serenity/Userland/Libraries/LibJS/Heap/Heap.cpp

/*
 * Copyright (c) 2020-2022, Andreas Kling <kling@serenityos.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/Badge.h>
#include <AK/Debug.h>
#include <AK/HashTable.h>
#include <AK/StackInfo.h>
#include <AK/TemporaryChange.h>
#include <LibCore/ElapsedTimer.h>
#include <LibJS/Heap/CellAllocator.h>
#include <LibJS/Heap/Handle.h>
#include <LibJS/Heap/Heap.h>
#include <LibJS/Heap/HeapBlock.h>
#include <LibJS/Interpreter.h>
#include <LibJS/Runtime/Object.h>
#include <LibJS/Runtime/WeakContainer.h>
#include <setjmp.h>

#ifdef __serenity__
#    include <serenity.h>
#endif

namespace JS {

#ifdef __serenity__
static int gc_perf_string_id;
#endif

Heap::Heap(VM& vm)
    : m_vm(vm)
{
#ifdef __serenity__
    auto gc_signpost_string = "Garbage collection"sv;
    gc_perf_string_id = perf_register_string(gc_signpost_string.characters_without_null_termination(), gc_signpost_string.length());
#endif

    if constexpr (HeapBlock::min_possible_cell_size <= 16) {
        m_allocators.append(make<CellAllocator>(16));
    }
    static_assert(HeapBlock::min_possible_cell_size <= 24, "Heap Cell tracking uses too much data!");
    m_allocators.append(make<CellAllocator>(32));
    m_allocators.append(make<CellAllocator>(64));
    m_allocators.append(make<CellAllocator>(96));
    m_allocators.append(make<CellAllocator>(128));
    m_allocators.append(make<CellAllocator>(256));
    m_allocators.append(make<CellAllocator>(512));
    m_allocators.append(make<CellAllocator>(1024));
    m_allocators.append(make<CellAllocator>(3072));
}

Heap::~Heap()
{
    vm().string_cache().clear();
    collect_garbage(CollectionType::CollectEverything);
}

ALWAYS_INLINE CellAllocator& Heap::allocator_for_size(size_t cell_size)
{
    for (auto& allocator : m_allocators) {
        if (allocator->cell_size() >= cell_size)
            return *allocator;
    }
    dbgln("Cannot get CellAllocator for cell size {}, largest available is {}!", cell_size, m_allocators.last()->cell_size());
    VERIFY_NOT_REACHED();
}

Cell* Heap::allocate_cell(size_t size)
{
    if (should_collect_on_every_allocation()) {
        collect_garbage();
    } else if (m_allocations_since_last_gc > m_max_allocations_between_gc) {
        m_allocations_since_last_gc = 0;
        collect_garbage();
    } else {
        ++m_allocations_since_last_gc;
    }

    auto& allocator = allocator_for_size(size);
    return allocator.allocate_cell(*this);
}

void Heap::collect_garbage(CollectionType collection_type, bool print_report)
{
    VERIFY(!m_collecting_garbage);
    TemporaryChange change(m_collecting_garbage, true);

#ifdef __serenity__
    static size_t global_gc_counter = 0;
    perf_event(PERF_EVENT_SIGNPOST, gc_perf_string_id, global_gc_counter++);
#endif

    auto collection_measurement_timer = Core::ElapsedTimer::start_new();
    if (collection_type == CollectionType::CollectGarbage) {
        if (m_gc_deferrals) {
            m_should_gc_when_deferral_ends = true;
            return;
        }
        HashTable<Cell*> roots;
        gather_roots(roots);
        mark_live_cells(roots);
    }
    sweep_dead_cells(print_report, collection_measurement_timer);
}

void Heap::gather_roots(HashTable<Cell*>& roots)
{
    vm().gather_roots(roots);
    gather_conservative_roots(roots);

    for (auto& handle : m_handles)
        roots.set(handle.cell());

    for (auto& vector : m_marked_vectors)
        vector.gather_roots(roots);

    if constexpr (HEAP_DEBUG) {
        dbgln("gather_roots:");
        for (auto* root : roots)
            dbgln("  + {}", root);
    }
}

__attribute__((no_sanitize("address"))) void Heap::gather_conservative_roots(HashTable<Cell*>& roots)
{
    FlatPtr dummy;

    dbgln_if(HEAP_DEBUG, "gather_conservative_roots:");

    jmp_buf buf;
    setjmp(buf);

    HashTable<FlatPtr> possible_pointers;

    auto* raw_jmp_buf = reinterpret_cast<FlatPtr const*>(buf);

    auto add_possible_value = [&](FlatPtr data) {
        if constexpr (sizeof(FlatPtr*) == sizeof(Value)) {
            // Because Value stores pointers in non-canonical form we have to check if the top bytes
            // match any pointer-backed tag, in that case we have to extract the pointer to its
            // canonical form and add that as a possible pointer.
            if ((data & SHIFTED_IS_CELL_PATTERN) == SHIFTED_IS_CELL_PATTERN)
                possible_pointers.set((u64)(((i64)data << 16) >> 16));
            else
                possible_pointers.set(data);
        } else {
            static_assert((sizeof(Value) % sizeof(FlatPtr*)) == 0);
            // In the 32-bit case we will look at the top and bottom part of Value separately we just
            // add both the upper and lower bytes as possible pointers.
            possible_pointers.set(data);
        }
    };

    for (size_t i = 0; i < ((size_t)sizeof(buf)) / sizeof(FlatPtr); i += sizeof(FlatPtr))
        add_possible_value(raw_jmp_buf[i]);

    auto stack_reference = bit_cast<FlatPtr>(&dummy);
    auto& stack_info = m_vm.stack_info();

    for (FlatPtr stack_address = stack_reference; stack_address < stack_info.top(); stack_address += sizeof(FlatPtr)) {
        auto data = *reinterpret_cast<FlatPtr*>(stack_address);
        add_possible_value(data);
    }

    HashTable<HeapBlock*> all_live_heap_blocks;
    for_each_block([&](auto& block) {
        all_live_heap_blocks.set(&block);
        return IterationDecision::Continue;
    });

    for (auto possible_pointer : possible_pointers) {
        if (!possible_pointer)
            continue;
        dbgln_if(HEAP_DEBUG, "  ? {}", (void const*)possible_pointer);
        auto* possible_heap_block = HeapBlock::from_cell(reinterpret_cast<Cell const*>(possible_pointer));
        if (all_live_heap_blocks.contains(possible_heap_block)) {
            if (auto* cell = possible_heap_block->cell_from_possible_pointer(possible_pointer)) {
                if (cell->state() == Cell::State::Live) {
                    dbgln_if(HEAP_DEBUG, "  ?-> {}", (void const*)cell);
                    roots.set(cell);
                } else {
                    dbgln_if(HEAP_DEBUG, "  #-> {}", (void const*)cell);
                }
            }
        }
    }
}

class MarkingVisitor final : public Cell::Visitor {
public:
    MarkingVisitor() = default;

    virtual void visit_impl(Cell& cell) override
    {
        if (cell.is_marked())
            return;
        dbgln_if(HEAP_DEBUG, "  ! {}", &cell);

        cell.set_marked(true);
        cell.visit_edges(*this);
    }
};

void Heap::mark_live_cells(HashTable<Cell*> const& roots)
{
    dbgln_if(HEAP_DEBUG, "mark_live_cells:");

    MarkingVisitor visitor;
    for (auto* root : roots)
        visitor.visit(root);

    for (auto& inverse_root : m_uprooted_cells)
        inverse_root->set_marked(false);

    m_uprooted_cells.clear();
}

void Heap::sweep_dead_cells(bool print_report, Core::ElapsedTimer const& measurement_timer)
{
    dbgln_if(HEAP_DEBUG, "sweep_dead_cells:");
    Vector<HeapBlock*, 32> empty_blocks;
    Vector<HeapBlock*, 32> full_blocks_that_became_usable;

    size_t collected_cells = 0;
    size_t live_cells = 0;
    size_t collected_cell_bytes = 0;
    size_t live_cell_bytes = 0;

    for_each_block([&](auto& block) {
        bool block_has_live_cells = false;
        bool block_was_full = block.is_full();
        block.template for_each_cell_in_state<Cell::State::Live>([&](Cell* cell) {
            if (!cell->is_marked()) {
                dbgln_if(HEAP_DEBUG, "  ~ {}", cell);
                block.deallocate(cell);
                ++collected_cells;
                collected_cell_bytes += block.cell_size();
            } else {
                cell->set_marked(false);
                block_has_live_cells = true;
                ++live_cells;
                live_cell_bytes += block.cell_size();
            }
        });
        if (!block_has_live_cells)
            empty_blocks.append(&block);
        else if (block_was_full != block.is_full())
            full_blocks_that_became_usable.append(&block);
        return IterationDecision::Continue;
    });

    for (auto& weak_container : m_weak_containers)
        weak_container.remove_dead_cells({});

    for (auto* block : empty_blocks) {
        dbgln_if(HEAP_DEBUG, " - HeapBlock empty @ {}: cell_size={}", block, block->cell_size());
        allocator_for_size(block->cell_size()).block_did_become_empty({}, *block);
    }

    for (auto* block : full_blocks_that_became_usable) {
        dbgln_if(HEAP_DEBUG, " - HeapBlock usable again @ {}: cell_size={}", block, block->cell_size());
        allocator_for_size(block->cell_size()).block_did_become_usable({}, *block);
    }

    if constexpr (HEAP_DEBUG) {
        for_each_block([&](auto& block) {
            dbgln(" > Live HeapBlock @ {}: cell_size={}", &block, block.cell_size());
            return IterationDecision::Continue;
        });
    }

    int time_spent = measurement_timer.elapsed();

    if (print_report) {
        size_t live_block_count = 0;
        for_each_block([&](auto&) {
            ++live_block_count;
            return IterationDecision::Continue;
        });

        dbgln("Garbage collection report");
        dbgln("=============================================");
        dbgln("     Time spent: {} ms", time_spent);
        dbgln("     Live cells: {} ({} bytes)", live_cells, live_cell_bytes);
        dbgln("Collected cells: {} ({} bytes)", collected_cells, collected_cell_bytes);
        dbgln("    Live blocks: {} ({} bytes)", live_block_count, live_block_count * HeapBlock::block_size);
        dbgln("   Freed blocks: {} ({} bytes)", empty_blocks.size(), empty_blocks.size() * HeapBlock::block_size);
        dbgln("=============================================");
    }
}

void Heap::did_create_handle(Badge<HandleImpl>, HandleImpl& impl)
{
    VERIFY(!m_handles.contains(impl));
    m_handles.append(impl);
}

void Heap::did_destroy_handle(Badge<HandleImpl>, HandleImpl& impl)
{
    VERIFY(m_handles.contains(impl));
    m_handles.remove(impl);
}

void Heap::did_create_marked_vector(Badge<MarkedVectorBase>, MarkedVectorBase& vector)
{
    VERIFY(!m_marked_vectors.contains(vector));
    m_marked_vectors.append(vector);
}

void Heap::did_destroy_marked_vector(Badge<MarkedVectorBase>, MarkedVectorBase& vector)
{
    VERIFY(m_marked_vectors.contains(vector));
    m_marked_vectors.remove(vector);
}

void Heap::did_create_weak_container(Badge<WeakContainer>, WeakContainer& set)
{
    VERIFY(!m_weak_containers.contains(set));
    m_weak_containers.append(set);
}

void Heap::did_destroy_weak_container(Badge<WeakContainer>, WeakContainer& set)
{
    VERIFY(m_weak_containers.contains(set));
    m_weak_containers.remove(set);
}

void Heap::defer_gc(Badge<DeferGC>)
{
    ++m_gc_deferrals;
}

void Heap::undefer_gc(Badge<DeferGC>)
{
    VERIFY(m_gc_deferrals > 0);
    --m_gc_deferrals;

    if (!m_gc_deferrals) {
        if (m_should_gc_when_deferral_ends)
            collect_garbage();
        m_should_gc_when_deferral_ends = false;
    }
}

void Heap::uproot_cell(Cell* cell)
{
    m_uprooted_cells.append(cell);
}

}