RFR(L): 8031755: Type speculation should be used to optimize explicit null checks

Roland Westrelin roland.westrelin at oracle.com
Fri Feb 28 09:43:58 PST 2014


Most Type profiling points record whether a null reference was seen but type speculation doesn’t currently take advantage of it. With this change, not only is the type seen at a profiling point feed to type speculation but also whether a null pointer was seen or not. This is then used to optimize null checks.

The _speculative field becomes:
const TypePtr*   _speculative;

so that if all we know from profiling is that a reference is not null:
_speculative = TypePtr::NOT NULL;

When a type is met with a TypePtr (NULL_PTR for instance), the meet must be applied to the speculative types as well. To keep the type system symmetric, the _speculative field had to be moved to class TypePtr. I also moved the _inline_depth there to keep all speculative stuff together but with the current code it could have stayed in class TypeOopPtr.

Traps caused by a failed speculative null check are recorded with speculative traps, similarly to what is done for a failed class check.

I made the following change:

*** 3561,3571 ****

      // Since klasses are different, we require a LCA in the Java
      // class hierarchy - which means we have to fall to at least NotNull.
      if( ptr == TopPTR || ptr == AnyNull || ptr == Constant )
        ptr = NotNull;

-     instance_id = InstanceBot;

      // Now we find the LCA of Java classes
      ciKlass* k = this_klass->least_common_ancestor(tinst_klass);
      return make(ptr, k, false, NULL, off, instance_id, speculative, depth);
    } // End of case InstPtr

--- 3706,3715 ——

because I hit a type not symmetric failure that I think it causes:

=== Meet Not Symmetric ===
t   =                   javax/management/openmbean/OpenType:AnyNull * (inline_depth=-2)
this=                   javax/management/openmbean/ArrayType:TopPTR *,iid=top (inline_depth=InlineDepthTop)
mt=(t meet this)=       javax/management/openmbean/ArrayType:AnyNull * (inline_depth=-2)
t_dual=                 javax/management/openmbean/OpenType:NotNull *,iid=top (inline_depth=2)
this_dual=              javax/management/openmbean/ArrayType *
mt_dual=                javax/management/openmbean/ArrayType:NotNull *,iid=top (inline_depth=2)
mt_dual meet t_dual=    javax/management/openmbean/OpenType:NotNull * (inline_depth=2)
mt_dual meet this_dual= javax/management/openmbean/ArrayType *


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