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diff --git a/js/src/jit/shared/Assembler-shared.h b/js/src/jit/shared/Assembler-shared.h
new file mode 100644
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+++ b/js/src/jit/shared/Assembler-shared.h
@@ -0,0 +1,632 @@
+/* -*- 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_shared_Assembler_shared_h
+#define jit_shared_Assembler_shared_h
+
+#include "mozilla/CheckedInt.h"
+
+#include <limits.h>
+
+#include "gc/Barrier.h"
+#include "jit/AtomicOp.h"
+#include "jit/JitAllocPolicy.h"
+#include "jit/JitCode.h"
+#include "jit/JitContext.h"
+#include "jit/Label.h"
+#include "jit/Registers.h"
+#include "jit/RegisterSets.h"
+#include "js/ScalarType.h"  // js::Scalar::Type
+#include "vm/HelperThreads.h"
+#include "vm/NativeObject.h"
+#include "wasm/WasmTypes.h"
+
+#if defined(JS_CODEGEN_ARM) || defined(JS_CODEGEN_ARM64) || \
+    defined(JS_CODEGEN_MIPS32) || defined(JS_CODEGEN_MIPS64)
+// Push return addresses callee-side.
+#  define JS_USE_LINK_REGISTER
+#endif
+
+#if defined(JS_CODEGEN_MIPS32) || defined(JS_CODEGEN_MIPS64) || \
+    defined(JS_CODEGEN_ARM64)
+// JS_CODELABEL_LINKMODE gives labels additional metadata
+// describing how Bind() should patch them.
+#  define JS_CODELABEL_LINKMODE
+#endif
+
+namespace js {
+namespace jit {
+
+enum class FrameType;
+
+namespace Disassembler {
+class HeapAccess;
+}  // namespace Disassembler
+
+static constexpr uint32_t Simd128DataSize = 4 * sizeof(int32_t);
+static_assert(Simd128DataSize == 4 * sizeof(int32_t),
+              "SIMD data should be able to contain int32x4");
+static_assert(Simd128DataSize == 4 * sizeof(float),
+              "SIMD data should be able to contain float32x4");
+static_assert(Simd128DataSize == 2 * sizeof(double),
+              "SIMD data should be able to contain float64x2");
+
+enum Scale { TimesOne = 0, TimesTwo = 1, TimesFour = 2, TimesEight = 3 };
+
+static_assert(sizeof(JS::Value) == 8,
+              "required for TimesEight and 3 below to be correct");
+static const Scale ValueScale = TimesEight;
+static const size_t ValueShift = 3;
+
+static inline unsigned ScaleToShift(Scale scale) { return unsigned(scale); }
+
+static inline bool IsShiftInScaleRange(int i) {
+  return i >= TimesOne && i <= TimesEight;
+}
+
+static inline Scale ShiftToScale(int i) {
+  MOZ_ASSERT(IsShiftInScaleRange(i));
+  return Scale(i);
+}
+
+static inline Scale ScaleFromElemWidth(int shift) {
+  switch (shift) {
+    case 1:
+      return TimesOne;
+    case 2:
+      return TimesTwo;
+    case 4:
+      return TimesFour;
+    case 8:
+      return TimesEight;
+  }
+
+  MOZ_CRASH("Invalid scale");
+}
+
+// Used for 32-bit immediates which do not require relocation.
+struct Imm32 {
+  int32_t value;
+
+  explicit Imm32(int32_t value) : value(value) {}
+  explicit Imm32(FrameType type) : Imm32(int32_t(type)) {}
+
+  static inline Imm32 ShiftOf(enum Scale s) {
+    switch (s) {
+      case TimesOne:
+        return Imm32(0);
+      case TimesTwo:
+        return Imm32(1);
+      case TimesFour:
+        return Imm32(2);
+      case TimesEight:
+        return Imm32(3);
+    };
+    MOZ_CRASH("Invalid scale");
+  }
+
+  static inline Imm32 FactorOf(enum Scale s) {
+    return Imm32(1 << ShiftOf(s).value);
+  }
+};
+
+// Pointer-sized integer to be embedded as an immediate in an instruction.
+struct ImmWord {
+  uintptr_t value;
+
+  explicit ImmWord(uintptr_t value) : value(value) {}
+};
+
+// Used for 64-bit immediates which do not require relocation.
+struct Imm64 {
+  uint64_t value;
+
+  explicit Imm64(int64_t value) : value(value) {}
+
+  Imm32 low() const { return Imm32(int32_t(value)); }
+
+  Imm32 hi() const { return Imm32(int32_t(value >> 32)); }
+
+  inline Imm32 firstHalf() const;
+  inline Imm32 secondHalf() const;
+};
+
+#ifdef DEBUG
+static inline bool IsCompilingWasm() {
+  return GetJitContext()->isCompilingWasm();
+}
+#endif
+
+// Pointer to be embedded as an immediate in an instruction.
+struct ImmPtr {
+  void* value;
+
+  struct NoCheckToken {};
+
+  explicit ImmPtr(void* value, NoCheckToken) : value(value) {
+    // A special unchecked variant for contexts where we know it is safe to
+    // use an immptr. This is assuming the caller knows what they're doing.
+  }
+
+  explicit ImmPtr(const void* value) : value(const_cast<void*>(value)) {
+    // To make code serialization-safe, wasm compilation should only
+    // compile pointer immediates using a SymbolicAddress.
+    MOZ_ASSERT(!IsCompilingWasm());
+  }
+
+  template <class R>
+  explicit ImmPtr(R (*pf)()) : value(JS_FUNC_TO_DATA_PTR(void*, pf)) {
+    MOZ_ASSERT(!IsCompilingWasm());
+  }
+
+  template <class R, class A1>
+  explicit ImmPtr(R (*pf)(A1)) : value(JS_FUNC_TO_DATA_PTR(void*, pf)) {
+    MOZ_ASSERT(!IsCompilingWasm());
+  }
+
+  template <class R, class A1, class A2>
+  explicit ImmPtr(R (*pf)(A1, A2)) : value(JS_FUNC_TO_DATA_PTR(void*, pf)) {
+    MOZ_ASSERT(!IsCompilingWasm());
+  }
+
+  template <class R, class A1, class A2, class A3>
+  explicit ImmPtr(R (*pf)(A1, A2, A3)) : value(JS_FUNC_TO_DATA_PTR(void*, pf)) {
+    MOZ_ASSERT(!IsCompilingWasm());
+  }
+
+  template <class R, class A1, class A2, class A3, class A4>
+  explicit ImmPtr(R (*pf)(A1, A2, A3, A4))
+      : value(JS_FUNC_TO_DATA_PTR(void*, pf)) {
+    MOZ_ASSERT(!IsCompilingWasm());
+  }
+};
+
+// The same as ImmPtr except that the intention is to patch this
+// instruction. The initial value of the immediate is 'addr' and this value is
+// either clobbered or used in the patching process.
+struct PatchedImmPtr {
+  void* value;
+
+  explicit PatchedImmPtr() : value(nullptr) {}
+  explicit PatchedImmPtr(const void* value) : value(const_cast<void*>(value)) {}
+};
+
+class AssemblerShared;
+class ImmGCPtr;
+
+// Used for immediates which require relocation.
+class ImmGCPtr {
+ public:
+  const gc::Cell* value;
+
+  explicit ImmGCPtr(const gc::Cell* ptr) : value(ptr) {
+    // Nursery pointers can't be used if the main thread might be currently
+    // performing a minor GC.
+    MOZ_ASSERT_IF(ptr && !ptr->isTenured(), !CurrentThreadIsIonCompiling());
+
+    // wasm shouldn't be creating GC things
+    MOZ_ASSERT(!IsCompilingWasm());
+  }
+
+ private:
+  ImmGCPtr() : value(0) {}
+};
+
+// Pointer to trampoline code. Trampoline code is kept alive until the runtime
+// is destroyed, so does not need to be traced.
+struct TrampolinePtr {
+  uint8_t* value;
+
+  TrampolinePtr() : value(nullptr) {}
+  explicit TrampolinePtr(uint8_t* value) : value(value) { MOZ_ASSERT(value); }
+};
+
+// Pointer to be embedded as an immediate that is loaded/stored from by an
+// instruction.
+struct AbsoluteAddress {
+  void* addr;
+
+  explicit AbsoluteAddress(const void* addr) : addr(const_cast<void*>(addr)) {
+    MOZ_ASSERT(!IsCompilingWasm());
+  }
+
+  AbsoluteAddress offset(ptrdiff_t delta) {
+    return AbsoluteAddress(((uint8_t*)addr) + delta);
+  }
+};
+
+// The same as AbsoluteAddress except that the intention is to patch this
+// instruction. The initial value of the immediate is 'addr' and this value is
+// either clobbered or used in the patching process.
+struct PatchedAbsoluteAddress {
+  void* addr;
+
+  explicit PatchedAbsoluteAddress() : addr(nullptr) {}
+  explicit PatchedAbsoluteAddress(const void* addr)
+      : addr(const_cast<void*>(addr)) {}
+  explicit PatchedAbsoluteAddress(uintptr_t addr)
+      : addr(reinterpret_cast<void*>(addr)) {}
+};
+
+// Specifies an address computed in the form of a register base and a constant,
+// 32-bit offset.
+struct Address {
+  RegisterOrSP base;
+  int32_t offset;
+
+  Address(Register base, int32_t offset)
+      : base(RegisterOrSP(base)), offset(offset) {}
+
+#ifdef JS_HAS_HIDDEN_SP
+  Address(RegisterOrSP base, int32_t offset) : base(base), offset(offset) {}
+#endif
+
+  Address() = delete;
+};
+
+#if JS_BITS_PER_WORD == 32
+
+static inline Address LowWord(const Address& address) {
+  using mozilla::CheckedInt;
+
+  CheckedInt<int32_t> offset =
+      CheckedInt<int32_t>(address.offset) + INT64LOW_OFFSET;
+  MOZ_ALWAYS_TRUE(offset.isValid());
+  return Address(address.base, offset.value());
+}
+
+static inline Address HighWord(const Address& address) {
+  using mozilla::CheckedInt;
+
+  CheckedInt<int32_t> offset =
+      CheckedInt<int32_t>(address.offset) + INT64HIGH_OFFSET;
+  MOZ_ALWAYS_TRUE(offset.isValid());
+  return Address(address.base, offset.value());
+}
+
+#endif
+
+// Specifies an address computed in the form of a register base, a register
+// index with a scale, and a constant, 32-bit offset.
+struct BaseIndex {
+  RegisterOrSP base;
+  Register index;
+  Scale scale;
+  int32_t offset;
+
+  BaseIndex(Register base, Register index, Scale scale, int32_t offset = 0)
+      : base(RegisterOrSP(base)), index(index), scale(scale), offset(offset) {}
+
+#ifdef JS_HAS_HIDDEN_SP
+  BaseIndex(RegisterOrSP base, Register index, Scale scale, int32_t offset = 0)
+      : base(base), index(index), scale(scale), offset(offset) {}
+#endif
+
+  BaseIndex() = delete;
+};
+
+#if JS_BITS_PER_WORD == 32
+
+static inline BaseIndex LowWord(const BaseIndex& address) {
+  using mozilla::CheckedInt;
+
+  CheckedInt<int32_t> offset =
+      CheckedInt<int32_t>(address.offset) + INT64LOW_OFFSET;
+  MOZ_ALWAYS_TRUE(offset.isValid());
+  return BaseIndex(address.base, address.index, address.scale, offset.value());
+}
+
+static inline BaseIndex HighWord(const BaseIndex& address) {
+  using mozilla::CheckedInt;
+
+  CheckedInt<int32_t> offset =
+      CheckedInt<int32_t>(address.offset) + INT64HIGH_OFFSET;
+  MOZ_ALWAYS_TRUE(offset.isValid());
+  return BaseIndex(address.base, address.index, address.scale, offset.value());
+}
+
+#endif
+
+// A BaseIndex used to access Values.  Note that |offset| is *not* scaled by
+// sizeof(Value).  Use this *only* if you're indexing into a series of Values
+// that aren't object elements or object slots (for example, values on the
+// stack, values in an arguments object, &c.).  If you're indexing into an
+// object's elements or slots, don't use this directly!  Use
+// BaseObject{Element,Slot}Index instead.
+struct BaseValueIndex : BaseIndex {
+  BaseValueIndex(Register base, Register index, int32_t offset = 0)
+      : BaseIndex(RegisterOrSP(base), index, ValueScale, offset) {}
+
+#ifdef JS_HAS_HIDDEN_SP
+  BaseValueIndex(RegisterOrSP base, Register index, int32_t offset = 0)
+      : BaseIndex(base, index, ValueScale, offset) {}
+#endif
+};
+
+// Specifies the address of an indexed Value within object elements from a
+// base.  The index must not already be scaled by sizeof(Value)!
+struct BaseObjectElementIndex : BaseValueIndex {
+  BaseObjectElementIndex(Register base, Register index, int32_t offset = 0)
+      : BaseValueIndex(base, index, offset) {
+    NativeObject::elementsSizeMustNotOverflow();
+  }
+
+#ifdef JS_HAS_HIDDEN_SP
+  BaseObjectElementIndex(RegisterOrSP base, Register index, int32_t offset = 0)
+      : BaseValueIndex(base, index, offset) {
+    NativeObject::elementsSizeMustNotOverflow();
+  }
+#endif
+};
+
+// Like BaseObjectElementIndex, except for object slots.
+struct BaseObjectSlotIndex : BaseValueIndex {
+  BaseObjectSlotIndex(Register base, Register index)
+      : BaseValueIndex(base, index) {
+    NativeObject::slotsSizeMustNotOverflow();
+  }
+
+#ifdef JS_HAS_HIDDEN_SP
+  BaseObjectSlotIndex(RegisterOrSP base, Register index)
+      : BaseValueIndex(base, index) {
+    NativeObject::slotsSizeMustNotOverflow();
+  }
+#endif
+};
+
+enum class RelocationKind {
+  // The target is immovable, so patching is only needed if the source
+  // buffer is relocated and the reference is relative.
+  HARDCODED,
+
+  // The target is the start of a JitCode buffer, which must be traced
+  // during garbage collection. Relocations and patching may be needed.
+  JITCODE
+};
+
+class CodeOffset {
+  size_t offset_;
+
+  static const size_t NOT_BOUND = size_t(-1);
+
+ public:
+  explicit CodeOffset(size_t offset) : offset_(offset) {}
+  CodeOffset() : offset_(NOT_BOUND) {}
+
+  size_t offset() const {
+    MOZ_ASSERT(bound());
+    return offset_;
+  }
+
+  void bind(size_t offset) {
+    MOZ_ASSERT(!bound());
+    offset_ = offset;
+    MOZ_ASSERT(bound());
+  }
+  bool bound() const { return offset_ != NOT_BOUND; }
+
+  void offsetBy(size_t delta) {
+    MOZ_ASSERT(bound());
+    MOZ_ASSERT(offset_ + delta >= offset_, "no overflow");
+    offset_ += delta;
+  }
+};
+
+// A code label contains an absolute reference to a point in the code. Thus, it
+// cannot be patched until after linking.
+// When the source label is resolved into a memory address, this address is
+// patched into the destination address.
+// Some need to distinguish between multiple ways of patching that address.
+// See JS_CODELABEL_LINKMODE.
+class CodeLabel {
+  // The destination position, where the absolute reference should get
+  // patched into.
+  CodeOffset patchAt_;
+
+  // The source label (relative) in the code to where the destination should
+  // get patched to.
+  CodeOffset target_;
+
+#ifdef JS_CODELABEL_LINKMODE
+ public:
+  enum LinkMode { Uninitialized = 0, RawPointer, MoveImmediate, JumpImmediate };
+
+ private:
+  LinkMode linkMode_ = Uninitialized;
+#endif
+
+ public:
+  CodeLabel() = default;
+  explicit CodeLabel(const CodeOffset& patchAt) : patchAt_(patchAt) {}
+  CodeLabel(const CodeOffset& patchAt, const CodeOffset& target)
+      : patchAt_(patchAt), target_(target) {}
+  CodeOffset* patchAt() { return &patchAt_; }
+  CodeOffset* target() { return &target_; }
+  CodeOffset patchAt() const { return patchAt_; }
+  CodeOffset target() const { return target_; }
+#ifdef JS_CODELABEL_LINKMODE
+  LinkMode linkMode() const { return linkMode_; }
+  void setLinkMode(LinkMode value) { linkMode_ = value; }
+#endif
+};
+
+typedef Vector<CodeLabel, 0, SystemAllocPolicy> CodeLabelVector;
+
+class CodeLocationLabel {
+  uint8_t* raw_ = nullptr;
+
+ public:
+  CodeLocationLabel(JitCode* code, CodeOffset base) {
+    MOZ_ASSERT(base.offset() < code->instructionsSize());
+    raw_ = code->raw() + base.offset();
+  }
+  explicit CodeLocationLabel(JitCode* code) { raw_ = code->raw(); }
+  explicit CodeLocationLabel(uint8_t* raw) {
+    MOZ_ASSERT(raw);
+    raw_ = raw;
+  }
+
+  ptrdiff_t operator-(const CodeLocationLabel& other) {
+    return raw_ - other.raw_;
+  }
+
+  uint8_t* raw() const { return raw_; }
+};
+
+}  // namespace jit
+
+namespace wasm {
+
+// Represents an instruction to be patched and the intended pointee. These
+// links are accumulated in the MacroAssembler, but patching is done outside
+// the MacroAssembler (in Module::staticallyLink).
+
+struct SymbolicAccess {
+  SymbolicAccess(jit::CodeOffset patchAt, SymbolicAddress target)
+      : patchAt(patchAt), target(target) {}
+
+  jit::CodeOffset patchAt;
+  SymbolicAddress target;
+};
+
+typedef Vector<SymbolicAccess, 0, SystemAllocPolicy> SymbolicAccessVector;
+
+// Describes a single wasm or asm.js memory access for the purpose of generating
+// code and metadata.
+
+class MemoryAccessDesc {
+  uint32_t offset_;
+  uint32_t align_;
+  Scalar::Type type_;
+  jit::Synchronization sync_;
+  wasm::BytecodeOffset trapOffset_;
+  wasm::SimdOp widenOp_;
+  enum { Plain, ZeroExtend, Splat, Widen } loadOp_;
+
+ public:
+  explicit MemoryAccessDesc(
+      Scalar::Type type, uint32_t align, uint32_t offset,
+      BytecodeOffset trapOffset,
+      const jit::Synchronization& sync = jit::Synchronization::None())
+      : offset_(offset),
+        align_(align),
+        type_(type),
+        sync_(sync),
+        trapOffset_(trapOffset),
+        widenOp_(wasm::SimdOp::Limit),
+        loadOp_(Plain) {
+    MOZ_ASSERT(mozilla::IsPowerOfTwo(align));
+  }
+
+  uint32_t offset() const { return offset_; }
+  uint32_t align() const { return align_; }
+  Scalar::Type type() const { return type_; }
+  unsigned byteSize() const { return Scalar::byteSize(type()); }
+  const jit::Synchronization& sync() const { return sync_; }
+  BytecodeOffset trapOffset() const { return trapOffset_; }
+  wasm::SimdOp widenSimdOp() const {
+    MOZ_ASSERT(isWidenSimd128Load());
+    return widenOp_;
+  }
+  bool isAtomic() const { return !sync_.isNone(); }
+  bool isZeroExtendSimd128Load() const { return loadOp_ == ZeroExtend; }
+  bool isSplatSimd128Load() const { return loadOp_ == Splat; }
+  bool isWidenSimd128Load() const { return loadOp_ == Widen; }
+
+  void setZeroExtendSimd128Load() {
+    MOZ_ASSERT(type() == Scalar::Float32 || type() == Scalar::Float64);
+    MOZ_ASSERT(!isAtomic());
+    MOZ_ASSERT(loadOp_ == Plain);
+    loadOp_ = ZeroExtend;
+  }
+
+  void setSplatSimd128Load() {
+    MOZ_ASSERT(type() == Scalar::Float64);
+    MOZ_ASSERT(!isAtomic());
+    MOZ_ASSERT(loadOp_ == Plain);
+    loadOp_ = Splat;
+  }
+
+  void setWidenSimd128Load(wasm::SimdOp op) {
+    MOZ_ASSERT(type() == Scalar::Float64);
+    MOZ_ASSERT(!isAtomic());
+    MOZ_ASSERT(loadOp_ == Plain);
+    widenOp_ = op;
+    loadOp_ = Widen;
+  }
+
+  void clearOffset() { offset_ = 0; }
+  void setOffset(uint32_t offset) { offset_ = offset; }
+};
+
+}  // namespace wasm
+
+namespace jit {
+
+// The base class of all Assemblers for all archs.
+class AssemblerShared {
+  wasm::CallSiteVector callSites_;
+  wasm::CallSiteTargetVector callSiteTargets_;
+  wasm::TrapSiteVectorArray trapSites_;
+  wasm::SymbolicAccessVector symbolicAccesses_;
+
+ protected:
+  CodeLabelVector codeLabels_;
+
+  bool enoughMemory_;
+  bool embedsNurseryPointers_;
+
+ public:
+  AssemblerShared() : enoughMemory_(true), embedsNurseryPointers_(false) {}
+
+  void propagateOOM(bool success) { enoughMemory_ &= success; }
+
+  void setOOM() { enoughMemory_ = false; }
+
+  bool oom() const { return !enoughMemory_; }
+
+  bool embedsNurseryPointers() const { return embedsNurseryPointers_; }
+
+  void addCodeLabel(CodeLabel label) {
+    propagateOOM(codeLabels_.append(label));
+  }
+  size_t numCodeLabels() const { return codeLabels_.length(); }
+  CodeLabel codeLabel(size_t i) { return codeLabels_[i]; }
+  CodeLabelVector& codeLabels() { return codeLabels_; }
+
+  // WebAssembly metadata emitted by masm operations accumulated on the
+  // MacroAssembler, and swapped into a wasm::CompiledCode after finish().
+
+  template <typename... Args>
+  void append(const wasm::CallSiteDesc& desc, CodeOffset retAddr,
+              Args&&... args) {
+    enoughMemory_ &= callSites_.emplaceBack(desc, retAddr.offset());
+    enoughMemory_ &= callSiteTargets_.emplaceBack(std::forward<Args>(args)...);
+  }
+  void append(wasm::Trap trap, wasm::TrapSite site) {
+    enoughMemory_ &= trapSites_[trap].append(site);
+  }
+  void append(const wasm::MemoryAccessDesc& access, uint32_t pcOffset) {
+    appendOutOfBoundsTrap(access.trapOffset(), pcOffset);
+  }
+  void appendOutOfBoundsTrap(wasm::BytecodeOffset trapOffset,
+                             uint32_t pcOffset) {
+    append(wasm::Trap::OutOfBounds, wasm::TrapSite(pcOffset, trapOffset));
+  }
+  void append(wasm::SymbolicAccess access) {
+    enoughMemory_ &= symbolicAccesses_.append(access);
+  }
+
+  wasm::CallSiteVector& callSites() { return callSites_; }
+  wasm::CallSiteTargetVector& callSiteTargets() { return callSiteTargets_; }
+  wasm::TrapSiteVectorArray& trapSites() { return trapSites_; }
+  wasm::SymbolicAccessVector& symbolicAccesses() { return symbolicAccesses_; }
+};
+
+}  // namespace jit
+}  // namespace js
+
+#endif /* jit_shared_Assembler_shared_h */