RFR: JDK-8283249: CompressedClassPointers.java fails on ppc with 'Narrow klass shift: 0' missing

[Thomas Stuefe]

These tests are highly vulnerable against ASLR. On PPC, we apparently don't manage to map the heap into the lower 4G region, therefore CCS does not live there either.

The test requires for "-UseCompressedOops -Xshare:off" and a heap size of 128m and a ccs size of 1G ccs to live in the lower 4G. That is an optimistic assumption.

The VM attaches instead CCS to 0x100000000 (4GB) and calculates Narrow Base/Shift as 0/3, which is totally correct.

As a stop-gap measure, the test should be less strict for PPC and PPCLE.

---

Idle thoughts:

A future solution would be to revise this test. It is maintenance intensive and fragile. Arguably its also not that important, since it tests with CDS disabled, which is not a realistic scenario.

To me, it is not clear to me what it is supposed to test:
1) the ability of the kernel to give us memory segments at preferred addresses?
2) the ability of the VM to make the best of it, i.e. to calculate narrow klass base and shift that make sense?
3) the ability of the VM to decode/encode with exotic base/shift addresses

IMHO, 
- testing (1) makes no sense since we are at the mercy of the kernel.
- testing (2) makes a bit sense, but honestly, the logic to derive narrow klass base and shift is so simple that I don't think we need to test that. 
- testing (3) makes a lot of sense, since the CPU dependent logic for decoding and encoding can be complicated and usually only runs with a given set of base and shift (we had e.g. hidden bugs on s390 for years). But that one can be tested way better with our new option `-XX:CompressedClassSpaceBaseAddress`.

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

Commit messages:
 - start

Changes: https://git.openjdk.java.net/jdk/pull/7837/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7837&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283249
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7837.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7837/head:pull/7837

PR: https://git.openjdk.java.net/jdk/pull/7837