diff --git a/js/src/jit/JitFrames.cpp b/js/src/jit/JitFrames.cpp
index addcd05e015..4d297f43270 100644
--- a/js/src/jit/JitFrames.cpp
+++ b/js/src/jit/JitFrames.cpp
@@ -1738,6 +1738,7 @@ bool SnapshotIterator::allocationReadable(const RValueAllocation& alloc,
     case RValueAllocation::INT64_REG:
       return hasRegister(alloc.reg());
     case RValueAllocation::INT64_STACK:
+    case RValueAllocation::INT64_INT32_STACK:
       return hasStack(alloc.stackOffset());
 #endif
 
@@ -1848,6 +1849,7 @@ Value SnapshotIterator::allocationValue(const RValueAllocation& alloc,
 #elif defined(JS_PUNBOX64)
     case RValueAllocation::INT64_REG:
     case RValueAllocation::INT64_STACK:
+    case RValueAllocation::INT64_INT32_STACK:
 #endif
       MOZ_CRASH("Can't read Int64 as Value");
 
@@ -1904,6 +1906,7 @@ bool SnapshotIterator::readMaybeUnpackedBigInt(JSContext* cx,
 #elif defined(JS_PUNBOX64)
     case RValueAllocation::INT64_REG:
     case RValueAllocation::INT64_STACK:
+    case RValueAllocation::INT64_INT32_STACK:
 #endif
     {
       auto* bigInt = JS::BigInt::createFromInt64(cx, allocationInt64(alloc));
@@ -1971,6 +1974,9 @@ int64_t SnapshotIterator::allocationInt64(const RValueAllocation& alloc) {
     case RValueAllocation::INT64_STACK: {
       return static_cast<int64_t>(fromStack(alloc.stackOffset()));
     }
+    case RValueAllocation::INT64_INT32_STACK: {
+      return static_cast<int64_t>(ReadFrameInt32Slot(fp_, alloc.stackOffset()));
+    }
 #endif
     default:
       break;
@@ -2044,6 +2050,7 @@ void SnapshotIterator::writeAllocationValuePayload(
 #elif defined(JS_PUNBOX64)
     case RValueAllocation::INT64_REG:
     case RValueAllocation::INT64_STACK:
+    case RValueAllocation::INT64_INT32_STACK:
 #endif
       MOZ_CRASH("Not a GC thing: Unexpected write");
       break;
diff --git a/js/src/jit/Snapshots.cpp b/js/src/jit/Snapshots.cpp
index 541efeb5eb6..41560a62080 100644
--- a/js/src/jit/Snapshots.cpp
+++ b/js/src/jit/Snapshots.cpp
@@ -154,6 +154,9 @@ using namespace js::jit;
 //           register/stack-offset correspond to the low 32-bits, and the
 //           second correspond to the high 32-bits.
 //
+//         INT64_INT32_STACK  [STACK_OFFSET]: (64-bit platform)
+//           Unpacked Int64 value stored in int32_t. Payload is stored at an
+//           offset on the stack.
 
 const RValueAllocation::Layout& RValueAllocation::layoutFromMode(Mode mode) {
   switch (mode) {
@@ -308,6 +311,12 @@ const RValueAllocation::Layout& RValueAllocation::layoutFromMode(Mode mode) {
           PAYLOAD_STACK_OFFSET, PAYLOAD_NONE, "unpacked int64"};
       return layout;
     }
+
+    case INT64_INT32_STACK: {
+      static const RValueAllocation::Layout layout = {
+          PAYLOAD_STACK_OFFSET, PAYLOAD_NONE, "unpacked int64 (int32)"};
+      return layout;
+    }
 #endif
 
     default: {
diff --git a/js/src/jit/Snapshots.h b/js/src/jit/Snapshots.h
index b7e08b4cf71..5fe49806b98 100644
--- a/js/src/jit/Snapshots.h
+++ b/js/src/jit/Snapshots.h
@@ -82,6 +82,7 @@ class RValueAllocation {
 #elif defined(JS_PUNBOX64)
     INT64_REG = 0x31,
     INT64_STACK = 0x32,
+    INT64_INT32_STACK = 0x33,
 #endif
 
     // This mask can be used with any other valid mode. When this flag is
@@ -348,6 +349,11 @@ class RValueAllocation {
   static RValueAllocation Int64(int32_t stackOffset) {
     return RValueAllocation(INT64_STACK, payloadOfStackOffset(stackOffset));
   }
+
+  static RValueAllocation Int64Int32(int32_t stackOffset) {
+    return RValueAllocation(INT64_INT32_STACK,
+                            payloadOfStackOffset(stackOffset));
+  }
 #endif
 
   void setNeedSideEffect() {
diff --git a/js/src/jit/shared/CodeGenerator-shared.cpp b/js/src/jit/shared/CodeGenerator-shared.cpp
index 64d5178194c..c31517c7da8 100644
--- a/js/src/jit/shared/CodeGenerator-shared.cpp
+++ b/js/src/jit/shared/CodeGenerator-shared.cpp
@@ -599,9 +599,14 @@ void CodeGeneratorShared::encodeAllocation(LSnapshot* snapshot,
       if (payload->isGeneralReg()) {
         alloc = RValueAllocation::Int64(ToRegister(payload));
       } else if (payload->isStackSlot()) {
-        MOZ_ASSERT(payload->toStackSlot()->width() ==
-                   LStackSlot::width(LDefinition::GENERAL));
-        alloc = RValueAllocation::Int64(ToStackIndex(payload));
+        LStackSlot::Width width = payload->toStackSlot()->width();
+        MOZ_ASSERT(width == LStackSlot::width(LDefinition::GENERAL) ||
+                   width == LStackSlot::width(LDefinition::INT32));
+        if (width == LStackSlot::width(LDefinition::GENERAL)) {
+          alloc = RValueAllocation::Int64(ToStackIndex(payload));
+        } else {
+          alloc = RValueAllocation::Int64Int32(ToStackIndex(payload));
+        }
       } else {
         MOZ_CRASH("Unexpected payload type.");
       }
diff --git a/js/src/jsapi-tests/testJitRValueAlloc.cpp b/js/src/jsapi-tests/testJitRValueAlloc.cpp
index 78e716e0ff0..9e114a7d982 100644
--- a/js/src/jsapi-tests/testJitRValueAlloc.cpp
+++ b/js/src/jsapi-tests/testJitRValueAlloc.cpp
@@ -382,3 +382,12 @@ BEGIN_TEST(testJitRValueAlloc_IntPtrInt32Stack) {
   return true;
 }
 END_TEST(testJitRValueAlloc_IntPtrInt32Stack)
+
+BEGIN_TEST(testJitRValueAlloc_Int64Int32Stack) {
+  for (auto i : Fibonacci{}) {
+    auto s = RValueAllocation::Int64Int32(i);
+    CHECK(s == Read(s));
+  }
+  return true;
+}
+END_TEST(testJitRValueAlloc_Int64Int32Stack)