Delivered presentation at JJUG (Japan Java Users Group) CCC 2017 Fall.

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Another compilation method in Java
AOT (Ahead of Time) Compilation
Akihiro Nishikawa
Oracle Corporation Japan
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2. Copyright © 2017, Oracle and/or its affiliates. All rights reserved.
Safe Harbor Statement
The following is intended to outline our general product direction. It is intended for
information purposes only, and may not be incorporated into any contract. It is not a
commitment to deliver any material, code, or functionality, and should not be relied upon
in making purchasing decisions. The development, release, and timing of any features or
functionality described for Oracle’s products remains at the sole discretion of Oracle.
2

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Agenda
JIT (Just-in-Time) Compilation
AOT (Ahead-of-Time) Compilation
Limitations
Summary
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2
3
4
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In this session, I mainly
cover HotSpot.

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JIT (Just in Time) Compilation
Source
Code
(*.java)
javac
Class file
(*.class)
HotSpot
Compile

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For improvement of warm up time
Tiered compilation
C1 (-client/client use) C2 (-server/server use)
• Longer compilation time and
longer startup time, but
generated code runs faster.
• Heavy optimization
• In case of prioritizing
performance after invocation...
• Shorter compilation time and
shorter startup time, but
generated code runs slow.
• Less optimization
• In case of prioritizing startup
time...

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Compilation Level
• 4: C2• 0: Interpreter
C2Interpreter C1
• 1: C1 full optimization
(no profiling)
• 2: C1 with profiling
about invocation and
back-edge only
• 3: C1 full profiling
(level2 + MDO)
1,500 10,000

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AOT – Ahead Of Time compilation
• Generate native code in advance
• JEP 295
• Based on Graal
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AOT Compilation
Source
Code
(*.java)
javac
Class file
(*.class)
HotSpot
jaotc
Shared
object file
(*.so)

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*.java
javac
*.class HotSpot
jaotc
*.so
javac Compile

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*.so
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Able to share “*.so” file among JVMs
HotSpot
HotSpot
HotSpot
HotSpot

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AOT looks like
CDS/AppCDS…
!

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CDS/AppCDS (Class Data Sharing)
• CDS is the scheme for sharing Java SE class library only.
• In case of AppCDS, application classes are also included.
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Class Data
classes.jsa
HotSpot
HotSpot
HotSpot
HotSpot

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Works for...
• Quicker warmup
• Lower total memory footprint
Does not work for...
• No feature to persist or share
machine (native) code.
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CDS/AppCDS

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Is AOT a new
compilation method ?
!

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$ gcc -o HelloAoT HelloAoT.c
In case of C...

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How about other Java
implementations?
!

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AOT Java Compilers
• IBM Java SDK for AIX
• IBM Java SDK for z/OS
• Oracle Java ME Embedded Client
• WebSphere Real Time
• Gluon VM
etc.
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Does AOT support tiered
compilation (C1/C2)?
!

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AOT Supports Tiered Compilation
• Non-tiered compilation mode (default)
–Similar to C1 in Client VM
–No collecting profiling information
–No JIT recompilation if AOT code is not deoptimized.
• Tied compilation mode (--compile-for-tiered)
–Profiling level is as same as C1 Level 2.
–If hitting AOT invocation thresholds, methods are recompiled by C1 at
Level 3 first for gathering full profiling information.
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What does AOT
work for?
!

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Benefits might be gained from AOT...
• No compilation overhead at runtime.
• Improve startup time and able to achieve peak performance
faster.
• Able to run on the platform where native code cannot be not
generated at runtime (e.g. iOS, embedded).
• Density improvement - Able to share native code.
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$ jaotc --output libHelloAOT.so HelloAOT.class
$ jaotc --output libjava.base.so --module
java.base
$ jaotc --output libmyapp.so --jar myapp.jar
$ jaotc -J-XX:+UseCompressedOops --output
libHelloAOT.so HelloAOT.class
How to use jaotc

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jaotc does not resolve all referenced classes
• jaotc does not resolve referenced classes which are not system
classes or part of compiled classes.
1. Have to add referenced classes to class path.
2. Specify additional java modules
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jaotc --output=libfoo.so --jar foo.jar ¥
-J-cp -J./
jaotc --output=libactivation.so --module ¥
java.activation -J--add-module=java.se.ee

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$ java -XX:AOTLibrary=./libHelloAOT.so HelloAOT
Hello AOT!
$ java -XX:+PrintAOT ¥
-XX:AOTLibrary=./libHelloAoT.so HelloAoT
13 1 loaded ./libHelloAoT.so aot library
76 1 aot[ 1] HelloAoT.<init>()V
76 2 aot[ 1] HelloAoT.main([Ljava/lang/String;)V
Hello AOT!
Run AOT compiled code

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On JVM startup...
• AOT initialization code looks for well-known AOT libraries in well-
known location ($JAVA_HOME/lib) or libraries specified using
-XX:AOTLibrary.
• JVM knows AOT library name for the following Java modules.
–java.base
–jdk.compiler (javac)
–jdk.scripting.nashorn (Nashorn)
–jdk.internal.vm.ci (JVMCI)
–jdk.internal.vm.compiler (Graal)
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VM Options should be synchronized.
• libjava.base.so
-XX:-UseCompressedOops -XX:+UseG1GC
• libjava.base-coop.so
-XX:+UseCompressedOops -XX:+UseG1GC
• libjava.base-nong1.so
-XX:-UseCompressedOops -XX:+UseParallelGC
• libjava.base-coop-nong1.so
-XX:+UseCompressedOops -XX:+UseParallelGC
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Limitations
• Experimental release
• No support
• No official document (except for JEP)
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First of all...

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Limitations
• Available only when using either Parallel or G1 GC on Linux x64
with libelf.so.
• AOT compilation must be executed on the same system or a
system with the same configuration on which AOT code will be
used by Java application.
• Unable to compile java code using dynamically generated classes
and bytecode (lambda expressions, InvokeDynamic)
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These limitations are in Java 9, and may be addressed in future releases.

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When and where to use AOT?
• Event-driven services such as Functions
• Embedded, IoT, mobile
• Not often invoked classes... (not hot code)
• Application code which should be protected from decompilers
...etc.
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If AOT is officially supported and is available on several platforms...

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Summary
• Since Java 9 (HotSpot), AOT compilation is available, but
positioned as experimental release and no official support.
• AOT have several characteristics and is expected to improve
performance of applications, especially such short-lived objects as
functions
• Working to remove current limitation and improve AOT.
32

33. Copyright © 2017, Oracle and/or its affiliates. All rights reserved.
Safe Harbor Statement
The preceding is intended to outline our general product direction. It is intended for
information purposes only, and may not be incorporated into any contract. It is not a
commitment to deliver any material, code, or functionality, and should not be relied upon
in making purchasing decisions. The development, release, and timing of any features or
functionality described for Oracle’s products remains at the sole discretion of Oracle.
33