diff --git a/js/src/gc/Barrier.cpp b/js/src/gc/Barrier.cpp
index 5e5f34b12a2a..dad97b85bd40 100644
--- a/js/src/gc/Barrier.cpp
+++ b/js/src/gc/Barrier.cpp
@@ -18,6 +18,8 @@
 #include "vm/SymbolType.h"
 #include "wasm/WasmJS.h"
 
+#include "gc/Zone-inl.h"
+
 namespace js {
 
 bool
diff --git a/js/src/gc/GC.cpp b/js/src/gc/GC.cpp
index 03eed7f633c7..6dcdccfcccbd 100644
--- a/js/src/gc/GC.cpp
+++ b/js/src/gc/GC.cpp
@@ -251,6 +251,7 @@
 #include "gc/Marking-inl.h"
 #include "gc/Nursery-inl.h"
 #include "gc/PrivateIterators-inl.h"
+#include "gc/Zone-inl.h"
 #include "vm/GeckoProfiler-inl.h"
 #include "vm/JSObject-inl.h"
 #include "vm/JSScript-inl.h"
diff --git a/js/src/gc/Marking.cpp b/js/src/gc/Marking.cpp
index 0bf06defc8a7..b5c18f5ccba4 100644
--- a/js/src/gc/Marking.cpp
+++ b/js/src/gc/Marking.cpp
@@ -37,6 +37,7 @@
 #include "gc/GC-inl.h"
 #include "gc/Nursery-inl.h"
 #include "gc/PrivateIterators-inl.h"
+#include "gc/Zone-inl.h"
 #include "vm/NativeObject-inl.h"
 #include "vm/Realm-inl.h"
 #include "vm/StringType-inl.h"
diff --git a/js/src/gc/Nursery.cpp b/js/src/gc/Nursery.cpp
index e1890b98f2f3..277f4721f262 100644
--- a/js/src/gc/Nursery.cpp
+++ b/js/src/gc/Nursery.cpp
@@ -32,6 +32,7 @@
 #include "vm/TypeInference.h"
 
 #include "gc/Marking-inl.h"
+#include "gc/Zone-inl.h"
 #include "vm/NativeObject-inl.h"
 
 using namespace js;
diff --git a/js/src/gc/Zone-inl.h b/js/src/gc/Zone-inl.h
new file mode 100644
index 000000000000..4e9d93bb75cd
--- /dev/null
+++ b/js/src/gc/Zone-inl.h
@@ -0,0 +1,161 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * 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 gc_Zone_inl_h
+#define gc_Zone_inl_h
+
+#include "gc/Zone.h"
+
+#ifdef DEBUG
+inline bool
+JS::Zone::requireGCTracer() const
+{
+    JSRuntime* rt = runtimeFromAnyThread();
+    return RuntimeHeapIsMajorCollecting() && !rt->gc.isHeapCompacting() && gcState_ != NoGC;
+}
+#endif
+
+inline void
+JS::Zone::updateAllGCMallocCountersOnGCStart()
+{
+    gcMallocCounter.updateOnGCStart();
+    jitCodeCounter.updateOnGCStart();
+}
+
+inline void
+JS::Zone::updateAllGCMallocCountersOnGCEnd(const js::AutoLockGC& lock)
+{
+    auto& gc = runtimeFromAnyThread()->gc;
+    gcMallocCounter.updateOnGCEnd(gc.tunables, lock);
+    jitCodeCounter.updateOnGCEnd(gc.tunables, lock);
+}
+
+inline js::gc::TriggerKind
+JS::Zone::shouldTriggerGCForTooMuchMalloc()
+{
+    auto& gc = runtimeFromAnyThread()->gc;
+    return std::max(gcMallocCounter.shouldTriggerGC(gc.tunables),
+                    jitCodeCounter.shouldTriggerGC(gc.tunables));
+}
+
+
+/* static */ inline js::HashNumber
+JS::Zone::UniqueIdToHash(uint64_t uid)
+{
+    return mozilla::HashGeneric(uid);
+}
+
+inline bool
+JS::Zone::getHashCode(js::gc::Cell* cell, js::HashNumber* hashp)
+{
+    uint64_t uid;
+    if (!getOrCreateUniqueId(cell, &uid))
+        return false;
+    *hashp = UniqueIdToHash(uid);
+    return true;
+}
+
+inline bool
+JS::Zone::maybeGetUniqueId(js::gc::Cell* cell, uint64_t* uidp)
+{
+    MOZ_ASSERT(uidp);
+    MOZ_ASSERT(js::CurrentThreadCanAccessZone(this));
+
+    // Get an existing uid, if one has been set.
+    auto p = uniqueIds().lookup(cell);
+    if (p)
+        *uidp = p->value();
+
+    return p.found();
+}
+
+inline bool
+JS::Zone::getOrCreateUniqueId(js::gc::Cell* cell, uint64_t* uidp)
+{
+    MOZ_ASSERT(uidp);
+    MOZ_ASSERT(js::CurrentThreadCanAccessZone(this) || js::CurrentThreadIsPerformingGC());
+
+    // Get an existing uid, if one has been set.
+    auto p = uniqueIds().lookupForAdd(cell);
+    if (p) {
+        *uidp = p->value();
+        return true;
+    }
+
+    MOZ_ASSERT(js::CurrentThreadCanAccessZone(this));
+
+    // Set a new uid on the cell.
+    *uidp = js::gc::NextCellUniqueId(runtimeFromAnyThread());
+    if (!uniqueIds().add(p, cell, *uidp))
+        return false;
+
+    // If the cell was in the nursery, hopefully unlikely, then we need to
+    // tell the nursery about it so that it can sweep the uid if the thing
+    // does not get tenured.
+    if (IsInsideNursery(cell) &&
+        !runtimeFromMainThread()->gc.nursery().addedUniqueIdToCell(cell))
+    {
+        uniqueIds().remove(cell);
+        return false;
+    }
+
+    return true;
+}
+
+inline js::HashNumber
+JS::Zone::getHashCodeInfallible(js::gc::Cell* cell)
+{
+    return UniqueIdToHash(getUniqueIdInfallible(cell));
+}
+
+inline uint64_t
+JS::Zone::getUniqueIdInfallible(js::gc::Cell* cell)
+{
+    uint64_t uid;
+    js::AutoEnterOOMUnsafeRegion oomUnsafe;
+    if (!getOrCreateUniqueId(cell, &uid))
+        oomUnsafe.crash("failed to allocate uid");
+    return uid;
+}
+
+inline bool
+JS::Zone::hasUniqueId(js::gc::Cell* cell)
+{
+    MOZ_ASSERT(js::CurrentThreadCanAccessZone(this) || js::CurrentThreadIsPerformingGC());
+    return uniqueIds().has(cell);
+}
+
+inline void
+JS::Zone::transferUniqueId(js::gc::Cell* tgt, js::gc::Cell* src)
+{
+    MOZ_ASSERT(src != tgt);
+    MOZ_ASSERT(!IsInsideNursery(tgt));
+    MOZ_ASSERT(js::CurrentThreadCanAccessRuntime(runtimeFromMainThread()));
+    MOZ_ASSERT(js::CurrentThreadCanAccessZone(this));
+    MOZ_ASSERT(!uniqueIds().has(tgt));
+    uniqueIds().rekeyIfMoved(src, tgt);
+}
+
+inline void
+JS::Zone::removeUniqueId(js::gc::Cell* cell)
+{
+    MOZ_ASSERT(js::CurrentThreadCanAccessZone(this));
+    uniqueIds().remove(cell);
+}
+
+inline void
+JS::Zone::adoptUniqueIds(JS::Zone* source)
+{
+    js::AutoEnterOOMUnsafeRegion oomUnsafe;
+    for (js::gc::UniqueIdMap::Enum e(source->uniqueIds()); !e.empty(); e.popFront()) {
+        MOZ_ASSERT(!uniqueIds().has(e.front().key()));
+        if (!uniqueIds().put(e.front().key(), e.front().value()))
+            oomUnsafe.crash("failed to transfer unique ids from off-thread");
+    }
+    source->uniqueIds().clear();
+}
+
+#endif // gc_Zone_inl_h
diff --git a/js/src/gc/Zone.cpp b/js/src/gc/Zone.cpp
index 725f73f0d09d..c79423e210d5 100644
--- a/js/src/gc/Zone.cpp
+++ b/js/src/gc/Zone.cpp
@@ -4,7 +4,7 @@
  * 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/. */
 
-#include "gc/Zone.h"
+#include "gc/Zone-inl.h"
 
 #include "gc/FreeOp.h"
 #include "gc/Policy.h"
@@ -465,6 +465,41 @@ Zone::traceAtomCache(JSTracer* trc)
     }
 }
 
+void*
+Zone::onOutOfMemory(js::AllocFunction allocFunc, size_t nbytes, void* reallocPtr)
+{
+    if (!js::CurrentThreadCanAccessRuntime(runtime_))
+        return nullptr;
+    return runtimeFromMainThread()->onOutOfMemory(allocFunc, nbytes, reallocPtr);
+}
+
+void
+Zone::reportAllocationOverflow()
+{
+    js::ReportAllocationOverflow(nullptr);
+}
+
+void
+JS::Zone::maybeTriggerGCForTooMuchMalloc(js::gc::MemoryCounter& counter, TriggerKind trigger)
+{
+    JSRuntime* rt = runtimeFromAnyThread();
+
+    if (!js::CurrentThreadCanAccessRuntime(rt))
+        return;
+
+    bool wouldInterruptGC = rt->gc.isIncrementalGCInProgress() && !isCollecting();
+    if (wouldInterruptGC && !counter.shouldResetIncrementalGC(rt->gc.tunables))
+        return;
+
+    if (!rt->gc.triggerZoneGC(this, JS::gcreason::TOO_MUCH_MALLOC,
+                              counter.bytes(), counter.maxBytes()))
+    {
+        return;
+    }
+
+    counter.recordTrigger(trigger);
+}
+
 ZoneList::ZoneList()
   : head(nullptr), tail(nullptr)
 {}
diff --git a/js/src/gc/Zone.h b/js/src/gc/Zone.h
index 61f03330934b..1a1b5bec9228 100644
--- a/js/src/gc/Zone.h
+++ b/js/src/gc/Zone.h
@@ -11,7 +11,6 @@
 #include "mozilla/HashFunctions.h"
 
 #include "gc/FindSCCs.h"
-#include "gc/GCRuntime.h"
 #include "js/GCHashTable.h"
 #include "vm/MallocProvider.h"
 #include "vm/Runtime.h"
@@ -218,12 +217,8 @@ class Zone : public JS::shadow::Zone,
     }
 
     MOZ_MUST_USE void* onOutOfMemory(js::AllocFunction allocFunc, size_t nbytes,
-                                     void* reallocPtr = nullptr) {
-        if (!js::CurrentThreadCanAccessRuntime(runtime_))
-            return nullptr;
-        return runtimeFromMainThread()->onOutOfMemory(allocFunc, nbytes, reallocPtr);
-    }
-    void reportAllocationOverflow() { js::ReportAllocationOverflow(nullptr); }
+                                     void* reallocPtr = nullptr);
+    void reportAllocationOverflow();
 
     void beginSweepTypes(bool releaseTypes);
 
@@ -259,13 +254,6 @@ class Zone : public JS::shadow::Zone,
             return needsIncrementalBarrier();
     }
 
-    // If this returns true, all object tracing must be done with a GC marking
-    // tracer.
-    bool requireGCTracer() const {
-        JSRuntime* rt = runtimeFromAnyThread();
-        return RuntimeHeapIsMajorCollecting() && !rt->gc.isHeapCompacting() && gcState_ != NoGC;
-    }
-
     bool shouldMarkInZone() const {
         return needsIncrementalBarrier() || isGCMarking();
     }
@@ -274,12 +262,6 @@ class Zone : public JS::shadow::Zone,
     // possibly at other times too.
     uint64_t gcNumber();
 
-    bool compileBarriers() const { return compileBarriers(needsIncrementalBarrier()); }
-    bool compileBarriers(bool needsIncrementalBarrier) const {
-        return needsIncrementalBarrier ||
-               runtimeFromMainThread()->hasZealMode(js::gc::ZealMode::VerifierPre);
-    }
-
     void setNeedsIncrementalBarrier(bool needs);
     const uint32_t* addressOfNeedsIncrementalBarrier() const { return &needsIncrementalBarrier_; }
 
@@ -292,6 +274,10 @@ class Zone : public JS::shadow::Zone,
     void prepareForCompacting();
 
 #ifdef DEBUG
+    // If this returns true, all object tracing must be done with a GC marking
+    // tracer.
+    bool requireGCTracer() const;
+
     // For testing purposes, return the index of the sweep group which this zone
     // was swept in in the last GC.
     unsigned lastSweepGroupIndex() { return gcLastSweepGroupIndex; }
@@ -443,22 +429,12 @@ class Zone : public JS::shadow::Zone,
         if (MOZ_LIKELY(trigger == js::gc::NoTrigger) || trigger <= counter.triggered())
             return;
 
-        if (!js::CurrentThreadCanAccessRuntime(rt))
-            return;
-
-        bool wouldInterruptGC = rt->gc.isIncrementalGCInProgress() && !isCollecting();
-        if (wouldInterruptGC && !counter.shouldResetIncrementalGC(rt->gc.tunables))
-            return;
-
-        if (!rt->gc.triggerZoneGC(this, JS::gcreason::TOO_MUCH_MALLOC,
-                                  counter.bytes(), counter.maxBytes()))
-        {
-            return;
-        }
-
-        counter.recordTrigger(trigger);
+        maybeTriggerGCForTooMuchMalloc(counter, trigger);
     }
 
+    void maybeTriggerGCForTooMuchMalloc(js::gc::MemoryCounter& counter,
+                                        js::gc::TriggerKind trigger);
+
     js::MainThreadData<js::UniquePtr<js::RegExpZone>> regExps_;
 
   public:
@@ -484,20 +460,9 @@ class Zone : public JS::shadow::Zone,
         updateMemoryCounter(jitCodeCounter, nbytes);
     }
 
-    void updateAllGCMallocCountersOnGCStart() {
-        gcMallocCounter.updateOnGCStart();
-        jitCodeCounter.updateOnGCStart();
-    }
-    void updateAllGCMallocCountersOnGCEnd(const js::AutoLockGC& lock) {
-        auto& gc = runtimeFromAnyThread()->gc;
-        gcMallocCounter.updateOnGCEnd(gc.tunables, lock);
-        jitCodeCounter.updateOnGCEnd(gc.tunables, lock);
-    }
-    js::gc::TriggerKind shouldTriggerGCForTooMuchMalloc() {
-        auto& gc = runtimeFromAnyThread()->gc;
-        return std::max(gcMallocCounter.shouldTriggerGC(gc.tunables),
-                        jitCodeCounter.shouldTriggerGC(gc.tunables));
-    }
+    void updateAllGCMallocCountersOnGCStart();
+    void updateAllGCMallocCountersOnGCEnd(const js::AutoLockGC& lock);
+    js::gc::TriggerKind shouldTriggerGCForTooMuchMalloc();
 
     void keepAtoms() {
         keepAtomsCount++;
@@ -606,111 +571,34 @@ class Zone : public JS::shadow::Zone,
     js::MainThreadData<unsigned> gcLastSweepGroupIndex;
 #endif
 
-    static js::HashNumber UniqueIdToHash(uint64_t uid) {
-        return mozilla::HashGeneric(uid);
-    }
+    static js::HashNumber UniqueIdToHash(uint64_t uid);
 
     // Creates a HashNumber based on getUniqueId. Returns false on OOM.
-    MOZ_MUST_USE bool getHashCode(js::gc::Cell* cell, js::HashNumber* hashp) {
-        uint64_t uid;
-        if (!getOrCreateUniqueId(cell, &uid))
-            return false;
-        *hashp = UniqueIdToHash(uid);
-        return true;
-    }
+    MOZ_MUST_USE bool getHashCode(js::gc::Cell* cell, js::HashNumber* hashp);
 
     // Gets an existing UID in |uidp| if one exists.
-    MOZ_MUST_USE bool maybeGetUniqueId(js::gc::Cell* cell, uint64_t* uidp) {
-        MOZ_ASSERT(uidp);
-        MOZ_ASSERT(js::CurrentThreadCanAccessZone(this));
-
-        // Get an existing uid, if one has been set.
-        auto p = uniqueIds().lookup(cell);
-        if (p)
-            *uidp = p->value();
-
-        return p.found();
-    }
+    MOZ_MUST_USE bool maybeGetUniqueId(js::gc::Cell* cell, uint64_t* uidp);
 
     // Puts an existing UID in |uidp|, or creates a new UID for this Cell and
     // puts that into |uidp|. Returns false on OOM.
-    MOZ_MUST_USE bool getOrCreateUniqueId(js::gc::Cell* cell, uint64_t* uidp) {
-        MOZ_ASSERT(uidp);
-        MOZ_ASSERT(js::CurrentThreadCanAccessZone(this) || js::CurrentThreadIsPerformingGC());
+    MOZ_MUST_USE bool getOrCreateUniqueId(js::gc::Cell* cell, uint64_t* uidp);
 
-        // Get an existing uid, if one has been set.
-        auto p = uniqueIds().lookupForAdd(cell);
-        if (p) {
-            *uidp = p->value();
-            return true;
-        }
-
-        MOZ_ASSERT(js::CurrentThreadCanAccessZone(this));
-
-        // Set a new uid on the cell.
-        *uidp = js::gc::NextCellUniqueId(runtimeFromAnyThread());
-        if (!uniqueIds().add(p, cell, *uidp))
-            return false;
-
-        // If the cell was in the nursery, hopefully unlikely, then we need to
-        // tell the nursery about it so that it can sweep the uid if the thing
-        // does not get tenured.
-        if (IsInsideNursery(cell) &&
-            !runtimeFromMainThread()->gc.nursery().addedUniqueIdToCell(cell))
-        {
-            uniqueIds().remove(cell);
-            return false;
-        }
-
-        return true;
-    }
-
-    js::HashNumber getHashCodeInfallible(js::gc::Cell* cell) {
-        return UniqueIdToHash(getUniqueIdInfallible(cell));
-    }
-
-    uint64_t getUniqueIdInfallible(js::gc::Cell* cell) {
-        uint64_t uid;
-        js::AutoEnterOOMUnsafeRegion oomUnsafe;
-        if (!getOrCreateUniqueId(cell, &uid))
-            oomUnsafe.crash("failed to allocate uid");
-        return uid;
-    }
+    js::HashNumber getHashCodeInfallible(js::gc::Cell* cell);
+    uint64_t getUniqueIdInfallible(js::gc::Cell* cell);
 
     // Return true if this cell has a UID associated with it.
-    MOZ_MUST_USE bool hasUniqueId(js::gc::Cell* cell) {
-        MOZ_ASSERT(js::CurrentThreadCanAccessZone(this) || js::CurrentThreadIsPerformingGC());
-        return uniqueIds().has(cell);
-    }
+    MOZ_MUST_USE bool hasUniqueId(js::gc::Cell* cell);
 
     // Transfer an id from another cell. This must only be called on behalf of a
     // moving GC. This method is infallible.
-    void transferUniqueId(js::gc::Cell* tgt, js::gc::Cell* src) {
-        MOZ_ASSERT(src != tgt);
-        MOZ_ASSERT(!IsInsideNursery(tgt));
-        MOZ_ASSERT(js::CurrentThreadCanAccessRuntime(runtimeFromMainThread()));
-        MOZ_ASSERT(js::CurrentThreadCanAccessZone(this));
-        MOZ_ASSERT(!uniqueIds().has(tgt));
-        uniqueIds().rekeyIfMoved(src, tgt);
-    }
+    void transferUniqueId(js::gc::Cell* tgt, js::gc::Cell* src);
 
     // Remove any unique id associated with this Cell.
-    void removeUniqueId(js::gc::Cell* cell) {
-        MOZ_ASSERT(js::CurrentThreadCanAccessZone(this));
-        uniqueIds().remove(cell);
-    }
+    void removeUniqueId(js::gc::Cell* cell);
 
     // When finished parsing off-thread, transfer any UIDs we created in the
     // off-thread zone into the target zone.
-    void adoptUniqueIds(JS::Zone* source) {
-        js::AutoEnterOOMUnsafeRegion oomUnsafe;
-        for (js::gc::UniqueIdMap::Enum e(source->uniqueIds()); !e.empty(); e.popFront()) {
-            MOZ_ASSERT(!uniqueIds().has(e.front().key()));
-            if (!uniqueIds().put(e.front().key(), e.front().value()))
-                oomUnsafe.crash("failed to transfer unique ids from off-thread");
-        }
-        source->uniqueIds().clear();
-    }
+    void adoptUniqueIds(JS::Zone* source);
 
 #ifdef JSGC_HASH_TABLE_CHECKS
     // Assert that the UniqueId table has been redirected successfully.
diff --git a/js/src/jsapi-tests/testGCUniqueId.cpp b/js/src/jsapi-tests/testGCUniqueId.cpp
index 9e78a56a900d..945c4c7a6bc1 100644
--- a/js/src/jsapi-tests/testGCUniqueId.cpp
+++ b/js/src/jsapi-tests/testGCUniqueId.cpp
@@ -11,6 +11,8 @@
 
 #include "jsapi-tests/tests.h"
 
+#include "gc/Zone-inl.h"
+
 static void
 MinimizeHeap(JSContext* cx)
 {