英文:
Why does the format of JVM tableswitch/lookupswitch instruction has between 0 and 3 bytes padding？
问题
我已经理解了tableswitch/lookupswitch的格式。
但是，在0和3个字节之间的填充有什么目的呢？
英文:
I have understanded the format of tableswitch/lookupswitch.
But,what is the purpose of between 0 and 3 bytes padding?
答案1
得分: 4
为了历史原因而存在填充。最早的Java虚拟机没有JIT编译，它们逐条解释字节码指令。为了允许解释器直接从字节码流中读取32位偏移量，这些偏移量是32位对齐的，它们的地址是4字节的精确倍数。
RISC处理器（例如SPARC、ARMv5等）仅允许对齐的内存访问。例如，要用单个CPU指令从内存中读取32位值，地址必须是32位对齐的。如果地址不对齐，则获取32位值需要进行四次8位内存读取，这显然会更慢。
现今，这种优化已不再有用。
英文:
The padding exists for historical reasons. The first Java Virtual Machines did not have JIT compilation, they interpreted bytecode instructions one by one. To allow interpreters read 32-bit offsets directly from the bytecode stream, the offsets are 32-bit aligned - their addresses are exact multiple of 4 bytes.
RISC processors (like SPARC, ARMv5 and earlier, etc.) permitted only aligned memory access. E.g. to read a 32-bit value from memory with a single CPU instruction, the address must be 32-bit aligned. If the address is not aligned, getting a 32-bit value requires four 8-bit memory reads, which is, of course, slower.
Nowadays the optimization is not useful anymore.
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