--- a/js/src/jit/arm64/MacroAssembler-arm64.cpp
+++ b/js/src/jit/arm64/MacroAssembler-arm64.cpp
@@ -14,16 +14,31 @@
 #include "util/Memory.h"
 #include "vm/JitActivation.h"  // js::jit::JitActivation
 
 #include "jit/MacroAssembler-inl.h"
 
 namespace js {
 namespace jit {
 
+void MacroAssemblerCompat::boxValue(JSValueType type, Register src,
+                                    Register dest) {
+#ifdef DEBUG
+  if (type == JSVAL_TYPE_INT32 || type == JSVAL_TYPE_BOOLEAN) {
+    Label upper32BitsZeroed;
+    movePtr(ImmWord(UINT32_MAX), dest);
+    asMasm().branchPtr(Assembler::BelowOrEqual, src, dest, &upper32BitsZeroed);
+    breakpoint();
+    bind(&upper32BitsZeroed);
+  }
+#endif
+  Orr(ARMRegister(dest, 64), ARMRegister(src, 64),
+      Operand(ImmShiftedTag(type).value));
+}
+
 void MacroAssembler::clampDoubleToUint8(FloatRegister input, Register output) {
   ARMRegister dest(output, 32);
   Fcvtns(dest, ARMFPRegister(input, 64));
 
   {
     vixl::UseScratchRegisterScope temps(this);
     const ARMRegister scratch32 = temps.AcquireW();
 

--- a/js/src/jit/arm64/MacroAssembler-arm64.h
+++ b/js/src/jit/arm64/MacroAssembler-arm64.h
@@ -388,20 +388,18 @@ class MacroAssemblerCompat : public vixl
     BufferOffset off = immPool64(ARMRegister(dest, 64), imm.value);
     return CodeOffset(off.getOffset());
   }
   CodeOffset movWithPatch(ImmPtr imm, Register dest) {
     BufferOffset off = immPool64(ARMRegister(dest, 64), uint64_t(imm.value));
     return CodeOffset(off.getOffset());
   }
 
-  void boxValue(JSValueType type, Register src, Register dest) {
-    Orr(ARMRegister(dest, 64), ARMRegister(src, 64),
-        Operand(ImmShiftedTag(type).value));
-  }
+  void boxValue(JSValueType type, Register src, Register dest);
+
   void splitSignExtTag(Register src, Register dest) {
     sbfx(ARMRegister(dest, 64), ARMRegister(src, 64), JSVAL_TAG_SHIFT,
          (64 - JSVAL_TAG_SHIFT));
   }
   MOZ_MUST_USE Register extractTag(const Address& address, Register scratch) {
     loadPtr(address, scratch);
     splitSignExtTag(scratch, scratch);
     return scratch;