RFR: 8267988: C2: assert(!addp->is_AddP() || addp->in(AddPNode::Base)->is_top() || addp->in(AddPNode::Base) == n->in(AddPNode::Base)) failed: Base pointers must match (addp 1301)

[Roland Westrelin]

The failure occurs because, in a loop body, two AddP nodes are
chained:
(AddPNode base (AddPNode base ..) ..)

One can be sunk out of loop but not the other which results in:
(AddPNode (CastPP base) (AddPNode base ..) ..)

And the 2 AddP nodes no longer have the same base.

The fix I propose is to make sure sinking nodes out of loop preserve
the invariant that 2 chained AddP nodes have the same base. It's not
straightfoward given the 2 AddP nodes are sunk one after the other and
when sinking the first one, it's hard to tell whether the second one
can be sunk as well.

With the fix:

- when the first AddP is sunk, no CastPP node is added to pin the AddP
  out of the loop so if sinking the second AddP fails, the 2 AddP
  nodes still have the same base.

- when the second AddP is sunk, a CastPP node is created and wired so
  both AddP base is updated to the new CastPP.

This change also addresses another unrelated minor issue. If, for
instance the loop has a null check, the loop body already includes a
CastPP. So if that CastPP is used by an AddP that is sunk, we end up
with:

(AddPNode (CastPP (CastPP...

Then code in ConstraintCastNode::dominating_cast(), causes the CastPP
added to pin the AddP out of loop to be replaced by the CastPP in the
loop so sinking the AddP has no effect. To prevent that, I tweaked the
way a ConstrainedCast is marked as carrying a dependency so
ConstraintCastNode::dominating_cast() doesn't trigger.

-------------

Commit messages:
 - fix

Changes: https://git.openjdk.java.net/jdk/pull/4387/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4387&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267988
  Stats: 134 lines in 8 files changed: 76 ins; 1 del; 57 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4387.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4387/head:pull/4387

PR: https://git.openjdk.java.net/jdk/pull/4387