LinuxPerfAsmProfiler显示适用于Java 8的Java代码对应的汇编热点，而不适用于Java 14

如何解决LinuxPerfAsmProfiler显示适用于Java 8的Java代码对应的汇编热点，而不适用于Java 14

在调查与Spring的org.springframework.util.ConcurrentReferenceHashMap（自spring-core-5.1.3.RELEASE起）的实例化有关的问题时，我使用JMH随附的LinuxPerfAsmProfiler来剖析生成的程序集。

我只是运行这个

@Benchmark
public Object measureInit() {
  return new ConcurrentReferenceHashMap<>();
}

通过JDK 8上的基准测试，可以识别出非显而易见的热点之一：

  0.61%        0x00007f32d92772ea: lock addl $0x0,(%rsp)     ;*putfield count
                                                             ; - org.springframework.util.ConcurrentReferenceHashMap$Segment::<init>@11 (line 476)
                                                             ; - org.springframework.util.ConcurrentReferenceHashMap::<init>@141 (line 184)
 15.81%        0x00007f32d92772ef: mov    0x60(%r15),%rdx

这将不必要的默认值分配给了volatile字段：

protected final class Segment extends ReentrantLock {
  private volatile int count = 0;
}

和Segment依次在CCRHM的构造函数中循环实例化：

public ConcurrentReferenceHashMap(
    int initialCapacity,float loadFactor,int concurrencyLevel,ReferenceType referenceType) {
  this.loadFactor = loadFactor;
  this.shift = calculateShift(concurrencyLevel,MAXIMUM_CONCURRENCY_LEVEL);
  int size = 1 << this.shift;
  this.referenceType = referenceType;
  int roundedUpSegmentCapacity = (int) ((initialCapacity + size - 1L) / size);
  this.segments = (Segment[]) Array.newInstance(Segment.class,size);
  for (int i = 0; i < this.segments.length; i++) {
   this.segments[i] = new Segment(roundedUpSegmentCapacity);
  }
}

因此，该说明可能真的很热门。程序集的完整布局可以在我的gist

中找到

然后，我在JDK 14上运行相同的基准测试，然后再次使用LinuxPerfAsmProfiler，但是现在我没有任何明确指向volatile int count = 0 in captured assembly的指针。

正在寻找lock addl $0x0前缀下的0分配的lock指令，我发现了这一点：

  0.08%                          │  0x00007f3717d46187:   lock addl $0x0,-0x40(%rsp)
 23.74%                          │  0x00007f3717d4618d:   mov    0x120(%r15),%rbx

可能与volatile int count = 0相对应，因为它遵循Segment的超类ReentrantLock的构造函数调用：

  0.77%                          │  0x00007f3717d46140:   movq   $0x0,0x18(%rax)              ;*new {reexecute=0 rethrow=0 return_oop=0}
                                 │                                                            ; - java.util.concurrent.locks.ReentrantLock::<init>@5 (line 294)
                                 │                                                            ; - org.springframework.util.ConcurrentReferenceHashMap$Segment::<init>@6 (line 484)
                                 │                                                            ; - org.springframework.util.ConcurrentReferenceHashMap::<init>@141 (line 184)
  0.06%                          │  0x00007f3717d46148:   mov    %r8,%rcx
  0.05%                          │  0x00007f3717d4614b:   mov    %rax,%rbx
  0.03%                          │  0x00007f3717d4614e:   shr    $0x3,%rbx
  0.74%                          │  0x00007f3717d46152:   mov    %ebx,0xc(%r8)
  0.06%                          │  0x00007f3717d46156:   mov    %rax,%rbx
  0.05%                          │  0x00007f3717d46159:   xor    %rcx,%rbx
  0.02%                          │  0x00007f3717d4615c:   shr    $0x14,%rbx
  0.72%                          │  0x00007f3717d46160:   test   %rbx,%rbx
                             ╭   │  0x00007f3717d46163:   je     0x00007f3717d4617f
                             │   │  0x00007f3717d46165:   shr    $0x9,%rcx
                             │   │  0x00007f3717d46169:   movabs $0x7f370a872000,%rdi
                             │   │  0x00007f3717d46173:   add    %rcx,%rdi
                             │   │  0x00007f3717d46176:   cmpb   $0x8,(%rdi)
  0.00%                      │   │  0x00007f3717d46179:   jne    0x00007f3717d46509
  0.04%                      ↘   │  0x00007f3717d4617f:   movl   $0x0,0x14(%r8)
  0.08%                          │  0x00007f3717d46187:   lock addl $0x0,%rbx

问题是我在生成的程序集中根本没有提到putfield count。

谁能解释为什么我看不到它？