code review request for MacOS X compressed oops work around (7118648)

[Daniel D. Daugherty]

Greetings,

The UseCompressedOops flag is enabled ergonomically in a 64-bit VM.
With MacOS X, we're seeing intermittent crashes in our automated
build/test system (JPRT) that look like this:

Internal Error 
(/private/tmp/jprt/P1/050642.jmelvin/source/src/share/vm/runtime/virtualspace.cpp:460), 
pid=53634, tid=4531154944
#  assert((_noaccess_prefix != 0) == (UseCompressedOops && _base != NULL 
&& (size_t(_base + _size) > OopEncodingHeapMax) && 
Universe::narrow_oop_use_implicit_null_checks())) failed: 
noaccess_prefix should be used only with non zero based compressed oops

I have filed the following bug to track that failure mode:

     7118647 3/3 compressed oops crashes on MacOS X with JPRT GCBasher test

In order to make more reliable progress on getting MacOS X fully
enabled in JPRT, I'm planning to disable compressed oops by default
in the 64-bit MacOS X VM until 7118647 can be fixed.

The work around is trivial so I'm including it here rather than
generate a webrev:

diff -r a5a9db0e0203 src/share/vm/runtime/arguments.cpp
--- a/src/share/vm/runtime/arguments.cpp        Tue Dec 06 07:23:42 2011 
-0800
+++ b/src/share/vm/runtime/arguments.cpp        Tue Dec 06 15:29:17 2011 
-0800
@@ -1359,9 +1359,12 @@
    // by ergonomics.
    if (MaxHeapSize <= max_heap_for_compressed_oops()) {
  #if !defined(COMPILER1) || defined(TIERED)
+#ifndef __APPLE__
+    // disable UseCompressedOops by default on MacOS X until 7118647 is 
fixed
      if (FLAG_IS_DEFAULT(UseCompressedOops)) {
        FLAG_SET_ERGO(bool, UseCompressedOops, true);
      }
+#endif // !__APPLE__
  #endif
  #ifdef _WIN64
      if (UseLargePages && UseCompressedOops) {


What the above code does is prevent the UseCompressedOops flag
from being set to true ergonomically on MacOS X. It can still be
set to true with the '-XX:+UseCompressedOops' option.

I'm planning to push this fix to RT_Baseline for the HSX-23-B08
snapshot. Jim Melvin has already reviewed the fix so I just need
one more reviewer.

Thanks, in advance, for any additional reviews.

Dan