RFR: 8341976: C2: use_mem_state != load->find_exact_control(load->in(0)) assert failure [v8]

[Roland Westrelin]

> The `arraycopy` writes to a non escaping array so its `ArrayCopy` node
> is marked as having a narrow memory effect. One of the loads from the
> destination after the copy is transformed into a load from the source
> array (the rationale being that if there's no load from the
> destination of the copy, the `arraycopy` is not needed). The load from
> the source has the input memory state of the `ArrayCopy` as memory
> input. That load is then sunk out of the loop and its control is
> updated to be after the `ArrayCopy`. That's legal because the
> `ArrayCopy` only has a narrow memory effect and can't modify the
> source. The `ArrayCopy` can't be eliminated and is expanded. In the
> process, a `MemBar` that has a wide memory effect is added. The load
> from the source has control after the membar but memory state before
> and because the membar has a wide memory effect, the load is anti
> dependent on the membar: the graph is broken (the load can't be pinned
> after the membar and anti dependent on it).
> 
> In short, the problem is that the graph is transformed under the
> assumption that the `ArrayCopy` has a narrow effect but the
> `ArrayCopy` is expanded to a subgraph that has a wide memory
> effect. The fix I propose is to not insert a membar with a wide memory
> effect. We still need a membar when the destination is non escaping
> because the expanded `ArrayCopy`, if it writes to a tighly allocated
> array, writes to raw memory and not to the destination memory slice.

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

  review

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23465/files
  - new: https://git.openjdk.org/jdk/pull/23465/files/9b21648d..a76839de

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23465&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23465&range=06-07

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23465.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23465/head:pull/23465

PR: https://git.openjdk.org/jdk/pull/23465