Java为什么有IINC字节码指令? [英] Why does Java have an IINC bytecode instruction?

问题描述

为什么Java具有 IINC 字节码指令?已经有一个 IADD 字节码指令可以用来完成相同的操作.

那为什么 IINC 存在?

解决方案

只有Java的原始设计者才能回答他们做出特定设计决策的原因.但是，我们可以推测:

IINC 不允许您执行 ILOAD / SIPUSH / IADD

code>/ ISTORE 组合.区别在于 IINC 是单个指令，仅占用3或6个字节，而这4个指令序列显然更长.因此， IINC 会稍微减小使用它的字节码的大小.

除此之外，Java的早期版本使用了解释器，其中每条指令在执行过程中都有开销.在这种情况下，使用一条 IINC 指令可能比等效的替代字节码序列要快.请注意，JITting使得这基本上无关紧要，但是 IINC 可以追溯到Java的原始版本.

Why does Java have an IINC bytecode instruction? There is already an IADD bytecode instruction that can be used to accomplish the same.

So why does IINC exist?

解决方案

Only the original designers of Java can answer why they made particular design decisions. However, we can speculate:

IINC does not let you do anything that can't already be accomplished by a ILOAD/SIPUSH/IADD/ISTORE combo. The difference is that IINC is a single instruction, which only takes 3 or 6 bytes, while the 4 instruction sequence is obviously longer. So IINC slightly reduces the size of bytecode that uses it.

Apart from that, early versions of Java used an interpreter, where every instruction has overhead during execution. In this case, using a single IINC instruction could be faster than the equivalent alternative bytecode sequence. Note that JITting has made this largely irrelevant, but IINC dates back to the original version of Java.

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