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Java JIT Inline

Java JIT Inline

This is a follow-up blog that I write after answering a related question on SO. Through this question, we can understand why we need to know the internal of machine/os/language etc.

Introduction of JIT

The JIT (just-in-time) compiler is a important component of JVM which is used to optimize the performance of Java. As it name indicates, JIT can perform just-in-time compilation, which is the compilation process from byte code into native code of specific platform to make program run faster.
And JIT compiler can not only makes standard compilation, but also it can performs optimizations like OSR (on stack replacement), class hierarchy analysis etc.

Today, we will see how JIT do inline optimization and how it affects the performance of program.

Inline Function

First, we get to understand what is inline function. To inline a function is to substitute the function call with the function body. Compiler can inline some small hot functions into caller which address the overhead of a function invocation, which is common in the optimization of compiled language, like c/c++.

Then, we want see when inline will work in JIT. We already know that the JIT will try to inline frequently called methods in order to avoid the overhead of method invocation. But we should not forget that the heuristic it uses depends on both how often a method is invoked and also on how big it is. Remembering this two criteria, we won’t miss optimization when it is possible.

Inline in JIT

Now, we learn how inline affect the performance of a program from an example.

The following is the main part of a simple program which calculates fib number uses recursion. I have two version of test: one use a fixed number; second use a random number to avoid some optimizations of JIT (they get similar results, so I just show one version of results for brief).

private static int fib(int i) {
    if (i == 0 || i == 1) {
        return 1;
    }
    return fib(i - 1) + fib(i - 2);
}

public static void main(String[] args) {
    Random random = new Random();
    long startTime = System.nanoTime();
    for (int i = 0; i < 1000; i++) {
//            fib(random.nextInt(5) + 25);
        fib(28);
    }
    long endTime = System.nanoTime();
    System.out.format("%.2f seconds elapsed.\n", (endTime - startTime) / 1000.0 / 1000 / 1000);
}

Run program with the following JVM options, we can print some JIT compiler info to assist our understanding of this process:

-XX:+CITime -XX:+UnlockDiagnosticVMOptions -XX:+PrintInlining -XX:+PrintCompilation
  • -XX:+PrintCompilation: logs when JIT standard compilation happens
  • -XX:+UnlockDiagnosticVMOptions: enables other developer flags like -XX:+PrintInlining
  • -XX:+PrintInlining: prints what methods get inlined and where
  • -XX:+CITime: print various statistics about compilations

Soon, we get the following results:

  task-id
    152   36       4       opt.InlineTest::fib (25 bytes)
                              @ 14   opt.InlineTest::fib (25 bytes)   inline (hot)
                                @ 14   opt.InlineTest::fib (25 bytes)   recursive inlining is too deep
                                @ 20   opt.InlineTest::fib (25 bytes)   recursive inlining is too deep
...
   2429   42       3       sun.misc.FDBigInteger::trimLeadingZeros (57 bytes)
2.28 seconds elapsed.

Accumulated compiler times (for compiled methods only)
------------------------------------------------
  Total compilation time   :  0.015 s
    Standard compilation   :  0.015 s, Average : 0.000
...

We can see two statistic from results which can be used to compare with following test without JIT inline:

  • Program runs 2.28s
  • JIT compiler costs 0.014s

Using same program, we can run with the following compiler commands, in which

  • XX:CompileCommand=dontinline,opt.InlineTest::fib will forbid the inline, which can be seen from the output of compiler;
-XX:+CITime -XX:+UnlockDiagnosticVMOptions -XX:+PrintInlining -XX:+PrintCompilation -XX:CompileCommand=dontinline,opt.InlineTest::fib

Output is like following:

  task-id
    149   35       4       opt.InlineTest::fib (25 bytes)
                              @ 14   opt.InlineTest::fib (25 bytes)   disallowed by CompilerOracle
                              @ 20   opt.InlineTest::fib (25 bytes)   disallowed by CompilerOracle
...
   3062   41       3       sun.misc.FDBigInteger::trimLeadingZeros (57 bytes)
2.91 seconds elapsed.
Accumulated compiler times (for compiled methods only)
------------------------------------------------
  Total compilation time   :  0.015 s
    Standard compilation   :  0.015 s, Average : 0.000
...

Compare these two results, we can see:

  • Compiler task id is larger when we forbid inline, which shows compiler do more work when we forbid inline. This shows inline will not cause too much work of JIT;
  • Program spent more time (2.91s compares to 2.28s) which proves that inline hot function indeed saves us time;
  • The time JIT compiler costs almost same when we disable inline and enable it, which verifies inline function is indeed a useful technique not taking extra overhead;

Ref

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