58 lines
2.3 KiB
C
58 lines
2.3 KiB
C
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// Copyright 2023 The Chromium Authors
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef BASE_MEMORY_STACK_ALLOCATED_H_
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#define BASE_MEMORY_STACK_ALLOCATED_H_
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#if defined(__clang__)
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#define STACK_ALLOCATED_IGNORE(reason) \
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__attribute__((annotate("stack_allocated_ignore")))
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#else // !defined(__clang__)
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#define STACK_ALLOCATED_IGNORE(reason)
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#endif // !defined(__clang__)
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// If a class or one of its ancestor classes is annotated with STACK_ALLOCATED()
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// in its class definition, then instances of the class may not be allocated on
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// the heap or as a member variable of a non-stack-allocated class.
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#define STACK_ALLOCATED() \
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public: \
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using IsStackAllocatedTypeMarker [[maybe_unused]] = int; \
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\
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private: \
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void* operator new(size_t) = delete; \
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void* operator new(size_t, ::base::NotNullTag, void*) = delete; \
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void* operator new(size_t, void*) = delete
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namespace base {
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// NotNullTag was originally added to WebKit here:
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// https://trac.webkit.org/changeset/103243/webkit
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// ...with the stated goal of improving the performance of the placement new
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// operator and potentially enabling the -fomit-frame-pointer compiler flag.
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//
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// TODO(szager): The placement new operator which uses this tag is currently
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// defined in third_party/blink/renderer/platform/wtf/allocator/allocator.h,
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// in the global namespace. It should probably move to /base.
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//
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// It's unknown at the time of writing whether it still provides any benefit
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// (or if it ever did). It is used by placing the kNotNull tag before the
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// address of the object when calling placement new.
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//
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// If the kNotNull tag is specified to placement new for a null pointer,
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// Undefined Behaviour can result.
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//
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// Example:
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//
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// union { int i; } u;
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//
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// // Typically placement new looks like this.
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// new (&u.i) int(3);
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// // But we can promise `&u.i` is not null like this.
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// new (base::NotNullTag::kNotNull, &u.i) int(3);
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enum class NotNullTag { kNotNull };
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} // namespace base
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#endif // BASE_MEMORY_STACK_ALLOCATED_H_
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