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diff --git a/js/src/jit/Snapshots.h b/js/src/jit/Snapshots.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef jit_Snapshot_h
+#define jit_Snapshot_h
+
+#include "mozilla/Attributes.h"
+
+#include "jit/CompactBuffer.h"
+#include "jit/IonTypes.h"
+#include "jit/Registers.h"
+#include "js/AllocPolicy.h"
+#include "js/HashTable.h"
+#include "js/TypeDecls.h"
+
+namespace js {
+class JS_PUBLIC_API GenericPrinter;
+
+namespace jit {
+
+class RValueAllocation;
+
+// A Recover Value Allocation mirror what is known at compiled time as being the
+// MIRType and the LAllocation.  This is read out of the snapshot to recover the
+// value which would be there if this frame was an interpreter frame instead of
+// an Ion frame.
+//
+// It is used with the SnapshotIterator to recover a Value from the stack,
+// spilled registers or the list of constant of the compiled script.
+//
+// Unit tests are located in jsapi-tests/testJitRValueAlloc.cpp.
+class RValueAllocation {
+ public:
+  // See RValueAllocation encoding in Snapshots.cpp
+  enum Mode {
+    CONSTANT = 0x00,
+    CST_UNDEFINED = 0x01,
+    CST_NULL = 0x02,
+    DOUBLE_REG = 0x03,
+    ANY_FLOAT_REG = 0x04,
+    ANY_FLOAT_STACK = 0x05,
+#if defined(JS_NUNBOX32)
+    UNTYPED_REG_REG = 0x06,
+    UNTYPED_REG_STACK = 0x07,
+    UNTYPED_STACK_REG = 0x08,
+    UNTYPED_STACK_STACK = 0x09,
+#elif defined(JS_PUNBOX64)
+    UNTYPED_REG = 0x06,
+    UNTYPED_STACK = 0x07,
+#endif
+
+    // Recover instructions.
+    RECOVER_INSTRUCTION = 0x0a,
+    RI_WITH_DEFAULT_CST = 0x0b,
+
+    // The JSValueType is packed in the Mode.
+    TYPED_REG_MIN = 0x10,
+    TYPED_REG_MAX = 0x1f,
+    TYPED_REG = TYPED_REG_MIN,
+
+    // The JSValueType is packed in the Mode.
+    TYPED_STACK_MIN = 0x20,
+    TYPED_STACK_MAX = 0x2f,
+    TYPED_STACK = TYPED_STACK_MIN,
+
+    // This mask can be used with any other valid mode. When this flag is
+    // set on the mode, this inform the snapshot iterator that even if the
+    // allocation is readable, the content of if might be incomplete unless
+    // all side-effects are executed.
+    RECOVER_SIDE_EFFECT_MASK = 0x80,
+
+    // This mask represents the set of bits which can be used to encode a
+    // value in a snapshot. The mode is used to determine how to interpret
+    // the union of values and how to pack the value in memory.
+    MODE_BITS_MASK = 0x17f,
+
+    INVALID = 0x100,
+  };
+
+  enum { PACKED_TAG_MASK = 0x0f };
+
+  // See Payload encoding in Snapshots.cpp
+  enum PayloadType {
+    PAYLOAD_NONE,
+    PAYLOAD_INDEX,
+    PAYLOAD_STACK_OFFSET,
+    PAYLOAD_GPR,
+    PAYLOAD_FPU,
+    PAYLOAD_PACKED_TAG
+  };
+
+  struct Layout {
+    PayloadType type1;
+    PayloadType type2;
+    const char* name;
+  };
+
+ private:
+  Mode mode_;
+
+  // Additional information to recover the content of the allocation.
+  struct FloatRegisterBits {
+    uint32_t data;
+    bool operator==(const FloatRegisterBits& other) const {
+      return data == other.data;
+    }
+    uint32_t code() const { return data; }
+    const char* name() const {
+      FloatRegister tmp = FloatRegister::FromCode(data);
+      return tmp.name();
+    }
+  };
+
+  union Payload {
+    uint32_t index;
+    int32_t stackOffset;
+    Register gpr;
+    FloatRegisterBits fpu;
+    JSValueType type;
+
+    Payload() : index(0) {
+      static_assert(sizeof(index) == sizeof(Payload),
+                    "All Payload bits are initialized.");
+    }
+  };
+
+  Payload arg1_;
+  Payload arg2_;
+
+  static Payload payloadOfIndex(uint32_t index) {
+    Payload p;
+    p.index = index;
+    return p;
+  }
+  static Payload payloadOfStackOffset(int32_t offset) {
+    Payload p;
+    p.stackOffset = offset;
+    return p;
+  }
+  static Payload payloadOfRegister(Register reg) {
+    Payload p;
+    p.gpr = reg;
+    return p;
+  }
+  static Payload payloadOfFloatRegister(FloatRegister reg) {
+    Payload p;
+    FloatRegisterBits b;
+    b.data = reg.code();
+    p.fpu = b;
+    return p;
+  }
+  static Payload payloadOfValueType(JSValueType type) {
+    Payload p;
+    p.type = type;
+    return p;
+  }
+
+  static const Layout& layoutFromMode(Mode mode);
+
+  static void readPayload(CompactBufferReader& reader, PayloadType t,
+                          uint8_t* mode, Payload* p);
+  static void writePayload(CompactBufferWriter& writer, PayloadType t,
+                           Payload p);
+  static void writePadding(CompactBufferWriter& writer);
+#ifdef JS_JITSPEW
+  static void dumpPayload(GenericPrinter& out, PayloadType t, Payload p);
+#endif
+  static bool equalPayloads(PayloadType t, Payload lhs, Payload rhs);
+
+  RValueAllocation(Mode mode, Payload a1, Payload a2)
+      : mode_(mode), arg1_(a1), arg2_(a2) {}
+
+  RValueAllocation(Mode mode, Payload a1) : mode_(mode), arg1_(a1) {
+    arg2_.index = 0;
+  }
+
+  explicit RValueAllocation(Mode mode) : mode_(mode) {
+    arg1_.index = 0;
+    arg2_.index = 0;
+  }
+
+ public:
+  RValueAllocation() : mode_(INVALID) {
+    arg1_.index = 0;
+    arg2_.index = 0;
+  }
+
+  // DOUBLE_REG
+  static RValueAllocation Double(FloatRegister reg) {
+    return RValueAllocation(DOUBLE_REG, payloadOfFloatRegister(reg));
+  }
+
+  // ANY_FLOAT_REG or ANY_FLOAT_STACK
+  static RValueAllocation AnyFloat(FloatRegister reg) {
+    return RValueAllocation(ANY_FLOAT_REG, payloadOfFloatRegister(reg));
+  }
+  static RValueAllocation AnyFloat(int32_t offset) {
+    return RValueAllocation(ANY_FLOAT_STACK, payloadOfStackOffset(offset));
+  }
+
+  // TYPED_REG or TYPED_STACK
+  static RValueAllocation Typed(JSValueType type, Register reg) {
+    MOZ_ASSERT(type != JSVAL_TYPE_DOUBLE && type != JSVAL_TYPE_MAGIC &&
+               type != JSVAL_TYPE_NULL && type != JSVAL_TYPE_UNDEFINED);
+    return RValueAllocation(TYPED_REG, payloadOfValueType(type),
+                            payloadOfRegister(reg));
+  }
+  static RValueAllocation Typed(JSValueType type, int32_t offset) {
+    MOZ_ASSERT(type != JSVAL_TYPE_MAGIC && type != JSVAL_TYPE_NULL &&
+               type != JSVAL_TYPE_UNDEFINED);
+    return RValueAllocation(TYPED_STACK, payloadOfValueType(type),
+                            payloadOfStackOffset(offset));
+  }
+
+  // UNTYPED
+#if defined(JS_NUNBOX32)
+  static RValueAllocation Untyped(Register type, Register payload) {
+    return RValueAllocation(UNTYPED_REG_REG, payloadOfRegister(type),
+                            payloadOfRegister(payload));
+  }
+
+  static RValueAllocation Untyped(Register type, int32_t payloadStackOffset) {
+    return RValueAllocation(UNTYPED_REG_STACK, payloadOfRegister(type),
+                            payloadOfStackOffset(payloadStackOffset));
+  }
+
+  static RValueAllocation Untyped(int32_t typeStackOffset, Register payload) {
+    return RValueAllocation(UNTYPED_STACK_REG,
+                            payloadOfStackOffset(typeStackOffset),
+                            payloadOfRegister(payload));
+  }
+
+  static RValueAllocation Untyped(int32_t typeStackOffset,
+                                  int32_t payloadStackOffset) {
+    return RValueAllocation(UNTYPED_STACK_STACK,
+                            payloadOfStackOffset(typeStackOffset),
+                            payloadOfStackOffset(payloadStackOffset));
+  }
+
+#elif defined(JS_PUNBOX64)
+  static RValueAllocation Untyped(Register reg) {
+    return RValueAllocation(UNTYPED_REG, payloadOfRegister(reg));
+  }
+
+  static RValueAllocation Untyped(int32_t stackOffset) {
+    return RValueAllocation(UNTYPED_STACK, payloadOfStackOffset(stackOffset));
+  }
+#endif
+
+  // common constants.
+  static RValueAllocation Undefined() {
+    return RValueAllocation(CST_UNDEFINED);
+  }
+  static RValueAllocation Null() { return RValueAllocation(CST_NULL); }
+
+  // CONSTANT's index
+  static RValueAllocation ConstantPool(uint32_t index) {
+    return RValueAllocation(CONSTANT, payloadOfIndex(index));
+  }
+
+  // Recover instruction's index
+  static RValueAllocation RecoverInstruction(uint32_t index) {
+    return RValueAllocation(RECOVER_INSTRUCTION, payloadOfIndex(index));
+  }
+  static RValueAllocation RecoverInstruction(uint32_t riIndex,
+                                             uint32_t cstIndex) {
+    return RValueAllocation(RI_WITH_DEFAULT_CST, payloadOfIndex(riIndex),
+                            payloadOfIndex(cstIndex));
+  }
+
+  void setNeedSideEffect() {
+    MOZ_ASSERT(!needSideEffect() && mode_ != INVALID);
+    mode_ = Mode(mode_ | RECOVER_SIDE_EFFECT_MASK);
+  }
+
+  void writeHeader(CompactBufferWriter& writer, JSValueType type,
+                   uint32_t regCode) const;
+
+ public:
+  static RValueAllocation read(CompactBufferReader& reader);
+  void write(CompactBufferWriter& writer) const;
+
+ public:
+  bool valid() const { return mode_ != INVALID; }
+  Mode mode() const { return Mode(mode_ & MODE_BITS_MASK); }
+  bool needSideEffect() const { return mode_ & RECOVER_SIDE_EFFECT_MASK; }
+
+  uint32_t index() const {
+    MOZ_ASSERT(layoutFromMode(mode()).type1 == PAYLOAD_INDEX);
+    return arg1_.index;
+  }
+  int32_t stackOffset() const {
+    MOZ_ASSERT(layoutFromMode(mode()).type1 == PAYLOAD_STACK_OFFSET);
+    return arg1_.stackOffset;
+  }
+  Register reg() const {
+    MOZ_ASSERT(layoutFromMode(mode()).type1 == PAYLOAD_GPR);
+    return arg1_.gpr;
+  }
+  FloatRegister fpuReg() const {
+    MOZ_ASSERT(layoutFromMode(mode()).type1 == PAYLOAD_FPU);
+    FloatRegisterBits b = arg1_.fpu;
+    return FloatRegister::FromCode(b.data);
+  }
+  JSValueType knownType() const {
+    MOZ_ASSERT(layoutFromMode(mode()).type1 == PAYLOAD_PACKED_TAG);
+    return arg1_.type;
+  }
+
+  uint32_t index2() const {
+    MOZ_ASSERT(layoutFromMode(mode()).type2 == PAYLOAD_INDEX);
+    return arg2_.index;
+  }
+  int32_t stackOffset2() const {
+    MOZ_ASSERT(layoutFromMode(mode()).type2 == PAYLOAD_STACK_OFFSET);
+    return arg2_.stackOffset;
+  }
+  Register reg2() const {
+    MOZ_ASSERT(layoutFromMode(mode()).type2 == PAYLOAD_GPR);
+    return arg2_.gpr;
+  }
+
+ public:
+#ifdef JS_JITSPEW
+  void dump(GenericPrinter& out) const;
+#endif
+
+  bool operator==(const RValueAllocation& rhs) const {
+    // Note, this equality compares the verbatim content of the payload,
+    // which is made possible because we ensure that the payload content is
+    // fully initialized during the creation.
+    static_assert(sizeof(int32_t) == sizeof(Payload),
+                  "All Payload bits are compared.");
+    return mode_ == rhs.mode_ && arg1_.index == rhs.arg1_.index &&
+           arg2_.index == rhs.arg2_.index;
+  }
+
+  HashNumber hash() const;
+
+  struct Hasher {
+    using Key = RValueAllocation;
+    using Lookup = Key;
+    static HashNumber hash(const Lookup& v) { return v.hash(); }
+    static bool match(const Key& k, const Lookup& l) { return k == l; }
+  };
+};
+
+class RecoverWriter;
+
+// Collects snapshots in a contiguous buffer, which is copied into IonScript
+// memory after code generation.
+class SnapshotWriter {
+  CompactBufferWriter writer_;
+  CompactBufferWriter allocWriter_;
+
+  // Map RValueAllocations to an offset in the allocWriter_ buffer.  This is
+  // useful as value allocations are repeated frequently.
+  using RVA = RValueAllocation;
+  typedef HashMap<RVA, uint32_t, RVA::Hasher, SystemAllocPolicy> RValueAllocMap;
+  RValueAllocMap allocMap_;
+
+  // This is only used to assert sanity.
+  uint32_t allocWritten_;
+
+  // Used to report size of the snapshot in the spew messages.
+  SnapshotOffset lastStart_;
+
+ public:
+  SnapshotWriter();
+
+  SnapshotOffset startSnapshot(RecoverOffset recoverOffset, BailoutKind kind);
+#ifdef TRACK_SNAPSHOTS
+  void trackSnapshot(uint32_t pcOpcode, uint32_t mirOpcode, uint32_t mirId,
+                     uint32_t lirOpcode, uint32_t lirId);
+#endif
+  [[nodiscard]] bool add(const RValueAllocation& slot);
+
+  uint32_t allocWritten() const { return allocWritten_; }
+  void endSnapshot();
+
+  bool oom() const {
+    return writer_.oom() || writer_.length() >= MAX_BUFFER_SIZE ||
+           allocWriter_.oom() || allocWriter_.length() >= MAX_BUFFER_SIZE;
+  }
+
+  size_t listSize() const { return writer_.length(); }
+  const uint8_t* listBuffer() const { return writer_.buffer(); }
+
+  size_t RVATableSize() const { return allocWriter_.length(); }
+  const uint8_t* RVATableBuffer() const { return allocWriter_.buffer(); }
+};
+
+class MNode;
+
+class RecoverWriter {
+  CompactBufferWriter writer_;
+
+  uint32_t instructionCount_;
+  uint32_t instructionsWritten_;
+
+ public:
+  SnapshotOffset startRecover(uint32_t instructionCount);
+
+  void writeInstruction(const MNode* rp);
+
+  void endRecover();
+
+  size_t size() const { return writer_.length(); }
+  const uint8_t* buffer() const { return writer_.buffer(); }
+
+  bool oom() const {
+    return writer_.oom() || writer_.length() >= MAX_BUFFER_SIZE;
+  }
+};
+
+class RecoverReader;
+
+// A snapshot reader reads the entries out of the compressed snapshot buffer in
+// a script. These entries describe the equivalent interpreter frames at a given
+// position in JIT code. Each entry is an Ion's value allocations, used to
+// recover the corresponding Value from an Ion frame.
+class SnapshotReader {
+  CompactBufferReader reader_;
+  CompactBufferReader allocReader_;
+  const uint8_t* allocTable_;
+
+  BailoutKind bailoutKind_;
+  uint32_t allocRead_;           // Number of slots that have been read.
+  RecoverOffset recoverOffset_;  // Offset of the recover instructions.
+
+#ifdef TRACK_SNAPSHOTS
+ private:
+  uint32_t pcOpcode_;
+  uint32_t mirOpcode_;
+  uint32_t mirId_;
+  uint32_t lirOpcode_;
+  uint32_t lirId_;
+
+ public:
+  void readTrackSnapshot();
+  void spewBailingFrom() const;
+#endif
+
+ private:
+  void readSnapshotHeader();
+  uint32_t readAllocationIndex();
+
+ public:
+  SnapshotReader(const uint8_t* snapshots, uint32_t offset,
+                 uint32_t RVATableSize, uint32_t listSize);
+
+  RValueAllocation readAllocation();
+  void skipAllocation() { readAllocationIndex(); }
+
+  BailoutKind bailoutKind() const { return bailoutKind_; }
+  RecoverOffset recoverOffset() const { return recoverOffset_; }
+
+  uint32_t numAllocationsRead() const { return allocRead_; }
+  void resetNumAllocationsRead() { allocRead_ = 0; }
+};
+
+class MOZ_NON_PARAM RInstructionStorage {
+  static constexpr size_t Size = 4 * sizeof(uint32_t);
+
+  // This presumes all RInstructionStorage are safely void*-alignable.
+  // RInstruction::readRecoverData asserts that no RInstruction subclass
+  // has stricter alignment requirements than RInstructionStorage.
+  static constexpr size_t Alignment = alignof(void*);
+
+  alignas(Alignment) unsigned char mem[Size];
+
+ public:
+  const void* addr() const { return mem; }
+  void* addr() { return mem; }
+
+  RInstructionStorage() = default;
+
+  // Making a copy of raw bytes holding a RInstruction instance would be a
+  // strict aliasing violation: see bug 1269319 for an instance of bytewise
+  // copying having caused crashes.
+  RInstructionStorage(const RInstructionStorage&) = delete;
+  RInstructionStorage& operator=(const RInstructionStorage& other) = delete;
+};
+
+class RInstruction;
+
+class RecoverReader {
+  CompactBufferReader reader_;
+
+  // Number of encoded instructions.
+  uint32_t numInstructions_;
+
+  // Number of instruction read.
+  uint32_t numInstructionsRead_;
+
+  // Space is reserved as part of the RecoverReader to avoid allocations of
+  // data which is needed to decode the current instruction.
+  RInstructionStorage rawData_;
+
+ private:
+  void readRecoverHeader();
+  void readInstruction();
+
+ public:
+  RecoverReader(SnapshotReader& snapshot, const uint8_t* recovers,
+                uint32_t size);
+  explicit RecoverReader(const RecoverReader& rr);
+  RecoverReader& operator=(const RecoverReader& rr);
+
+  uint32_t numInstructions() const { return numInstructions_; }
+  uint32_t numInstructionsRead() const { return numInstructionsRead_; }
+
+  bool moreInstructions() const {
+    return numInstructionsRead_ < numInstructions_;
+  }
+  void nextInstruction() { readInstruction(); }
+
+  const RInstruction* instruction() const {
+    return reinterpret_cast<const RInstruction*>(rawData_.addr());
+  }
+};
+
+}  // namespace jit
+}  // namespace js
+
+#endif /* jit_Snapshot_h */