Puzzled by a performance glitch? You might have to look at the generated code.
Examining generated code
The following HotSpot options (with an -XX: prefix on the command line) require OpenJDK 7 and an externally loadable disassembler plugin:
+PrintAssemblyprint assembly code for bytecoded and native methods
+PrintNMethodsprint nmethods as they are generated
+PrintNativeNMethodsprint native method wrappers as they are generated
+PrintSignatureHandlersprint native method signature handlers
+PrintAdapterHandlersprint adapters (i2c, c2i) as they are generated
+PrintStubCodeprint stubs: deopt, uncommon trap, exception, safepoint, runtime support
+PrintInterpreterprint interpreter code
These flags are "diagnostic", meaning that they must be preceded by
-XX:+UnlockDiagnosticVMOptions. On the command line, they must all be preceded by
-XX:. They may also be placed in a flags file,
.hotspotrc by default, or configurable as
The disassembly output is annotated with various kinds of debugging information, such as field names and source locations. The quality of this information improved markedly in January 2010 (bug fix 6912062).
Complete file: sample-disassembly.txt
There are 2 implementations around:
OpenJDK sources (defines the plugin API)
This version of the plugin requires the Gnu disassembler, which is available separately as part of the binutils project.
Kenai project base-hsdis
This is a from-scratch implementation which uses code from the Bastard project at SourceForge. The copyrights on this code are non-restrictive.
The Kenai project offers binary downloads.
Installing the Plugin
Once you succeed in building or downloading the
hsdis binary library (in the following named DLL), you have to install it next to your libjvm.so (jvm.dll on Windows), in the same folder. (Alternatively, you can put it anywhere on your
LD_LIBRARY_PATH.) The DLL must be given the name that the JVM will be looking for. The core of the name will be
hsdis-i386 for 32-bit Intel JVMs. Other names in use are
hsdis-sparcv9. A prefix and/or suffix will be required, according to system-dependent rules for naming DLLs.
The last line (with
myloopingbenchmark) is most typical, since it uses the batch execution mode common with benchmarks. The
-XX:+PrintCompilation flag will let you know which (if any) methods are being compiled.
-XX:+PrintAssembly option prints everything. If that's too much, drop it and use one of the following options.
Individual methods may be printed:
CompileCommand=print,*MyClass.myMethodprints assembly for just one method
CompileCommand=option,*MyClass.myMethod,PrintOptoAssembly(debug build only) produces the old print command output
CompileCommand=option,*MyClass.myMethod,PrintNMethodsproduces method dumps
These options accumulate.
If you get no output, use
-XX:+PrintCompilation to verify that your method is getting compiled at all.
Reading the compiler's mind
-XX:+LogCompilation flag produces a low-level XML file about compiler and runtime decisions, which may be interesting to some. The
-XX:+UnlockDiagnosticVMOptions must come first. The dump is to
hotspot.log in the current directory; use
-XX:LogFile=foo.log to change this.
LogCompilation output is basic line-oriented XML. It can usefully be read in a text editor, and there are also tools for parsing and scanning it.