diff --git a/src/backend_x64/emit_x64.cpp b/src/backend_x64/emit_x64.cpp
index 76afd2a3..ef453062 100644
--- a/src/backend_x64/emit_x64.cpp
+++ b/src/backend_x64/emit_x64.cpp
@@ -278,16 +278,18 @@ void EmitX64::EmitGetOverflowFromOp(IR::Block&, IR::Inst*) {
 }
 
 void EmitX64::EmitPack2x32To1x64(IR::Block&, IR::Inst* inst) {
-    auto lo = reg_alloc.UseRegister(inst->GetArg(0), any_gpr);
-    auto hi = reg_alloc.UseDefRegister(inst->GetArg(1), inst, any_gpr);
-    code->SHL(64, R(hi), Imm8(32));
-    code->OR(64, R(hi), R(lo));
+    OpArg hi;
+    X64Reg result;
+    std::tie(hi, result) = reg_alloc.UseDefOpArg(inst->GetArg(1), inst, any_gpr);
+    OpArg lo = reg_alloc.UseOpArg(inst->GetArg(0), any_gpr);
+
+    code->MOVZX(64, 32, result, hi);
+    code->SHL(64, R(result), Imm8(32));
+    code->OR(64, R(result), lo);
 }
 
 void EmitX64::EmitLeastSignificantWord(IR::Block&, IR::Inst* inst) {
-    // TODO: Optimize
-    auto u64 = reg_alloc.UseDefRegister(inst->GetArg(0), inst, any_gpr);
-    code->MOVZX(64, 32, u64, R(u64));
+    reg_alloc.RegisterAddDef(inst, inst->GetArg(0));
 }
 
 void EmitX64::EmitMostSignificantWord(IR::Block&, IR::Inst* inst) {
@@ -738,7 +740,7 @@ void EmitX64::EmitAdd64(IR::Block& block, IR::Inst* inst) {
     IR::Value b = inst->GetArg(1);
 
     X64Reg result = reg_alloc.UseDefRegister(a, inst, any_gpr);
-    OpArg op_arg = R(reg_alloc.UseRegister(b, any_gpr));
+    OpArg op_arg = reg_alloc.UseOpArg(b, any_gpr);
 
     code->ADD(64, R(result), op_arg);
 }
@@ -791,11 +793,12 @@ void EmitX64::EmitMul(IR::Block&, IR::Inst* inst) {
     IR::Value b = inst->GetArg(1);
     if (a.IsImmediate())
         std::swap(a, b);
+
     X64Reg result = reg_alloc.UseDefRegister(a, inst, any_gpr);
     if (b.IsImmediate()) {
         code->IMUL(32, result, R(result), Imm32(b.GetU32()));
     } else {
-        OpArg op_arg = R(reg_alloc.UseRegister(b.GetInst(), any_gpr));
+        OpArg op_arg = reg_alloc.UseOpArg(b, any_gpr);
         code->IMUL(32, result, op_arg);
     }
 }
@@ -803,8 +806,10 @@ void EmitX64::EmitMul(IR::Block&, IR::Inst* inst) {
 void EmitX64::EmitMul64(IR::Block&, IR::Inst* inst) {
     IR::Value a = inst->GetArg(0);
     IR::Value b = inst->GetArg(1);
+
     X64Reg result = reg_alloc.UseDefRegister(a, inst, any_gpr);
-    OpArg op_arg = R(reg_alloc.UseRegister(b.GetInst(), any_gpr));
+    OpArg op_arg = reg_alloc.UseOpArg(b, any_gpr);
+
     code->IMUL(64, result, op_arg);
 }
 
@@ -853,10 +858,11 @@ void EmitX64::EmitNot(IR::Block&, IR::Inst* inst) {
 }
 
 void EmitX64::EmitSignExtendWordToLong(IR::Block&, IR::Inst* inst) {
-    // TODO: Remove unnecessary mov that may occur here
-    X64Reg result = reg_alloc.UseDefRegister(inst->GetArg(0), inst, any_gpr);
+    OpArg source;
+    X64Reg result;
+    std::tie(source, result) = reg_alloc.UseDefOpArg(inst->GetArg(0), inst, any_gpr);
 
-    code->MOVSX(64, 32, result, R(result));
+    code->MOVSX(64, 32, result, source);
 }
 
 void EmitX64::EmitSignExtendHalfToWord(IR::Block&, IR::Inst* inst) {
@@ -876,10 +882,11 @@ void EmitX64::EmitSignExtendByteToWord(IR::Block&, IR::Inst* inst) {
 }
 
 void EmitX64::EmitZeroExtendWordToLong(IR::Block&, IR::Inst* inst) {
-    // TODO: Remove unnecessary mov that may occur here
-    X64Reg result = reg_alloc.UseDefRegister(inst->GetArg(0), inst, any_gpr);
+    OpArg source;
+    X64Reg result;
+    std::tie(source, result) = reg_alloc.UseDefOpArg(inst->GetArg(0), inst, any_gpr);
 
-    code->MOVZX(64, 32, result, R(result));
+    code->MOVZX(64, 32, result, source);
 }
 
 void EmitX64::EmitZeroExtendHalfToWord(IR::Block&, IR::Inst* inst) {