查看JIT工作情况
Java代码
- public class VolatileBarrierExample {
- long a;
- volatile long v1=1;
- volatile long v2=1;
-
- void readAndWrite(){
- long j=v1;
- long i=v2;
- a=i+j;
- v1=i+1;
- long v=v1;
- v2=j*2;
- }
-
- public static void main(String[] args){
- final VolatileBarrierExample ex=new VolatileBarrierExample();
- for(int i=0;i<50000;i++)
- ex.readAndWrite();
- }
- }
运行命令：
- java -XX:+PrintCompilation VolatileBarrierExample
输出：
- VM option ‘+PrintCompilation‘
- 242 1 VolatileBarrierExample::readAndWrite (40 bytes)
- 249 1 % VolatileBarrierExample::main @ 10 (27 bytes)
用如下命令行可以更多地了解内联优化的实际情况以及优化发生的级别：
- java -XX:+PrintCompilation -XX:+UnlockDiagnosticVMOptions -XX:+PrintInlining -XX:+TieredCompilation VolatileBarrierExample
查看JIT生成的汇编代码
环境说明：
1. CPU：Intel i7
2. 操作系统：Ubuntu 12.04-amd64
3. JDK：openjdkversion "1.7.0-internal-fastdebug"，我使用的是fastdebug版本，可以直接使用openjdk。
4. hsdis 下载地址：https://kenai.com/projects/base-hsdis/downloads，也可自行下载源代码编译，将hsdis-*.so放在目录$JAVA_HOME/jre/lib/amd64/server下.
运行命令：
- java -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly VolatileBarrierExample
产生的汇编代码
- VM option ‘+UnlockDiagnosticVMOptions‘
- VM option ‘+PrintAssembly‘
- OpenJDK 64-Bit Server VM warning: PrintAssembly is enabled; turning on DebugNonSafepoints to gain additional output
- Loaded disassembler from hsdis-amd64.so
- Decoding compiled method 0x00007fae110f5890:
- Code:
- [Disassembling for mach=‘i386:x86-64‘]
- [Entry Point]
- [Constants]
- # {method} ‘readAndWrite‘ ‘()V‘ in ‘VolatileBarrierExample‘
- # [sp+0x20] (sp of caller)
- 0x00007fae110f59c0: mov 0x8(%rsi),%r10d
- 0x00007fae110f59c4: cmp %r10,%rax
- 0x00007fae110f59c7: jne 0x00007fae110cd920 ; {runtime_call}
- 0x00007fae110f59cd: nop
- 0x00007fae110f59ce: nop
- 0x00007fae110f59cf: nop
- [Verified Entry Point]
- 0x00007fae110f59d0: push %rbp
- 0x00007fae110f59d1: sub $0x10,%rsp
- 0x00007fae110f59d5: nop ;*synchronization entry
- ; - VolatileBarrierExample::readAndWrite@-1 (line 13)
- 0x00007fae110f59d6: mov 0x18(%rsi),%r10 ;*getfield v1
- ; - VolatileBarrierExample::readAndWrite@1 (line 13)
- 0x00007fae110f59da: mov 0x20(%rsi),%r11 ;*getfield v2
- ; - VolatileBarrierExample::readAndWrite@6 (line 14)
- 0x00007fae110f59de: mov %r10,%r8
- 0x00007fae110f59e1: add %r11,%r8
- 0x00007fae110f59e4: mov %r8,0x10(%rsi)
- 0x00007fae110f59e8: shl %r10
- 0x00007fae110f59eb: add $0x1,%r11
- 0x00007fae110f59ef: mov %r11,0x18(%rsi)
- 0x00007fae110f59f3: mov %r10,0x20(%rsi) ;*putfield v2
- ; - VolatileBarrierExample::readAndWrite@36 (line 18)
- 0x00007fae110f59f7: add $0x10,%rsp
- 0x00007fae110f59fb: pop %rbp
- 0x00007fae110f59fc: test %eax,0xa6875fe(%rip) # 0x00007fae1b77d000
- ; {poll_return}
- ……….
如果只查看某个方法产生的汇编代码，可以使用命令：
- java -XX:+UnlockDiagnosticVMOptions -XX:CompileCommand=print,*VolatileBarrierExample.readAndWrite VolatileBarrierExample
如果运行命令是出现此错误：
Could not load hsdis-amd64.so; library notloadable; PrintAssembly is disabled
可以设置
- export LD_LIBRARY_PATH=$JAVA_HOME/jre/lib/amd64/server/
参考资料：
1. http://chaoslawful.info/archives/387
2. https://wikis.oracle.com/display/HotSpotInternals/PrintAssembly
