RFR(M): 8016697: PPC64 (part 5): Use stubs to implement safefetch

[Lindenmaier, Goetz]

Hi,

the PPC64 port uses stub routines to implement safefetch. We had and
have problems with inline assembly, especially with non-gcc
compilers. Stub routines allow to implement safefetch without
depending on OS or compiler, as it's the case with the current
implementation. This also allows to use a single implementation if an
architecture is supported on several os platforms.

http://cr.openjdk.java.net/~goetz/webrevs/8016697-safefetch/

Currently, we guard the code with
  #ifdef SAFEFETCH_STUBS
which is set in ppc64.make.

I could also imagine an implementation that uses a pd_debug flag
or a const flag set in os_<os_cpu>.hpp that allows the C-compiler to
optimize, and writing the code like this:

extern "C" int pd_SafeFetch32 (int * adr, int errValue) ;
extern "C" intptr_t pd_SafeFetchN (intptr_t * adr, intptr_t errValue) ;
inline int SafeFetch32(int* adr, int errValue) {
  if (UseSafefetchStub) {
    assert(StubRoutines::SafeFetch32_stub(), "stub not yet generated");
    return StubRoutines::SafeFetch32_stub()(adr, errValue);
  } else {
    pd_SafeFetch32(adr, errValue);
  }
}

Unfortunately this requires
  1) setting the flag on all platforms
  2) renaming the safefetch function on all platfoms.

Actually I would prefer this second solution as it avoids the preprocessor,
and am happy to edit the sources accordingly if this finds acceptance.

For the implementation of a safefetch_stub see
http://hg.openjdk.java.net/ppc-aix-port/jdk8/hotspot/file/fce884e5ba0b/src/cpu/ppc/vm/stubGenerator_ppc.cpp

Could I get a review on this change, please?

Thanks,
  Goetz.
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