RFR: 8283408: Fix a C2 crash when filling arrays with unsafe

[Pengfei Li]

We recently found a segmentation fault issue in C2 compiler with some
code that uses Java Unsafe API to initialize an array in a loop. It can
be reproduced by below code snippet compiled by C2 on AArch64. It's also
reproducible on x86 with an additional VM option "-XX:+OptimizeFill".

  byte[] arr = new byte[size];
  int offset = unsafe.arrayBaseOffset(byte[].class);
  for (int i = offset; i < offset + size; i++) {
    unsafe.putByte(arr, i, val);
  }

This issue is caused by a NULL pointer in a C2 loop optimization phase
called intrinsify_fill. In this phase, array filling loop patterns are
recognized and replaced by some intrinsics. But filling operations with
Unsafe API call are not handled very well. From C2 mid-end's point of
view, a difference between an Unsafe call and a normal array access like
`arr[i] = val` is element addressing. For normal array accesses, C2 uses
two AddP nodes for computing an element's address - one for adding array
header size and another for adding the element's relative offset from
the header. But Unsafe calls may have only one AddP node for adding an
absolute offset of an element from the array base. In current code, the
intrinsify_fill phase creates an AddP node but with NULL input for above
case and eventually causes a segmentation fault.

In this patch, we add a check to allow one AddP node in array filling
patterns to be optional. After this fix, the case above can be optimized
by intrinsify_fill as well. We know that the Unsafe call is rarely used
in Java application code and developers should use it at their own risk.
But we still propose this fix because C2 crashes even Unsafe is used in
a correct way.

Jtreg hotspot::hotspot_all_no_apps, jdk::tier1~3 and langtools::tier1
are tested and no issue is found. We also create a new jtreg case within
this patch.

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

Commit messages:
 - 8283408: Fix a C2 crash when filling arrays with unsafe

Changes: https://git.openjdk.java.net/jdk/pull/7884/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7884&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283408
  Stats: 70 lines in 2 files changed: 67 ins; 2 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7884.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7884/head:pull/7884

PR: https://git.openjdk.java.net/jdk/pull/7884