Coroutines in LLVM

Warning

This is a work in progress. Compatibility across LLVM releases is not guaranteed.

Introduction

LLVM coroutines are functions that have one or more suspend points. When a suspend point is reached, the execution of a coroutine is suspended and control is returned back to its caller. A suspended coroutine can be resumed to continue execution from the last suspend point or it can be destroyed.

In the following example, we call function f (which may or may not be a coroutine itself) that returns a handle to a suspended coroutine (coroutine handle) that is used by main to resume the coroutine twice and then destroy it:

define i32 @main() {
entry:
  %hdl = call i8* @f(i32 4)
  call void @llvm.coro.resume(i8* %hdl)
  call void @llvm.coro.resume(i8* %hdl)
  call void @llvm.coro.destroy(i8* %hdl)
  ret i32 0
}

In addition to the function stack frame which exists when a coroutine is executing, there is an additional region of storage that contains objects that keep the coroutine state when a coroutine is suspended. This region of storage is called the coroutine frame. It is created when a coroutine is called and destroyed when a coroutine either runs to completion or is destroyed while suspended.

LLVM currently supports two styles of coroutine lowering. These styles support substantially different sets of features, have substantially different ABIs, and expect substantially different patterns of frontend code generation. However, the styles also have a great deal in common.

In all cases, an LLVM coroutine is initially represented as an ordinary LLVM function that has calls to coroutine intrinsics defining the structure of the coroutine. The coroutine function is then, in the most general case, rewritten by the coroutine lowering passes to become the “ramp function”, the initial entrypoint of the coroutine, which executes until a suspend point is first reached. The remainder of the original coroutine function is split out into some number of “resume functions”. Any state which must persist across suspensions is stored in the coroutine frame. The resume functions must somehow be able to handle either a “normal” resumption, which continues the normal execution of the coroutine, or an “abnormal” resumption, which must unwind the coroutine without attempting to suspend it.

Switched-Resume Lowering

In LLVM’s standard switched-resume lowering, signaled by the use of llvm.coro.id, the coroutine frame is stored as part of a “coroutine object” which represents a handle to a particular invocation of the coroutine. All coroutine objects support a common ABI allowing certain features to be used without knowing anything about the coroutine’s implementation:

• A coroutine object can be queried to see if it has reached completion with llvm.coro.done.

• A coroutine object can be resumed normally if it has not already reached completion with llvm.coro.resume.

• A coroutine object can be destroyed, invalidating the coroutine object, with llvm.coro.destroy. This must be done separately even if the coroutine has reached completion normally.

• “Promise” storage, which is known to have a certain size and alignment, can be projected out of the coroutine object with llvm.coro.promise. The coroutine implementation must have been compiled to define a promise of the same size and alignment.

In general, interacting with a coroutine object in any of these ways while it is running has undefined behavior.

The coroutine function is split into three functions, representing three different ways that control can enter the coroutine:

1. the ramp function that is initially invoked, which takes arbitrary arguments and returns a pointer to the coroutine object;

2. a coroutine resume function that is invoked when the coroutine is resumed, which takes a pointer to the coroutine object and returns void;

3. a coroutine destroy function that is invoked when the coroutine is destroyed, which takes a pointer to the coroutine object and returns void.

Because the resume and destroy functions are shared across all suspend points, suspend points must store the index of the active suspend in the coroutine object, and the resume/destroy functions must switch over that index to get back to the correct point. Hence the name of this lowering.

Pointers to the resume and destroy functions are stored in the coroutine object at known offsets which are fixed for all coroutines. A completed coroutine is represented with a null resume function.

There is a somewhat complex protocol of intrinsics for allocating and deallocating the coroutine object. It is complex in order to allow the allocation to be elided due to inlining. This protocol is discussed in further detail below.

The frontend may generate code to call the coroutine function directly; this will become a call to the ramp function and will return a pointer to the coroutine object. The frontend should always resume or destroy the coroutine using the corresponding intrinsics.

Returned-Continuation Lowering

In returned-continuation lowering, signaled by the use of llvm.coro.id.retcon or llvm.coro.id.retcon.once, some aspects of the ABI must be handled more explicitly by the frontend.

In this lowering, every suspend point takes a list of “yielded values” which are returned back to the caller along with a function pointer, called the continuation function. The coroutine is resumed by simply calling this continuation function pointer. The original coroutine is divided into the ramp function and then an arbitrary number of these continuation functions, one for each suspend point.

LLVM actually supports two closely-related returned-continuation lowerings:

• In normal returned-continuation lowering, the coroutine may suspend itself multiple times. This means that a continuation function itself returns another continuation pointer, as well as a list of yielded values.

  The coroutine indicates that it has run to completion by returning a null continuation pointer. Any yielded values will be undef should be ignored.

• In yield-once returned-continuation lowering, the coroutine must suspend itself exactly once (or throw an exception). The ramp function returns a continuation function pointer and yielded values, but the continuation function simply returns void when the coroutine has run to completion.

The coroutine frame is maintained in a fixed-size buffer that is passed to the coro.id intrinsic, which guarantees a certain size and alignment statically. The same buffer must be passed to the continuation function(s). The coroutine will allocate memory if the buffer is insufficient, in which case it will need to store at least that pointer in the buffer; therefore the buffer must always be at least pointer-sized. How the coroutine uses the buffer may vary between suspend points.

In addition to the buffer pointer, continuation functions take an argument indicating whether the coroutine is being resumed normally (zero) or abnormally (non-zero).

LLVM is currently ineffective at statically eliminating allocations after fully inlining returned-continuation coroutines into a caller. This may be acceptable if LLVM’s coroutine support is primarily being used for low-level lowering and inlining is expected to be applied earlier in the pipeline.

Async Lowering

In async-continuation lowering, signaled by the use of llvm.coro.id.async, handling of control-flow must be handled explicitly by the frontend.

In this lowering, a coroutine is assumed to take the current async context as one of its arguments (the argument position is determined by llvm.coro.id.async). It is used to marshal arguments and return values of the coroutine. Therefore an async coroutine returns void.

define swiftcc void @async_coroutine(i8* %async.ctxt, i8*, i8*) {
}

Values live across a suspend point need to be stored in the coroutine frame to be available in the continuation function. This frame is stored as a tail to the async context.

Every suspend point takes an context projection function argument which describes how-to obtain the continuations async context and every suspend point has an associated resume function denoted by the llvm.coro.async.resume intrinsic. The coroutine is resumed by calling this resume function passing the async context as the one of its arguments argument. The resume function can restore its (the caller’s) async context by applying a context projection function that is provided by the frontend as a parameter to the llvm.coro.suspend.async intrinsic.

// For example:
struct async_context {
  struct async_context *caller_context;
  ...
}

char *context_projection_function(struct async_context *callee_ctxt) {
   return callee_ctxt->caller_context;
}

%resume_func_ptr = call i8* @llvm.coro.async.resume()
call {i8*, i8*, i8*} (i8*, i8*, ...) @llvm.coro.suspend.async(
                                            i8* %resume_func_ptr,
                                            i8* %context_projection_function

The frontend should provide a async function pointer struct associated with each async coroutine by llvm.coro.id.async’s argument. The initial size and alignment of the async context must be provided as arguments to the llvm.coro.id.async intrinsic. Lowering will update the size entry with the coroutine frame requirements. The frontend is responsible for allocating the memory for the async context but can use the async function pointer struct to obtain the required size.

struct async_function_pointer {
  uint32_t relative_function_pointer_to_async_impl;
  uint32_t context_size;
}

Lowering will split an async coroutine into a ramp function and one resume function per suspend point.

How control-flow is passed between caller, suspension point, and back to resume function is left up to the frontend.

The suspend point takes a function and its arguments. The function is intended to model the transfer to the callee function. It will be tail called by lowering and therefore must have the same signature and calling convention as the async coroutine.

call {i8*, i8*, i8*} (i8*, i8*, ...) @llvm.coro.suspend.async(
                 i8* %resume_func_ptr,
                 i8* %context_projection_function,
                 i8* (bitcast void (i8*, i8*, i8*)* to i8*) %suspend_function,
                 i8* %arg1, i8* %arg2, i8 %arg3)

Coroutines by Example

The examples below are all of switched-resume coroutines.

Coroutine Representation

Let’s look at an example of an LLVM coroutine with the behavior sketched by the following pseudo-code.

void *f(int n) {
   for(;;) {
     print(n++);
     <suspend> // returns a coroutine handle on first suspend
   }
}

This coroutine calls some function print with value n as an argument and suspends execution. Every time this coroutine resumes, it calls print again with an argument one bigger than the last time. This coroutine never completes by itself and must be destroyed explicitly. If we use this coroutine with a main shown in the previous section. It will call print with values 4, 5 and 6 after which the coroutine will be destroyed.

The LLVM IR for this coroutine looks like this:

define i8* @f(i32 %n) {
entry:
  %id = call token @llvm.coro.id(i32 0, i8* null, i8* null, i8* null)
  %size = call i32 @llvm.coro.size.i32()
  %alloc = call i8* @malloc(i32 %size)
  %hdl = call noalias i8* @llvm.coro.begin(token %id, i8* %alloc)
  br label %loop
loop:
  %n.val = phi i32 [ %n, %entry ], [ %inc, %loop ]
  %inc = add nsw i32 %n.val, 1
  call void @print(i32 %n.val)
  %0 = call i8 @llvm.coro.suspend(token none, i1 false)
  switch i8 %0, label %suspend [i8 0, label %loop
                                i8 1, label %cleanup]
cleanup:
  %mem = call i8* @llvm.coro.free(token %id, i8* %hdl)
  call void @free(i8* %mem)
  br label %suspend
suspend:
  %unused = call i1 @llvm.coro.end(i8* %hdl, i1 false)
  ret i8* %hdl
}

The entry block establishes the coroutine frame. The coro.size intrinsic is lowered to a constant representing the size required for the coroutine frame. The coro.begin intrinsic initializes the coroutine frame and returns the coroutine handle. The second parameter of coro.begin is given a block of memory to be used if the coroutine frame needs to be allocated dynamically. The coro.id intrinsic serves as coroutine identity useful in cases when the coro.begin intrinsic get duplicated by optimization passes such as jump-threading.

The cleanup block destroys the coroutine frame. The coro.free intrinsic, given the coroutine handle, returns a pointer of the memory block to be freed or null if the coroutine frame was not allocated dynamically. The cleanup block is entered when coroutine runs to completion by itself or destroyed via call to the coro.destroy intrinsic.

The suspend block contains code to be executed when coroutine runs to completion or suspended. The coro.end intrinsic marks the point where a coroutine needs to return control back to the caller if it is not an initial invocation of the coroutine.

The loop blocks represents the body of the coroutine. The coro.suspend intrinsic in combination with the following switch indicates what happens to control flow when a coroutine is suspended (default case), resumed (case 0) or destroyed (case 1).

Coroutine Transformation

One of the steps of coroutine lowering is building the coroutine frame. The def-use chains are analyzed to determine which objects need be kept alive across suspend points. In the coroutine shown in the previous section, use of virtual register %inc is separated from the definition by a suspend point, therefore, it cannot reside on the stack frame since the latter goes away once the coroutine is suspended and control is returned back to the caller. An i32 slot is allocated in the coroutine frame and %inc is spilled and reloaded from that slot as needed.

We also store addresses of the resume and destroy functions so that the coro.resume and coro.destroy intrinsics can resume and destroy the coroutine when its identity cannot be determined statically at compile time. For our example, the coroutine frame will be:

%f.frame = type { void (%f.frame*)*, void (%f.frame*)*, i32 }

After resume and destroy parts are outlined, function f will contain only the code responsible for creation and initialization of the coroutine frame and execution of the coroutine until a suspend point is reached:

define i8* @f(i32 %n) {
entry:
  %id = call token @llvm.coro.id(i32 0, i8* null, i8* null, i8* null)
  %alloc = call noalias i8* @malloc(i32 24)
  %0 = call noalias i8* @llvm.coro.begin(token %id, i8* %alloc)
  %frame = bitcast i8* %0 to %f.frame*
  %1 = getelementptr %f.frame, %f.frame* %frame, i32 0, i32 0
  store void (%f.frame*)* @f.resume, void (%f.frame*)** %1
  %2 = getelementptr %f.frame, %f.frame* %frame, i32 0, i32 1
  store void (%f.frame*)* @f.destroy, void (%f.frame*)** %2

  %inc = add nsw i32 %n, 1
  %inc.spill.addr = getelementptr inbounds %f.Frame, %f.Frame* %FramePtr, i32 0, i32 2
  store i32 %inc, i32* %inc.spill.addr
  call void @print(i32 %n)

  ret i8* %frame
}

Outlined resume part of the coroutine will reside in function f.resume:

define internal fastcc void @f.resume(%f.frame* %frame.ptr.resume) {
entry:
  %inc.spill.addr = getelementptr %f.frame, %f.frame* %frame.ptr.resume, i64 0, i32 2
  %inc.spill = load i32, i32* %inc.spill.addr, align 4
  %inc = add i32 %n.val, 1
  store i32 %inc, i32* %inc.spill.addr, align 4
  tail call void @print(i32 %inc)
  ret void
}

Whereas function f.destroy will contain the cleanup code for the coroutine:

define internal fastcc void @f.destroy(%f.frame* %frame.ptr.destroy) {
entry:
  %0 = bitcast %f.frame* %frame.ptr.destroy to i8*
  tail call void @free(i8* %0)
  ret void
}

Avoiding Heap Allocations

A particular coroutine usage pattern, which is illustrated by the main function in the overview section, where a coroutine is created, manipulated and destroyed by the same calling function, is common for coroutines implementing RAII idiom and is suitable for allocation elision optimization which avoid dynamic allocation by storing the coroutine frame as a static alloca in its caller.

In the entry block, we will call coro.alloc intrinsic that will return true when dynamic allocation is required, and false if dynamic allocation is elided.

entry:
  %id = call token @llvm.coro.id(i32 0, i8* null, i8* null, i8* null)
  %need.dyn.alloc = call i1 @llvm.coro.alloc(token %id)
  br i1 %need.dyn.alloc, label %dyn.alloc, label %coro.begin
dyn.alloc:
  %size = call i32 @llvm.coro.size.i32()
  %alloc = call i8* @CustomAlloc(i32 %size)
  br label %coro.begin
coro.begin:
  %phi = phi i8* [ null, %entry ], [ %alloc, %dyn.alloc ]
  %hdl = call noalias i8* @llvm.coro.begin(token %id, i8* %phi)

In the cleanup block, we will make freeing the coroutine frame conditional on coro.free intrinsic. If allocation is elided, coro.free returns null thus skipping the deallocation code:

cleanup:
  %mem = call i8* @llvm.coro.free(token %id, i8* %hdl)
  %need.dyn.free = icmp ne i8* %mem, null
  br i1 %need.dyn.free, label %dyn.free, label %if.end
dyn.free:
  call void @CustomFree(i8* %mem)
  br label %if.end
if.end:
  ...

With allocations and deallocations represented as described as above, after coroutine heap allocation elision optimization, the resulting main will be:

define i32 @main() {
entry:
  call void @print(i32 4)
  call void @print(i32 5)
  call void @print(i32 6)
  ret i32 0
}

Multiple Suspend Points

Let’s consider the coroutine that has more than one suspend point:

void *f(int n) {
   for(;;) {
     print(n++);
     <suspend>
     print(-n);
     <suspend>
   }
}

Matching LLVM code would look like (with the rest of the code remaining the same as the code in the previous section):

loop:
  %n.addr = phi i32 [ %n, %entry ], [ %inc, %loop.resume ]
  call void @print(i32 %n.addr) #4
  %2 = call i8 @llvm.coro.suspend(token none, i1 false)
  switch i8 %2, label %suspend [i8 0, label %loop.resume
                                i8 1, label %cleanup]
loop.resume:
  %inc = add nsw i32 %n.addr, 1
  %sub = xor i32 %n.addr, -1
  call void @print(i32 %sub)
  %3 = call i8 @llvm.coro.suspend(token none, i1 false)
  switch i8 %3, label %suspend [i8 0, label %loop
                                i8 1, label %cleanup]

In this case, the coroutine frame would include a suspend index that will indicate at which suspend point the coroutine needs to resume. The resume function will use an index to jump to an appropriate basic block and will look as follows:

define internal fastcc void @f.Resume(%f.Frame* %FramePtr) {
entry.Resume:
  %index.addr = getelementptr inbounds %f.Frame, %f.Frame* %FramePtr, i64 0, i32 2
  %index = load i8, i8* %index.addr, align 1
  %switch = icmp eq i8 %index, 0
  %n.addr = getelementptr inbounds %f.Frame, %f.Frame* %FramePtr, i64 0, i32 3
  %n = load i32, i32* %n.addr, align 4
  br i1 %switch, label %loop.resume, label %loop

loop.resume:
  %sub = xor i32 %n, -1
  call void @print(i32 %sub)
  br label %suspend
loop:
  %inc = add nsw i32 %n, 1
  store i32 %inc, i32* %n.addr, align 4
  tail call void @print(i32 %inc)
  br label %suspend

suspend:
  %storemerge = phi i8 [ 0, %loop ], [ 1, %loop.resume ]
  store i8 %storemerge, i8* %index.addr, align 1
  ret void
}

If different cleanup code needs to get executed for different suspend points, a similar switch will be in the f.destroy function.

Note

Using suspend index in a coroutine state and having a switch in f.resume and f.destroy is one of the possible implementation strategies. We explored another option where a distinct f.resume1, f.resume2, etc. are created for every suspend point, and instead of storing an index, the resume and destroy function pointers are updated at every suspend. Early testing showed that the current approach is easier on the optimizer than the latter so it is a lowering strategy implemented at the moment.

Distinct Save and Suspend

In the previous example, setting a resume index (or some other state change that needs to happen to prepare a coroutine for resumption) happens at the same time as a suspension of a coroutine. However, in certain cases, it is necessary to control when coroutine is prepared for resumption and when it is suspended.

In the following example, a coroutine represents some activity that is driven by completions of asynchronous operations async_op1 and async_op2 which get a coroutine handle as a parameter and resume the coroutine once async operation is finished.

void g() {
   for (;;)
     if (cond()) {
        async_op1(<coroutine-handle>); // will resume once async_op1 completes
        <suspend>
        do_one();
     }
     else {
        async_op2(<coroutine-handle>); // will resume once async_op2 completes
        <suspend>
        do_two();
     }
   }
}

In this case, coroutine should be ready for resumption prior to a call to async_op1 and async_op2. The coro.save intrinsic is used to indicate a point when coroutine should be ready for resumption (namely, when a resume index should be stored in the coroutine frame, so that it can be resumed at the correct resume point):

if.true:
  %save1 = call token @llvm.coro.save(i8* %hdl)
  call void @async_op1(i8* %hdl)
  %suspend1 = call i1 @llvm.coro.suspend(token %save1, i1 false)
  switch i8 %suspend1, label %suspend [i8 0, label %resume1
                                       i8 1, label %cleanup]
if.false:
  %save2 = call token @llvm.coro.save(i8* %hdl)
  call void @async_op2(i8* %hdl)
  %suspend2 = call i1 @llvm.coro.suspend(token %save2, i1 false)
  switch i8 %suspend1, label %suspend [i8 0, label %resume2
                                       i8 1, label %cleanup]

Coroutine Promise

A coroutine author or a frontend may designate a distinguished alloca that can be used to communicate with the coroutine. This distinguished alloca is called coroutine promise and is provided as the second parameter to the coro.id intrinsic.

The following coroutine designates a 32 bit integer promise and uses it to store the current value produced by a coroutine.

define i8* @f(i32 %n) {
entry:
  %promise = alloca i32
  %pv = bitcast i32* %promise to i8*
  %id = call token @llvm.coro.id(i32 0, i8* %pv, i8* null, i8* null)
  %need.dyn.alloc = call i1 @llvm.coro.alloc(token %id)
  br i1 %need.dyn.alloc, label %dyn.alloc, label %coro.begin
dyn.alloc:
  %size = call i32 @llvm.coro.size.i32()
  %alloc = call i8* @malloc(i32 %size)
  br label %coro.begin
coro.begin:
  %phi = phi i8* [ null, %entry ], [ %alloc, %dyn.alloc ]
  %hdl = call noalias i8* @llvm.coro.begin(token %id, i8* %phi)
  br label %loop
loop:
  %n.val = phi i32 [ %n, %coro.begin ], [ %inc, %loop ]
  %inc = add nsw i32 %n.val, 1
  store i32 %n.val, i32* %promise
  %0 = call i8 @llvm.coro.suspend(token none, i1 false)
  switch i8 %0, label %suspend [i8 0, label %loop
                                i8 1, label %cleanup]
cleanup:
  %mem = call i8* @llvm.coro.free(token %id, i8* %hdl)
  call void @free(i8* %mem)
  br label %suspend
suspend:
  %unused = call i1 @llvm.coro.end(i8* %hdl, i1 false)
  ret i8* %hdl
}

A coroutine consumer can rely on the coro.promise intrinsic to access the coroutine promise.

define i32 @main() {
entry:
  %hdl = call i8* @f(i32 4)
  %promise.addr.raw = call i8* @llvm.coro.promise(i8* %hdl, i32 4, i1 false)
  %promise.addr = bitcast i8* %promise.addr.raw to i32*
  %val0 = load i32, i32* %promise.addr
  call void @print(i32 %val0)
  call void @llvm.coro.resume(i8* %hdl)
  %val1 = load i32, i32* %promise.addr
  call void @print(i32 %val1)
  call void @llvm.coro.resume(i8* %hdl)
  %val2 = load i32, i32* %promise.addr
  call void @print(i32 %val2)
  call void @llvm.coro.destroy(i8* %hdl)
  ret i32 0
}

After example in this section is compiled, result of the compilation will be:

define i32 @main() {
entry:
  tail call void @print(i32 4)
  tail call void @print(i32 5)
  tail call void @print(i32 6)
  ret i32 0
}

Final Suspend

A coroutine author or a frontend may designate a particular suspend to be final, by setting the second argument of the coro.suspend intrinsic to true. Such a suspend point has two properties:

• it is possible to check whether a suspended coroutine is at the final suspend point via coro.done intrinsic;

• a resumption of a coroutine stopped at the final suspend point leads to undefined behavior. The only possible action for a coroutine at a final suspend point is destroying it via coro.destroy intrinsic.

From the user perspective, the final suspend point represents an idea of a coroutine reaching the end. From the compiler perspective, it is an optimization opportunity for reducing number of resume points (and therefore switch cases) in the resume function.

The following is an example of a function that keeps resuming the coroutine until the final suspend point is reached after which point the coroutine is destroyed:

define i32 @main() {
entry:
  %hdl = call i8* @f(i32 4)
  br label %while
while:
  call void @llvm.coro.resume(i8* %hdl)
  %done = call i1 @llvm.coro.done(i8* %hdl)
  br i1 %done, label %end, label %while
end:
  call void @llvm.coro.destroy(i8* %hdl)
  ret i32 0
}

Usually, final suspend point is a frontend injected suspend point that does not correspond to any explicitly authored suspend point of the high level language. For example, for a Python generator that has only one suspend point:

def coroutine(n):
  for i in range(n):
    yield i

Python frontend would inject two more suspend points, so that the actual code looks like this:

void* coroutine(int n) {
  int current_value;
  <designate current_value to be coroutine promise>
  <SUSPEND> // injected suspend point, so that the coroutine starts suspended
  for (int i = 0; i < n; ++i) {
    current_value = i; <SUSPEND>; // corresponds to "yield i"
  }
  <SUSPEND final=true> // injected final suspend point
}

and python iterator __next__ would look like:

int __next__(void* hdl) {
  coro.resume(hdl);
  if (coro.done(hdl)) throw StopIteration();
  return *(int*)coro.promise(hdl, 4, false);
}

Intrinsics

Coroutine Manipulation Intrinsics

Intrinsics described in this section are used to manipulate an existing coroutine. They can be used in any function which happen to have a pointer to a coroutine frame or a pointer to a coroutine promise.

‘llvm.coro.destroy’ Intrinsic

Syntax:

declare void @llvm.coro.destroy(i8* <handle>)

Overview:

The ‘llvm.coro.destroy’ intrinsic destroys a suspended switched-resume coroutine.

Arguments:

The argument is a coroutine handle to a suspended coroutine.

Semantics:

When possible, the coro.destroy intrinsic is replaced with a direct call to the coroutine destroy function. Otherwise it is replaced with an indirect call based on the function pointer for the destroy function stored in the coroutine frame. Destroying a coroutine that is not suspended leads to undefined behavior.

‘llvm.coro.resume’ Intrinsic

declare void @llvm.coro.resume(i8* <handle>)

Overview:

The ‘llvm.coro.resume’ intrinsic resumes a suspended switched-resume coroutine.

Arguments:

The argument is a handle to a suspended coroutine.

Semantics:

When possible, the coro.resume intrinsic is replaced with a direct call to the coroutine resume function. Otherwise it is replaced with an indirect call based on the function pointer for the resume function stored in the coroutine frame. Resuming a coroutine that is not suspended leads to undefined behavior.

‘llvm.coro.done’ Intrinsic

declare i1 @llvm.coro.done(i8* <handle>)

Overview:

The ‘llvm.coro.done’ intrinsic checks whether a suspended switched-resume coroutine is at the final suspend point or not.

Arguments:

The argument is a handle to a suspended coroutine.

Semantics:

Using this intrinsic on a coroutine that does not have a final suspend point or on a coroutine that is not suspended leads to undefined behavior.

‘llvm.coro.promise’ Intrinsic

declare i8* @llvm.coro.promise(i8* <ptr>, i32 <alignment>, i1 <from>)

Overview:

The ‘llvm.coro.promise’ intrinsic obtains a pointer to a coroutine promise given a switched-resume coroutine handle and vice versa.

Arguments:

The first argument is a handle to a coroutine if from is false. Otherwise, it is a pointer to a coroutine promise.

The second argument is an alignment requirements of the promise. If a frontend designated %promise = alloca i32 as a promise, the alignment argument to coro.promise should be the alignment of i32 on the target platform. If a frontend designated %promise = alloca i32, align 16 as a promise, the alignment argument should be 16. This argument only accepts constants.

The third argument is a boolean indicating a direction of the transformation. If from is true, the intrinsic returns a coroutine handle given a pointer to a promise. If from is false, the intrinsics return a pointer to a promise from a coroutine handle. This argument only accepts constants.

Semantics:

Using this intrinsic on a coroutine that does not have a coroutine promise leads to undefined behavior. It is possible to read and modify coroutine promise of the coroutine which is currently executing. The coroutine author and a coroutine user are responsible to makes sure there is no data races.

Example:

define i8* @f(i32 %n) {
entry:
  %promise = alloca i32
  %pv = bitcast i32* %promise to i8*
  ; the second argument to coro.id points to the coroutine promise.
  %id = call token @llvm.coro.id(i32 0, i8* %pv, i8* null, i8* null)
  ...
  %hdl = call noalias i8* @llvm.coro.begin(token %id, i8* %alloc)
  ...
  store i32 42, i32* %promise ; store something into the promise
  ...
  ret i8* %hdl
}

define i32 @main() {
entry:
  %hdl = call i8* @f(i32 4) ; starts the coroutine and returns its handle
  %promise.addr.raw = call i8* @llvm.coro.promise(i8* %hdl, i32 4, i1 false)
  %promise.addr = bitcast i8* %promise.addr.raw to i32*
  %val = load i32, i32* %promise.addr ; load a value from the promise
  call void @print(i32 %val)
  call void @llvm.coro.destroy(i8* %hdl)
  ret i32 0
}

Coroutine Structure Intrinsics

Intrinsics described in this section are used within a coroutine to describe the coroutine structure. They should not be used outside of a coroutine.

‘llvm.coro.size’ Intrinsic

declare i32 @llvm.coro.size.i32()
declare i64 @llvm.coro.size.i64()

Overview:

The ‘llvm.coro.size’ intrinsic returns the number of bytes required to store a coroutine frame. This is only supported for switched-resume coroutines.

Arguments:

None

Semantics:

The coro.size intrinsic is lowered to a constant representing the size of the coroutine frame.

‘llvm.coro.begin’ Intrinsic

declare i8* @llvm.coro.begin(token <id>, i8* <mem>)

Overview:

The ‘llvm.coro.begin’ intrinsic returns an address of the coroutine frame.

Arguments:

The first argument is a token returned by a call to ‘llvm.coro.id’ identifying the coroutine.

The second argument is a pointer to a block of memory where coroutine frame will be stored if it is allocated dynamically. This pointer is ignored for returned-continuation coroutines.

Semantics:

Depending on the alignment requirements of the objects in the coroutine frame and/or on the codegen compactness reasons the pointer returned from coro.begin may be at offset to the %mem argument. (This could be beneficial if instructions that express relative access to data can be more compactly encoded with small positive and negative offsets).

A frontend should emit exactly one coro.begin intrinsic per coroutine.

‘llvm.coro.free’ Intrinsic

declare i8* @llvm.coro.free(token %id, i8* <frame>)

Overview:

The ‘llvm.coro.free’ intrinsic returns a pointer to a block of memory where coroutine frame is stored or null if this instance of a coroutine did not use dynamically allocated memory for its coroutine frame. This intrinsic is not supported for returned-continuation coroutines.

Arguments:

The first argument is a token returned by a call to ‘llvm.coro.id’ identifying the coroutine.

The second argument is a pointer to the coroutine frame. This should be the same pointer that was returned by prior coro.begin call.

Example (custom deallocation function):

cleanup:
  %mem = call i8* @llvm.coro.free(token %id, i8* %frame)
  %mem_not_null = icmp ne i8* %mem, null
  br i1 %mem_not_null, label %if.then, label %if.end
if.then:
  call void @CustomFree(i8* %mem)
  br label %if.end
if.end:
  ret void

Example (standard deallocation functions):

cleanup:
  %mem = call i8* @llvm.coro.free(token %id, i8* %frame)
  call void @free(i8* %mem)
  ret void

‘llvm.coro.alloc’ Intrinsic

declare i1 @llvm.coro.alloc(token <id>)

Overview:

The ‘llvm.coro.alloc’ intrinsic returns true if dynamic allocation is required to obtain a memory for the coroutine frame and false otherwise. This is not supported for returned-continuation coroutines.

Arguments:

The first argument is a token returned by a call to ‘llvm.coro.id’ identifying the coroutine.

Semantics:

A frontend should emit at most one coro.alloc intrinsic per coroutine. The intrinsic is used to suppress dynamic allocation of the coroutine frame when possible.

Example:

entry:
  %id = call token @llvm.coro.id(i32 0, i8* null, i8* null, i8* null)
  %dyn.alloc.required = call i1 @llvm.coro.alloc(token %id)
  br i1 %dyn.alloc.required, label %coro.alloc, label %coro.begin

coro.alloc:
  %frame.size = call i32 @llvm.coro.size()
  %alloc = call i8* @MyAlloc(i32 %frame.size)
  br label %coro.begin

coro.begin:
  %phi = phi i8* [ null, %entry ], [ %alloc, %coro.alloc ]
  %frame = call i8* @llvm.coro.begin(token %id, i8* %phi)

‘llvm.coro.noop’ Intrinsic

declare i8* @llvm.coro.noop()

Overview:

The ‘llvm.coro.noop’ intrinsic returns an address of the coroutine frame of a coroutine that does nothing when resumed or destroyed.

Arguments:

None

Semantics:

This intrinsic is lowered to refer to a private constant coroutine frame. The resume and destroy handlers for this frame are empty functions that do nothing. Note that in different translation units llvm.coro.noop may return different pointers.

‘llvm.coro.frame’ Intrinsic

declare i8* @llvm.coro.frame()

Overview:

The ‘llvm.coro.frame’ intrinsic returns an address of the coroutine frame of the enclosing coroutine.

Arguments:

None

Semantics:

This intrinsic is lowered to refer to the coro.begin instruction. This is a frontend convenience intrinsic that makes it easier to refer to the coroutine frame.

‘llvm.coro.id’ Intrinsic

declare token @llvm.coro.id(i32 <align>, i8* <promise>, i8* <coroaddr>,
                                                        i8* <fnaddrs>)

Overview:

The ‘llvm.coro.id’ intrinsic returns a token identifying a switched-resume coroutine.

Arguments:

The first argument provides information on the alignment of the memory returned by the allocation function and given to coro.begin by the first argument. If this argument is 0, the memory is assumed to be aligned to 2 * sizeof(i8*). This argument only accepts constants.

The second argument, if not null, designates a particular alloca instruction to be a coroutine promise.

The third argument is null coming out of the frontend. The CoroEarly pass sets this argument to point to the function this coro.id belongs to.

The fourth argument is null before coroutine is split, and later is replaced to point to a private global constant array containing function pointers to outlined resume and destroy parts of the coroutine.

Semantics:

The purpose of this intrinsic is to tie together coro.id, coro.alloc and coro.begin belonging to the same coroutine to prevent optimization passes from duplicating any of these instructions unless entire body of the coroutine is duplicated.

A frontend should emit exactly one coro.id intrinsic per coroutine.

‘llvm.coro.id.async’ Intrinsic

declare token @llvm.coro.id.async(i32 <context size>, i32 <align>,
                                  i8* <context arg>,
                                  i8* <async function pointer>)

Overview:

The ‘llvm.coro.id.async’ intrinsic returns a token identifying an async coroutine.

Arguments:

The first argument provides the initial size of the async context as required from the frontend. Lowering will add to this size the size required by the frame storage and store that value to the async function pointer.

The second argument, is the alignment guarantee of the memory of the async context. The frontend guarantees that the memory will be aligned by this value.

The third argument is the async context argument in the current coroutine.

The fourth argument is the address of the async function pointer struct. Lowering will update the context size requirement in this struct by adding the coroutine frame size requirement to the initial size requirement as specified by the first argument of this intrinsic.

Semantics:

A frontend should emit exactly one coro.id.async intrinsic per coroutine.

‘llvm.coro.id.retcon’ Intrinsic

declare token @llvm.coro.id.retcon(i32 <size>, i32 <align>, i8* <buffer>,
                                   i8* <continuation prototype>,
                                   i8* <alloc>, i8* <dealloc>)

Overview:

The ‘llvm.coro.id.retcon’ intrinsic returns a token identifying a multiple-suspend returned-continuation coroutine.

The ‘result-type sequence’ of the coroutine is defined as follows:

• if the return type of the coroutine function is void, it is the empty sequence;

• if the return type of the coroutine function is a struct, it is the element types of that struct in order;

• otherwise, it is just the return type of the coroutine function.

The first element of the result-type sequence must be a pointer type; continuation functions will be coerced to this type. The rest of the sequence are the ‘yield types’, and any suspends in the coroutine must take arguments of these types.

Arguments:

The first and second arguments are the expected size and alignment of the buffer provided as the third argument. They must be constant.

The fourth argument must be a reference to a global function, called the ‘continuation prototype function’. The type, calling convention, and attributes of any continuation functions will be taken from this declaration. The return type of the prototype function must match the return type of the current function. The first parameter type must be a pointer type. The second parameter type must be an integer type; it will be used only as a boolean flag.

The fifth argument must be a reference to a global function that will be used to allocate memory. It may not fail, either by returning null or throwing an exception. It must take an integer and return a pointer.

The sixth argument must be a reference to a global function that will be used to deallocate memory. It must take a pointer and return void.

‘llvm.coro.id.retcon.once’ Intrinsic

declare token @llvm.coro.id.retcon.once(i32 <size>, i32 <align>, i8* <buffer>,
                                        i8* <prototype>,
                                        i8* <alloc>, i8* <dealloc>)

Overview:

The ‘llvm.coro.id.retcon.once’ intrinsic returns a token identifying a unique-suspend returned-continuation coroutine.

Arguments:

As for llvm.core.id.retcon, except that the return type of the continuation prototype must be void instead of matching the coroutine’s return type.

‘llvm.coro.end’ Intrinsic

declare i1 @llvm.coro.end(i8* <handle>, i1 <unwind>)

Overview:

The ‘llvm.coro.end’ marks the point where execution of the resume part of the coroutine should end and control should return to the caller.

Arguments:

The first argument should refer to the coroutine handle of the enclosing coroutine. A frontend is allowed to supply null as the first parameter, in this case coro-early pass will replace the null with an appropriate coroutine handle value.

The second argument should be true if this coro.end is in the block that is part of the unwind sequence leaving the coroutine body due to an exception and false otherwise.

Semantics:

The purpose of this intrinsic is to allow frontends to mark the cleanup and other code that is only relevant during the initial invocation of the coroutine and should not be present in resume and destroy parts.

In returned-continuation lowering, llvm.coro.end fully destroys the coroutine frame. If the second argument is false, it also returns from the coroutine with a null continuation pointer, and the next instruction will be unreachable. If the second argument is true, it falls through so that the following logic can resume unwinding. In a yield-once coroutine, reaching a non-unwind llvm.coro.end without having first reached a llvm.coro.suspend.retcon has undefined behavior.

The remainder of this section describes the behavior under switched-resume lowering.

This intrinsic is lowered when a coroutine is split into the start, resume and destroy parts. In the start part, it is a no-op, in resume and destroy parts, it is replaced with ret void instruction and the rest of the block containing coro.end instruction is discarded. In landing pads it is replaced with an appropriate instruction to unwind to caller. The handling of coro.end differs depending on whether the target is using landingpad or WinEH exception model.

For landingpad based exception model, it is expected that frontend uses the coro.end intrinsic as follows:

ehcleanup:
  %InResumePart = call i1 @llvm.coro.end(i8* null, i1 true)
  br i1 %InResumePart, label %eh.resume, label %cleanup.cont

cleanup.cont:
  ; rest of the cleanup

eh.resume:
  %exn = load i8*, i8** %exn.slot, align 8
  %sel = load i32, i32* %ehselector.slot, align 4
  %lpad.val = insertvalue { i8*, i32 } undef, i8* %exn, 0
  %lpad.val29 = insertvalue { i8*, i32 } %lpad.val, i32 %sel, 1
  resume { i8*, i32 } %lpad.val29

The CoroSpit pass replaces coro.end with True in the resume functions, thus leading to immediate unwind to the caller, whereas in start function it is replaced with False, thus allowing to proceed to the rest of the cleanup code that is only needed during initial invocation of the coroutine.

For Windows Exception handling model, a frontend should attach a funclet bundle referring to an enclosing cleanuppad as follows:

ehcleanup:
  %tok = cleanuppad within none []
  %unused = call i1 @llvm.coro.end(i8* null, i1 true) [ "funclet"(token %tok) ]
  cleanupret from %tok unwind label %RestOfTheCleanup

The CoroSplit pass, if the funclet bundle is present, will insert cleanupret from %tok unwind to caller before the coro.end intrinsic and will remove the rest of the block.

The following table summarizes the handling of coro.end intrinsic.

		In Start Function	In Resume/Destroy Functions
unwind=false		nothing	ret void
unwind=true	WinEH	nothing	cleanupret unwind to caller
	Landingpad	nothing	nothing

‘llvm.coro.end.async’ Intrinsic

declare i1 @llvm.coro.end.async(i8* <handle>, i1 <unwind>, ...)

Overview:

The ‘llvm.coro.end.async’ marks the point where execution of the resume part of the coroutine should end and control should return to the caller. As part of its variable tail arguments this instruction allows to specify a function and the function’s arguments that are to be tail called as the last action before returning.

Arguments:

The first argument should refer to the coroutine handle of the enclosing coroutine. A frontend is allowed to supply null as the first parameter, in this case coro-early pass will replace the null with an appropriate coroutine handle value.

The second argument should be true if this coro.end is in the block that is part of the unwind sequence leaving the coroutine body due to an exception and false otherwise.

The third argument if present should specify a function to be called.

If the third argument is present, the remaining arguments are the arguments to the function call.

call i1 (i8*, i1, ...) @llvm.coro.end.async(
                         i8* %hdl, i1 0,
                         void (i8*, %async.task*, %async.actor*)* @must_tail_call_return,
                         i8* %ctxt, %async.task* %task, %async.actor* %actor)
unreachable

‘llvm.coro.suspend’ Intrinsic

declare i8 @llvm.coro.suspend(token <save>, i1 <final>)

Overview:

The ‘llvm.coro.suspend’ marks the point where execution of a switched-resume coroutine is suspended and control is returned back to the caller. Conditional branches consuming the result of this intrinsic lead to basic blocks where coroutine should proceed when suspended (-1), resumed (0) or destroyed (1).

Arguments:

The first argument refers to a token of coro.save intrinsic that marks the point when coroutine state is prepared for suspension. If none token is passed, the intrinsic behaves as if there were a coro.save immediately preceding the coro.suspend intrinsic.

The second argument indicates whether this suspension point is final. The second argument only accepts constants. If more than one suspend point is designated as final, the resume and destroy branches should lead to the same basic blocks.

Example (normal suspend point):

%0 = call i8 @llvm.coro.suspend(token none, i1 false)
switch i8 %0, label %suspend [i8 0, label %resume
                              i8 1, label %cleanup]

Example (final suspend point):

while.end:
  %s.final = call i8 @llvm.coro.suspend(token none, i1 true)
  switch i8 %s.final, label %suspend [i8 0, label %trap
                                      i8 1, label %cleanup]
trap:
  call void @llvm.trap()
  unreachable

Semantics:

If a coroutine that was suspended at the suspend point marked by this intrinsic is resumed via coro.resume the control will transfer to the basic block of the 0-case. If it is resumed via coro.destroy, it will proceed to the basic block indicated by the 1-case. To suspend, coroutine proceed to the default label.

If suspend intrinsic is marked as final, it can consider the true branch unreachable and can perform optimizations that can take advantage of that fact.

‘llvm.coro.save’ Intrinsic

declare token @llvm.coro.save(i8* <handle>)

Overview:

The ‘llvm.coro.save’ marks the point where a coroutine need to update its state to prepare for resumption to be considered suspended (and thus eligible for resumption).

Arguments:

The first argument points to a coroutine handle of the enclosing coroutine.

Semantics:

Whatever coroutine state changes are required to enable resumption of the coroutine from the corresponding suspend point should be done at the point of coro.save intrinsic.

Example:

Separate save and suspend points are necessary when a coroutine is used to represent an asynchronous control flow driven by callbacks representing completions of asynchronous operations.

In such a case, a coroutine should be ready for resumption prior to a call to async_op function that may trigger resumption of a coroutine from the same or a different thread possibly prior to async_op call returning control back to the coroutine:

%save1 = call token @llvm.coro.save(i8* %hdl)
call void @async_op1(i8* %hdl)
%suspend1 = call i1 @llvm.coro.suspend(token %save1, i1 false)
switch i8 %suspend1, label %suspend [i8 0, label %resume1
                                     i8 1, label %cleanup]

‘llvm.coro.suspend.async’ Intrinsic

declare {i8*, i8*, i8*} @llvm.coro.suspend.async(
                           i8* <resume function>,
                           i8* <context projection function>,
                           ... <function to call>
                           ... <arguments to function>)

Overview:

The ‘llvm.coro.suspend.async’ intrinsic marks the point where execution of a async coroutine is suspended and control is passed to a callee.

Arguments:

The first argument should be the result of the llvm.coro.async.resume intrinsic. Lowering will replace this intrinsic with the resume function for this suspend point.

The second argument is the context projection function. It should describe how-to restore the async context in the continuation function from the first argument of the continuation function. Its type is i8* (i8*).

The third argument is the function that models transfer to the callee at the suspend point. It should take 3 arguments. Lowering will musttail call this function.

The fourth to six argument are the arguments for the third argument.

Semantics:

The result of the intrinsic are mapped to the arguments of the resume function. Execution is suspended at this intrinsic and resumed when the resume function is called.

‘llvm.coro.prepare.async’ Intrinsic

declare i8* @llvm.coro.prepare.async(i8* <coroutine function>)

Overview:

The ‘llvm.coro.prepare.async’ intrinsic is used to block inlining of the async coroutine until after coroutine splitting.

Arguments:

The first argument should be an async coroutine of type void (i8*, i8*, i8*). Lowering will replace this intrinsic with its coroutine function argument.

‘llvm.coro.suspend.retcon’ Intrinsic

declare i1 @llvm.coro.suspend.retcon(...)

Overview:

The ‘llvm.coro.suspend.retcon’ intrinsic marks the point where execution of a returned-continuation coroutine is suspended and control is returned back to the caller.

llvm.coro.suspend.retcon` does not support separate save points; they are not useful when the continuation function is not locally accessible. That would be a more appropriate feature for a passcon lowering that is not yet implemented.

Arguments:

The types of the arguments must exactly match the yielded-types sequence of the coroutine. They will be turned into return values from the ramp and continuation functions, along with the next continuation function.

Semantics:

The result of the intrinsic indicates whether the coroutine should resume abnormally (non-zero).

In a normal coroutine, it is undefined behavior if the coroutine executes a call to llvm.coro.suspend.retcon after resuming abnormally.

In a yield-once coroutine, it is undefined behavior if the coroutine executes a call to llvm.coro.suspend.retcon after resuming in any way.

‘llvm.coro.param’ Intrinsic

declare i1 @llvm.coro.param(i8* <original>, i8* <copy>)

Overview:

The ‘llvm.coro.param’ is used by a frontend to mark up the code used to construct and destruct copies of the parameters. If the optimizer discovers that a particular parameter copy is not used after any suspends, it can remove the construction and destruction of the copy by replacing corresponding coro.param with i1 false and replacing any use of the copy with the original.

Arguments:

The first argument points to an alloca storing the value of a parameter to a coroutine.

The second argument points to an alloca storing the value of the copy of that parameter.

Semantics:

The optimizer is free to always replace this intrinsic with i1 true.

The optimizer is also allowed to replace it with i1 false provided that the parameter copy is only used prior to control flow reaching any of the suspend points. The code that would be DCE’d if the coro.param is replaced with i1 false is not considered to be a use of the parameter copy.

The frontend can emit this intrinsic if its language rules allow for this optimization.

Example:

Consider the following example. A coroutine takes two parameters a and b that has a destructor and a move constructor.

struct A { ~A(); A(A&&); bool foo(); void bar(); };

task<int> f(A a, A b) {
  if (a.foo())
    return 42;

  a.bar();
  co_await read_async(); // introduces suspend point
  b.bar();
}

Note that, uses of b is used after a suspend point and thus must be copied into a coroutine frame, whereas a does not have to, since it never used after suspend.

A frontend can create parameter copies for a and b as follows:

task<int> f(A a', A b') {
  a = alloca A;
  b = alloca A;
  // move parameters to its copies
  if (coro.param(a', a)) A::A(a, A&& a');
  if (coro.param(b', b)) A::A(b, A&& b');
  ...
  // destroy parameters copies
  if (coro.param(a', a)) A::~A(a);
  if (coro.param(b', b)) A::~A(b);
}

The optimizer can replace coro.param(a’,a) with i1 false and replace all uses of a with a’, since it is not used after suspend.

The optimizer must replace coro.param(b’, b) with i1 true, since b is used after suspend and therefore, it has to reside in the coroutine frame.

Coroutine Transformation Passes

CoroEarly

The pass CoroEarly lowers coroutine intrinsics that hide the details of the structure of the coroutine frame, but, otherwise not needed to be preserved to help later coroutine passes. This pass lowers coro.frame, coro.done, and coro.promise intrinsics.

CoroSplit

The pass CoroSplit buides coroutine frame and outlines resume and destroy parts into separate functions.

CoroElide

The pass CoroElide examines if the inlined coroutine is eligible for heap allocation elision optimization. If so, it replaces coro.begin intrinsic with an address of a coroutine frame placed on its caller and replaces coro.alloc and coro.free intrinsics with false and null respectively to remove the deallocation code. This pass also replaces coro.resume and coro.destroy intrinsics with direct calls to resume and destroy functions for a particular coroutine where possible.

CoroCleanup

This pass runs late to lower all coroutine related intrinsics not replaced by earlier passes.

Areas Requiring Attention

1. When coro.suspend returns -1, the coroutine is suspended, and it’s possible that the coroutine has already been destroyed (hence the frame has been freed). We cannot access anything on the frame on the suspend path. However there is nothing that prevents the compiler from moving instructions along that path (e.g. LICM), which can lead to use-after-free. At the moment we disabled LICM for loops that have coro.suspend, but the general problem still exists and requires a general solution.

2. Take advantage of the lifetime intrinsics for the data that goes into the coroutine frame. Leave lifetime intrinsics as is for the data that stays in allocas.

3. The CoroElide optimization pass relies on coroutine ramp function to be inlined. It would be beneficial to split the ramp function further to increase the chance that it will get inlined into its caller.

4. Design a convention that would make it possible to apply coroutine heap elision optimization across ABI boundaries.

5. Cannot handle coroutines with inalloca parameters (used in x86 on Windows).

6. Alignment is ignored by coro.begin and coro.free intrinsics.

7. Make required changes to make sure that coroutine optimizations work with LTO.

8. More tests, more tests, more tests