使用hsdis查看jit生成的彙編代碼

 http://blog.csdn.net/unei66/article/details/26477629

JVM 有 HotSpot引擎能夠對熱代碼路徑進行有效的 JIT優化，大幅度提高計算密集代碼的性能。默認一個方法至少被調用10k次以上纔可能被JIT優化。

 

查看JIT工做狀況

 

Java代碼

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();
    }
}

運行命令：

 

java -XX:+PrintCompilation VolatileBarrierExample

 

 

輸出：

VM option '+PrintCompilation'
242    1             VolatileBarrierExample::readAndWrite (40 bytes)
249    1 %           VolatileBarrierExample::main @ 10 (27 bytes)

 

用以下命令行能夠更多地瞭解內聯優化的實際狀況以及優化發生的級別：

 

java -XX:+PrintCompilation -XX:+UnlockDiagnosticVMOptions -XX:+PrintInlining -XX:+TieredCompilation VolatileBarrierExample

 

 

查看JIT生成的彙編代碼

環境說明：

1.      CPU：Intel i7

2.      操做系統：Ubuntu 12.04-amd64

3.      JDK：openjdkversion "1.7.0-internal-fastdebug"，我使用的是fastdebug版本，能夠直接使用openjdk。

4.      hsdis  下載地址：https://kenai.com/projects/base-hsdis/downloads，也可自行下載源代碼編譯，將hsdis-*.so放在目錄$JAVA_HOME/jre/lib/amd64/server下.

 

運行命令：

java -XX:+UnlockDiagnosticVMOptions -XX:+PrintAssembly VolatileBarrierExample

產生的彙編代碼

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}
……….

若是隻查看某個方法產生的彙編代碼，能夠使用命令：

java -XX:+UnlockDiagnosticVMOptions -XX:CompileCommand=print,*VolatileBarrierExample.readAndWrite VolatileBarrierExample

若是運行命令是出現此錯誤：

Could not load hsdis-amd64.so; library notloadable; PrintAssembly is disabled

能夠設置

export LD_LIBRARY_PATH=$JAVA_HOME/jre/lib/amd64/server/

參考資料：

1. http://chaoslawful.info/archives/387

2. https://wikis.oracle.com/display/HotSpotInternals/PrintAssembly