TABLESWITCH性能提高了吗？

Question

Say I have the following switch statement:

switch (i)
{
    case 0: ...; return ...;
    case 1: ...; return ...;
    case 2: ...; return ...;
    case 400: ...; return ...;
    case 401: ...; return ...;
    case 402: ...; return ...;
}

Since the gaps are too large for the compiler to generate a reasonable TABLESWITCH ( O(1) complexity) instruction here, it uses a LOOKUPSWITCH ( O(log n) complexity). Would it be possible to increase the performance of this code by splitting the switch into two like this:

switch (i)
{
    case 0: ...; return ...;
    case 1: ...; return ...;
    case 2: ...; return ...;
}

switch (i)
{
    case 400: ...; return ...;
    case 401: ...; return ...;
    case 402: ...; return ...;
}

This would cause the compiler to generate two TABLESWITCH instead of one LOOKUPSWITCH .

Answer 1

Do not spend much time trying to optimize the bytecode. The bytecode does not necessarily reflect the performance of JIT-compiled methods. Why don't you take JMH and check the actual performance of both cases yourself?

In fact, HotSpot C2 compiler treats tableswitch and lookupswitch in a similar way, and it takes care of lookupswitch that has sequential labels with gaps very well.

Both cases are translated into a sequence of compare and conditional jump instructions in a binary search-like manner and work almost identically performance-wise.