Zero is an interpreter-only port of HotSpot that uses no assembler and
can trivially be built on any Linux system. The following webrev adds
Zero support to OpenJDK:

 http://cr.openjdk.java.net/~gbenson/zero-10/

Building is largely the same as the previous webrev (zero-07) except
for the following changes:

 - The variable CORE_BUILD is no longer used.
 - The variable ZERO_BITSPERWORD has been replaced with ARCH_DATA_MODEL

Summary of changes
==================

- The code has been reformatted to better match HotSpot's coding
  conventions. Notably, all opening braces have been moved to the
  end of the previous line, rather than being on their own lines.

- The variable CORE_BUILD is no longer required. Setting ZERO_BUILD
  is all that is required to enable Zero.

- The Zero build process no longer hijacks the core rules in the
  makefiles, but provides its own rules. A consequence of this is
  that the build directory is now linux_zero_zero rather than
  linux_zero_core. Another consequence is that Zero now has its
  own includeDB file, into which all direct #includes have been
  moved.

- The variable ZERO_BITSPERWORD has been replaced with the existing
  ARCH_DATA_MODEL.

- The type definition ZeroEntry::method_entry_t has been renamed.
  Tom Rodriguez suggested MethodEntryFunc, but Zero now supports
  OSR, which requires a different calling convention, so there is
  now ZeroEntry::NormalEntryFunc and ZeroEntry::OSREntryFunc.

- Zero no longer hijacks C2_BASE_DIR in hotspot/make/Makefile.
  ZERO_BASE_DIR is used instead.

- A duplicate definition of STATIC_CXX was removed.

- Zero's ICache now overrides some AbstractICache methods with
  empty methods, avoiding conditionals in icache.cpp. Comments
  in icache_zero.hpp explain this rationale.

- JNIHandles::clear is now protected on all platforms, not just Zero.

- Code from the ZeroStackPrinter class has been migrated to methods
  in ZeroFrame and its subclasses. stackPrinter_zero.hpp (previously
  directly included from vmError.cpp) has been removed, and most of
  the Zero-specific code in VMError::report has likewise been
  replaced. There is still a small amount of conditional code in
  VMError::report, but this could be removed entirely if it is not
  acceptable (at the expense of losing the super-detailed stack
  traces that Zero emits, eg http://tinyurl.com/y9rp4r3).

- frame::frame uses find_blob_unsafe instead of find_blob.

- All stubs contain ShouldNotCallThis() instead of Unimplemented().

Summary of non-changes
======================

- The variable ZERO_ENDIANNESS remains unchanged. I haven't renamed
  it as ENDIANNESS because it's only used during Zero builds. It's
  possible that the makefiles could be refactored such that, for
  example, existing platforms set ENDIANNESS (leaving Zero requiring
  it to be set externally) but I wasn't sure I should be doing this
  in this patch.

- The build directory is linux_zero_zero (or linux_zero_shark) rather
  than linux_$(ARCH)_zero. There is no variable defining the name of
  the build directory as such, it's always referenced as
  $(OSNAME)_$(BUILDARCH)_whatever, and in addition to specifiying the
  build directory, BUILDARCH is used in conditionals in various
  makefiles to add extra compiler flags, etc. If BUILDARCH is set to
  "i486", for example, then extra compiler flags are added which are
  unnecessary (and possibly problematic) for Zero.

- The variable ZERO_LIBARCH remains. It's used in several different
  places, and I couldn't figure out a place to put the logic to
  convert a uname string to the correct ZERO_LIBARCH value where the
  result would be accessible everywhere it is required.

Build Instructions
==================

 There are a number of environment variables that need setting before
 running make, but if you are using jdk/make/jdk_generic_profile.sh
 to set up your environment then you only need to set one, and a build
 can be as simple as:

   export ZERO_BUILD=true
   . jdk/make/jdk_generic_profile.sh
   gmake sanity && gmake

 The mandatory environment variable does the following:

   ZERO_BUILD
     Setting ZERO_BUILD to "true" will cause the Zero interpreter to
     be used. If ZERO_BUILD is unset, or set to any other value than
     "true", the standard, platform-specific interpreter will be used.

 Of the other environment variables the following are required when
 ZERO_BUILD is set to "true". These are set by
 jdk/make/jdk_generic_profile.sh based on the result of uname -m:

   ZERO_LIBARCH
     This is the name of the architecture-specific subdirectory under
     $JAVA_HOME/jre/lib. Typically this will be the same as the output
     of uname -m, although there are some exceptions: "amd64" instead
     of "x86_64", for example, and "i386" instead of "i686".

   ZERO_ARCHDEF
     The value of ZERO_ARCHDEF will be passed to the C++ compiler using
     -D${ZERO_ARCHDEF} to allow conditionalized platform-specific code.
     This is typically set to ZERO_LIBARCH converted to uppercase but,
     again, there are exceptions. "i386" becomes "IA32", to match what
     HotSpot already does, and on platforms with both 32- and 64-bit
     variants ZERO_ARCHDEF corresponds to the 32-bit version, so both
     ppc and ppc64 have ZERO_ARCHDEF set to "PPC".

   ZERO_ENDIANNESS
     This is set to "little" or "big".

   ARCH_DATA_MODEL
     This is set to "32" or "64".

   ZERO_ARCHFLAG
     This is a flag that will be passed to the C++ compiler and to the
     linker to instruct them to generate code for the particular
     platform. This is required for platforms with both 32- and 64-bit
     variants where the compiler needs to be told which variant to
     build for. ZERO_ARCHFLAG will typically be set to "-m32" or
     "-m64", except on 31-bit zSeries where it will be set to "-m31".

 Zero uses one external library, libffi, for JNI method invocation.
 The following two variables are used to tell the compiler and linker
 how to find libffi. These can be set by the user, but if left unset
 then jdk/make/jdk_generic_profile.sh will attempt to set them using
 pkg-config or, failing that, by supplying defaults pointing to the
 standard locations:

   LIBFFI_CFLAGS
     Flags to be passed to the C++ compiler to build against libffi.
     If unset (and pkg-config is not installed, or if libffi is not
     built with pkg-config) then this defaults to "".

   LIBFFI_LIBS
     Flags to be passed to the linker to link against libffi. If
     unset (and pkg-config is not installed, or if libffi is not
     built with pkg-config) then this defaults to "-lffi".