Shark: fix ldc(Class)

[Gary Benson]

Andrew Haley wrote:
> Gary Benson wrote:
> > Andrew Haley wrote:
> > > 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...
> 
> Many of the CPUs we use won't be affected.  SPARC apparently can use
> either Total Store ordering (TSO) or Partial Store Order (PSO) or
> Relaxed Memory Order (RMO).  :-)
> 
> http://developers.sun.com/solaris/articles/atomic_sparc/
> 
> From what you say, I think the Java VM assumes TSO.
> 
> TSO is, approximately, what we have on x86_64 too.
> 
> PowerPC uses, as far as I can see, Relaxed Store Ordering.
> So, Load Acquire requires a sync.
[snip]

That's... unpleasant.  Changing Zero ought not to be too difficult;
orderAccess_linux_zero.inline.hpp has acquire() and release()
functions defined, right at the top, so presumably load_acquire et al
just need conditionalized calls to these inserting.  Shark on the
other hand... there will be other places in Shark aside from the one
you mentioned where I've inlined OrderAccess calls as simple loads and
stores.  There may not be any way around this other than an audit.

I filed http://icedtea.classpath.org/bugzilla/show_bug.cgi?id=282
about this so it doesn't get lost.

Cheers,
Gary

-- 
http://gbenson.net/