Value Types Optimization Dependencies

[Karen Kinnear]

Value Types Optimization Dependencies

Optimizations in the JVM runtime for Value Types depend on three different kinds of characteristics:

Type Information:
  - Value Type vs. Identity Type
All the optimizations below depend on the static type information, specifically
the key type properties of being identity-less and immutable.

It is important to ensure Value Type Consistency to enable these optimizations.
This relies on disallowing value types to ever migrate to become identity types.
Value-Based-Classes migrating to value types are a corner case which will require
special handling.

Non-nullability/flattenability
Some optimizations additionally require non-nullability which enables flattenability.
 This could be thought of as container properties.

Runtime dynamic information
	Some optimizations additionally require dynamic information such as:
	is_flattened  (is flattened in heap containers)
                (requires flattenable, implementation-dependent)
            non-null (either non-nullable or dynamically non-null)

*1: Calling convention optimizations will require additional exploration. 
Here are some possibilities.

JIT optimizations have been destabilized with the introduction of the null reference for value types. Current implementation switches back to interpreted mode when a null reference to a value type is detected, it is primordial to have value types optimization re-enabled whenever possible. Considered explorations include:
null profiling to scalarize only value types that are non-null most of the time
support for multiple nmethods
use of a new non-null emotional type/carrier type to provide non-nullability guarantees
New JIT optimizations might also be required to fully optimize the different kind of value arrays.


Optimization
Dependencies
Value Types

value type:
identity-less & immutable
non-nullable
flattenable
dynamic non-null
is_flattened
flattening in containers
required
required
N/A
Implementation-specific
scalarization invocation: 
input arguments
required
not required
i2c adapter
null filter *1
not required
scalarization invocation: return
required
not required
caller check: if null, deopt
not required
scalarization field access
required
required

type check with null filter
required
scalarization
inside method
required
not required
type check
with null filter
not required





> On Jun 15, 2018, at 10:06 AM, Remi Forax <forax at univ-mlv.fr> wrote:
> 
> Hi Karen,
> the pdf was scrubbed.
> 
> Rémi
> 
> ----- Mail original -----
>> De: "Karen Kinnear" <karen.kinnear at oracle.com>
>> À: "valhalla-dev" <valhalla-dev at openjdk.java.net>
>> Envoyé: Vendredi 15 Juin 2018 16:00:08
>> Objet: Re: Value Types Optimization Dependencies
> 
>> resend again - Attaching the .pdf - I had cut-and-pasted from preview - sorry
>> that didn’t translate.
>> Feedback contents requested.
>> 
>> thanks,
>> Karen
>> 
>>> 
>>> Goal of the exercise is to clarify which optimizations depend on which
>>> characteristics, in part to
>>> understand the impact of null on optimizations.