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439edbce1f
Change the code generation patterns for `async let` bindings to use an ABI based on the following functions: - `swift_asyncLet_begin`, which starts an `async let` child task, but which additionally now associates the `async let` with a caller-owned buffer to receive the result of the task. This is intended to allow the task to emplace its result in caller-owned memory, allowing the child task to be deallocated after completion without invalidating the result buffer. - `swift_asyncLet_get[_throwing]`, which replaces `swift_asyncLet_wait[_throwing]`. Instead of returning a copy of the value, this entry point concerns itself with populating the local buffer. If the buffer hasn't been populated, then it awaits completion of the task and emplaces the result in the buffer; otherwise, it simply returns. The caller can then read the result out of its owned memory. These entry points are intended to be used before every read from the `async let` binding, after which point the local buffer is guaranteed to contain an initialized value. - `swift_asyncLet_finish`, which replaces `swift_asyncLet_end`. Unlike `_end`, this variant is async and will suspend the parent task after cancelling the child to ensure it finishes before cleaning up. The local buffer will also be deinitialized if necessary. This is intended to be used on exit from an `async let` scope, to handle cleaning up the local buffer if necessary as well as cancelling, awaiting, and deallocating the child task. - `swift_asyncLet_consume[_throwing]`, which combines `get` and `finish`. This will await completion of the task, leaving the result value in the result buffer (or propagating the error, if it throws), while destroying and deallocating the child task. This is intended as an optimization for reading `async let` variables that are read exactly once by their parent task. To avoid an epoch break with existing swiftinterfaces and ABI clients, the old builtins and entry points are kept intact for now, but SILGen now only generates code using the new interface. This new interface fixes several issues with the old async let codegen, including use-after-free crashes if the `async let` was never awaited, and the inability to read from an `async let` variable more than once. rdar://77855176
67 lines
2.3 KiB
C++
67 lines
2.3 KiB
C++
//===--- AsyncLet.h - ABI structures for async let -00-----------*- C++ -*-===//
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//
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// This source file is part of the Swift.org open source project
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//
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// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
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// Licensed under Apache License v2.0 with Runtime Library Exception
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//
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// See https://swift.org/LICENSE.txt for license information
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// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
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//
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//===----------------------------------------------------------------------===//
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//
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// Swift ABI describing task groups.
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//
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//===----------------------------------------------------------------------===//
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#ifndef SWIFT_ABI_TASK_ASYNC_LET_H
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#define SWIFT_ABI_TASK_ASYNC_LET_H
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#include "swift/ABI/Task.h"
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#include "swift/ABI/HeapObject.h"
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#include "swift/Runtime/Concurrency.h"
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#include "swift/Runtime/Config.h"
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#include "swift/Basic/RelativePointer.h"
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#include "swift/Basic/STLExtras.h"
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namespace swift {
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/// Represents an in-flight `async let`, i.e. the Task that is computing the
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/// result of the async let, along with the awaited status and other metadata.
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class alignas(Alignment_AsyncLet) AsyncLet {
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public:
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// These constructors do not initialize the AsyncLet instance, and the
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// destructor does not destroy the AsyncLet instance; you must call
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// swift_asyncLet_{start,end} yourself.
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constexpr AsyncLet()
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: PrivateData{} {}
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void *PrivateData[NumWords_AsyncLet];
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// TODO: we could offer a "was awaited on" check here
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/// Returns the child task that is associated with this async let.
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/// The tasks completion is used to fulfil the value represented by this async let.
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AsyncTask *getTask() const;
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// The compiler preallocates a large fixed space for the `async let`, with the
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// intent that most of it be used for the child task context. The next two
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// methods return the address and size of that space.
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/// Return a pointer to the unused space within the async let block.
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void *getPreallocatedSpace();
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/// Return the size of the unused space within the async let block.
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static size_t getSizeOfPreallocatedSpace();
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/// Was the task allocated out of the parent's allocator?
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bool didAllocateFromParentTask();
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/// Flag that the task was allocated from the parent's allocator.
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void setDidAllocateFromParentTask(bool value = true);
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};
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} // end namespace swift
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#endif // SWIFT_ABI_TASK_ASYNC_LET_H
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