RFR: 8272562: C2: assert(false) failed: Bad graph detected in build_loop_late

[Roland Westrelin]

A counted loop has an array access:

(LoadI (AddP (AddP (ConvI2L (CastII (AddI (Phi ...)))))))

that is

array[iv - 1]

The Phi is the iv Phi. The ConvI2L/CastII are from a range check
and capture type: 0..maxint-1 The loop is unrolled once, the
LoadI is cloned.

array[iv - 1]
array[iv]

The first LoadI is only used out of loop and is sunk with 2
clones. One of the clones is on the IfFalse branch of a test
for iv != 0.

(LoadI (AddP (AddP (ConvI2L (CastII (AddI (CastII ...)))))))

The second CastII pins the nodes out of the loop. The ConvI2L and
CastII are pushed thru the AddI (for -1). As a result the ConvI2L
has type:
1..maxint-1

The CastII, because it has the same input as the input for iv != 0,
becomes 0 which is not part of 1..maxint-1. The ConvI2L and all its
uses including the LoadI become top. The use of the LoadI is a Phi
that is transformed into its remaining input and the graph is broken.

The root cause is that the loop body initially contains:

if (iv - 1 >=u array.length) { // range check
  trap();
}

if (iv == 0) {
  // path where nodes are sunk later on
}

And obviously if iv - 1 >= 0 then iv == 0 is always false but c2 fails
to prove it. I tried to implement a simple fix for this issue but
while it fixes this bug, I couldn't convince myself that it was robust
enough.

So instead I propose following the suggestion Christian and Vladimir I.
made in:

https://github.com/openjdk/jdk/pull/5199

that is to more generally exclude cast nodes from sinking as a
workaround for now.

I've been looking for a more general solution to this problem and I
have a prototype that fixes this failure but is a lot more
complicated. I'll revisit this workaround when it's ready.

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

Commit messages:
 - fix & test

Changes: https://git.openjdk.java.net/jdk/pull/5716/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5716&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272562
  Stats: 55 lines in 2 files changed: 54 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5716.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5716/head:pull/5716

PR: https://git.openjdk.java.net/jdk/pull/5716