code which created
an inaccurate REG_UNUSED note would have to be reconsidered if we were to make
the change Andreas suggested.

  > For example, consider a PARALLEL that sets an UNUSED register to a constant.
  > This constant could be needed for a reg_equiv_constant or secondary address
  > reload of a later instruction, and thus the previously unused register coul> d
  > be made used by reload inheritance or reload_cse_regs.
That would be a problem.  Depending on what we decide to do with Andreas's
patch we may (or may not) have to disable that kind of optimization if the
register has a REG_UNUSED note.  Or have reload remove the inaccurate note.
Or find another way to fix Andreas's problem.



Jeff