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This matches how dispatch_once works in C, dramatically cutting the cost of a global accessor by avoiding the runtime call in the hot path and giving the global a unique branch for the CPU to predict away. For now, only do this for Darwin; non-ObjC platforms don't necessarily expose their "done" value as ABI like ours do. While we're here, change "once" to take a thin function pointer. We don't ever emit global initializers with context dependencies, and this simplifies the runtime glue between swift_once and dispatch_once/std::call_once a bit. Swift SVN r28166
53 lines
2.1 KiB
C++
53 lines
2.1 KiB
C++
//===--- Once.cpp - Runtime support for lazy initialization ----------------==//
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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 - 2015 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 http://swift.org/LICENSE.txt for license information
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// See http://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 runtime functions in support of lazy initialization.
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//
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//===----------------------------------------------------------------------===//
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#include "swift/Runtime/Once.h"
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#include "Debug.h"
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#include <type_traits>
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using namespace swift;
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#ifdef __APPLE__
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// On OS X and and iOS, swift_once is implemented using GCD.
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#include <dispatch/dispatch.h>
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static_assert(std::is_same<swift_once_t, dispatch_once_t>::value,
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"swift_once_t and dispatch_once_t must stay in sync");
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#endif
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// The compiler generates the swift_once_t values as word-sized zero-initialized
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// variables, so we want to make sure swift_once_t isn't larger than the
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// platform word or the function below might overwrite something it shouldn't.
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static_assert(sizeof(swift_once_t) <= sizeof(void*),
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"swift_once_t must be no larger than the platform word");
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/// Runs the given function with the given context argument exactly once.
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/// The predicate argument must point to a global or static variable of static
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/// extent of type swift_once_t.
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void swift::swift_once(swift_once_t *predicate, void (*fn)(void *)) {
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#if __APPLE__
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dispatch_once_f(predicate, nullptr, fn);
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#else
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// FIXME: We're relying here on the coincidence that libstdc++ uses pthread's
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// pthread_once, and that on glibc pthread_once follows a compatible init
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// process (the token is a word that is atomically incremented from 0 to
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// 1 to 2 during initialization) to work. We should implement our own version
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// that we can rely on to continue to work that way.
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// For more information, see rdar://problem/18499385
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std::call_once(*predicate, [fn]() { fn(nullptr); });
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#endif
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}
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