Source/WTF/wtf/StackAllocation.h

/*
 * Copyright (C) 2026 Apple Inc. All rights reserved.
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 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
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#pragma once

#include <cstddef>
#include <cstdint>
#include <span>
#include <wtf/Assertions.h>

namespace WTF {

class StackAllocationSpecification {
public:
    enum class Kind : std::uint8_t {
        Default = 0, // OS provided stack, OS default size
        SizeOnly, // OS provided stack, specified size
        SizeAndLocation, // Preallocated stack with known size
    };

    StackAllocationSpecification() = default;

    static StackAllocationSpecification RequestSize(std::size_t bytes)
    {
        StackAllocationSpecification s(Kind::SizeOnly);
        s.storage.sizeBytes = bytes;
        return s;
    }

    static StackAllocationSpecification CustomStack(std::span<std::byte> stack)
    {
        StackAllocationSpecification s(Kind::SizeAndLocation);
        s.storage.stack = stack;
        return s;
    }

    Kind kind() const { return m_kind; }
    bool isKind(Kind kind) const { return this->kind() == kind; }

    std::size_t sizeBytes() const
    {
        RELEASE_ASSERT(m_kind == Kind::SizeOnly || m_kind == Kind::SizeAndLocation);
        if (m_kind == Kind::SizeOnly)
            return storage.sizeBytes;
        if (m_kind == Kind::SizeAndLocation)
            return storage.stack.size_bytes();
        RELEASE_ASSERT_NOT_REACHED();
    }

    std::span<std::byte> stackSpan() const
    {
        RELEASE_ASSERT(m_kind == Kind::SizeAndLocation);
        return storage.stack;
    }

private:
    explicit StackAllocationSpecification(Kind k)
        : m_kind(k) { }

    Kind m_kind { Kind::Default };

    union Storage {
        constexpr Storage() : sizeBytes(0) { }
        std::size_t sizeBytes;
        std::span<std::byte> stack;
    } storage { };
};

} // namespace WTF

using WTF::StackAllocationSpecification;