Source/JavaScriptCore/bytecode/IterationModeMetadata.h

/*
 * Copyright (C) 2020 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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 * 1. Redistributions of source code must retain the above copyright
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 *
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#pragma once

#include <cstdint>
#include <wtf/EnumClassOperatorOverloads.h>
#include <wtf/StdLibExtras.h>

namespace JSC {

enum class IterationMode : uint16_t {
    Generic = 1 << 0,
    FastArray = 1 << 1,
    FastMap = 1 << 2,
    FastSet = 1 << 3,
    FastString = 1 << 4,
    FastArrayValues = 1 << 5,
    FastArrayKeys = 1 << 6,
    FastArrayEntries = 1 << 7,
    FastMapKeys = 1 << 8,
    FastMapValues = 1 << 9,
    FastMapEntries = 1 << 10,
    FastSetValues = 1 << 11,
    FastSetEntries = 1 << 12,
    FastAsyncGenerator = 1 << 13,
    AsyncFromSync = 1 << 14,
};

constexpr unsigned numberOfIterationModes = 15;

// To keep the amount of code emitted for one iteration site reasonable, we only allow this many
// distinct fast iteration modes per site. Once the limit is reached, newly observed iterable
// types are recorded and handled as Generic instead.
constexpr unsigned maxNumberOfFastIterationModes = 2;

OVERLOAD_BITWISE_OPERATORS_FOR_ENUM_CLASS_WITH_INTERGRALS(IterationMode);

inline bool canUseFastIterationMode(uint16_t seenModes, IterationMode mode)
{
    ASSERT(mode != IterationMode::Generic);
    uint16_t seenFastModes = seenModes & ~static_cast<uint16_t>(IterationMode::Generic);
    if (seenFastModes & static_cast<uint16_t>(mode))
        return true;
    return static_cast<unsigned>(std::popcount(seenFastModes)) < maxNumberOfFastIterationModes;
}

struct IterationModeMetadata {
    uint16_t seenModes { 0 };
    static constexpr ptrdiff_t offsetOfSeenModes() { return OBJECT_OFFSETOF(IterationModeMetadata, seenModes); }
    static_assert(sizeof(decltype(seenModes)) == sizeof(IterationMode));
};

} // namespace JSC