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The Ultimate Breakdown of Kotlin Coroutines. Part 5. Suspend Functions.

Previous parts covered suspend lambdas and the machinery, which makes coroutines tick - suspend and resume. Also, I covered continuations, state-machines and interception. There is only one part remaining - suspend functions. I will omit coroutines inlining, which despite having complicated implementation, has an easy mental model - inlining does not affect behavior of coroutines. All the inlining is done prior to state-machine transformation. Finally, I will omit optimizations for the same reason - they do not change the behavior. Suspend Functions As explained in the continuation-passing style section, every suspending function's signature is changed: the compiler adds a continuation parameter and changes the return type to Any? . The tricky part becomes when we try to make it suspendable, in other words, when we build a state machine and generate a continuation. Unlike suspend lambdas, we cannot reuse an existing class for the continuation, since the suspend function can be s...
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The Ultimate Breakdown of Kotlin Coroutines. Part 4. Create, Start, Suspend, Intercept.

The previous parts mostly explained what the compiler does with coroutines. However, there is the other side of a coin - runtime support. In Kotlin's case, standard library functions, including intrinsic one, which are responsible for all the essential operations - creation, starting, suspension and interception. There is also a part responsible for resumption, but I already explained it in part 2. Coroutine Intrinsics Previous examples had an elementary coroutine builder. They used so-called empty continuation. Let us now recreate kotlinx.coroutines' async function, which runs a coroutine on another thread and then, upon its completion, returns the result to the main thread. First, we need a class which waits on the main thread: class Async < T > { suspend fun await (): T = TODO() } then the root continuation: class AsyncContinuation < T >: Continuation < T > { override val context = EmptyCoroutineContext override fun resumeWith (r...
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The Ultimate Breakdown of Kotlin Coroutines. Part 3. Variable Spilling.

In the previous parts of the breakdown I covered all the essential parts of coroutines, or, as I like to call them - three-headed hydra - suspension, resumption and state-machine. This time I will explain local variables spilling - a process of saving a coroutine state before suspension and restoring them after resumption. Variables Spilling All the previous examples did not have local variables, and there is a reason for it. When a coroutine suspends, we should save its local variables. Otherwise, when it resumes, the values of them are lost. So, before the suspension, which can be on each suspend call (more generally, on each suspension point), we save them, and after the resumption, we restore them. There is no reason to restore them right after the call if the call did not return COROUTINE_SUSPENDED : their values are still in local variable slots. Let us consider a simple example: import kotlin.coroutines.* data class A ( val i: Int ) var c: Continuation< Unit >? ...
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The Ultimate Breakdown of Kotlin Coroutines. Part 2. Continuation Passing Style and Resumption.

This is a continuation of series about coroutines internals. In the first part I explained how the compiler turns sequential code into suspendable by turing it into state-machines and how suspension works. This part covers continuation passing style - the reason for two-world model, and the counterpart of suspension - resumption. Continuation Passing Style In the first blogpost I touched upon the COROUTINE_SUSPENDED marker and said that suspending functions and lambdas return the marker when they suspend. Consequently, every suspend function return returnType | COROUTINE_SUSPENDED union type. However, since neither Kotlin nor JVM support union types, every coroutine's return type is Any? (also known as java.lang.Object ) at runtime. Let's now look to resume process closely. Suppose we have a couple of coroutines, one of them calls the other: fun main () { val a: suspend () -> Unit = { suspendMe() } val b: suspend () -> Unit = { a() } builder { b()...
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The Ultimate Breakdown of Kotlin Coroutines. Part 1. State-Machines and Suspension.

Introduction Throughout 2020 I wrote a document on coroutines codegen, which aimed mostly at my colleagues at JetBrains and Google to help them understand the pitfalls of the codegen and which design decisions we made. These blog posts are a simplified and trimmed down version of the documentation for a larger audience of experienced Kotlin programmers, who just want to know how the compiler turns sequential code into suspendable and become more proficient at their jobs. The blogposts will not cover kotlinx.coroutines internals. However, I think, with the knowledge of coroutines internals, it will be easier to understand, for example, how Flow's backpressure works and why it is essential for Flow operators to be inline with crossinline lambda parameter (spoilers: this is an optimization). I decided to write the blogposts now, before Kotlin 1.5 release, since Kotlin 1.5 brings new JVM_IR back-end with a lot of improvements in code generations, tail-call optimization of suspend fu...
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