RFR: 8286625: C2 fails with assert(!n->is_Store() && !n->is_LoadStore()) failed: no node with a side effect [v2]

[Roland Westrelin]

> It's another case where because of overunrolling, the main loop is
> never executed but not optimized out and the type of some
> CastII/ConvI2L for a range check conflicts with the type of its input
> resulting in a broken graph for the main loop.
> 
> This is supposed to have been solved by skeleton predicates. There's
> indeed a predicate that should catch that the loop is unreachable but
> it doesn't constant fold. The shape of the predicate is:
> 
> (CmpUL (SubL 15 (ConvI2L (AddI (CastII int:>=1) 15) minint..maxint)) 16)
> 
> I propose adding a CastII, that is in this case:
> 
> (CmpUL (SubL 15 (ConvI2L (CastII (AddI (CastII int:>=1) 15) 0..max-1) minint..maxint)) 16)
> 
> The justification for the CastII is that the skeleton predicate is a
> predicate for a specific iteration of the loop. That iteration of the
> loop must be in the range of the iv Phi.
> 
> With the extra CastII, the AddI can be pushed through the CastII and
> ConvI2L and the check constant folds. Actually, with the extra CastII,
> the predicate is not implemented with a CmpUL but a CmpU because the
> code can tell there's no risk of overflow (I did force the use of
> CmpUL as an experiment and the CmpUL does constant fold)

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

  Update test/hotspot/jtreg/compiler/loopopts/TestOverUnrolling2.java
  
  Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8996/files
  - new: https://git.openjdk.java.net/jdk/pull/8996/files/8f4a999a..9c9120ef

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8996&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8996&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8996.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8996/head:pull/8996

PR: https://git.openjdk.java.net/jdk/pull/8996