Introduction to JVM Intrinsics

1. Introduction

In this article, we're going to learn what intrinsics are and how they work in Java and other JVM-based languages.

2. What Are Intrinsics?

An intrinsic function is a function that has special handling by the compiler or interpreter for our programming language. More specifically, it's a special case where the compiler or interpreter can replace the function with an alternative implementation for various reasons.

The programming language typically handles this by understanding that a specific method call is special, and whenever we call this method, then the resulting behavior is different. This then allows our code to look no different from normal, but the programming language's implementation can intervene in special cases to give additional benefits.

The exact way that it works varies between programming languages and also between operating systems and hardware. However, because these are handled for us, we typically don't need to know any of these details.

Intrinsics can give various benefits. Replacing particular algorithms with native code can make them perform better or even leverage the operating system's specific features or underlying hardware.

3. Intrinsics on the JVM

The JVM implements intrinsics by replacing the exact method call on an exact class with an alternative version. The JVM handles this itself, so it will only work for core classes and particular architectures. It also allows only certain methods to be swapped out, rather than entire classes.

Exactly how this works will vary between JVMs. This includes not only different versions of the JVM – Java 8 vs. Java 11, for example. This also includes different JVM targets – Linux vs. Windows, for example – and especially JVM vendors – Oracle vs. IBM. In some cases, certain command-line flags passed to the JVM can affect them.

This variety means that there's no way to determine, based only on the application, which methods will be replaced with intrinsics and which won't. It'll be different based on the JVM running the application. But this can lead to surprising results in some cases – including significant performance benefits achieved simply by changing the JVM used.

4. Performance Benefits

Intrinsics are often used to implement a more efficient version of the same code, for example, by leveraging implementation details of the running OS or CPU. Sometimes this is because it can use a more efficient implementation, and other times it can go as far as using hardware-specific functionality.

For example, the HotSpot JDK has an intrinsic implementation for many of the methods in java.lang.Math. Depending on the exact JVM, these are potentially implemented using CPU instructions to do the exact calculations required.

A simple test will demonstrate this. For example, take java.lang.Math.sqrt(). We can write a test:

for (int a = 0; a < 100000; ++a) {
    double result = Math.sqrt(a);
}

This test is performing a square root operation 100,000 times, which takes approx 123ms. However, if we replace this code with a copy of the implementation of Math.sqrt() instead:

double result = StrictMath.sqrt(a);

This code does the same thing but executes in 166ms instead. That's an increase of 35% by copying the implementation instead of allowing the JVM to replace it with the intrinsic version.

5. Impossible Implementations

In other cases, intrinsics are used for situations where the code can't be implemented in Java. These are typically reserved for very low-level cases.

For example, let's look at the method onSpinWait() in the java.lang.Thread class. This method indicates that this thread is currently performing no work and that CPU time can be given to another thread. To implement this, it needs to work at the lowest level possible.

The HotSpot JDK for x86 architectures implements this directly on the CPU, using the PAUSE opcode. The only other way to achieve this would've been to use a JNI call to native code, and the overhead involved in this would defeat the benefits of the call in the first place.

6. Identifying Intrinsics in Java

There is, unfortunately, no guaranteed way to identify methods that might be replaced with intrinsic versions. This is because different JVMs or even the same JVM on different platforms will do this for different methods.

However, when using Hotspot JVM as of Java 9, the @HotSpotIntrinsicCandidate annotation is used on all methods that may be replaced. Adding this annotation doesn't automatically cause the method to be replaced. In reality, that happens within the underlying JVM. Instead, JVM developers know that these methods are special and to be careful with them.

Other JVMs might handle this differently if they are identified at all. This includes the Hotspot JVM in Java 8 or older.

7. Summary

We can't write our programs to rely on the presence of intrinsics because there's no way to know if they'll be available or not on the runtime JVM. However, they are a compelling approach that the JVM can use to improve the way that programs will work.

These intrinsics can be – and often are – added to newer versions of the JVM. This, then, allows for improvements to our already running code simply by upgrading the JVM that we're running on, so this is another reason to ensure that we stay up-to-date with our dependencies and runtime.