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◆ main openmsx / src / utils / monotonic_allocator.hh
1.8 KiB · C++ · last changed chore(): initial commit 1 month ago source blame history raw
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#ifndef MEMORY_RESOURCE
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#define MEMORY_RESOURCE
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#include <algorithm>
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#include <cassert>
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#include <cstddef>
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#include <cstdlib>
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#include <memory>
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#include <new>
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// A minimal re-implementation of std::monotonic_buffer_resource.
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// Because libc++ doesn't have this yet, even though it's part of c++17.
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class monotonic_allocator
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{
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public:
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	monotonic_allocator() = default;
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	explicit monotonic_allocator(size_t initialSize)
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		: nextSize(initialSize)
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	{
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		assert(initialSize != 0);
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	}
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	monotonic_allocator(void* buffer, size_t bufferSize)
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		: current(buffer)
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		, available(bufferSize)
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		, nextSize(2 * bufferSize)
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	{
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		assert(buffer || bufferSize == 0);
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	}
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	monotonic_allocator(const monotonic_allocator&) = delete;
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	monotonic_allocator(monotonic_allocator&&) = delete;
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	monotonic_allocator& operator=(const monotonic_allocator&) = delete;
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	monotonic_allocator& operator=(monotonic_allocator&&) = delete;
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	~monotonic_allocator()
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	{
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		void* p = head;
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		while (p) {
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			void* next = *static_cast<void**>(p);
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			free(p);
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			p = next;
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		}
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	}
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	[[nodiscard]] void* allocate(size_t bytes, size_t alignment)
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	{
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		assert(alignment <= alignof(max_align_t));
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		if (bytes == 0) bytes = 1;
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		void* p = std::align(alignment, bytes, current, available);
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		if (!p) {
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			newBuffer(bytes);
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			p = current;
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		}
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		current = static_cast<char*>(current) + bytes;
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		available -= bytes;
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		return p;
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	}
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private:
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	void newBuffer(size_t bytes)
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	{
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		size_t n = std::max(nextSize, bytes);
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		void* newBuf = malloc(n + sizeof(void*));
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		if (!newBuf) {
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			throw std::bad_alloc();
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		}
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		auto** p = static_cast<void**>(newBuf);
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		p[0] = head;
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		current = static_cast<void*>(&p[1]);
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		available = n;
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		nextSize = 2 * n;
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		head = newBuf;
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	}
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private:
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	void* current = nullptr;
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	size_t available = 0;
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	size_t nextSize = 1024;
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	void* head = nullptr;
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};
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#endif