mirror of
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3fe61868a9
The `llvm.coro.suspend.async` intrinsic takes a function pointer as its
argument that describes how-to restore the current continuation's
context from the context argument of the continuation function. Before
we assumed that the current context can be restored by loading from the
context arguments first pointer field (`first_arg->caller_context`).
This allows for defining suspension points that reuse the current
context for example.
Also:
llvm.coro.id.async lowering: Add llvm.coro.preprare.async intrinsic
Blocks inlining until after the async coroutine was split.
Also, change the async function pointer's context size position
struct async_function_pointer {
uint32_t relative_function_pointer_to_async_impl;
uint32_t context_size;
}
And make the position of the `async context` argument configurable. The
position is specified by the `llvm.coro.id.async` intrinsic.
rdar://70097093
Differential Revision: https://reviews.llvm.org/D90783
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57 KiB
ReStructuredText
1740 lines
57 KiB
ReStructuredText
=====================================
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Coroutines in LLVM
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=====================================
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.. contents::
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:local:
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:depth: 3
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.. warning::
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This is a work in progress. Compatibility across LLVM releases is not
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guaranteed.
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Introduction
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============
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.. _coroutine handle:
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LLVM coroutines are functions that have one or more `suspend points`_.
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When a suspend point is reached, the execution of a coroutine is suspended and
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control is returned back to its caller. A suspended coroutine can be resumed
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to continue execution from the last suspend point or it can be destroyed.
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In the following example, we call function `f` (which may or may not be a
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coroutine itself) that returns a handle to a suspended coroutine
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(**coroutine handle**) that is used by `main` to resume the coroutine twice and
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then destroy it:
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.. code-block:: llvm
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define i32 @main() {
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entry:
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%hdl = call i8* @f(i32 4)
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call void @llvm.coro.resume(i8* %hdl)
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call void @llvm.coro.resume(i8* %hdl)
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call void @llvm.coro.destroy(i8* %hdl)
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ret i32 0
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}
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.. _coroutine frame:
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In addition to the function stack frame which exists when a coroutine is
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executing, there is an additional region of storage that contains objects that
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keep the coroutine state when a coroutine is suspended. This region of storage
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is called the **coroutine frame**. It is created when a coroutine is called
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and destroyed when a coroutine either runs to completion or is destroyed
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while suspended.
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LLVM currently supports two styles of coroutine lowering. These styles
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support substantially different sets of features, have substantially
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different ABIs, and expect substantially different patterns of frontend
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code generation. However, the styles also have a great deal in common.
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In all cases, an LLVM coroutine is initially represented as an ordinary LLVM
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function that has calls to `coroutine intrinsics`_ defining the structure of
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the coroutine. The coroutine function is then, in the most general case,
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rewritten by the coroutine lowering passes to become the "ramp function",
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the initial entrypoint of the coroutine, which executes until a suspend point
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is first reached. The remainder of the original coroutine function is split
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out into some number of "resume functions". Any state which must persist
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across suspensions is stored in the coroutine frame. The resume functions
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must somehow be able to handle either a "normal" resumption, which continues
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the normal execution of the coroutine, or an "abnormal" resumption, which
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must unwind the coroutine without attempting to suspend it.
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Switched-Resume Lowering
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------------------------
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In LLVM's standard switched-resume lowering, signaled by the use of
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`llvm.coro.id`, the coroutine frame is stored as part of a "coroutine
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object" which represents a handle to a particular invocation of the
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coroutine. All coroutine objects support a common ABI allowing certain
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features to be used without knowing anything about the coroutine's
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implementation:
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- A coroutine object can be queried to see if it has reached completion
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with `llvm.coro.done`.
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- A coroutine object can be resumed normally if it has not already reached
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completion with `llvm.coro.resume`.
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- A coroutine object can be destroyed, invalidating the coroutine object,
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with `llvm.coro.destroy`. This must be done separately even if the
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coroutine has reached completion normally.
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- "Promise" storage, which is known to have a certain size and alignment,
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can be projected out of the coroutine object with `llvm.coro.promise`.
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The coroutine implementation must have been compiled to define a promise
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of the same size and alignment.
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In general, interacting with a coroutine object in any of these ways while
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it is running has undefined behavior.
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The coroutine function is split into three functions, representing three
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different ways that control can enter the coroutine:
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1. the ramp function that is initially invoked, which takes arbitrary
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arguments and returns a pointer to the coroutine object;
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2. a coroutine resume function that is invoked when the coroutine is resumed,
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which takes a pointer to the coroutine object and returns `void`;
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3. a coroutine destroy function that is invoked when the coroutine is
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destroyed, which takes a pointer to the coroutine object and returns
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`void`.
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Because the resume and destroy functions are shared across all suspend
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points, suspend points must store the index of the active suspend in
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the coroutine object, and the resume/destroy functions must switch over
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that index to get back to the correct point. Hence the name of this
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lowering.
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Pointers to the resume and destroy functions are stored in the coroutine
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object at known offsets which are fixed for all coroutines. A completed
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coroutine is represented with a null resume function.
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There is a somewhat complex protocol of intrinsics for allocating and
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deallocating the coroutine object. It is complex in order to allow the
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allocation to be elided due to inlining. This protocol is discussed
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in further detail below.
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The frontend may generate code to call the coroutine function directly;
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this will become a call to the ramp function and will return a pointer
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to the coroutine object. The frontend should always resume or destroy
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the coroutine using the corresponding intrinsics.
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Returned-Continuation Lowering
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------------------------------
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In returned-continuation lowering, signaled by the use of
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`llvm.coro.id.retcon` or `llvm.coro.id.retcon.once`, some aspects of
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the ABI must be handled more explicitly by the frontend.
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In this lowering, every suspend point takes a list of "yielded values"
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which are returned back to the caller along with a function pointer,
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called the continuation function. The coroutine is resumed by simply
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calling this continuation function pointer. The original coroutine
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is divided into the ramp function and then an arbitrary number of
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these continuation functions, one for each suspend point.
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LLVM actually supports two closely-related returned-continuation
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lowerings:
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- In normal returned-continuation lowering, the coroutine may suspend
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itself multiple times. This means that a continuation function
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itself returns another continuation pointer, as well as a list of
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yielded values.
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The coroutine indicates that it has run to completion by returning
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a null continuation pointer. Any yielded values will be `undef`
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should be ignored.
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- In yield-once returned-continuation lowering, the coroutine must
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suspend itself exactly once (or throw an exception). The ramp
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function returns a continuation function pointer and yielded
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values, but the continuation function simply returns `void`
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when the coroutine has run to completion.
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The coroutine frame is maintained in a fixed-size buffer that is
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passed to the `coro.id` intrinsic, which guarantees a certain size
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and alignment statically. The same buffer must be passed to the
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continuation function(s). The coroutine will allocate memory if the
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buffer is insufficient, in which case it will need to store at
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least that pointer in the buffer; therefore the buffer must always
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be at least pointer-sized. How the coroutine uses the buffer may
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vary between suspend points.
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In addition to the buffer pointer, continuation functions take an
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argument indicating whether the coroutine is being resumed normally
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(zero) or abnormally (non-zero).
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LLVM is currently ineffective at statically eliminating allocations
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after fully inlining returned-continuation coroutines into a caller.
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This may be acceptable if LLVM's coroutine support is primarily being
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used for low-level lowering and inlining is expected to be applied
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earlier in the pipeline.
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Async Lowering
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--------------
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In async-continuation lowering, signaled by the use of `llvm.coro.id.async`,
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handling of control-flow must be handled explicitly by the frontend.
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In this lowering, a coroutine is assumed to take the current `async context` as
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one of its arguments (the argument position is determined by
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`llvm.coro.id.async`). It is used to marshal arguments and return values of the
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coroutine. Therefore an async coroutine returns `void`.
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.. code-block:: llvm
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define swiftcc void @async_coroutine(i8* %async.ctxt, i8*, i8*) {
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}
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Values live accross a suspend point need to be stored in the coroutine frame to
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be available in the continuation function. This frame is stored as a tail to the
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`async context`.
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Every suspend point takes an `context projection function` argument which
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describes how-to obtain the continuations `async context` and every suspend
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point has an associated `resume function` denoted by the
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`llvm.coro.async.resume` intrinsic. The coroutine is resumed by calling this
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`resume function` passing the `async context` as the one of its arguments
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argument. The `resume function` can restore its (the caller's) `async context`
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by applying a `context projection function` that is provided by the frontend as
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a parameter to the `llvm.coro.suspend.async` intrinsic.
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.. code-block:: c
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// For example:
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struct async_context {
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struct async_context *caller_context;
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...
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}
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char *context_projection_function(struct async_context *callee_ctxt) {
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return callee_ctxt->caller_context;
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}
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.. code-block:: llvm
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%resume_func_ptr = call i8* @llvm.coro.async.resume()
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call {i8*, i8*, i8*} (i8*, i8*, ...) @llvm.coro.suspend.async(
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i8* %resume_func_ptr,
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i8* %context_projection_function
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The frontend should provide a `async function pointer` struct associated with
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each async coroutine by `llvm.coro.id.async`'s argument. The initial size and
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alignment of the `async context` must be provided as arguments to the
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`llvm.coro.id.async` intrinsic. Lowering will update the size entry with the
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coroutine frame requirements. The frontend is responsible for allocating the
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memory for the `async context` but can use the `async function pointer` struct
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to obtain the required size.
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.. code-block:: c
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struct async_function_pointer {
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uint32_t relative_function_pointer_to_async_impl;
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uint32_t context_size;
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}
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Lowering will split an async coroutine into a ramp function and one resume
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function per suspend point.
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How control-flow is passed between caller, suspension point, and back to
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resume function is left up to the frontend.
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The suspend point takes a function and its arguments. The function is intended
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to model the transfer to the callee function. It will be tail called by
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lowering and therefore must have the same signature and calling convention as
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the async coroutine.
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.. code-block:: llvm
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call {i8*, i8*, i8*} (i8*, i8*, ...) @llvm.coro.suspend.async(
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i8* %resume_func_ptr,
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i8* %context_projection_function,
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i8* (bitcast void (i8*, i8*, i8*)* to i8*) %suspend_function,
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i8* %arg1, i8* %arg2, i8 %arg3)
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Coroutines by Example
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=====================
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The examples below are all of switched-resume coroutines.
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Coroutine Representation
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------------------------
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Let's look at an example of an LLVM coroutine with the behavior sketched
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by the following pseudo-code.
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.. code-block:: c++
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void *f(int n) {
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for(;;) {
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print(n++);
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<suspend> // returns a coroutine handle on first suspend
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}
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}
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This coroutine calls some function `print` with value `n` as an argument and
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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
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a `main` shown in the previous section. It will call `print` with values 4, 5
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and 6 after which the coroutine will be destroyed.
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The LLVM IR for this coroutine looks like this:
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.. code-block:: llvm
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define i8* @f(i32 %n) {
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entry:
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%id = call token @llvm.coro.id(i32 0, i8* null, i8* null, i8* null)
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%size = call i32 @llvm.coro.size.i32()
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%alloc = call i8* @malloc(i32 %size)
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%hdl = call noalias i8* @llvm.coro.begin(token %id, i8* %alloc)
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br label %loop
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loop:
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%n.val = phi i32 [ %n, %entry ], [ %inc, %loop ]
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%inc = add nsw i32 %n.val, 1
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call void @print(i32 %n.val)
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%0 = call i8 @llvm.coro.suspend(token none, i1 false)
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switch i8 %0, label %suspend [i8 0, label %loop
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i8 1, label %cleanup]
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cleanup:
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%mem = call i8* @llvm.coro.free(token %id, i8* %hdl)
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call void @free(i8* %mem)
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br label %suspend
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suspend:
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%unused = call i1 @llvm.coro.end(i8* %hdl, i1 false)
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ret i8* %hdl
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}
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The `entry` block establishes the coroutine frame. The `coro.size`_ intrinsic is
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lowered to a constant representing the size required for the coroutine frame.
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The `coro.begin`_ intrinsic initializes the coroutine frame and returns the
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coroutine handle. The second parameter of `coro.begin` is given a block of memory
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to be used if the coroutine frame needs to be allocated dynamically.
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The `coro.id`_ intrinsic serves as coroutine identity useful in cases when the
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`coro.begin`_ intrinsic get duplicated by optimization passes such as
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jump-threading.
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The `cleanup` block destroys the coroutine frame. The `coro.free`_ intrinsic,
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given the coroutine handle, returns a pointer of the memory block to be freed or
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`null` if the coroutine frame was not allocated dynamically. The `cleanup`
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block is entered when coroutine runs to completion by itself or destroyed via
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call to the `coro.destroy`_ intrinsic.
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The `suspend` block contains code to be executed when coroutine runs to
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completion or suspended. The `coro.end`_ intrinsic marks the point where
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a coroutine needs to return control back to the caller if it is not an initial
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invocation of the coroutine.
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The `loop` blocks represents the body of the coroutine. The `coro.suspend`_
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intrinsic in combination with the following switch indicates what happens to
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control flow when a coroutine is suspended (default case), resumed (case 0) or
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destroyed (case 1).
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Coroutine Transformation
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------------------------
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One of the steps of coroutine lowering is building the coroutine frame. The
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def-use chains are analyzed to determine which objects need be kept alive across
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suspend points. In the coroutine shown in the previous section, use of virtual register
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`%inc` is separated from the definition by a suspend point, therefore, it
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cannot reside on the stack frame since the latter goes away once the coroutine
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is suspended and control is returned back to the caller. An i32 slot is
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allocated in the coroutine frame and `%inc` is spilled and reloaded from that
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slot as needed.
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We also store addresses of the resume and destroy functions so that the
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`coro.resume` and `coro.destroy` intrinsics can resume and destroy the coroutine
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when its identity cannot be determined statically at compile time. For our
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example, the coroutine frame will be:
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.. code-block:: llvm
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%f.frame = type { void (%f.frame*)*, void (%f.frame*)*, i32 }
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After resume and destroy parts are outlined, function `f` will contain only the
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code responsible for creation and initialization of the coroutine frame and
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execution of the coroutine until a suspend point is reached:
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.. code-block:: llvm
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define i8* @f(i32 %n) {
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entry:
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%id = call token @llvm.coro.id(i32 0, i8* null, i8* null, i8* null)
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%alloc = call noalias i8* @malloc(i32 24)
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%0 = call noalias i8* @llvm.coro.begin(token %id, i8* %alloc)
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%frame = bitcast i8* %0 to %f.frame*
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%1 = getelementptr %f.frame, %f.frame* %frame, i32 0, i32 0
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store void (%f.frame*)* @f.resume, void (%f.frame*)** %1
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%2 = getelementptr %f.frame, %f.frame* %frame, i32 0, i32 1
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store void (%f.frame*)* @f.destroy, void (%f.frame*)** %2
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%inc = add nsw i32 %n, 1
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%inc.spill.addr = getelementptr inbounds %f.Frame, %f.Frame* %FramePtr, i32 0, i32 2
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store i32 %inc, i32* %inc.spill.addr
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call void @print(i32 %n)
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ret i8* %frame
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}
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Outlined resume part of the coroutine will reside in function `f.resume`:
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.. code-block:: llvm
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define internal fastcc void @f.resume(%f.frame* %frame.ptr.resume) {
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entry:
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%inc.spill.addr = getelementptr %f.frame, %f.frame* %frame.ptr.resume, i64 0, i32 2
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%inc.spill = load i32, i32* %inc.spill.addr, align 4
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%inc = add i32 %n.val, 1
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store i32 %inc, i32* %inc.spill.addr, align 4
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tail call void @print(i32 %inc)
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ret void
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}
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Whereas function `f.destroy` will contain the cleanup code for the coroutine:
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.. code-block:: llvm
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define internal fastcc void @f.destroy(%f.frame* %frame.ptr.destroy) {
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entry:
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%0 = bitcast %f.frame* %frame.ptr.destroy to i8*
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tail call void @free(i8* %0)
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ret void
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}
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Avoiding Heap Allocations
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-------------------------
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A particular coroutine usage pattern, which is illustrated by the `main`
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function in the overview section, where a coroutine is created, manipulated and
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destroyed by the same calling function, is common for coroutines implementing
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RAII idiom and is suitable for allocation elision optimization which avoid
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dynamic allocation by storing the coroutine frame as a static `alloca` in its
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caller.
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In the entry block, we will call `coro.alloc`_ intrinsic that will return `true`
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when dynamic allocation is required, and `false` if dynamic allocation is
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elided.
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.. code-block:: llvm
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entry:
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%id = call token @llvm.coro.id(i32 0, i8* null, i8* null, i8* null)
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%need.dyn.alloc = call i1 @llvm.coro.alloc(token %id)
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br i1 %need.dyn.alloc, label %dyn.alloc, label %coro.begin
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dyn.alloc:
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%size = call i32 @llvm.coro.size.i32()
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%alloc = call i8* @CustomAlloc(i32 %size)
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br label %coro.begin
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coro.begin:
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%phi = phi i8* [ null, %entry ], [ %alloc, %dyn.alloc ]
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%hdl = call noalias i8* @llvm.coro.begin(token %id, i8* %phi)
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In the cleanup block, we will make freeing the coroutine frame conditional on
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`coro.free`_ intrinsic. If allocation is elided, `coro.free`_ returns `null`
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thus skipping the deallocation code:
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.. code-block:: llvm
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cleanup:
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%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:
|
|
|
|
.. 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++);
|
|
<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):
|
|
|
|
.. 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. 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(%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.
|
|
|
|
.. code-block:: text
|
|
|
|
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):
|
|
|
|
.. code-block:: llvm
|
|
|
|
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:
|
|
|
|
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 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.
|
|
|
|
.. code-block:: llvm
|
|
|
|
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:
|
|
|
|
.. 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 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:
|
|
|
|
.. 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;
|
|
<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:
|
|
|
|
.. 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(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.
|
|
|
|
.. _coro.resume:
|
|
|
|
'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.
|
|
|
|
.. _coro.done:
|
|
|
|
'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.
|
|
|
|
.. _coro.promise:
|
|
|
|
'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:
|
|
""""""""
|
|
|
|
.. code-block:: llvm
|
|
|
|
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 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.begin:
|
|
|
|
'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.
|
|
|
|
.. _coro.free:
|
|
|
|
'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):
|
|
"""""""""""""""""""""""""""""""""""""""
|
|
|
|
.. code-block:: llvm
|
|
|
|
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):
|
|
""""""""""""""""""""""""""""""""""""""""""
|
|
|
|
.. code-block:: llvm
|
|
|
|
cleanup:
|
|
%mem = call i8* @llvm.coro.free(token %id, i8* %frame)
|
|
call void @free(i8* %mem)
|
|
ret void
|
|
|
|
.. _coro.alloc:
|
|
|
|
'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:
|
|
""""""""
|
|
|
|
.. code-block:: llvm
|
|
|
|
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)
|
|
|
|
.. _coro.noop:
|
|
|
|
'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.
|
|
|
|
.. _coro.frame:
|
|
|
|
'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.
|
|
|
|
.. _coro.id:
|
|
|
|
'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.
|
|
|
|
.. _coro.id.async:
|
|
|
|
'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 intrinisc.
|
|
|
|
|
|
Semantics:
|
|
""""""""""
|
|
|
|
A frontend should emit exactly one `coro.id.async` intrinsic per coroutine.
|
|
|
|
.. _coro.id.retcon:
|
|
|
|
'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.
|
|
|
|
.. _coro.end:
|
|
|
|
'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:
|
|
|
|
.. code-block:: llvm
|
|
|
|
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:
|
|
|
|
.. code-block:: llvm
|
|
|
|
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`` |
|
|
+------------+-------------+-------------------+-------------------------------+
|
|
| | WinEH | nothing |``cleanupret unwind to caller``|
|
|
|unwind=true +-------------+-------------------+-------------------------------+
|
|
| | Landingpad | nothing | nothing |
|
|
+------------+-------------+-------------------+-------------------------------+
|
|
|
|
.. _coro.suspend:
|
|
.. _suspend points:
|
|
|
|
'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):
|
|
"""""""""""""""""""""""""""""""
|
|
|
|
.. 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(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:
|
|
|
|
.. code-block:: llvm
|
|
|
|
%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]
|
|
|
|
.. _coro.suspend.async:
|
|
|
|
'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 tranfer 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 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.
|
|
|
|
.. _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.param:
|
|
|
|
'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.
|
|
|
|
.. code-block:: c++
|
|
|
|
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:
|
|
|
|
.. code-block:: text
|
|
|
|
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:
|
|
|
|
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
|
|
=========================
|
|
#. 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
|