===================================== Coroutines in LLVM ===================================== .. contents:: :local: :depth: 3 .. warning:: Compatibility across LLVM releases is not guaranteed. Introduction ============ .. _coroutine handle: 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: .. code-block:: llvm define i32 @main() { entry: %hdl = call ptr @f(i32 4) call void @llvm.coro.resume(ptr %hdl) call void @llvm.coro.resume(ptr %hdl) call void @llvm.coro.destroy(ptr %hdl) ret i32 0 } .. _coroutine frame: 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, the continuation function may optionally return ordinary results 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`. .. code-block:: llvm define swiftcc void @async_coroutine(ptr %async.ctxt, ptr, ptr) { } 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. .. code-block:: c // For example: struct async_context { struct async_context *caller_context; ... } char *context_projection_function(struct async_context *callee_ctxt) { return callee_ctxt->caller_context; } .. code-block:: llvm %resume_func_ptr = call ptr @llvm.coro.async.resume() call {ptr, ptr, ptr} (ptr, ptr, ...) @llvm.coro.suspend.async( ptr %resume_func_ptr, ptr %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. .. code-block:: c 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. .. code-block:: llvm call {ptr, ptr, ptr} (ptr, ptr, ...) @llvm.coro.suspend.async( ptr %resume_func_ptr, ptr %context_projection_function, ptr %suspend_function, ptr %arg1, ptr %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. .. code-block:: c++ void *f(int n) { for(;;) { print(n++); // 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: .. code-block:: llvm define ptr @f(i32 %n) presplitcoroutine { entry: %id = call token @llvm.coro.id(i32 0, ptr null, ptr null, ptr null) %size = call i32 @llvm.coro.size.i32() %alloc = call ptr @malloc(i32 %size) %hdl = call noalias ptr @llvm.coro.begin(token %id, ptr %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 ptr @llvm.coro.free(token %id, ptr %hdl) call void @free(ptr %mem) br label %suspend suspend: %unused = call i1 @llvm.coro.end(ptr %hdl, i1 false, token none) ret ptr %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: .. code-block:: llvm %f.frame = type { ptr, ptr, 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: .. code-block:: llvm define ptr @f(i32 %n) { entry: %id = call token @llvm.coro.id(i32 0, ptr null, ptr null, ptr null) %alloc = call noalias ptr @malloc(i32 24) %frame = call noalias ptr @llvm.coro.begin(token %id, ptr %alloc) %1 = getelementptr %f.frame, ptr %frame, i32 0, i32 0 store ptr @f.resume, ptr %1 %2 = getelementptr %f.frame, ptr %frame, i32 0, i32 1 store ptr @f.destroy, ptr %2 %inc = add nsw i32 %n, 1 %inc.spill.addr = getelementptr inbounds %f.Frame, ptr %FramePtr, i32 0, i32 2 store i32 %inc, ptr %inc.spill.addr call void @print(i32 %n) ret ptr %frame } Outlined resume part of the coroutine will reside in function `f.resume`: .. code-block:: llvm define internal fastcc void @f.resume(ptr %frame.ptr.resume) { entry: %inc.spill.addr = getelementptr %f.frame, ptr %frame.ptr.resume, i64 0, i32 2 %inc.spill = load i32, ptr %inc.spill.addr, align 4 %inc = add i32 %inc.spill, 1 store i32 %inc, ptr %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: .. code-block:: llvm define internal fastcc void @f.destroy(ptr %frame.ptr.destroy) { entry: tail call void @free(ptr %frame.ptr.destroy) 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. .. code-block:: llvm entry: %id = call token @llvm.coro.id(i32 0, ptr null, ptr null, ptr 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 ptr @CustomAlloc(i32 %size) br label %coro.begin coro.begin: %phi = phi ptr [ null, %entry ], [ %alloc, %dyn.alloc ] %hdl = call noalias ptr @llvm.coro.begin(token %id, ptr %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: .. code-block:: llvm cleanup: %mem = call ptr @llvm.coro.free(token %id, ptr %hdl) %need.dyn.free = icmp ne ptr %mem, null br i1 %need.dyn.free, label %dyn.free, label %if.end dyn.free: call void @CustomFree(ptr %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: .. code-block:: llvm 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: .. code-block:: c++ void *f(int n) { for(;;) { print(n++); print(-n); } } Matching LLVM code would look like (with the rest of the code remaining the same as the code in the previous section): .. code-block:: llvm 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. .. code-block:: llvm %f.frame = type { ptr, ptr, i32, i32 } The resume function will use an index to jump to an appropriate basic block and will look as follows: .. code-block:: llvm define internal fastcc void @f.Resume(ptr %FramePtr) { entry.Resume: %index.addr = getelementptr inbounds %f.Frame, ptr %FramePtr, i64 0, i32 2 %index = load i8, ptr %index.addr, align 1 %switch = icmp eq i8 %index, 0 %n.addr = getelementptr inbounds %f.Frame, ptr %FramePtr, i64 0, i32 3 %n = load i32, ptr %n.addr, align 4 br i1 %switch, label %loop.resume, label %loop loop.resume: %sub = sub nsw i32 0, %n call void @print(i32 %sub) br label %suspend loop: %inc = add nsw i32 %n, 1 store i32 %inc, ptr %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, ptr %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. .. code-block:: text void g() { for (;;) if (cond()) { async_op1(); // will resume once async_op1 completes do_one(); } else { async_op2(); // will resume once async_op2 completes 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): .. code-block:: llvm if.true: %save1 = call token @llvm.coro.save(ptr %hdl) call void @async_op1(ptr %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(ptr %hdl) call void @async_op2(ptr %hdl) %suspend2 = call i1 @llvm.coro.suspend(token %save2, i1 false) switch i8 %suspend2, label %suspend [i8 0, label %resume2 i8 1, label %cleanup] .. _coroutine promise: 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. .. code-block:: llvm define ptr @f(i32 %n) { entry: %promise = alloca i32 %id = call token @llvm.coro.id(i32 0, ptr %promise, ptr null, ptr 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 ptr @malloc(i32 %size) br label %coro.begin coro.begin: %phi = phi ptr [ null, %entry ], [ %alloc, %dyn.alloc ] %hdl = call noalias ptr @llvm.coro.begin(token %id, ptr %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, ptr %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 ptr @llvm.coro.free(token %id, ptr %hdl) call void @free(ptr %mem) br label %suspend suspend: %unused = call i1 @llvm.coro.end(ptr %hdl, i1 false, token none) ret ptr %hdl } A coroutine consumer can rely on the `coro.promise`_ intrinsic to access the coroutine promise. .. code-block:: llvm define i32 @main() { entry: %hdl = call ptr @f(i32 4) %promise.addr = call ptr @llvm.coro.promise(ptr %hdl, i32 4, i1 false) %val0 = load i32, ptr %promise.addr call void @print(i32 %val0) call void @llvm.coro.resume(ptr %hdl) %val1 = load i32, ptr %promise.addr call void @print(i32 %val1) call void @llvm.coro.resume(ptr %hdl) %val2 = load i32, ptr %promise.addr call void @print(i32 %val2) call void @llvm.coro.destroy(ptr %hdl) ret i32 0 } After example in this section is compiled, result of the compilation will be: .. code-block:: llvm 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: .. _final suspend: 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: .. code-block:: llvm define i32 @main() { entry: %hdl = call ptr @f(i32 4) br label %while while: call void @llvm.coro.resume(ptr %hdl) %done = call i1 @llvm.coro.done(ptr %hdl) br i1 %done, label %end, label %while end: call void @llvm.coro.destroy(ptr %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: .. code-block:: python 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: .. code-block:: c void* coroutine(int n) { int current_value; // injected suspend point, so that the coroutine starts suspended for (int i = 0; i < n; ++i) { current_value = i; ; // corresponds to "yield i" } // injected final suspend point } and python iterator `__next__` would look like: .. code-block:: c++ 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`_. .. _coro.destroy: 'llvm.coro.destroy' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Syntax: """"""" :: declare void @llvm.coro.destroy(ptr ) 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. .. _coro.resume: 'llvm.coro.resume' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare void @llvm.coro.resume(ptr ) 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. .. _coro.done: 'llvm.coro.done' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare i1 @llvm.coro.done(ptr ) 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. .. _coro.promise: 'llvm.coro.promise' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare ptr @llvm.coro.promise(ptr , i32 , i1 ) 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: """""""" .. code-block:: llvm define ptr @f(i32 %n) { entry: %promise = alloca i32 ; the second argument to coro.id points to the coroutine promise. %id = call token @llvm.coro.id(i32 0, ptr %promise, ptr null, ptr null) ... %hdl = call noalias ptr @llvm.coro.begin(token %id, ptr %alloc) ... store i32 42, ptr %promise ; store something into the promise ... ret ptr %hdl } define i32 @main() { entry: %hdl = call ptr @f(i32 4) ; starts the coroutine and returns its handle %promise.addr = call ptr @llvm.coro.promise(ptr %hdl, i32 4, i1 false) %val = load i32, ptr %promise.addr ; load a value from the promise call void @print(i32 %val) call void @llvm.coro.destroy(ptr %hdl) ret i32 0 } .. _coroutine intrinsics: 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. .. _coro.size: '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. .. _coro.align: 'llvm.coro.align' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare i32 @llvm.coro.align.i32() declare i64 @llvm.coro.align.i64() Overview: """"""""" The '``llvm.coro.align``' intrinsic returns the alignment of a `coroutine frame`_. This is only supported for switched-resume coroutines. Arguments: """""""""" None Semantics: """""""""" The `coro.align` intrinsic is lowered to a constant representing the alignment of the coroutine frame. .. _coro.begin: 'llvm.coro.begin' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare ptr @llvm.coro.begin(token , ptr ) 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. .. _coro.free: 'llvm.coro.free' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare ptr @llvm.coro.free(token %id, ptr ) 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): """"""""""""""""""""""""""""""""""""""" .. code-block:: llvm cleanup: %mem = call ptr @llvm.coro.free(token %id, ptr %frame) %mem_not_null = icmp ne ptr %mem, null br i1 %mem_not_null, label %if.then, label %if.end if.then: call void @CustomFree(ptr %mem) br label %if.end if.end: ret void Example (standard deallocation functions): """""""""""""""""""""""""""""""""""""""""" .. code-block:: llvm cleanup: %mem = call ptr @llvm.coro.free(token %id, ptr %frame) call void @free(ptr %mem) ret void .. _coro.alloc: 'llvm.coro.alloc' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare i1 @llvm.coro.alloc(token ) 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: """""""" .. code-block:: llvm entry: %id = call token @llvm.coro.id(i32 0, ptr null, ptr null, ptr 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 ptr @MyAlloc(i32 %frame.size) br label %coro.begin coro.begin: %phi = phi ptr [ null, %entry ], [ %alloc, %coro.alloc ] %frame = call ptr @llvm.coro.begin(token %id, ptr %phi) .. _coro.noop: 'llvm.coro.noop' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare ptr @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. .. _coro.frame: 'llvm.coro.frame' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare ptr @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. .. _coro.id: 'llvm.coro.id' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare token @llvm.coro.id(i32 , ptr , ptr , ptr ) 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(ptr). 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. A frontend should emit function attribute `presplitcoroutine` for the coroutine. .. _coro.id.async: 'llvm.coro.id.async' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare token @llvm.coro.id.async(i32 , i32 , ptr , ptr ) 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. A frontend should emit function attribute `presplitcoroutine` for the coroutine. .. _coro.id.retcon: 'llvm.coro.id.retcon' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare token @llvm.coro.id.retcon(i32 , i32 , ptr , ptr , ptr , ptr ) 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``. Semantics: """""""""" A frontend should emit function attribute `presplitcoroutine` for the coroutine. 'llvm.coro.id.retcon.once' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare token @llvm.coro.id.retcon.once(i32 , i32 , ptr , ptr , ptr , ptr ) 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 represent the normal return type of the continuation (instead of matching the coroutine's return type). Semantics: """""""""" A frontend should emit function attribute `presplitcoroutine` for the coroutine. .. _coro.end: 'llvm.coro.end' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare i1 @llvm.coro.end(ptr , i1 , token ) 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. Non-trivial (non-none) token argument can only be specified for unique-suspend returned-continuation coroutines where it must be a token value produced by '``llvm.coro.end.results``' intrinsic. Only none token is allowed for coro.end calls in unwind sections 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: .. code-block:: llvm ehcleanup: %InResumePart = call i1 @llvm.coro.end(ptr null, i1 true, token none) br i1 %InResumePart, label %eh.resume, label %cleanup.cont cleanup.cont: ; rest of the cleanup eh.resume: %exn = load ptr, ptr %exn.slot, align 8 %sel = load i32, ptr %ehselector.slot, align 4 %lpad.val = insertvalue { ptr, i32 } undef, ptr %exn, 0 %lpad.val29 = insertvalue { ptr, i32 } %lpad.val, i32 %sel, 1 resume { ptr, 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: .. code-block:: llvm ehcleanup: %tok = cleanuppad within none [] %unused = call i1 @llvm.coro.end(ptr null, i1 true, token none) [ "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. In the unwind path (when the argument is `true`), `coro.end` will mark the coroutine as done, making it undefined behavior to resume the coroutine again and causing `llvm.coro.done` to return `true`. This is not necessary in the normal path because the coroutine will already be marked as done by the final suspend. The following table summarizes the handling of `coro.end`_ intrinsic. +--------------------------+------------------------+---------------------------------+ | | In Start Function | In Resume/Destroy Functions | +--------------------------+------------------------+---------------------------------+ |unwind=false | nothing |``ret void`` | +------------+-------------+------------------------+---------------------------------+ | | WinEH | mark coroutine as done || ``cleanupret unwind to caller``| | | | || mark coroutine done | |unwind=true +-------------+------------------------+---------------------------------+ | | Landingpad | mark coroutine as done | mark coroutine done | +------------+-------------+------------------------+---------------------------------+ .. _coro.end.results: 'llvm.coro.end.results' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare token @llvm.coro.end.results(...) Overview: """"""""" The '``llvm.coro.end.results``' intrinsic captures values to be returned from unique-suspend returned-continuation coroutines. Arguments: """""""""" The number of arguments must match the return type of the continuation function: - if the return type of the continuation function is ``void`` there must be no arguments - if the return type of the continuation function is a ``struct``, the arguments will be of element types of that ``struct`` in order; - otherwise, it is just the return value of the continuation function. .. code-block:: llvm define {ptr, ptr} @g(ptr %buffer, ptr %ptr, i8 %val) presplitcoroutine { entry: %id = call token @llvm.coro.id.retcon.once(i32 8, i32 8, ptr %buffer, ptr @prototype, ptr @allocate, ptr @deallocate) %hdl = call ptr @llvm.coro.begin(token %id, ptr null) ... cleanup: %tok = call token (...) @llvm.coro.end.results(i8 %val) call i1 @llvm.coro.end(ptr %hdl, i1 0, token %tok) unreachable ... declare i8 @prototype(ptr, i1 zeroext) 'llvm.coro.end.async' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare i1 @llvm.coro.end.async(ptr , i1 , ...) 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. .. code-block:: llvm call i1 (ptr, i1, ...) @llvm.coro.end.async( ptr %hdl, i1 0, ptr @must_tail_call_return, ptr %ctxt, ptr %task, ptr %actor) unreachable .. _coro.suspend: .. _suspend points: 'llvm.coro.suspend' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare i8 @llvm.coro.suspend(token , i1 ) 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): """"""""""""""""""""""""""""""" .. code-block:: llvm %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): """""""""""""""""""""""""""""" .. code-block:: llvm 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. .. _coro.save: 'llvm.coro.save' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare token @llvm.coro.save(ptr ) 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). It is illegal to merge two '``llvm.coro.save``' calls unless their '``llvm.coro.suspend``' users are also merged. So '``llvm.coro.save``' is currently tagged with the `no_merge` function attribute. 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: .. code-block:: llvm %save1 = call token @llvm.coro.save(ptr %hdl) call void @async_op1(ptr %hdl) %suspend1 = call i1 @llvm.coro.suspend(token %save1, i1 false) switch i8 %suspend1, label %suspend [i8 0, label %resume1 i8 1, label %cleanup] .. _coro.suspend.async: 'llvm.coro.suspend.async' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare {ptr, ptr, ptr} @llvm.coro.suspend.async( ptr , ptr , ... ... ) Overview: """"""""" The '``llvm.coro.suspend.async``' intrinsic marks the point where execution of an 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 `ptr (ptr)`. 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. .. _coro.prepare.async: 'llvm.coro.prepare.async' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare ptr @llvm.coro.prepare.async(ptr ) 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 (ptr, ptr, ptr)`. Lowering will replace this intrinsic with its coroutine function argument. .. _coro.suspend.retcon: '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. .. _coro.await.suspend.void: 'llvm.coro.await.suspend.void' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare void @llvm.coro.await.suspend.void( ptr , ptr , ptr ) Overview: """"""""" The '``llvm.coro.await.suspend.void``' intrinsic encapsulates C++ `await-suspend` block until it can't interfere with coroutine transform. The `await_suspend` block of `co_await` is essentially asynchronous to the execution of the coroutine. Inlining it normally into an unsplit coroutine can cause miscompilation because the coroutine CFG misrepresents the true control flow of the program: things that happen in the await_suspend are not guaranteed to happen prior to the resumption of the coroutine, and things that happen after the resumption of the coroutine (including its exit and the potential deallocation of the coroutine frame) are not guaranteed to happen only after the end of `await_suspend`. This version of intrinsic corresponds to '``void awaiter.await_suspend(...)``' variant. Arguments: """""""""" The first argument is a pointer to `awaiter` object. The second argument is a pointer to the current coroutine's frame. The third argument is a pointer to the wrapper function encapsulating `await-suspend` logic. Its signature must be .. code-block:: llvm declare void @await_suspend_function(ptr %awaiter, ptr %hdl) Semantics: """""""""" The intrinsic must be used between corresponding `coro.save`_ and `coro.suspend`_ calls. It is lowered to a direct `await_suspend_function` call during `CoroSplit`_ pass. Example: """""""" .. code-block:: llvm ; before lowering await.suspend: %save = call token @llvm.coro.save(ptr %hdl) call void @llvm.coro.await.suspend.void( ptr %awaiter, ptr %hdl, ptr @await_suspend_function) %suspend = call i8 @llvm.coro.suspend(token %save, i1 false) ... ; after lowering await.suspend: %save = call token @llvm.coro.save(ptr %hdl) ; the call to await_suspend_function can be inlined call void @await_suspend_function( ptr %awaiter, ptr %hdl) %suspend = call i8 @llvm.coro.suspend(token %save, i1 false) ... ; wrapper function example define void @await_suspend_function(ptr %awaiter, ptr %hdl) entry: %hdl.arg = ... ; construct std::coroutine_handle from %hdl call void @"Awaiter::await_suspend"(ptr %awaiter, ptr %hdl.arg) ret void .. _coro.await.suspend.bool: 'llvm.coro.await.suspend.bool' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare i1 @llvm.coro.await.suspend.bool( ptr , ptr , ptr ) Overview: """"""""" The '``llvm.coro.await.suspend.bool``' intrinsic encapsulates C++ `await-suspend` block until it can't interfere with coroutine transform. The `await_suspend` block of `co_await` is essentially asynchronous to the execution of the coroutine. Inlining it normally into an unsplit coroutine can cause miscompilation because the coroutine CFG misrepresents the true control flow of the program: things that happen in the await_suspend are not guaranteed to happen prior to the resumption of the coroutine, and things that happen after the resumption of the coroutine (including its exit and the potential deallocation of the coroutine frame) are not guaranteed to happen only after the end of `await_suspend`. This version of intrinsic corresponds to '``bool awaiter.await_suspend(...)``' variant. Arguments: """""""""" The first argument is a pointer to `awaiter` object. The second argument is a pointer to the current coroutine's frame. The third argument is a pointer to the wrapper function encapsulating `await-suspend` logic. Its signature must be .. code-block:: llvm declare i1 @await_suspend_function(ptr %awaiter, ptr %hdl) Semantics: """""""""" The intrinsic must be used between corresponding `coro.save`_ and `coro.suspend`_ calls. It is lowered to a direct `await_suspend_function` call during `CoroSplit`_ pass. If `await_suspend_function` call returns `true`, the current coroutine is immediately resumed. Example: """""""" .. code-block:: llvm ; before lowering await.suspend: %save = call token @llvm.coro.save(ptr %hdl) %resume = call i1 @llvm.coro.await.suspend.bool( ptr %awaiter, ptr %hdl, ptr @await_suspend_function) br i1 %resume, %await.suspend.bool, %await.ready await.suspend.bool: %suspend = call i8 @llvm.coro.suspend(token %save, i1 false) ... await.ready: call void @"Awaiter::await_resume"(ptr %awaiter) ... ; after lowering await.suspend: %save = call token @llvm.coro.save(ptr %hdl) ; the call to await_suspend_function can inlined %resume = call i1 @await_suspend_function( ptr %awaiter, ptr %hdl) br i1 %resume, %await.suspend.bool, %await.ready ... ; wrapper function example define i1 @await_suspend_function(ptr %awaiter, ptr %hdl) entry: %hdl.arg = ... ; construct std::coroutine_handle from %hdl %resume = call i1 @"Awaiter::await_suspend"(ptr %awaiter, ptr %hdl.arg) ret i1 %resume .. _coro.await.suspend.handle: 'llvm.coro.await.suspend.handle' Intrinsic ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: declare ptr @llvm.coro.await.suspend.handle( ptr , ptr , ptr ) Overview: """"""""" The '``llvm.coro.await.suspend.handle``' intrinsic encapsulates C++ `await-suspend` block until it can't interfere with coroutine transform. The `await_suspend` block of `co_await` is essentially asynchronous to the execution of the coroutine. Inlining it normally into an unsplit coroutine can cause miscompilation because the coroutine CFG misrepresents the true control flow of the program: things that happen in the await_suspend are not guaranteed to happen prior to the resumption of the coroutine, and things that happen after the resumption of the coroutine (including its exit and the potential deallocation of the coroutine frame) are not guaranteed to happen only after the end of `await_suspend`. This version of intrinsic corresponds to '``std::corouine_handle<> awaiter.await_suspend(...)``' variant. Arguments: """""""""" The first argument is a pointer to `awaiter` object. The second argument is a pointer to the current coroutine's frame. The third argument is a pointer to the wrapper function encapsulating `await-suspend` logic. Its signature must be .. code-block:: llvm declare ptr @await_suspend_function(ptr %awaiter, ptr %hdl) Semantics: """""""""" The intrinsic must be used between corresponding `coro.save`_ and `coro.suspend`_ calls. It is lowered to a direct `await_suspend_function` call during `CoroSplit`_ pass. `await_suspend_function` must return a pointer to a valid coroutine frame, which is immediately resumed Example: """""""" .. code-block:: llvm ; before lowering await.suspend: %save = call token @llvm.coro.save(ptr %hdl) %next = call ptr @llvm.coro.await.suspend.handle( ptr %awaiter, ptr %hdl, ptr @await_suspend_function) call void @llvm.coro.resume(%next) %suspend = call i8 @llvm.coro.suspend(token %save, i1 false) ... ; after lowering await.suspend: %save = call token @llvm.coro.save(ptr %hdl) ; the call to await_suspend_function can be inlined %next = call ptr @await_suspend_function( ptr %awaiter, ptr %hdl) call void @llvm.coro.resume(%next) %suspend = call i8 @llvm.coro.suspend(token %save, i1 false) ... ; wrapper function example define ptr @await_suspend_function(ptr %awaiter, ptr %hdl) entry: %hdl.arg = ... ; construct std::coroutine_handle from %hdl %hdl.raw = call ptr @"Awaiter::await_suspend"(ptr %awaiter, ptr %hdl.arg) %hdl.result = ... ; get address of returned coroutine handle ret ptr %hdl.result 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: CoroSplit --------- The pass CoroSplit builds coroutine frame and outlines resume and destroy parts into separate functions. This pass also lowers `coro.await.suspend.void`_, `coro.await.suspend.bool`_ and `coro.await.suspend.handle`_ intrinsics. 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. Attributes ========== coro_only_destroy_when_complete ------------------------------- When the coroutine are marked with coro_only_destroy_when_complete, it indicates the coroutine must reach the final suspend point when it get destroyed. This attribute only works for switched-resume coroutines now. Metadata ======== '``coro.outside.frame``' Metadata --------------------------------- ``coro.outside.frame`` metadata may be attached to an alloca instruction to to signify that it shouldn't be promoted to the coroutine frame, useful for filtering allocas out by the frontend when emitting internal control mechanisms. Additionally, this metadata is only used as a flag, so the associated node must be empty. .. code-block:: text %__coro_gro = alloca %struct.GroType, align 1, !coro.outside.frame !0 ... !0 = !{} Areas Requiring Attention ========================= #. 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. #. 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. #. 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. #. Design a convention that would make it possible to apply coroutine heap elision optimization across ABI boundaries. #. Cannot handle coroutines with `inalloca` parameters (used in x86 on Windows). #. Alignment is ignored by coro.begin and coro.free intrinsics. #. Make required changes to make sure that coroutine optimizations work with LTO. #. More tests, more tests, more tests