这个问题是不是他们之间的区别 - 我知道虚假的失败是什么,为什么会发生在LL/SC。我的问题是如果我在intel x86上并使用java-9(build 149),为什么它们的汇编代码有区别?weakCompareAndSwap VS比较并交换
public class WeakVsNonWeak {
static jdk.internal.misc.Unsafe UNSAFE = jdk.internal.misc.Unsafe.getUnsafe();
public static void main(String[] args) throws NoSuchFieldException, SecurityException {
Holder h = new Holder();
h.setValue(33);
Class<?> holderClass = Holder.class;
long valueOffset = UNSAFE.objectFieldOffset(holderClass.getDeclaredField("value"));
int result = 0;
for (int i = 0; i < 30_000; ++i) {
result = strong(h, valueOffset);
}
System.out.println(result);
}
private static int strong(Holder h, long offset) {
int sum = 0;
for (int i = 33; i < 11_000; ++i) {
boolean result = UNSAFE.weakCompareAndSwapInt(h, offset, i, i + 1);
if (!result) {
sum++;
}
}
return sum;
}
public static class Holder {
private int value;
public int getValue() {
return value;
}
public void setValue(int value) {
this.value = value;
}
}
}
与运行:
输出compareAndSwapInt的java -XX:-TieredCompilation
-XX:CICompilerCount=1
-XX:+UnlockDiagnosticVMOptions
-XX:+PrintIntrinsics
-XX:+PrintAssembly
--add-opens java.base/jdk.internal.misc=ALL-UNNAMED
WeakVsNonWeak
(相关部分):
输出的weakCompareAndSwapInt 0x0000000109f0f4b8: movabs $0x111927c18,%rsi ; {metadata({method} {0x0000000111927c18} 'compareAndSwapInt' '(Ljava/lang/Object;JII)Z' in 'jdk/internal/misc/Unsafe')}
0x0000000109f0f4c2: mov %r15,%rdi
0x0000000109f0f4c5: test $0xf,%esp
0x0000000109f0f4cb: je 0x0000000109f0f4e3
0x0000000109f0f4d1: sub $0x8,%rsp
0x0000000109f0f4d5: callq 0x00000001098569d2 ; {runtime_call SharedRuntime::dtrace_method_entry(JavaThread*, Method*)}
0x0000000109f0f4da: add $0x8,%rsp
0x0000000109f0f4de: jmpq 0x0000000109f0f4e8
0x0000000109f0f4e3: callq 0x00000001098569d2 ; {runtime_call SharedRuntime::dtrace_method_entry(JavaThread*, Method*)}
0x0000000109f0f4e8: pop %r9
0x0000000109f0f4ea: pop %r8
0x0000000109f0f4ec: pop %rcx
0x0000000109f0f4ed: pop %rdx
0x0000000109f0f4ee: pop %rsi
0x0000000109f0f4ef: lea 0x210(%r15),%rdi
0x0000000109f0f4f6: movl $0x4,0x288(%r15)
0x0000000109f0f501: callq 0x00000001098fee40 ; {runtime_call Unsafe_CompareAndSwapInt(JNIEnv_*, _jobject*, _jobject*, long, int, int)}
0x0000000109f0f506: vzeroupper
0x0000000109f0f509: and $0xff,%eax
0x0000000109f0f50f: setne %al
0x0000000109f0f512: movl $0x5,0x288(%r15)
0x0000000109f0f51d: lock addl $0x0,-0x40(%rsp)
0x0000000109f0f523: cmpl $0x0,-0x3f04dd(%rip) # 0x0000000109b1f050
:
0x000000010b698840: sub $0x18,%rsp
0x0000010b698847: mov %rbp,0x10(%rsp)
0x000000010b69884c: mov %r8d,%eax
0x000000010b69884f: lock cmpxchg %r9d,(%rdx,%rcx,1)
0x000000010b698855: sete %r11b
0x000000010b698859: movzbl %r11b,%r11d ;*invokevirtual compareAndSwapInt {reexecute=0 rethrow=0 return_oop=0}
; - jdk.internal.misc.Unsafe::[email protected] (line 1369)
我远远不够的多才多艺,了解整个输出,但绝对可以看到锁ADDL和锁CMPXCHG之间的差异。
编辑 彼得的回答让我想到了。让我们来看看比较并交换将是一个内在的呼叫:
-XX:+ PrintIntrinsics -XX:-PrintAssembly
@ 7 jdk.internal.misc.Unsafe::compareAndSwapInt (0 bytes) (intrinsic)
@ 20 jdk.internal.misc.Unsafe::weakCompareAndSwapInt (11 bytes) (intrinsic).
然后用/运行两次的例子,而不:
-XX:DisableIntrinsic = _compareAndSwapInt
This is sor奇怪的T,输出是完全一样的(完全相同的指令),唯一的变化是与使内在我得到这样的电话:
0x000000010c23e355: callq 0x00000001016569d2 ; {runtime_call SharedRuntime::dtrace_method_entry(JavaThread*, Method*)}
0x000000010c23e381: callq 0x00000001016fee40 ; {runtime_call Unsafe_CompareAndSwapInt(JNIEnv_*, _jobject*, _jobject*, long, int, int)}
和残疾人:
0x00000001109322d5: callq 0x0000000105c569d2 ; {runtime_call _ZN13SharedRuntime19dtrace_method_entryEP10JavaThreadP6Method}
0x00000001109322e3: callq 0x0000000105c569d2 ; {runtime_call _ZN13SharedRuntime19dtrace_method_entryEP10JavaThreadP6Method}
这是相当有趣,不应该内在的代码是不同的?
EDIT-2 the8472也有意义。
锁ADDL是MFENCE一个替代品刷新StoreBuffer在x86,据我所知,它有知名度,而不是原子确实做。此条目之前对,就是:
0x00000001133db6f6: movl $0x4,0x288(%r15)
0x00000001133db701: callq 0x00000001060fee40 ; {runtime_call Unsafe_CompareAndSwapInt(JNIEnv_*, _jobject*, _jobject*, long, int, int)}
0x00000001133db706: vzeroupper
0x00000001133db709: and $0xff,%eax
0x00000001133db70f: setne %al
0x00000001133db712: movl $0x5,0x288(%r15)
0x00000001133db71d: lock addl $0x0,-0x40(%rsp)
0x00000001133db723: cmpl $0x0,-0xd0bc6dd(%rip) # 0x000000010631f050
; {external_word}
如果你看看here是将委托给另一本地call to Atomic:: cmpxchg这似乎是原子做掉。
为什么这不是直接替代锁cmpxchg是我的一个谜。
与您的编辑和来自不同优化级别的众多汇编样本不太清楚你实际要求什么。 – the8472
因此,'sun.misc.Unsafe'仍然没有消失,但是移动到另一个包jdk.internal.misc中,证明它实际上不是一个兼容性问题,它使这个类保持活着? – Holger
@Holger它没有移动,现在有两个版本。正如Shipilev所说,sun.misc.Unsafe将被删除 - 这次肯定。在sun.misc.Unsafe过去很有用的* other *地方有多个增强功能,现在已经过时(比如AtomicFieldUpdater)。他们甚至将释放/获取语义直接添加到不安全! – Eugene