Static variable access of Java Reverse Foundation

This article refer to http://www.vuln.cn/7117

Note: Static variable statics can be assigned multiple times, and it is not possible to assign a final static value multiple times

The linear congruence pseudo-random number algorithm LCG algorithm is mathematically based on the formula:

X (n+1) = (A * X (n) + C)% m

Among them, the coefficients are:

Die m, M > 0

Coefficient A, 0 < a < m

Delta C, 0 <= C < m

Original value (Seed) 0 <= X (0) < M

Where the parameters C, M, a is more sensitive, or directly affects the quality of pseudo-random number generation.

In general, a high LCG m is an exponential power of 2 (generally 2^32 or 2^64), as this allows the modulo operation to truncate the right 32 or 64 bits. Most compiler libraries Use this theory to implement its pseudo-random number generator rand ().

Here m take 2^32,a take 1664525,c take 1013904223

LCG Algorithm Implementation Example

public class lcg{public static int rand_state;public void My_srand (int init) {rand_state=init;} public static int rng_a=1664525;public static int rng_c=1013904223;public int My_rand () {rand_state=rand_state*rng_a; Rand_state=rand_state+rng_c;return rand_state & 0x7FFF;}}

Compile

Javac Lcg.java

Anti-compilation

Javap-c-verbose Lcg.class

Initialization of static blocks

static {};d Escriptor: () vflags:acc_staticcode:stack=1, locals=0, ARGS_SIZE=00:LDC #5//INT 1 6645252:putstatic #3//Field RNG_A:I5:LDC #6//int 10139042237:putsta Tic #4//Field RNG_c:I10:return

LDC #5 take constant 1664525 stack

Putstatic #3 The value from the top of the stack and into the static variable rng_a:i

LDC #6 take constant 1013904223 stack

Putstatic #4 The value from the top of the stack and into the static variable rng_c:i

Here Putstatic implements the initialization value of the static variable.

The following My_srand () function stores the input values in the Rand_state

public void My_srand (int);d escriptor: (I) vflags:acc_publiccode:stack=1, locals=2, Args_size=20:iload_11:putstatic # 2//Field Rand_state:I4:return

Iload_1//Press the first parameter (init) into the stack, why not iload_0? Iload_0 has been used in the default build constructor method.

(The ILOAD_0 is pressed into the top of the stack in the construction method)

Putstatic #2//Remove the value of the Init at the top of the stack and save the value to a static variable rand_state:i

Here to add, do not know whether it is the translator or the original author of the problem, the stored value should be the initialization of the class when the stack points of the static variable storage area, not the second storage bit said

See My_rand ()

Public int my_rand ();d escriptor:  () iflags: acc_publiccode:stack=2, locals=1,  args_size=10: getstatic      #2                    // field rand_state:i3: getstatic       #3                    // Field RNG_a:I6: imul7: putstatic       #2                    // Field rand_state:I10: getstatic      #2                    // Field  rand_state:i13: getstatic      #4                    // Field RNG_c:I16: iadd17: putstatic       #2                    // Field rand_state:I20: getstatic      #2                    //  FIELD RAND_STATE:I23: SIPUSH        3276726: IAND27:  ireturn

The original text reviews 17 of putstatic #2和20的getstatic #2效率不高, which is actually just a translation of the Java statement

Try to

public int My_rand () {Rand_state=rand_state*rng_a;rand_state=rand_state+rng_c;return rand_state & 0x7fff;}

Change into

public int My_rand () {Rand_state=rand_state*rng_a+rng_c;return rand_state & 0x7fff;}

And look at this part of the anti-compilation

  public int my_rand ();    descriptor:  () I     flags: ACC_PUBLIC    Code:      stack=2,  locals=1, args_size=1         0: getstatic       #2                    // field rand_state:i         3:  getstatic      #3                    // Field RNG_a:I          6: imul         7: getstatic       #4                    //&nbsP field rng_c:i        10: iadd         11: putstatic      #2                    // Field rand_state:I         14: getstatic      #2                    // field rand_ state:i        17: sipush         32767        20: iand         21: ireturn

In this way, the original 20 line removed, this should be in the source of the optimization of the JVM to optimize it, the JVM over-optimized and said the source is not good

Static variable access of Java Reverse Foundation