Changeset 246423 in webkit

Timestamp:

Jun 13, 2019, 7:59:16 PM (7 years ago)

Author:

Kocsen Chung

Message:

Apply patch. rdar://problem/51656841

Location:

branches/safari-607-branch/Source/JavaScriptCore

Files:

• ChangeLog (modified) (1 diff)
• ftl/FTLLowerDFGToB3.cpp (modified) (1 diff)
• heap/SlotVisitor.cpp (modified) (2 diffs)
• jit/AssemblyHelpers.cpp (modified) (2 diffs)
• jit/AssemblyHelpers.h (modified) (3 diffs)
• jit/SpecializedThunkJIT.h (modified) (2 diffs)
• llint/LowLevelInterpreter.asm (modified) (1 diff)
• llint/LowLevelInterpreter64.asm (modified) (2 diffs)
• runtime/StructureIDTable.cpp (modified) (4 diffs)
• runtime/StructureIDTable.h (modified) (7 diffs)

branches/safari-607-branch/Source/JavaScriptCore/ChangeLog

@@ +1 @@
+2019-06-13  Kocsen Chung  <kocsen_chung@apple.com>
+
+        Apply patch. rdar://problem/51656841
+
+    2019-06-13  Mark Lam  <mark.lam@apple.com>
+
+            Misc cleanup in StructureIDTable after r242096.
+            https://bugs.webkit.org/show_bug.cgi?id=195063
+
+            Reviewed by Saam Barati.
+
+            * runtime/StructureIDTable.cpp:
+            (JSC::StructureIDTable::allocateID):
+            - RELEASE_ASSERT that the StructureID allocation will succeed.
+
+            * runtime/StructureIDTable.h:
+            (JSC::StructureIDTable::decode):
+            (JSC::StructureIDTable::encode):
+            - Add back a comment that Yusuke requested but was lost when the patch was rolled
+              out and relanded.
+            - Applied bitwise_casts that Saam requested.
+
+2019-02-26  Mark Lam  <mark.lam@apple.com>
+
+        [Re-landing] Add some randomness into the StructureID.
+        https://bugs.webkit.org/show_bug.cgi?id=194989
+        <rdar://problem/47975563>
+
+        Reviewed by Yusuke Suzuki.
+
+        1. On 64-bit, the StructureID will now be encoded as:
+
+            ----------------------------------------------------------------
+            | 1 Nuke Bit | 24 StructureIDTable index bits | 7 entropy bits |
+            ----------------------------------------------------------------
+
+           The entropy bits are chosen at random and assigned when a StructureID is
+           allocated.
+
+        2. Instead of Structure pointers, the StructureIDTable will now contain
+           encodedStructureBits, which is encoded as such:
+
+            ----------------------------------------------------------------
+            | 7 entropy bits |                   57 structure pointer bits |
+            ----------------------------------------------------------------
+
+           The entropy bits here are the same 7 bits used in the encoding of the
+           StructureID for this structure entry in the StructureIDTable.
+
+        3. Retrieval of the structure pointer given a StructureID is now computed as
+           follows:
+
+                index = structureID >> 7; // with arithmetic shift.
+                encodedStructureBits = structureIDTable[index];
+                structure = encodedStructureBits ^ (structureID << 57);
+
+            We use an arithmetic shift for the right shift because that will preserve
+            the nuke bit in the high bit of the index if the StructureID was not
+            decontaminated before use as expected.
+
+        4. Remove unused function loadArgumentWithSpecificClass() in SpecializedThunkJIT.
+
+        5. Define StructureIDTable::m_size to be the number of allocated StructureIDs
+           instead of always being the same as m_capacity.
+
+        6. Change StructureIDTable::s_unusedID's value to 0.
+
+           Its previous value of unusedPointer i.e. 0xd1e7beef, does not make sense for
+           StructureID on 64-bit.  Also, there was never any code that initializes unused
+           IDs to the s_unusedID.  The only meaningful value for s_unusedID is 0, which
+           is the ID we'll get when the freelist is empty, prompting a resize of the
+           structureIDTable.
+
+        This patch appears to be perf neutral on JetStream 2 run via the cli on a
+        11" MacBook Air, 13" MacBook Pro, iPhone 6S, and iPhone XR.
+
+        * ftl/FTLLowerDFGToB3.cpp:
+        (JSC::FTL::DFG::LowerDFGToB3::loadStructure):
+        * heap/SlotVisitor.cpp:
+        (JSC::SlotVisitor::appendJSCellOrAuxiliary):
+        * jit/AssemblyHelpers.cpp:
+        (JSC::AssemblyHelpers::emitLoadStructure):
+        * jit/AssemblyHelpers.h:
+        * jit/SpecializedThunkJIT.h:
+        (JSC::SpecializedThunkJIT::loadArgumentWithSpecificClass): Deleted.
+        * llint/LowLevelInterpreter.asm:
+        * llint/LowLevelInterpreter64.asm:
+        * runtime/StructureIDTable.cpp:
+        (JSC::StructureIDTable::StructureIDTable):
+        (JSC::StructureIDTable::makeFreeListFromRange):
+        (JSC::StructureIDTable::resize):
+        (JSC::StructureIDTable::allocateID):
+        (JSC::StructureIDTable::deallocateID):
+        * runtime/StructureIDTable.h:
+        (JSC::StructureIDTable::decode):
+        (JSC::StructureIDTable::encode):
+        (JSC::StructureIDTable::get):
+        (JSC::StructureIDTable::isValid):
+
+2019-02-13  Mark Lam  <mark.lam@apple.com>
+
+        Create a randomized free list for new StructureIDs on StructureIDTable resize.
+        https://bugs.webkit.org/show_bug.cgi?id=194566
+        <rdar://problem/47975502>
+
+        Reviewed by Michael Saboff.
+
+        Also isolate 32-bit implementation of StructureIDTable out more so the 64-bit
+        implementation is a little easier to read.
+
+        This patch appears to be perf neutral on JetStream2 (as run from the command line).
+
+        * runtime/StructureIDTable.cpp:
+        (JSC::StructureIDTable::StructureIDTable):
+        (JSC::StructureIDTable::makeFreeListFromRange):
+        (JSC::StructureIDTable::resize):
+        (JSC::StructureIDTable::allocateID):
+        (JSC::StructureIDTable::deallocateID):
+        * runtime/StructureIDTable.h:
+        (JSC::StructureIDTable::get):
+        (JSC::StructureIDTable::deallocateID):
+        (JSC::StructureIDTable::allocateID):
+        (JSC::StructureIDTable::flushOldTables):
+
+2019-02-11  Mark Lam  <mark.lam@apple.com>
+
+        Randomize insertion of deallocated StructureIDs into the StructureIDTable's free list.
+        https://bugs.webkit.org/show_bug.cgi?id=194512
+        <rdar://problem/47975465>
+
+        Reviewed by Yusuke Suzuki.
+
+        * runtime/StructureIDTable.cpp:
+        (JSC::StructureIDTable::StructureIDTable):
+        (JSC::StructureIDTable::allocateID):
+        (JSC::StructureIDTable::deallocateID):
+        * runtime/StructureIDTable.h:
+
 2019-06-13  Kocsen Chung  <kocsen_chung@apple.com>
 

branches/safari-607-branch/Source/JavaScriptCore/ftl/FTLLowerDFGToB3.cpp

@@ -16929 +16929 @@
     LValue loadStructure(LValue value)
     {
-        LValue tableIndex = m_out.load32(value, m_heaps.JSCell_structureID);
-        LValue tableBase = m_out.loadPtr(
-            m_out.absolute(vm().heap.structureIDTable().base()));
-        TypedPointer address = m_out.baseIndex(
-            m_heaps.structureTable, tableBase, m_out.zeroExtPtr(tableIndex));
-        return m_out.loadPtr(address);
+        LValue structureID = m_out.load32(value, m_heaps.JSCell_structureID);
+        LValue tableBase = m_out.loadPtr(m_out.absolute(vm().heap.structureIDTable().base()));
+        LValue tableIndex = m_out.aShr(structureID, m_out.constInt32(StructureIDTable::s_numberOfEntropyBits));
+        LValue entropyBits = m_out.shl(m_out.zeroExtPtr(structureID), m_out.constInt32(StructureIDTable::s_entropyBitsShiftForStructurePointer));
+        TypedPointer address = m_out.baseIndex(m_heaps.structureTable, tableBase, m_out.zeroExtPtr(tableIndex));
+        LValue encodedStructureBits = m_out.loadPtr(address);
+        return m_out.bitXor(encodedStructureBits, entropyBits);
     }
 

branches/safari-607-branch/Source/JavaScriptCore/heap/SlotVisitor.cpp

@@ -1 +1 @@
 /*
- * Copyright (C) 2012-2018 Apple Inc. All rights reserved.
+ * Copyright (C) 2012-2019 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -199 +199 @@
 #if USE(JSVALUE64)
         // This detects the worst of the badness.
-        if (structureID >= heap()->structureIDTable().size())
-            die("GC scan found corrupt object: structureID is out of bounds!\n");
+        if (!heap()->structureIDTable().isValid(structureID))
+            die("GC scan found corrupt object: structureID is invalid!\n");
 #endif
     };

branches/safari-607-branch/Source/JavaScriptCore/jit/AssemblyHelpers.cpp

@@ -1 +1 @@
 /*
- * Copyright (C) 2011-2018 Apple Inc. All rights reserved.
+ * Copyright (C) 2011-2019 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -370 +370 @@
 {
 #if USE(JSVALUE64)
+#if CPU(ARM64)
+    RegisterID scratch2 = dataTempRegister;
+#elif CPU(X86_64)
+    RegisterID scratch2 = scratchRegister();
+#else
+#error "Unsupported cpu"
+#endif
+
     ASSERT(dest != scratch);
-    load32(MacroAssembler::Address(source, JSCell::structureIDOffset()), dest);
+    ASSERT(dest != scratch2);
+    ASSERT(scratch != scratch2);
+
+    load32(MacroAssembler::Address(source, JSCell::structureIDOffset()), scratch2);
     loadPtr(vm.heap.structureIDTable().base(), scratch);
+    rshift32(scratch2, TrustedImm32(StructureIDTable::s_numberOfEntropyBits), dest);
     loadPtr(MacroAssembler::BaseIndex(scratch, dest, MacroAssembler::TimesEight), dest);
-#else
+    lshiftPtr(TrustedImm32(StructureIDTable::s_entropyBitsShiftForStructurePointer), scratch2);
+    xorPtr(scratch2, dest);
+#else // not USE(JSVALUE64)
     UNUSED_PARAM(scratch);
+    UNUSED_PARAM(scratch2);
     UNUSED_PARAM(vm);
     loadPtr(MacroAssembler::Address(source, JSCell::structureIDOffset()), dest);
-#endif
+#endif // not USE(JSVALUE64)
 }
 

branches/safari-607-branch/Source/JavaScriptCore/jit/AssemblyHelpers.h

@@ -1 +1 @@
 /*
- * Copyright (C) 2011-2018 Apple Inc. All rights reserved.
+ * Copyright (C) 2011-2019 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -1782 +1782 @@
     }
 
-    JumpList branchIfValue(VM&, JSValueRegs value, GPRReg scratch, GPRReg scratchIfShouldCheckMasqueradesAsUndefined, FPRReg, FPRReg, bool shouldCheckMasqueradesAsUndefined, JSGlobalObject*, bool negateResult);
+    JumpList branchIfValue(VM&, JSValueRegs, GPRReg scratch, GPRReg scratchIfShouldCheckMasqueradesAsUndefined, FPRReg, FPRReg, bool shouldCheckMasqueradesAsUndefined, JSGlobalObject*, bool negateResult);
     JumpList branchIfTruthy(VM& vm, JSValueRegs value, GPRReg scratch, GPRReg scratchIfShouldCheckMasqueradesAsUndefined, FPRReg scratchFPR0, FPRReg scratchFPR1, bool shouldCheckMasqueradesAsUndefined, JSGlobalObject* globalObject)
     {
@@ -1791 +1791 @@
         return branchIfValue(vm, value, scratch, scratchIfShouldCheckMasqueradesAsUndefined, scratchFPR0, scratchFPR1, shouldCheckMasqueradesAsUndefined, globalObject, true);
     }
-    void emitConvertValueToBoolean(VM&, JSValueRegs value, GPRReg result, GPRReg scratchIfShouldCheckMasqueradesAsUndefined, FPRReg, FPRReg, bool shouldCheckMasqueradesAsUndefined, JSGlobalObject*, bool negateResult = false);
+    void emitConvertValueToBoolean(VM&, JSValueRegs, GPRReg result, GPRReg scratchIfShouldCheckMasqueradesAsUndefined, FPRReg, FPRReg, bool shouldCheckMasqueradesAsUndefined, JSGlobalObject*, bool negateResult = false);
     
     template<typename ClassType>

branches/safari-607-branch/Source/JavaScriptCore/jit/SpecializedThunkJIT.h

@@ -1 +1 @@
 /*
- * Copyright (C) 2010-2018 Apple Inc. All rights reserved.
+ * Copyright (C) 2010-2019 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -72 +72 @@
         }
         
-        void loadArgumentWithSpecificClass(const ClassInfo* classInfo, int argument, RegisterID dst, RegisterID scratch)
-        {
-            loadCellArgument(argument, dst);
-            emitLoadStructure(*vm(), dst, scratch, dst);
-            appendFailure(branchPtr(NotEqual, Address(scratch, Structure::classInfoOffset()), TrustedImmPtr(PoisonedClassInfoPtr(classInfo).bits())));
-            // We have to reload the argument since emitLoadStructure clobbered it.
-            loadCellArgument(argument, dst);
-        }
-        
         void loadInt32Argument(int argument, RegisterID dst, Jump& failTarget)
         {

branches/safari-607-branch/Source/JavaScriptCore/llint/LowLevelInterpreter.asm

@@ -203 +203 @@
     const DeletedValueTag = constexpr JSValue::DeletedValueTag
     const LowestTag = constexpr JSValue::LowestTag
+end
+
+if JSVALUE64
+    const NumberOfStructureIDEntropyBits = constexpr StructureIDTable::s_numberOfEntropyBits
+    const StructureEntropyBitsShift = constexpr StructureIDTable::s_entropyBitsShiftForStructurePointer
 end
 

branches/safari-607-branch/Source/JavaScriptCore/llint/LowLevelInterpreter64.asm

@@ -536 +536 @@
     loadp CodeBlock::m_poisonedVM[scratch], scratch
     unpoison(_g_CodeBlockPoison, scratch, scratch2)
+    move structureIDThenStructure, scratch2
+    rshifti NumberOfStructureIDEntropyBits, scratch2
     loadp VM::heap + Heap::m_structureIDTable + StructureIDTable::m_table[scratch], scratch
-    loadp [scratch, structureIDThenStructure, PtrSize], structureIDThenStructure
+    loadp [scratch, scratch2, PtrSize], scratch2
+    lshiftp StructureEntropyBitsShift, structureIDThenStructure
+    xorp scratch2, structureIDThenStructure
 end
 
@@ -1176 +1180 @@
     return(t1)
 .masqueradesAsUndefined:
-    loadStructureWithScratch(t0, t3, t1, t5)
+    loadStructureWithScratch(t0, t3, t1, t2)
     loadp CodeBlock[cfr], t1
     loadp CodeBlock::m_globalObject[t1], t1

branches/safari-607-branch/Source/JavaScriptCore/runtime/StructureIDTable.cpp

@@ -1 +1 @@
 /*
- * Copyright (C) 2013 Apple Inc. All rights reserved.
+ * Copyright (C) 2013-2019 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -32 +32 @@
 namespace JSC {
 
+#if USE(JSVALUE64)
+
 StructureIDTable::StructureIDTable()
-    : m_firstFreeOffset(0)
-    , m_table(makeUniqueArray<StructureOrOffset>(s_initialSize))
-    , m_size(0)
+    : m_table(makeUniqueArray<StructureOrOffset>(s_initialSize))
+    , m_size(1)
     , m_capacity(s_initialSize)
 {
     // We pre-allocate the first offset so that the null Structure
     // can still be represented as the StructureID '0'.
-    allocateID(0);
+    table()[0].encodedStructureBits = 0;
+
+    makeFreeListFromRange(1, m_capacity - 1);
+}
+
+void StructureIDTable::makeFreeListFromRange(uint32_t first, uint32_t last)
+{
+    ASSERT(!m_firstFreeOffset);
+    ASSERT(!m_lastFreeOffset);
+
+    // Put all the new IDs on the free list sequentially.
+    uint32_t head = first;
+    uint32_t tail = last;
+    for (uint32_t i = first; i < last; ++i)
+        table()[i].offset = i + 1;
+    table()[last].offset = 0;
+
+    // Randomize the free list.
+    uint32_t size = last - first + 1;
+    uint32_t maxIterations = (size * 2) / 3;
+    for (uint32_t count = 0; count < maxIterations; ++count) {
+        // Move a random pick either to the head or the tail of the free list.
+        uint32_t random = m_weakRandom.getUint32();
+        uint32_t nodeBefore = first + (random % size);
+        uint32_t pick = table()[nodeBefore].offset;
+        if (pick) {
+            uint32_t nodeAfter = table()[pick].offset;
+            table()[nodeBefore].offset = nodeAfter;
+            if ((random & 1) || !nodeAfter) {
+                // Move to the head.
+                table()[pick].offset = head;
+                head = pick;
+                if (!nodeAfter)
+                    tail = nodeBefore;
+            } else {
+                // Move to the tail.
+                table()[pick].offset = 0;
+                table()[tail].offset = pick;
+                tail = pick;
+            }
+        }
+    }
+
+    // Cut list in half and swap halves.
+    uint32_t cut = first + (m_weakRandom.getUint32() % size);
+    uint32_t afterCut = table()[cut].offset;
+    if (afterCut) {
+        table()[tail].offset = head;
+        tail = cut;
+        head = afterCut;
+        table()[cut].offset = 0;
+    }
+
+    m_firstFreeOffset = head;
+    m_lastFreeOffset = tail;
 }
 
 void StructureIDTable::resize(size_t newCapacity)
 {
+    if (newCapacity > s_maximumNumberOfStructures)
+        newCapacity = s_maximumNumberOfStructures;
+
     // Create the new table.
     auto newTable = makeUniqueArray<StructureOrOffset>(newCapacity);
@@ -62 +120 @@
     // Update the capacity.
     m_capacity = newCapacity;
+
+    makeFreeListFromRange(m_size, m_capacity - 1);
 }
 
@@ -71 +131 @@
 StructureID StructureIDTable::allocateID(Structure* structure)
 {
-#if USE(JSVALUE64)
-    if (!m_firstFreeOffset) {
-        RELEASE_ASSERT(m_capacity <= UINT_MAX);
-        if (m_size == m_capacity)
-            resize(m_capacity * 2);
+    if (UNLIKELY(!m_firstFreeOffset)) {
+        RELEASE_ASSERT(m_capacity <= s_maximumNumberOfStructures);
+        ASSERT(m_size == m_capacity);
+        resize(m_capacity * 2);
         ASSERT(m_size < m_capacity);
-
-        StructureOrOffset newEntry;
-        newEntry.structure = structure;
-
-        if (m_size == s_unusedID) {
-            m_size++;
-            return allocateID(structure);
-        }
-
-        StructureID result = m_size;
-        table()[result] = newEntry;
-        m_size++;
-        ASSERT(!isNuked(result));
-        return result;
+        RELEASE_ASSERT(m_firstFreeOffset);
     }
 
-    ASSERT(m_firstFreeOffset != s_unusedID);
+    // entropyBits must not be zero. This ensures that if a corrupted
+    // structureID is encountered (with incorrect entropyBits), the decoded
+    // structure pointer for that ID will be always be a bad pointer with
+    // high bits set.
+    constexpr uint32_t entropyBitsMask = (1 << s_numberOfEntropyBits) - 1;
+    uint32_t entropyBits = m_weakRandom.getUint32() & entropyBitsMask;
+    if (UNLIKELY(!entropyBits)) {
+        constexpr uint32_t numberOfValuesToPickFrom = entropyBitsMask;
+        entropyBits = (m_weakRandom.getUint32() % numberOfValuesToPickFrom) + 1;
+    }
 
-    StructureID result = m_firstFreeOffset;
+    uint32_t structureIndex = m_firstFreeOffset;
     m_firstFreeOffset = table()[m_firstFreeOffset].offset;
-    table()[result].structure = structure;
+    if (!m_firstFreeOffset)
+        m_lastFreeOffset = 0;
+
+    StructureID result = (structureIndex << s_numberOfEntropyBits) | entropyBits;
+    table()[structureIndex].encodedStructureBits = encode(structure, result);
+    m_size++;
     ASSERT(!isNuked(result));
     return result;
-#else
-    ASSERT(!isNuked(structure));
-    return structure;
-#endif
 }
 
 void StructureIDTable::deallocateID(Structure* structure, StructureID structureID)
 {
-#if USE(JSVALUE64)
     ASSERT(structureID != s_unusedID);
-    RELEASE_ASSERT(table()[structureID].structure == structure);
-    table()[structureID].offset = m_firstFreeOffset;
-    m_firstFreeOffset = structureID;
-#else
-    UNUSED_PARAM(structure);
-    UNUSED_PARAM(structureID);
-#endif
+    uint32_t structureIndex = structureID >> s_numberOfEntropyBits;
+    ASSERT(structureIndex && structureIndex < s_maximumNumberOfStructures);
+    RELEASE_ASSERT(table()[structureIndex].encodedStructureBits == encode(structure, structureID));
+    m_size--;
+    if (!m_firstFreeOffset) {
+        table()[structureIndex].offset = 0;
+        m_firstFreeOffset = structureIndex;
+        m_lastFreeOffset = structureIndex;
+        return;
+    }
+
+    bool insertAtHead = m_weakRandom.getUint32() & 1;
+    if (insertAtHead) {
+        table()[structureIndex].offset = m_firstFreeOffset;
+        m_firstFreeOffset = structureIndex;
+    } else {
+        table()[structureIndex].offset = 0;
+        table()[m_lastFreeOffset].offset = structureIndex;
+        m_lastFreeOffset = structureIndex;
+    }
 }
 
+#endif // USE(JSVALUE64)
+
 } // namespace JSC

branches/safari-607-branch/Source/JavaScriptCore/runtime/StructureIDTable.h

@@ -1 +1 @@
 /*
- * Copyright (C) 2013 Apple Inc. All rights reserved.
+ * Copyright (C) 2013-2019 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -29 +29 @@
 #include <wtf/UniqueArray.h>
 #include <wtf/Vector.h>
+#include <wtf/WeakRandom.h>
 
 namespace JSC {
@@ -56 +57 @@
     return id & ~nukedStructureIDBit();
 }
-#else
+#else // not USE(JSVALUE64)
 typedef Structure* StructureID;
 
@@ -78 +79 @@
     return bitwise_cast<StructureID>(bitwise_cast<uintptr_t>(id) & ~bitwise_cast<uintptr_t>(nukedStructureIDBit()));
 }
-#endif
+#endif // not USE(JSVALUE64)
+
+#if USE(JSVALUE64)
+
+using EncodedStructureBits = uintptr_t;
 
 class StructureIDTable {
@@ -87 +92 @@
     void** base() { return reinterpret_cast<void**>(&m_table); }
 
+    bool isValid(StructureID);
     Structure* get(StructureID);
     void deallocateID(Structure*, StructureID);
@@ -97 +103 @@
 private:
     void resize(size_t newCapacity);
+    void makeFreeListFromRange(uint32_t first, uint32_t last);
 
     union StructureOrOffset {
         WTF_MAKE_FAST_ALLOCATED;
     public:
-        Structure* structure;
+        EncodedStructureBits encodedStructureBits;
         StructureID offset;
     };
@@ -107 +114 @@
     StructureOrOffset* table() const { return m_table.get(); }
     
-    static const size_t s_initialSize = 256;
+    static Structure* decode(EncodedStructureBits, StructureID);
+    static EncodedStructureBits encode(Structure*, StructureID);
+
+    static constexpr size_t s_initialSize = 512;
 
     Vector<UniqueArray<StructureOrOffset>> m_oldTables;
 
-    uint32_t m_firstFreeOffset;
+    uint32_t m_firstFreeOffset { 0 };
+    uint32_t m_lastFreeOffset { 0 };
     UniqueArray<StructureOrOffset> m_table;
 
-    size_t m_size;
+    size_t m_size { 0 };
     size_t m_capacity;
 
-#if USE(JSVALUE64)
-    static const StructureID s_unusedID = unusedPointer;
-#endif
+    WeakRandom m_weakRandom;
+
+    static constexpr StructureID s_unusedID = 0;
+
+public:
+    // 1. StructureID is encoded as:
+    //
+    //    ----------------------------------------------------------------
+    //    | 1 Nuke Bit | 24 StructureIDTable index bits | 7 entropy bits |
+    //    ----------------------------------------------------------------
+    //
+    //    The entropy bits are chosen at random and assigned when a StructureID
+    //    is allocated.
+    //
+    // 2. For each StructureID, the StructureIDTable stores encodedStructureBits
+    //    which are encoded from the structure pointer as such:
+    //
+    //    ----------------------------------------------------------------
+    //    | 7 entropy bits |                   57 structure pointer bits |
+    //    ----------------------------------------------------------------
+    //
+    //    The entropy bits here are the same 7 bits used in the encoding of the
+    //    StructureID for this structure entry in the StructureIDTable.
+
+    static constexpr uint32_t s_numberOfNukeBits = 1;
+    static constexpr uint32_t s_numberOfEntropyBits = 7;
+    static constexpr uint32_t s_entropyBitsShiftForStructurePointer = (sizeof(intptr_t) * 8) - s_numberOfEntropyBits;
+
+    static constexpr uint32_t s_maximumNumberOfStructures = 1 << (32 - s_numberOfEntropyBits - s_numberOfNukeBits);
 };
 
+ALWAYS_INLINE Structure* StructureIDTable::decode(EncodedStructureBits bits, StructureID structureID)
+{
+    return bitwise_cast<Structure*>(bits ^ (static_cast<uintptr_t>(structureID) << s_entropyBitsShiftForStructurePointer));
+}
+
+ALWAYS_INLINE EncodedStructureBits StructureIDTable::encode(Structure* structure, StructureID structureID)
+{
+    return bitwise_cast<EncodedStructureBits>(structure) ^ (static_cast<EncodedStructureBits>(structureID) << s_entropyBitsShiftForStructurePointer);
+}
+
 inline Structure* StructureIDTable::get(StructureID structureID)
 {
-#if USE(JSVALUE64)
     ASSERT_WITH_SECURITY_IMPLICATION(structureID);
     ASSERT_WITH_SECURITY_IMPLICATION(!isNuked(structureID));
-    ASSERT_WITH_SECURITY_IMPLICATION(structureID < m_capacity);
-    return table()[structureID].structure;
-#else
-    return structureID;
+    uint32_t structureIndex = structureID >> s_numberOfEntropyBits;
+    ASSERT_WITH_SECURITY_IMPLICATION(structureIndex < m_capacity);
+    return decode(table()[structureIndex].encodedStructureBits, structureID);
+}
+
+inline bool StructureIDTable::isValid(StructureID structureID)
+{
+    if (!structureID)
+        return false;
+    uint32_t structureIndex = structureID >> s_numberOfEntropyBits;
+    if (structureIndex >= m_capacity)
+        return false;
+#if CPU(ADDRESS64)
+    Structure* structure = decode(table()[structureIndex].encodedStructureBits, structureID);
+    if (reinterpret_cast<uintptr_t>(structure) >> s_entropyBitsShiftForStructurePointer)
+        return false;
 #endif
-}
+    return true;
+}
+
+#else // not USE(JSVALUE64)
+
+class StructureIDTable {
+    friend class LLIntOffsetsExtractor;
+public:
+    StructureIDTable() = default;
+
+    Structure* get(StructureID structureID) { return structureID; }
+    void deallocateID(Structure*, StructureID) { }
+    StructureID allocateID(Structure* structure)
+    {
+        ASSERT(!isNuked(structure));
+        return structure;
+    };
+
+    void flushOldTables() { }
+};
+
+#endif // not USE(JSVALUE64)
 
 } // namespace JSC