LibJS: Split Heap into per-cell-size allocators

Instead of keeping all the HeapBlocks in one big list, we now split it
into two levels:

- Heap has a set of Allocators, each with a specific cell size.
- Allocators have two lists of blocks, "full" and "usable".

Allocating a new cell no longer has to scan the entire set of blocks,
but instead just needs to find the right allocator and then pop a cell
from its freelist. If all the blocks in the allocator are full, a new
block will be created.

Blocks are moved from the "full" to "usable" list after sweeping has
determined that they are not completely empty and not completely full.

There are certainly many ways we can improve on this. This patch is
mostly about getting the new allocator architecture in place. :^)

Libraries/LibJS/CMakeLists.txt

@@ -1,6 +1,7 @@
 set(SOURCES
     AST.cpp
     Console.cpp
+    Heap/Allocator.cpp
     Heap/Handle.cpp
     Heap/HeapBlock.cpp
     Heap/Heap.cpp

Libraries/LibJS/Forward.h

@@ -95,6 +95,7 @@
 namespace JS {
 
 class ASTNode;
+class Allocator;
 class BigInt;
 class BoundFunction;
 class Cell;

Libraries/LibJS/Heap/Allocator.cpp

@@ -0,0 +1,76 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/Badge.h>
+#include <LibJS/Heap/Allocator.h>
+#include <LibJS/Heap/HeapBlock.h>
+
+namespace JS {
+
+Allocator::Allocator(size_t cell_size)
+    : m_cell_size(cell_size)
+{
+}
+
+Allocator::~Allocator()
+{
+}
+
+Cell* Allocator::allocate_cell(Heap& heap)
+{
+    if (m_usable_blocks.is_empty()) {
+        m_usable_blocks.append(HeapBlock::create_with_cell_size(heap, m_cell_size));
+    }
+
+    auto& block = *m_usable_blocks.last();
+    auto* cell = block.allocate();
+    ASSERT(cell);
+    if (block.is_full()) {
+        m_full_blocks.append(m_usable_blocks.take_last());
+    }
+    return cell;
+}
+
+void Allocator::block_did_become_empty(Badge<Heap>, HeapBlock& block)
+{
+    bool removed_something = false;
+    removed_something |= m_full_blocks.remove_first_matching([&block](auto& entry) { return entry == &block; });
+    removed_something |= m_usable_blocks.remove_first_matching([&block](auto& entry) { return entry == &block; });
+    ASSERT(removed_something);
+}
+
+void Allocator::block_did_become_usable(Badge<Heap>, HeapBlock& block)
+{
+    ASSERT(!block.is_full());
+    auto it = m_full_blocks.find([&](auto& entry) {
+        return entry == &block;
+    });
+    ASSERT(it != m_full_blocks.end());
+    auto owned_block = m_full_blocks.take(it.index());
+    m_usable_blocks.append(move(owned_block));
+}
+
+}

Libraries/LibJS/Heap/Allocator.h

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/NonnullOwnPtr.h>
+#include <AK/Vector.h>
+#include <LibJS/Forward.h>
+
+namespace JS {
+
+class Allocator {
+public:
+    Allocator(size_t cell_size);
+    ~Allocator();
+
+    size_t cell_size() const { return m_cell_size; }
+
+    Cell* allocate_cell(Heap&);
+
+    template<typename Callback>
+    IterationDecision for_each_block(Callback callback)
+    {
+        for (auto& block : m_full_blocks) {
+            if (callback(*block) == IterationDecision::Break)
+                return IterationDecision::Break;
+        }
+        for (auto& block : m_usable_blocks) {
+            if (callback(*block) == IterationDecision::Break)
+                return IterationDecision::Break;
+        }
+        return IterationDecision::Continue;
+    }
+
+    void block_did_become_empty(Badge<Heap>, HeapBlock&);
+    void block_did_become_usable(Badge<Heap>, HeapBlock&);
+
+private:
+    const size_t m_cell_size;
+
+    Vector<NonnullOwnPtr<HeapBlock>> m_full_blocks;
+    Vector<NonnullOwnPtr<HeapBlock>> m_usable_blocks;
+};
+
+}

Libraries/LibJS/Heap/Heap.cpp

@@ -27,6 +27,7 @@
 #include <AK/Badge.h>
 #include <AK/HashTable.h>
 #include <LibCore/ElapsedTimer.h>
+#include <LibJS/Heap/Allocator.h>
 #include <LibJS/Heap/Handle.h>
 #include <LibJS/Heap/Heap.h>
 #include <LibJS/Heap/HeapBlock.h>
@@ -50,6 +51,14 @@ namespace JS {
 Heap::Heap(VM& vm)
     : m_vm(vm)
 {
+    m_allocators.append(make<Allocator>(16));
+    m_allocators.append(make<Allocator>(32));
+    m_allocators.append(make<Allocator>(64));
+    m_allocators.append(make<Allocator>(128));
+    m_allocators.append(make<Allocator>(256));
+    m_allocators.append(make<Allocator>(512));
+    m_allocators.append(make<Allocator>(1024));
+    m_allocators.append(make<Allocator>(3172));
 }
 
 Heap::~Heap()
@@ -57,6 +66,15 @@ Heap::~Heap()
     collect_garbage(CollectionType::CollectEverything);
 }
 
+ALWAYS_INLINE Allocator& Heap::allocator_for_size(size_t cell_size)
+{
+    for (auto& allocator : m_allocators) {
+        if (allocator->cell_size() >= cell_size)
+            return *allocator;
+    }
+    ASSERT_NOT_REACHED();
+}
+
 Cell* Heap::allocate_cell(size_t size)
 {
     if (should_collect_on_every_allocation()) {
@@ -68,18 +86,8 @@ Cell* Heap::allocate_cell(size_t size)
         ++m_allocations_since_last_gc;
     }
 
-    for (auto& block : m_blocks) {
-        if (size > block->cell_size())
-            continue;
-        if (auto* cell = block->allocate())
-            return cell;
-    }
-
-    size_t cell_size = round_up_to_power_of_two(size, 16);
-    auto block = HeapBlock::create_with_cell_size(*this, cell_size);
-    auto* cell = block->allocate();
-    m_blocks.append(move(block));
-    return cell;
+    auto& allocator = allocator_for_size(size);
+    return allocator.allocate_cell(*this);
 }
 
 void Heap::collect_garbage(CollectionType collection_type, bool print_report)
@@ -203,7 +211,15 @@ void Heap::gather_conservative_roots(HashTable<Cell*>& roots)
 Cell* Heap::cell_from_possible_pointer(FlatPtr pointer)
 {
     auto* possible_heap_block = HeapBlock::from_cell(reinterpret_cast<const Cell*>(pointer));
-    if (m_blocks.find([possible_heap_block](auto& block) { return block.ptr() == possible_heap_block; }) == m_blocks.end())
+    bool found = false;
+    for_each_block([&](auto& block) {
+        if (&block == possible_heap_block) {
+            found = true;
+            return IterationDecision::Break;
+        }
+        return IterationDecision::Continue;
+    });
+    if (!found)
         return nullptr;
     return possible_heap_block->cell_from_possible_pointer(pointer);
 }
@@ -240,57 +256,77 @@ void Heap::sweep_dead_cells(bool print_report, const Core::ElapsedTimer& measure
     dbg() << "sweep_dead_cells:";
 #endif
     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 (auto& block : m_blocks) {
+    for_each_block([&](auto& block) {
         bool block_has_live_cells = false;
-        block->for_each_cell([&](Cell* cell) {
+        bool block_was_full = block.is_full();
+        block.for_each_cell([&](Cell* cell) {
             if (cell->is_live()) {
                 if (!cell->is_marked()) {
 #ifdef HEAP_DEBUG
                     dbg() << "  ~ " << cell;
 #endif
-                    block->deallocate(cell);
+                    block.deallocate(cell);
                     ++collected_cells;
-                    collected_cell_bytes += block->cell_size();
+                    collected_cell_bytes += block.cell_size();
                 } else {
                     cell->set_marked(false);
                     block_has_live_cells = true;
                     ++live_cells;
-                    live_cell_bytes += block->cell_size();
+                    live_cell_bytes += block.cell_size();
                 }
             }
         });
         if (!block_has_live_cells)
-            empty_blocks.append(block);
-    }
+            empty_blocks.append(&block);
+        else if (block_was_full != block.is_full())
+            full_blocks_that_became_usable.append(&block);
+        return IterationDecision::Continue;
+    });
 
     for (auto* block : empty_blocks) {
 #ifdef HEAP_DEBUG
-        dbg() << " - Reclaim HeapBlock @ " << block << ": cell_size=" << block->cell_size();
+        dbg() << " - HeapBlock empty @ " << block << ": cell_size=" << block->cell_size();
 #endif
-        m_blocks.remove_first_matching([block](auto& entry) { return entry == block; });
+        allocator_for_size(block->cell_size()).block_did_become_empty({}, *block);
     }
 
+    for (auto* block : full_blocks_that_became_usable) {
 #ifdef HEAP_DEBUG
-    for (auto& block : m_blocks) {
-        dbg() << " > Live HeapBlock @ " << block << ": cell_size=" << block->cell_size();
+        dbg() << " - HeapBlock usable again @ " << block << ": cell_size=" << block->cell_size();
+#endif
+        allocator_for_size(block->cell_size()).block_did_become_usable({}, *block);
     }
+
+#ifdef HEAP_DEBUG
+    for_each_block([&](auto& block) {
+        dbg() << " > Live HeapBlock @ " << &block << ": cell_size=" << block.cell_size();
+        return IterationDecision::Continue;
+    });
 #endif
 
     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)", m_blocks.size(), m_blocks.size() * HeapBlock::block_size);
+        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("=============================================");
     }

Libraries/LibJS/Heap/Heap.h

@@ -33,6 +33,7 @@
 #include <AK/Vector.h>
 #include <LibCore/Forward.h>
 #include <LibJS/Forward.h>
+#include <LibJS/Heap/Allocator.h>
 #include <LibJS/Heap/Handle.h>
 #include <LibJS/Runtime/Cell.h>
 #include <LibJS/Runtime/Object.h>
@@ -101,13 +102,25 @@ private:
 
     Cell* cell_from_possible_pointer(FlatPtr);
 
+    Allocator& allocator_for_size(size_t);
+
+    template<typename Callback>
+    void for_each_block(Callback callback)
+    {
+        for (auto& allocator : m_allocators) {
+            if (allocator->for_each_block(callback) == IterationDecision::Break)
+                return;
+        }
+    }
+
     size_t m_max_allocations_between_gc { 10000 };
     size_t m_allocations_since_last_gc { false };
 
     bool m_should_collect_on_every_allocation { false };
 
     VM& m_vm;
-    Vector<NonnullOwnPtr<HeapBlock>> m_blocks;
+
+    Vector<NonnullOwnPtr<Allocator>> m_allocators;
     HashTable<HandleImpl*> m_handles;
 
     HashTable<MarkedValueList*> m_marked_value_lists;

Libraries/LibJS/Heap/HeapBlock.h

@@ -33,6 +33,9 @@
 namespace JS {
 
 class HeapBlock {
+    AK_MAKE_NONCOPYABLE(HeapBlock);
+    AK_MAKE_NONMOVABLE(HeapBlock);
+
 public:
     static constexpr size_t block_size = 16 * KiB;
     static NonnullOwnPtr<HeapBlock> create_with_cell_size(Heap&, size_t);
@@ -41,6 +44,7 @@ public:
 
     size_t cell_size() const { return m_cell_size; }
     size_t cell_count() const { return (block_size - sizeof(HeapBlock)) / m_cell_size; }
+    bool is_full() const { return !m_freelist; }
 
     ALWAYS_INLINE Cell* allocate()
     {