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) [v4]

[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.

Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:

  review

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4387/files
  - new: https://git.openjdk.java.net/jdk/pull/4387/files/709809a9..29ffc393

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4387&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4387&range=02-03

  Stats: 9 lines in 1 file changed: 6 ins; 0 del; 3 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