如何使用 -XX：+UnlockDiagnosticVMOptions -XX：CompileCommand=print 选项与 JVM HotSpot

这些说明适用于 Linux （Ubuntu 10.04.4 LTS），但应该适用于您的操作系统。下载 Oracle JDK 7u3 并正确设置您的和环境变量后，请执行以下命令以检查可用选项：JAVA_HOMEPATH

java -XX:+AggressiveOpts -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+PrintFlagsFinal -version

您应该会看到 ，并且选项可用。使用该选项也将启用该选项。但是，您将需要HotSpot反汇编器插件才能工作;如果没有它，您可能会看到类似下面的内容：UnlockDiagnosticVMOptionsCompileCommandPrintAssemblyCompileCommandPrintAssemblyPrintAssembly

$ java -version
java version "1.7.0_03"
Java(TM) SE Runtime Environment (build 1.7.0_03-b04)
Java HotSpot(TM) Server VM (build 22.1-b02, mixed mode)
$ java -server -XX:+UnlockDiagnosticVMOptions '-XX:CompileCommand=print,*Main.main' Main
CompilerOracle: print *Main.main
Java HotSpot(TM) Server VM warning: printing of assembly code is enabled; turning on DebugNonSafepoints to gain additional output
Compiled method (c2)      68    1 %           Main::main @ 4 (49 bytes)
 total in heap  [0xb3a97548,0xb3a979ec] = 1188
 relocation     [0xb3a97610,0xb3a97624] = 20
 main code      [0xb3a97640,0xb3a97840] = 512
 stub code      [0xb3a97840,0xb3a97850] = 16
 oops           [0xb3a97850,0xb3a97858] = 8
 scopes data    [0xb3a97858,0xb3a97898] = 64
 scopes pcs     [0xb3a97898,0xb3a979e8] = 336
 dependencies   [0xb3a979e8,0xb3a979ec] = 4
Could not load hsdis-i386.so; library not loadable; PrintAssembly is disabled
OopMapSet contains 1 OopMaps

要获取 HotSpot 反汇编器插件，您需要构建它。查看OpenJDK 7u2源代码，hsdis插件自述文件说：

要将插件与JVM一起使用，您需要一个可以加载它的新版本。如果 JVM 的产品模式不接受 -XX：+PrintAssembly，则您没有足够新的版本。

要构建这个项目，你[需要]一个GNU binutils的副本来构建。

从理论上讲，这应该可以在Windows上构建，但是在Windows上获得一个有效的GNU构建环境已被证明是困难的。

我们在上面已经确认了Oracle JDK 7u3支持。我按照hsdis插件自述说明，下载了GNU binutils 2.22，将其放在hsdis目录中并运行。这最终产生了以下错误：PrintAssemblybuild/binutilsmake

hsdis.c:32:20: error: sysdep.h: No such file or directory

为了纠正这一点，我使用以下补丁更改了hsdis.c：

diff -r 6259c6d3bbb7 src/share/tools/hsdis/hsdis.c
--- a/src/share/tools/hsdis/hsdis.c Mon Dec 12 23:08:01 2011 -0800
+++ b/src/share/tools/hsdis/hsdis.c Thu Feb 23 09:26:37 2012 -0500
@@ -29,7 +29,7 @@

 #include "hsdis.h"

-#include <sysdep.h>
+#include <errno.h>
 #include <libiberty.h>
 #include <bfd.h>
 #include <dis-asm.h>

然后运行成功。现在只需将 hsdis 目录中的插件复制到 Oracle JDK 7u3 目录中即可。makehsdis-i386.sobuildjre/lib/i386

现在你可以看到反汇编的编译代码：

$ java -server -XX:+UnlockDiagnosticVMOptions '-XX:CompileCommand=print,*Main.main' Main
CompilerOracle: print *Main.main
Java HotSpot(TM) Server VM warning: printing of assembly code is enabled; turning on DebugNonSafepoints to gain additional output
Compiled method (c2)      68    1 %           Main::main @ 4 (49 bytes)
 total in heap  [0xb3999548,0xb39999ec] = 1188
 relocation     [0xb3999610,0xb3999624] = 20
 main code      [0xb3999640,0xb3999840] = 512
 stub code      [0xb3999840,0xb3999850] = 16
 oops           [0xb3999850,0xb3999858] = 8
 scopes data    [0xb3999858,0xb3999898] = 64
 scopes pcs     [0xb3999898,0xb39999e8] = 336
 dependencies   [0xb39999e8,0xb39999ec] = 4
Loaded disassembler from [snip]/jdk1.7.0_03/jre/lib/i386/hsdis-i386.so
Decoding compiled method 0xb3999548:
Code:
[Disassembling for mach='i386']
[Entry Point]
[Verified Entry Point]
[Constants]
  # {method} 'main' '([Ljava/lang/String;)V' in 'Main'
  0xb3999640: call   0xb6ff8510         ;   {runtime_call}
  0xb3999645: data32 xchg %ax,%ax
  0xb3999648: mov    %eax,-0x3000(%esp)
  0xb399964f: push   %ebp
  0xb3999650: sub    $0x38,%esp
  0xb3999656: mov    %ecx,%esi
  0xb3999658: mov    0x4(%esi),%ebp
  0xb399965b: mov    0x8(%esi),%edi
  0xb399965e: mov    (%ecx),%esi
  0xb3999660: mov    %ecx,(%esp)
  0xb3999663: call   0xb7078cf0         ;*iload_3
[snip]
  0xb399983e: hlt    
  0xb399983f: hlt    
[Exception Handler]
[Stub Code]
  0xb3999840: jmp    0xb39981e0         ;   {no_reloc}
[Deopt Handler Code]
  0xb3999845: push   $0xb3999845        ;   {section_word}
  0xb399984a: jmp    0xb397e220         ;   {runtime_call}
  0xb399984f: .byte 0x0
OopMapSet contains 1 OopMaps

#0 
OopMap{off=468}

我使用的测试类是：

public class Main {
    public static void main(final String[] args) {
        long x = 0;
        for (int i = 0; i < 1000000; i++) {
            x += calculate(i);
        }
        System.out.println("x=" + x);
    }

    private static long calculate(final int i) {
        return (long)i * (long)i;
    }
}

在我的情况下，看到反汇编的编译代码：

$ java -XX:CompileThreshold=1 -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly -XX:CompileCommand="compileonly pac/kage/MyClass myMethod" MyClass

在上面的例子中，有循环：，这就是为什么在我们的例子中没有1 000 000次迭代的情况下，我们需要选项（默认情况下，-server为10 000）来查看我们反汇编的编译代码。for (int i = 0; i < 1 000 000; i++) {...}-XX:CompileThreshold=1