type oddity

[Vladimir Kozlov]

This changes were made for next bug. Bug report has explanation.

4933273: Flattened alias type for [KlassPtr + offset] must be consistent

Vladimir

Tom Rodriguez wrote:
> While looking into a failure in the metadata repo I noticed this code in flatten_alias_type:
> 
>   // Klass pointers to object array klasses need some flattening                                                                                        
>   const TypeKlassPtr *tk = tj->isa_klassptr();
>   if( tk ) {
>     // If we are referencing a field within a Klass, we need                                                                                            
>     // to assume the worst case of an Object.  Both exact and                                                                                          
>     // inexact types must flatten to the same alias class.                                                                                              
>     // Since the flattened result for a klass is defined to be                                                                                          
>     // precisely java.lang.Object, use a constant ptr.                                                                                                  
>     if ( offset == Type::OffsetBot || (offset >= 0 && (size_t)offset < sizeof(Klass)) ) {
> 
>       tj = tk = TypeKlassPtr::make(TypePtr::Constant,
>                                    TypeKlassPtr::OBJECT->klass(),
>                                    offset);
>     }
> 
> It's followed by a couple checks for specific fields where the PTR is changes to NotNull for just those offsets.  It seems odd to me that Constant is used here since we're flattening a non constant type into a constant one for some fields but not for others.  We end up with a constant class for things like the java_mirror field which seems wrong:
> 
> precise klass java/lang/Object: 0x08142438:Constant:exact+64 *
> 
> For objArrayKlass we convert it back to NotNull.  The comment explaining why Constant is used doesn't really make sense either.  Anyone know why it's done this way?
> 
> tom