java - 根据执行历史记录，给定指令的操作数堆栈大小是否会有所不同？

Question

例如，对于方法

public int f() {
    int k = 1;
    for (int i = 0; i < 10; i++) {
        k += 2;
    }
    return k;
}

javac 生成以下字节码:

public int f();
Code:
   0: iconst_1
   1: istore_1
   2: iconst_0
   3: istore_2
   4: iload_2
   5: bipush        10
   7: if_icmpge     19
  10: iinc          1, 2
  13: iinc          2, 1
  16: goto          4
  19: iload_1
  20: ireturn

在标签 4 处，堆栈具有相同的大小 (0)，无论之前是哪条指令:3 或 16。

从 java 代码生成的字节码通常是这样吗？

Answer 1

要引用 Antimony 的回答，请参阅 JVM specification §4.9.2. Structural Constraints (谢谢霍尔格!):

• ...
• If an instruction can be executed along several different execution paths, the operand stack must have the same depth (§2.6.2) prior to the execution of the instruction, regardless of the path taken.

在这个答案的第一个版本中，我引用了 §4.10.2.1. The Process of Verification by Type Inference ，这是针对不包含 StackMapTable 属性的类文件(版本号为 49.0 或以下):

... The verifier ensures that at any given point in the program, no matter what code path is taken to reach that point, all of the following are true:

• The operand stack is always the same size and contains the same types of values.
• ...