[jdk17] RFR: 8269230: C2: main loop in micro benchmark never executed

[Roland Westrelin]

The benchmark loop is roughly:

for (int i = 0; i < limit; i++) {
  if (!(i < max_jint && i > min_jint)) {
    uncommon_trap();
  }
}

which IfNode::fold_compares() tranforms to:

for (int i = 0; i < limit; i++) {
  if (!(i - min_jint - 1 <u -2)) {
    uncommon_trap();
  }
}

Range elimination then adjusts loop limits so the unsigned comparison
can be removed. The new limit is:

min(limit, max(-2 + min_jint + 1, min_jint)) = min(limit, min_jint) = min_jint

and the main loop is never executed.

The logic that protects against overflows during range check
elimination is what causes the main loop to never be executed. As a
fix, I propose delaying IfNode::fold_compares() if the resulting test
compares with a potential negative number which could then cause RC
overflow code to trigger. I think that would preserve the most
interesting cases for IfNode::fold_compares() when it comes to range
check elimination:

i >= 0 && i < array.length

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

Commit messages:
 - fix

Changes: https://git.openjdk.java.net/jdk17/pull/156/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=156&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269230
  Stats: 27 lines in 1 file changed: 27 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/156.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/156/head:pull/156

PR: https://git.openjdk.java.net/jdk17/pull/156