--- a/js/src/jit/MIR.h
+++ b/js/src/jit/MIR.h
@@ -5402,31 +5402,36 @@ class MAdd : public MBinaryArithInstruct
 };
 
 class MSub : public MBinaryArithInstruction {
   MSub(MDefinition* left, MDefinition* right)
       : MBinaryArithInstruction(classOpcode, left, right) {
     setResultType(MIRType::Value);
   }
 
-  MSub(MDefinition* left, MDefinition* right, MIRType type,
-       bool mustPreserveNaN = false)
+  MSub(MDefinition* left, MDefinition* right, MIRType type)
       : MSub(left, right) {
     specialization_ = type;
     setResultType(type);
-    setMustPreserveNaN(mustPreserveNaN);
-    if (type == MIRType::Int32) {
-      setTruncateKind(Truncate);
-    }
   }
 
  public:
   INSTRUCTION_HEADER(Sub)
   TRIVIAL_NEW_WRAPPERS
 
+  static MSub* NewWasm(TempAllocator& alloc, MDefinition* left,
+                       MDefinition* right, MIRType type, bool mustPreserveNaN) {
+    auto* ret = new (alloc) MSub(left, right, type);
+    ret->setMustPreserveNaN(mustPreserveNaN);
+    if (type == MIRType::Int32) {
+      ret->setTruncateKind(Truncate);
+    }
+    return ret;
+  }
+
   double getIdentity() override { return 0; }
 
   bool isFloat32Commutative() const override { return true; }
 
   bool fallible() const;
   void computeRange(TempAllocator& alloc) override;
   bool needTruncation(TruncateKind kind) override;
   void truncate() override;

--- a/js/src/wasm/WasmIonCompile.cpp
+++ b/js/src/wasm/WasmIonCompile.cpp
@@ -354,17 +354,17 @@ class FunctionCompiler {
   }
 
   MDefinition* sub(MDefinition* lhs, MDefinition* rhs, MIRType type) {
     if (inDeadCode()) {
       return nullptr;
     }
 
     // wasm can't fold x - 0.0 because of NaN with custom payloads.
-    MSub* ins = MSub::New(alloc(), lhs, rhs, type, mustPreserveNaN(type));
+    MSub* ins = MSub::NewWasm(alloc(), lhs, rhs, type, mustPreserveNaN(type));
     curBlock_->add(ins);
     return ins;
   }
 
   MDefinition* nearbyInt(MDefinition* input, RoundingMode roundingMode) {
     if (inDeadCode()) {
       return nullptr;
     }