How to see JIT-compiled code in JVM?

Is there some way to see the native code produces by the JIT in a JVM?

###General usage

As explained by other answers, you can run with the following JVM options:

-XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly

###Filter on a specific method

You can also filter on a specific method with the following syntax:

-XX:+UnlockDiagnosticVMOptions -XX:CompileCommand=print,*MyClass.myMethod

Notes:

• you might need to put the second argument within quotes depending on OS etc.
• if the method gets inlined, you could miss some optimisations

###How to: Install the required libraries on Windows

If you are running Windows, this page has instructions on how to build and install hsdis-amd64.dll and hsdis-i386.dll which are required to make it work. We copy below and extend the content of that page* for reference:

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Where to get prebuilt binaries

You can download prebuilt binaries for Windows from the fcml project

• hsdis-amd64.dll
• hsdis-i386.dll

How to build hsdis-amd64.dll and hsdis-i386.dll on Windows

This version of the guide was prepared on Windows 8.1 64bit using 64-bit Cygwin and producing hsdis-amd64.dll

1. Install Cygwin. At the Select Packages screen, add the following packages (by expanding the Devel category, then clicking once on the Skip label next to each package name):

• make
• mingw64-x86_64-gcc-core (only needed for hsdis-amd64.dll)
• mingw64-i686-gcc-core (only needed for hsdis-i386.dll)
• diffutils (in Utils category)

2. Run the Cygwin Terminal. This can be done using the Desktop or Start Menu icon created by the installer, and will create your Cygwin home directory (C:\cygwin\home\<username>\ or C:\cygwin64\home\<username>\ by default).

3. Download the latest GNU binutils source package and extract its contents to your Cygwin home directory. At the time of writing, the latest package is binutils-2.25.tar.bz2. This should result in a directory named binutils-2.25 (or whatever the latest version is) in your Cygwin home directory.

4. Download the OpenJDK source by going to the JDK 8 Updates repository, selecting the tag corresponding to your installed JRE version, and clicking bz2. Extract the hsdis directory (found in src\share\tools) to your Cygwin home directory.

5. In the Cygwin Terminal, enter cd ~/hsdis.

6. To build hsdis-amd64.dll, enter

   make OS=Linux MINGW=x86_64-w64-mingw32 'AR=$(MINGW)-ar' BINUTILS=~/binutils-2.25

To build hsdis-i386.dll, enter

make OS=Linux MINGW=i686-w64-mingw32 'AR=$(MINGW)-ar' BINUTILS=~/binutils-2.25

In either case, replace 2.25 with the binutils version you downloaded. OS=Linux is necessary because, although Cygwin is a Linux-like environment, the hsdis makefile fails to recognize it as such. 7. The build will fail with messages ./chew: No such file or directory and gcc: command not found. Edit <Cygwin home directory>\hsdis\build\Linux-amd64\bfd\Makefile in a text editor like Wordpad or Notepad++ to change SUBDIRS = doc po (line 342, if using binutils 2.25) to SUBDIRS = po. Re-run the previous command.

The DLL can now be installed by copying it from hsdis\build\Linux-amd64 or hsdis\build\Linux-i586 to your JRE's bin\server or bin\client directory. You can find all such directories on your system by searching for java.dll.

Bonus tip: if you prefer Intel ASM syntax to AT&T, specify -XX:PrintAssemblyOptions=intel alongside any other PrintAssembly options you use.

_{*page license is Creative Commons}

Assuming you're using the Sun Hotspot JVM (i.e. the one provided on java.com by Oracle), you can add the flag

> -XX:+PrintOptoAssembly

when running your code. This will print out the optimized code generated by the JIT compiler and leaves out the rest.

If you want see the entire bytecode, including the unoptimized parts, add

> -XX:CompileThreshold=#

when you're running your code.

You can read more about this command and the functionality of JIT in general here.

You need an hsdis plugin to use PrintAssembly. A convenient choice is the hsdis plugin based on FCML library.

It can be compiled for UNIX-like systems and on Windows you can use pre-built libraries available in the FCML download section on Sourceforge:

To install in Windows:

• Extract the dll (it can be found in hsdis-1.1.2-win32-i386.zip and hsdis-1.1.2-win32-amd64.zip).
• Copy the dll to wherever exists java.dll (use Windows search). On my system, I found it at two locations:
  • C:\Program Files\Java\jre1.8.0_45\bin\server
• C:\Program Files\Java\jdk1.8.0_45\jre\bin\server

To install in Linux:

• Download source code, extract it
• cd <source code dir>
• ./configure && make && sudo make install
• cd example/hsdis && make && sudo make install
• sudo ln -s /usr/local/lib/libhsdis.so <JDK PATH>/lib/amd64/hsdis-amd64.so
• sudo ln -s /usr/local/lib/libhsdis.so <JDK PATH>/jre/lib/amd64/hsdis-amd64.so
• On my system, the JDK is in /usr/lib/jvm/java-8-oracle

How to run it:

java -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly 
-XX:+LogCompilation -XX:PrintAssemblyOptions=intel,mpad=10,cpad=10,code 
-jar fcml-test.jar

Additional configuration parameters:

code Print machine code before the mnemonic.
intel Use the Intel syntax.
gas Use the AT&T assembler syntax (GNU assembler compatible).
dec Prints IMM and displacement as decimal values.
mpad=XX Padding for the mnemonic part of the instruction.
cpad=XX Padding for the machine code.
seg Shows the default segment registers.
zeros Show leading zeros in case of HEX literals.

The Intel syntax is a default one in case of Windows, whereas the AT&T one is a default for the GNU/Linux.

For more details see the FCML Library Reference Manual

For the HotSpot (was Sun) JVM, even in product modes:

http://wikis.oracle.com/display/HotSpotInternals/PrintAssembly

Some assembly required: it needs a plugin.

I believe WinDbg would be helpful if you are running it on windows machine. I have just run one jar.

• Then I attached to the java process through Windbg

• Examined threads by ~ command; There were 11 threads, 0 thread was main worker thread

• Switched to 0-thread - ~0s

• Looked through unmanmaged callstack by kb there was:

  0008fba8 7c90e9c0 ntdll!KiFastSystemCallRet
  0008fbac 7c8025cb ntdll!ZwWaitForSingleObject+0xc
  0008fc10 7c802532 kernel32!WaitForSingleObjectEx+0xa8
  0008fc24 00403a13 kernel32!WaitForSingleObject+0x12
  0008fc40 00402f68 java+0x3a13
  0008fee4 004087b8 java+0x2f68
  0008ffc0 7c816fd7 java+0x87b8
  0008fff0 00000000 kernel32!BaseProcessStart+0x23
  

Highlighted lines is direct running JIT-ed code on JVM.

• Then we can look for method address:
  java+0x2f68 is 00402f68
  

• On WinDBG:
  Click View --> Disassembly.
  Click Edit --> Go to Address.
  Put 00402f68 there
  and got

  00402f68 55 push ebp
  00402f69 8bec mov ebp,esp
  00402f6b 81ec80020000 sub esp,280h
  00402f71 53 push ebx
  00402f72 56 push esi
  00402f73 57 push edi
  ... and so on

For additional info here is the Example how to trace back JIT-ed code from memory dumps using process explorer and WinDbg.

Another way to see machine code and some performance data is to use AMD's CodeAnalyst or OProfile, which have a Java plugin to visualize executing Java code as machine code.

Print the assembly of your hotspots with JMH's perfasm profilers (LinuxPerfAsmProfiler or WinPerfAsmProfiler). JMH does require the hsdis library since it relies on PrintAssembly.