Bug ID: JDK-8143542 C2 doesn't eliminate identical checks

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JDK 9
9 b108Fixed

There are cases when C2 fails to remove redundant check (has identical dominating check).  

Reported by Vitaly Davidovich:
  http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2015-November/019930.html

Final IR from attached test case:
RedundantNullCheck::doIt() {
  w = Load this._w
  If (w == NULL) deopt
  i = Load w._i
  If (i != NULL) {  // check #1
    If (!i instanceof C) deopt
  } v1 = Phi(T:1,F:-1);
  If (i != NULL) {  // check #2
    If (!i instanceof C) deopt
  } v2 = Phi(T:2,F:-1)
  return v1+v2
}

Checks #1 & #2 are identical. #1 dominates #2, but C2 doesn't remove #2.

Next transformations should be:
  w = Load this._w
  If (w == NULL) deopt
  i = Load w._i
  If (i != NULL) {
    If (!i instanceof C) deopt
    If (!i instanceof C) deopt
  } v1 = Phi(T:1,F:-1)
    v2 = Phi(T:2,F:-1)
  return v1+v2

and

  w = Load this._w
  If (w == NULL) deopt
  i = Load w._i
  If (i != NULL) {
    If (!i instanceof C) deopt
  } v1 = Phi(T:1,F:-1)
    v2 = Phi(T:2,F:-1)
  return i1+i2;

URL: http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/894c8b63e200 User: lana Date: 2016-03-02 19:34:38 +0000

02-03-2016

URL: http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/rev/894c8b63e200 User: roland Date: 2016-02-12 20:44:53 +0000

12-02-2016

Regarding lack of test case: affects code generation (and not correctness) that can't be observed from a test case.

12-02-2016

Relates :	JDK-8275610 - C2: Object field load floats above its null check resulting in a segfault
Relates :	JDK-8278228 - C2: Improve identical back-to-back if elimination
Relates :	JDK-8149797 - Compilation fails with "assert(in_hash) failed: node should be in igvn hash table"