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diff --git a/js/src/jit/shared/Disassembler-shared.h b/js/src/jit/shared/Disassembler-shared.h
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+++ b/js/src/jit/shared/Disassembler-shared.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_shared_Disassembler_shared_h
+#define jit_shared_Disassembler_shared_h
+
+#include "mozilla/Atomics.h"
+#include "mozilla/Attributes.h"
+
+#include <stdarg.h>
+#include <stddef.h>
+#include <stdint.h>
+
+#if defined(JS_DISASM_ARM) || defined(JS_DISASM_ARM64)
+#  define JS_DISASM_SUPPORTED
+#endif
+
+namespace js {
+
+class Sprinter;
+
+namespace jit {
+
+class Label;
+
+// A wrapper around spew/disassembly functionality.  The disassembler is built
+// on a per-instruction disassembler (as in our ARM, ARM64 back-ends) and
+// formats labels with meaningful names and literals with meaningful values, if
+// the assembler creates documentation (with provided helpers) at appropriate
+// points.
+
+class DisassemblerSpew {
+#ifdef JS_DISASM_SUPPORTED
+  struct Node {
+    const Label* key;  // Never dereferenced, only used for its value
+    uint32_t value;    // The printable label value
+    bool bound;        // If the label has been seen by spewBind()
+    Node* next;
+  };
+
+  Node* lookup(const Label* key);
+  Node* add(const Label* key, uint32_t value);
+  bool remove(const Label* key);
+
+  uint32_t probe(const Label* l);
+  uint32_t define(const Label* l);
+  uint32_t internalResolve(const Label* l);
+#endif
+
+  void spewVA(const char* fmt, va_list args) MOZ_FORMAT_PRINTF(2, 0);
+
+ public:
+  DisassemblerSpew();
+  ~DisassemblerSpew();
+
+#ifdef JS_DISASM_SUPPORTED
+  // Set indentation strings.  The spewer retains a reference to s.
+  void setLabelIndent(const char* s);
+  void setTargetIndent(const char* s);
+#endif
+
+  // Set the spew printer, which will always be used if it is set, regardless
+  // of whether the system spew channel is enabled or not.  The spewer retains
+  // a reference to sp.
+  void setPrinter(Sprinter* sp);
+
+  // Return true if disassembly spew is disabled and no additional printer is
+  // set.
+  bool isDisabled();
+
+  // Format and print text on the spew channel; output is suppressed if spew
+  // is disabled.  The output is not indented, and is terminated by a newline.
+  void spew(const char* fmt, ...) MOZ_FORMAT_PRINTF(2, 3);
+
+  // Documentation for a label reference.
+  struct LabelDoc {
+#ifdef JS_DISASM_SUPPORTED
+    LabelDoc() : doc(0), bound(false), valid(false) {}
+    LabelDoc(uint32_t doc, bool bound) : doc(doc), bound(bound), valid(true) {}
+    const uint32_t doc;
+    const bool bound;
+    const bool valid;
+#else
+    LabelDoc() = default;
+    LabelDoc(uint32_t, bool) {}
+#endif
+  };
+
+  // Documentation for a literal load.
+  struct LiteralDoc {
+#ifdef JS_DISASM_SUPPORTED
+    enum class Type { Patchable, I32, U32, I64, U64, F32, F64 };
+    const Type type;
+    union {
+      int32_t i32;
+      uint32_t u32;
+      int64_t i64;
+      uint64_t u64;
+      float f32;
+      double f64;
+    } value;
+    LiteralDoc() : type(Type::Patchable) {}
+    explicit LiteralDoc(int32_t v) : type(Type::I32) { value.i32 = v; }
+    explicit LiteralDoc(uint32_t v) : type(Type::U32) { value.u32 = v; }
+    explicit LiteralDoc(int64_t v) : type(Type::I64) { value.i64 = v; }
+    explicit LiteralDoc(uint64_t v) : type(Type::U64) { value.u64 = v; }
+    explicit LiteralDoc(float v) : type(Type::F32) { value.f32 = v; }
+    explicit LiteralDoc(double v) : type(Type::F64) { value.f64 = v; }
+#else
+    LiteralDoc() = default;
+    explicit LiteralDoc(int32_t) {}
+    explicit LiteralDoc(uint32_t) {}
+    explicit LiteralDoc(int64_t) {}
+    explicit LiteralDoc(uint64_t) {}
+    explicit LiteralDoc(float) {}
+    explicit LiteralDoc(double) {}
+#endif
+  };
+
+  // Reference a label, resolving it to a printable representation.
+  //
+  // NOTE: The printable representation depends on the state of the label, so
+  // if we call resolve() when emitting & disassembling a branch instruction
+  // then it should be called before the label becomes Used, if emitting the
+  // branch can change the label's state.
+  //
+  // If the disassembler is not defined this returns a structure that is
+  // marked not valid.
+  LabelDoc refLabel(const Label* l);
+
+#ifdef JS_DISASM_SUPPORTED
+  // Spew the label information previously gathered by refLabel(), at a point
+  // where the label is referenced.  The output is indented by targetIndent_
+  // and terminated by a newline.
+  void spewRef(const LabelDoc& target);
+
+  // Spew the label at the point where the label is bound.  The output is
+  // indented by labelIndent_ and terminated by a newline.
+  void spewBind(const Label* label);
+
+  // Spew a retarget directive at the point where the retarget is recorded.
+  // The output is indented by labelIndent_ and terminated by a newline.
+  void spewRetarget(const Label* label, const Label* target);
+
+  // Format a literal value into the buffer.  The buffer is always
+  // NUL-terminated even if this chops the formatted value.
+  void formatLiteral(const LiteralDoc& doc, char* buffer, size_t bufsize);
+
+  // Print any unbound labels, one per line, with normal label indent and with
+  // a comment indicating the label is not defined.  Labels can be referenced
+  // but unbound in some legitimate cases, normally for traps.  Printing them
+  // reduces confusion.
+  void spewOrphans();
+#endif
+
+ private:
+  Sprinter* printer_;
+#ifdef JS_DISASM_SUPPORTED
+  const char* labelIndent_;
+  const char* targetIndent_;
+  uint32_t spewNext_;
+  Node* nodes_;
+  uint32_t tag_;
+
+  // This global is used to disambiguate concurrently live assemblers, see
+  // comments in Disassembler-shared.cpp for why this is desirable.
+  //
+  // The variable is atomic to avoid any kind of complaint from thread
+  // sanitizers etc.  However, trying to look at disassembly without using
+  // --no-threads is basically insane, so you can ignore the multi-threading
+  // implications here.
+  static mozilla::Atomic<uint32_t> counter_;
+#endif
+};
+
+}  // namespace jit
+}  // namespace js
+
+#endif  // jit_shared_Disassembler_shared_h