Jato ToDo
=========
Pekka Enberg <penberg@kernel.org>

Introduction
------------
This document is intended to be a starting point for people interested in
hacking on Jato. It includes a list of open projects with a brief description
on each of them to help a new contributor get started.

Performance
-----------
[quote, Michael A. Jackson]
____
``The First Rule of Program Optimization: Don't do it. The Second Rule of
Program Optimization (for experts only!): Don't do it yet.''
____

Startup Time
~~~~~~~~~~~~
The purpose of this project is to improve VM startup time. There are two
different classes of applications to optimize: command line applications such
as Ant and Maven and graphical user interface applications such as Eclipse.
Both classes might require different kinds of hacks to speed up the start up
time. You might need to optimize GNU Classpath and external libraries Jato uses
during VM startup.

Required skills::
    C, Java
Difficulty::
    Medium

Inline Caching
~~~~~~~~~~~~~~
Inline caching is an optimization technique that is able to improve method
invocation performance for virtual and interface methods by turning an indirect
call into a cheap comparison instruction and a direct call for the common case.
For background material, please check out a blog post by
http://www.azulsystems.com/blog/cliff-click/2010-04-08-inline-caches-and-call-site-optimization[Cliff
Click] ("Inline Caches and Call Site Optimization").

Required skills::
    C, x86
Difficulty::
    Medium

Method Inlining
~~~~~~~~~~~~~~~
Method inlining is an optimization where a method invocation is replaced with
an inline copy of the invoked method body. As shown by <<Suganuma02>>, static
inlining decisions only make sense for tiny methods where the body of the
method is smaller than the invocation site footprint. The purpose of this
project is to implement method inlining for tiny static methods and optionally,
if inline cache support is added to the VM, inlining of tiny virtual and
interface methods to the inline cache.

Required skills::
    C, x86
Difficulty::
    Medium

Static Single Assignment Form (SSA)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The SSA form is a type of intermediate representation in which every variable
is assigned exactly once which simplifies many optimization algorithms. The
purpose of this project is to implement algorithms for converting LIR to SSA
form and back to normal form for code emission stage. You are encouraged to
implement the iterative version of dominance-frontier calculation <<Cooper01>>
as it's simpler and easier to understand.

You are also encouraged to implement a simple SSA-based compiler optimization,
such as dead code elimination, as a proof-of-concept that the SSA form works as
expected.

Required skills::
    C
Difficulty::
    Hard

Array Bounds Check Elimination
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Out-of-bounds array stores and loads are required to throw the
+IndexOutOfBoundsException+. Currently the JIT compiler generates a bounds
check for every array access which is slow as seen in the SciMark2 benchmark,
for example. Fortunately, much of the array bounds checks can be eliminated
which speeds up SciMark by 40% and SPECjvm98 2% on average <<Wuerthinger07>>.

Required skills::
    C
Difficulty::
    Medium

Tail Call Optimizations
~~~~~~~~~~~~~~~~~~~~~~~
Functional languages such as Scala and Clojure make heavy use of tail-calls.
The purpose of this project is to implement tail call optimizations to speed up
functional languages on JVM. For background material, check out HotSpot
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4726340[Request for
Enhancement] on tail-call optimizations and a related blog post by
http://blogs.sun.com/jrose/entry/tail_calls_in_the_vm[John Rose] ("Tail calls
in the VM").

Required skills::
    C, x86
Difficulty::
    Hard

Garbage Collector
-----------------
[quote, Robert Sewell]
____
``If Java had true garbage collection, most programs would delete themselves upon execution.''
____

Exact GC
~~~~~~~~
Jato uses Boehm GC as its garbage collector. While Boehm GC is stable and
reliable, it's unnecessarily slow because it's based on a conservative GC
algorithm. The first step for a fully integrated and fast garbage collector is
to implement an exact GC that uses safepoints (also known as GC points) to
stop-the-world and GC maps to enumerate the GC root set. There's a broken
implementation of safepoints in 'vm/gc.c' that serves as a starting point for
the project.

Required skills::
    C, x86
Difficulty::
    Medium

Compacting GC
~~~~~~~~~~~~~
This project is for implementing a compacting GC on top of the core GC to
reduce memory fragmentation and speed up object allocation in the GC.  Please
note that this requires support from the VM and the JIT compiler because you
need to be able to move objects during compacting phase.

Required skills::
    C, x86
Difficulty::
    Hard

Low Latency GC
~~~~~~~~~~~~~~
The stop-the-world part of a GC can cause latencies in the order of many
milliseconds in Java applications. While there are pauseless GCs for special
purpose hardware, the purpose of this project is to implement GC latency
monitoring tools for the VM and investigate and implement practical solutions
for reducing GC latency on commodity hardware.

Required skills::
    C, x86
Difficulty::
    Hard

x86
---
Legacy floating point support (x87)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Add instruction selection and code emission for floating point arithmetic using
the x87 FPU instruction such as +fadd+ and +fdiv+. The main difference to the
project idea above is that the x87 instructions do not operate on regular
registers but use a stack-based approach which makes instruction selection
harder. As the SSE2 approach is more performant, the x87 support is only for
compatilibity with older x86 CPUs that do not support SSE.

Required skills::
    C, x86
Difficulty::
    Medium

Portability
-----------
[quote, Jim McCarthy]
____
``Portability is for canoes.''
____

Machine Architectures
~~~~~~~~~~~~~~~~~~~~~
Jato currently runs on IA-32 and AMD64 machine architectures but there are
other important architectures out there (e.g. ARM and PowerPC) that we want to
support. To port the VM to a new architecture, you need to introduce a new
'arch/<target>' directory that implements the following machine architecture
specific parts:

- Instruction encoding (see 'arch/x86/emit.c' and 'arch/x86/encoding.c')
- Instruction selection (see 'arch/x86/insn-selector.brg')
- Exception handling (see 'arch/x86/unwind_{32,64}.S')
- Signal bottom halves (see 'arch/x86/signal.c' and 'arch/x86/signal-bh.sh')
- +sun/misc/Unsafe+ locking primitives (see 'arch/x86/include/arch/cmpxchg*.h')

You also need to make sure the core code in 'vm', 'jit', and 'runtime'
directories works on your architecture. Special attention needs to be paid when
porting to big endian CPUs because of our x86 centric heritage.

Required skills::
    C, target machine architecture
Difficulty::
    Hard

Operating Systems
~~~~~~~~~~~~~~~~~
Linux is the only operating system currently supported by Jato. If you're
interested in adding support for other operating systems, you probably need to
hack on the following things:

- ABI issues (e.g. Microsoft x64 calling conventions)
- Thread-local storage (TLS)
- Signal handling

Porting to operating systems such as Darwin and BSDs that support recent
versions of GCC and that support POSIX is easier than porting to non-POSIX
operating systems such as Windows.

Required skills::
    C, target operating system
Difficulty::
    Medium to Hard

Clang
~~~~~
Clang is an interesting new compiler built on top of LLVM that claims faster
compilation times and better code generation. The purpose of this project is to
fix blatant GCC-ism in Jato to make it compile with Clang. Please note that you
might need to dive into LLVM and Clang sources if you run into Clang crashes
while compiling the VM.

Required skills::
    C, possibly LLVM
Difficulty::
    Medium to Hard

JVM
---

Support for OpenJDK
~~~~~~~~~~~~~~~~~~~
Jato uses GNU Classpath to provide essential Java APIs. Unfortunately the
development of GNU Classpath seems to have slowed after OpenJDK was released.
The purpose of this project is to OpenJDK as an alternative for providing
essential APIs for better Java compatibility. One possible way to do that is to
use The OpenJDK Common Virtual Machine Interface
http://fuseyism.com/openjdk/cvmi/[CVMI] as a starting point.

Required skills::
    C, Java
Difficulty::
    Medium

Java Native Interface (JNI) support
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The VM has partial support for Java Native Interface (JNI) API. The goal of
this project is to finish the API.

Required skills::
    C, Java
Difficulty::
    Medium

JSR 292: invokedynamic
~~~~~~~~~~~~~~~~~~~~~~
The goal of this project is to add support for the new +invokedynamic+ bytecode
instruction specified in JSR 292 that's designed to improve execution
performance of dynamic languages such as JRuby.

Required skills::
    C, Java
Difficulty::
    Medium

JVM Tool Interface (JVM TI)
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The JVM Tool Inteface is a native programming interface that's used by
development tool such as commercial YourKit Java Profiler.

Required skills::
    C, Java
Difficulty::
    Medium

Tools Support
-------------

Valgrind
~~~~~~~~
You can run Jato under Valgrind to detect bugs in the VM. However, to
accomplish that, Jato generates slightly different code because using signals
for lazy class initialization interracts badly with Valgrind. Furthermore,
throwing a +NullPointerException+ through +SIGSEGV+ also upsets Valgrind.

The goal of this project is to fix up Jato and Valgrind interractions. This
might require changes possibly in both projects.

Required skills::
    C, x86
Difficulty::
    Medium


[bibliography]
References
----------

[bibliography]

- [[[Cooper01]]] Keith Cooper et al. A Simple, Fast Dominance Algorithm. 2001.
  http://www.cs.rice.edu/~keith/EMBED/dom.pdf[URL]
- [[[Suganuma02]]] Toshio Suganuma et al. An Empirical Study of Method Inlining
  for a Java Just-In-Time Compiler. http://www.usenix.org/events/javavm02/suganuma/suganuma_html/[URL]
- [[[Wuerthinger07]]] Thomas Würthinger and Christian Wimmer. Array Bounds
  Check Elimination for the Java HotSpot™ Client Compiler. 2007.
  http://www.ssw.uni-linz.ac.at/Research/Papers/Wuerthinger07/[URL]
