RFR(S): 8224580: Matcher can cause oop field/array element to be reloaded

[Roland Westrelin]

Hi Dean,

Thanks for looking at this.

> It seems to do what you want, but I'm curious, what's the worse that can 
> happen if we call set_shared() on a node that doesn't need it?

Some complex match rules wouldn't trigger anymore. Let's say we have
(CmpN (LoadN ... and we erroneously call set_shared on the LoadN then
this:

instruct testN_mem(rFlagsReg cr, memory mem, immN0 zero)
%{
  predicate(CompressedOops::base() != NULL);
  match(Set cr (CmpN (LoadN mem) zero));

won't match and instead of a single instruction we would have a load
instruction followed by a comparison.

So worse case, correctness is not affected but code quality can be.

Roland.