Shark: fix ldc(Class)

[Gary Benson]

Andrew Haley wrote:
> Gary Benson wrote:
> > Andrew Haley wrote:
> > > I've been trying to track down a mysterious bug in Shark for
> > > a couple of weeks, and I finally got it.  It turns out that
> > > there's no code generated for ldc(Class).  What fun!
> > >
> > > Oh well, I learned a lot...  :-)
> > 
> > Was it not hitting an Unimplemented() or similar?
> 
> It simply generated a read from the constant pool.  The cpool entry
> points to a classOopDesc, so code has to be generated to get from
> the classOopDesc to the Class.

Yuk :(

> Incidentally, I think there has to me a memory barrier between
> reading the tag and reading the constant pool entry.  The c++
> interpeter does this by calling byte_at_acquire().

A lot of those OrderAccess functions are barrierless on every platform:

 hotspot/src/share/vm/oops/constantPoolOop.hpp: constantTag tag_at(int which) const { return (constantTag) tags()->byte_at_acquire(which); }
  ...
 hotspot/src/share/vm/oops/typeArrayOop.hpp: jbyte byte_at_acquire(int which) const { return OrderAccess::load_acquire(byte_at_addr(which)); }
  ...
 hotspot/src/os_cpu/linux_sparc/vm/orderAccess_linux_sparc.inline.hpp:     inline jbyte OrderAccess::load_acquire(volatile jbyte* p) { return *p; }
 hotspot/src/os_cpu/linux_x86/vm/orderAccess_linux_x86.inline.hpp:         inline jbyte OrderAccess::load_acquire(volatile jbyte* p) { return *p; }
 hotspot/src/os_cpu/solaris_sparc/vm/orderAccess_solaris_sparc.inline.hpp: inline jbyte OrderAccess::load_acquire(volatile jbyte* p) { return *p; }
 hotspot/src/os_cpu/solaris_x86/vm/orderAccess_solaris_x86.inline.hpp:     inline jbyte OrderAccess::load_acquire(volatile jbyte* p) { return *p; }
 hotspot/src/os_cpu/windows_x86/vm/orderAccess_windows_x86.inline.hpp:     inline jbyte OrderAccess::load_acquire(volatile jbyte* p) { return *p; }

I just did the same for Zero.  It seemed hacky, but I wasn't sure what
else to do...

Cheers,
Gary

--
http://gbenson.net/