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diff --git a/js/src/wasm/WasmDebugFrame.h b/js/src/wasm/WasmDebugFrame.h
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+++ b/js/src/wasm/WasmDebugFrame.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:
+ *
+ * Copyright 2021 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef wasm_debugframe_h
+#define wasm_debugframe_h
+
+#include "mozilla/Assertions.h"
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "wasm/WasmCodegenConstants.h"
+#include "wasm/WasmFrame.h"
+#include "wasm/WasmValType.h"
+#include "wasm/WasmValue.h"
+
+namespace js {
+
+class GlobalObject;
+
+namespace wasm {
+
+class Instance;
+
+// A DebugFrame is a Frame with additional fields that are added after the
+// normal function prologue by the baseline compiler. If a Module is compiled
+// with debugging enabled, then all its code creates DebugFrames on the stack
+// instead of just Frames. These extra fields are used by the Debugger API.
+
+class DebugFrame {
+  // The register results field.  Initialized only during the baseline
+  // compiler's return sequence to allow the debugger to inspect and
+  // modify the return values of a frame being debugged.
+  union SpilledRegisterResult {
+   private:
+    int32_t i32_;
+    int64_t i64_;
+    float f32_;
+    double f64_;
+#ifdef ENABLE_WASM_SIMD
+    V128 v128_;
+#endif
+    AnyRef anyref_;
+
+#ifdef DEBUG
+    // Should we add a new value representation, this will remind us to update
+    // SpilledRegisterResult.
+    static inline void assertAllValueTypesHandled(ValType type) {
+      switch (type.kind()) {
+        case ValType::I32:
+        case ValType::I64:
+        case ValType::F32:
+        case ValType::F64:
+        case ValType::V128:
+        case ValType::Ref:
+          return;
+      }
+    }
+#endif
+  };
+  SpilledRegisterResult registerResults_[MaxRegisterResults];
+
+  // The returnValue() method returns a HandleValue pointing to this field.
+  JS::Value cachedReturnJSValue_;
+
+  // If the function returns multiple results, this field is initialized
+  // to a pointer to the stack results.
+  void* stackResultsPointer_;
+
+  // The function index of this frame. Technically, this could be derived
+  // given a PC into this frame (which could lookup the CodeRange which has
+  // the function index), but this isn't always readily available.
+  uint32_t funcIndex_;
+
+  // Flags whose meaning are described below.
+  union Flags {
+    struct {
+      uint32_t observing : 1;
+      uint32_t isDebuggee : 1;
+      uint32_t prevUpToDate : 1;
+      uint32_t hasCachedSavedFrame : 1;
+      uint32_t hasCachedReturnJSValue : 1;
+      uint32_t hasSpilledRefRegisterResult : MaxRegisterResults;
+    };
+    uint32_t allFlags;
+  } flags_;
+
+  // Avoid -Wunused-private-field warnings.
+ protected:
+#if defined(JS_CODEGEN_ARM) || defined(JS_CODEGEN_X86) || defined(__wasi__)
+  // See alignmentStaticAsserts().  For ARM32 and X86 DebugFrame is only
+  // 4-byte aligned, so we add another word to get up to 8-byte
+  // alignment.
+  uint32_t padding_;
+#endif
+#if defined(ENABLE_WASM_SIMD) && defined(JS_CODEGEN_ARM64)
+  uint64_t padding_;
+#endif
+
+ private:
+  // The Frame goes at the end since the stack grows down.
+  Frame frame_;
+
+ public:
+  static DebugFrame* from(Frame* fp);
+  Frame& frame() { return frame_; }
+  uint32_t funcIndex() const { return funcIndex_; }
+  Instance* instance();
+  const Instance* instance() const;
+  GlobalObject* global();
+  bool hasGlobal(const GlobalObject* global) const;
+  JSObject* environmentChain();
+  bool getLocal(uint32_t localIndex, JS::MutableHandleValue vp);
+
+  // The return value must be written from the unboxed representation in the
+  // results union into cachedReturnJSValue_ by updateReturnJSValue() before
+  // returnValue() can return a Handle to it.
+
+  bool hasCachedReturnJSValue() const { return flags_.hasCachedReturnJSValue; }
+  [[nodiscard]] bool updateReturnJSValue(JSContext* cx);
+  void discardReturnJSValue();
+  JS::HandleValue returnValue() const;
+  void clearReturnJSValue();
+
+  // Once the debugger observes a frame, it must be notified via
+  // onLeaveFrame() before the frame is popped. Calling observe() ensures the
+  // leave frame traps are enabled. Both methods are idempotent so the caller
+  // doesn't have to worry about calling them more than once.
+
+  void observe(JSContext* cx);
+  void leave(JSContext* cx);
+
+  // The 'isDebugge' bit is initialized to false and set by the WebAssembly
+  // runtime right before a frame is exposed to the debugger, as required by
+  // the Debugger API. The bit is then used for Debugger-internal purposes
+  // afterwards.
+
+  bool isDebuggee() const { return flags_.isDebuggee; }
+  void setIsDebuggee() { flags_.isDebuggee = true; }
+  void unsetIsDebuggee() { flags_.isDebuggee = false; }
+
+  // These are opaque boolean flags used by the debugger to implement
+  // AbstractFramePtr. They are initialized to false and not otherwise read or
+  // written by wasm code or runtime.
+
+  bool prevUpToDate() const { return flags_.prevUpToDate; }
+  void setPrevUpToDate() { flags_.prevUpToDate = true; }
+  void unsetPrevUpToDate() { flags_.prevUpToDate = false; }
+
+  bool hasCachedSavedFrame() const { return flags_.hasCachedSavedFrame; }
+  void setHasCachedSavedFrame() { flags_.hasCachedSavedFrame = true; }
+  void clearHasCachedSavedFrame() { flags_.hasCachedSavedFrame = false; }
+
+  bool hasSpilledRegisterRefResult(size_t n) const {
+    uint32_t mask = hasSpilledRegisterRefResultBitMask(n);
+    return (flags_.allFlags & mask) != 0;
+  }
+
+  // DebugFrame is accessed directly by JIT code.
+
+  static constexpr size_t offsetOfRegisterResults() {
+    return offsetof(DebugFrame, registerResults_);
+  }
+  static constexpr size_t offsetOfRegisterResult(size_t n) {
+    MOZ_ASSERT(n < MaxRegisterResults);
+    return offsetOfRegisterResults() + n * sizeof(SpilledRegisterResult);
+  }
+  static constexpr size_t offsetOfCachedReturnJSValue() {
+    return offsetof(DebugFrame, cachedReturnJSValue_);
+  }
+  static constexpr size_t offsetOfStackResultsPointer() {
+    return offsetof(DebugFrame, stackResultsPointer_);
+  }
+  static constexpr size_t offsetOfFlags() {
+    return offsetof(DebugFrame, flags_);
+  }
+  static constexpr uint32_t hasSpilledRegisterRefResultBitMask(size_t n) {
+    MOZ_ASSERT(n < MaxRegisterResults);
+    union Flags flags = {.allFlags = 0};
+    flags.hasSpilledRefRegisterResult = 1 << n;
+    MOZ_ASSERT(flags.allFlags != 0);
+    return flags.allFlags;
+  }
+  static constexpr size_t offsetOfFuncIndex() {
+    return offsetof(DebugFrame, funcIndex_);
+  }
+  static constexpr size_t offsetOfFrame() {
+    return offsetof(DebugFrame, frame_);
+  }
+
+  // DebugFrames are aligned to 8-byte aligned, allowing them to be placed in
+  // an AbstractFramePtr.
+
+  static const unsigned Alignment = 8;
+  static void alignmentStaticAsserts();
+};
+
+}  // namespace wasm
+}  // namespace js
+
+#endif  // wasm_debugframe_h