假设我有一个(基数,偏移量)元组指向的值。
e.g。
class Data{
int x = 0;
}
class Accessor{
public Data data;
public Object base$x;
public long off$x;
public static final Unsafe unsafe;
public void run(){
data = new Data();
base$x = data;
off$x = 12;
unsafe.putInt(base$x,off$x,1);
assert(data.x == 1);
}
static{
try {
Constructor<Unsafe> unsafeConstructor = Unsafe.class.getDeclaredConstructor();
unsafeConstructor.setAccessible(true);
unsafe = unsafeConstructor.newInstance();
} catch (NoSuchMethodException | InvocationTargetException | IllegalAccessException | InstantiationException e) {
throw new RuntimeException(e);
}
}
}
(可运行的版本:https://ideone.com/OasQrh)
我现在想执行
unsafe.setInt(base$x,off$x,1);
C
中的。
为什么呢?因为不同的流程可能会移动数据并将base
和off
更改为指向新位置。我想利用intel RTM来确保移动数据不会干扰对该字段的定期访问。
所以让我们创建一个新类
class Transactionally{
static{
System.loadLibrary("RTM_transact");
}
public static native void setInt(Object target, String fieldname);
}
并替换
class Accessor{
public Data data;
public Object base;
public long off;
public static final Unsafe unsafe;
public void run(){
data = new Data();
base$x = data;
off$x = 12;
Transactionally.setInt(this,"x",1);
assert(data.x == 1);
}
static{
try {
Constructor<Unsafe> unsafeConstructor = Unsafe.class.getDeclaredConstructor();
unsafeConstructor.setAccessible(true);
unsafe = unsafeConstructor.newInstance();
} catch (NoSuchMethodException | InvocationTargetException | IllegalAccessException | InstantiationException e) {
throw new RuntimeException(e);
}
}
}
我们通过javah运行以获取标头,将方法签名复制到RTM_transct.cpp
并获得类似
#include<jni.h>
#include "rtmbenchmark_Transactionally.h"
#include <string>
std::string fldBase("base$");
std::string fldOff("off$");
/*
* Class: rtmbenchmark_Transactionally
* Method: setInt
* Signature: (Ljava/lang/Object;Ljava/lang/String;I)V
*/
JNIEXPORT void JNICALL Java_rtmbenchmark_Transactionally_setInt(JNIEnv *env, jclass _obsolete, jobject target, jstring fieldname, jint value){
jclass targetClass = (*env)->GetObjClass(env,target);
jfieldID fidFLDbase = (*env)->GetFieldID(env, targetClass, fldBase + fieldname, "Ljava.lang.Object");
jfieldid fidFLDoff = (*env)->GetFieldID(env, targetClass, fldOff + fieldname, "J");
volatile int tries = 0;
volatile boolean success = 0;
while(!success){/*TODO: maybe switch to a different strategy when tries grows too large?*/
__asm__ __volatile__ (
"xbegin 1f" /*1f: local label 1, look forward to find first*/
:"+rm"(tries) /*artificial dependency to prevent re-ordering*/
);
++tries;
jobject base = (*env)->GetIntField(env,targetClass,fidFLDbase);
jlong offset = (*env)->GetLongField(env,targetClass,fidFLDoff);
//??? ==> unsafe.setLong(base,offset,value)
__asm__ __volatile__ (
"xend\n\t"
"incl %0\n" /*increment success ==> break out of loop*/
"jmp 2f\n" /*jump to end of loop*/
"1:\n\t" /*local label 1 (jumped to when transaction is aborted)*/
"2:" /*local label 2 (jumped to after transaction succeeded)*/
:"+rm"(success)
:"rm"(tries) /*artificial dependency*/
);
}
}
我在交易体中使用什么不安全的东西?
交易的“心脏”是这三条线(应该是) 对应于Accessor过去做的事情:
jobject base = (*env)->GetIntField(env,targetClass,fidFLDbase);
jlong offset = (*env)->GetLongField(env,targetClass,fidFLDoff);
//??? ==> unsafe.setInt(base,offset,value)
)
理想情况下,当然,我想做类似base[offset] = value
的事情,但我非常怀疑这会有效。
我在这里为unsafe.setInt做什么?