weakCompareAndSwap vs compareAndSwap
2022-09-04 20:19:10
这个问题不是关于它们之间的区别 - 我知道什么是虚假故障以及为什么它会发生在LL / SC上。我的问题是，如果我使用的是intel x86并使用java-9（build 149），为什么它们的汇编代码之间有区别？
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;
}
}
}
运行方式：
java -XX:-TieredCompilation
-XX:CICompilerCount=1
-XX:+UnlockDiagnosticVMOptions
-XX:+PrintIntrinsics
-XX:+PrintAssembly
--add-opens java.base/jdk.internal.misc=ALL-UNNAMED
WeakVsNonWeak
CompareAndSwapInt（相关部分）的输出：
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
弱比较和交换输出：
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::weakCompareAndSwapInt@7 (line 1369)
到目前为止，我的通用性不足以理解整个输出，但可以肯定地看到锁定addl和锁定cmpxchg之间的区别。
编辑彼得的回答让我思考。让我们看看compareAndSwap是否是一个内在调用：
-XX：+打印智能 -XX：-打印组件
@ 7 jdk.internal.misc.Unsafe::compareAndSwapInt (0 bytes) (intrinsic)
@ 20 jdk.internal.misc.Unsafe::weakCompareAndSwapInt (11 bytes) (intrinsic).
然后运行该示例两次，使用/不使用：
-XX：禁用网络 = _compareAndSwapInt
这有点奇怪，输出完全相同（相同的确切指令），唯一的区别是启用内部函数，我得到这样的调用：
0x000000010c23e355: callq 0x00000001016569d2 ; {runtime_call SharedRuntime::dtrace_method_entry(JavaThread*, Method*)}
0x000000010c23e381: callq 0x00000001016fee40 ; {runtime_call Unsafe_CompareAndSwapInt(JNIEnv_*, _jobject*, _jobject*, long, int, int)}
和禁用：
0x00000001109322d5: callq 0x0000000105c569d2 ; {runtime_call _ZN13SharedRuntime19dtrace_method_entryEP10JavaThreadP6Method}
0x00000001109322e3: callq 0x0000000105c569d2 ; {runtime_call _ZN13SharedRuntime19dtrace_method_entryEP10JavaThreadP6Method}
这相当耐人寻味，内在代码不应该有所不同吗？
EDIT-2 the8472也是有道理的。
lock addl是mfence的替代品，据我所知，它在x86上刷新StoreBuffer，它与可见性有关，而不是原子性。在此条目之前，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}
如果你看这里，将委托给另一个对Atomic：：cmpxchg的本机调用，它似乎正在原子地进行交换。
为什么这不能替代直接锁定cmpxchg对我来说是一个谜。
