RFR: 8280600: C2: assert(!had_error) failed: bad dominance

[Roland Westrelin]

For the inner loop of the test, c2, first, creates a counted loop and
assigns the type -400..1 to the trip count Phi. The loop body has a
range check that guards a CastII on the trip count Phi. That CastII
has type: 0..1 as a result.

Pre/main/post loops are then created. In the process,
PhaseIdealLoop::cast_incr_before_loop() inserts a CastII right before
the main loop. That CastII's input is the add node of the pre
loop. The loop is also unrolled once. The CastII on the loop entry is
pushed through the add node and replaced by the range check CastII
that has the same input and dominates. As a result, the main loop
tripcount Phi has type -400..1 but init value, 1..2. This causes the
Phi to constant fold to 1. The amount of unrolling and the main loop
bounds are inconsistent (the loop shouldn't be unrolled if it executes
for one iteration) but when the unrolling decision is made the type of
the lower bound of the main loop is not yet accurately known.

The actual crash is the result of a chain of nodes sunk out of the
inner loop and pinned out of the loop with a CastII node. The type of
the CastII and the type of its input after unrolling conflict.

The fix I propose is to change the type of the CastII that's added
before the main loop so it can't be replaced by a dominating
CastII. Given that CastII's role is to carry dependencies for nodes in
the loop body on the loop entry test, I think it's the right thing to
do. I've also been working on 8275202 (C2: optimize out more redundant
conditions) as a way to avoid type inconsistencies like this one.

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

Commit messages:
 - test
 - fix

Changes: https://git.openjdk.java.net/jdk/pull/7307/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7307&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280600
  Stats: 54 lines in 2 files changed: 53 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7307.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7307/head:pull/7307

PR: https://git.openjdk.java.net/jdk/pull/7307