diff --git a/lib/singlepass-backend/src/emitter_x64.rs b/lib/singlepass-backend/src/emitter_x64.rs
index 4bea37d3e..83099f259 100644
--- a/lib/singlepass-backend/src/emitter_x64.rs
+++ b/lib/singlepass-backend/src/emitter_x64.rs
@@ -97,6 +97,7 @@ pub trait Emitter {
     fn emit_movsx(&mut self, sz_src: Size, src: Location, sz_dst: Size, dst: Location);
     fn emit_xchg(&mut self, sz: Size, src: Location, dst: Location);
     fn emit_lock_xadd(&mut self, sz: Size, src: Location, dst: Location);
+    fn emit_lock_cmpxchg(&mut self, sz: Size, src: Location, dst: Location);
 
     fn emit_btc_gpr_imm8_32(&mut self, src: u8, dst: GPR);
     fn emit_btc_gpr_imm8_64(&mut self, src: u8, dst: GPR);
@@ -858,6 +859,24 @@ impl Emitter for Assembler {
         }
     }
 
+    fn emit_lock_cmpxchg(&mut self, sz: Size, src: Location, dst: Location) {
+        match (sz, src, dst) {
+            (Size::S8, Location::GPR(src), Location::Memory(dst, disp)) => {
+                dynasm!(self ; lock cmpxchg [Rq(dst as u8) + disp], Rb(src as u8));
+            }
+            (Size::S16, Location::GPR(src), Location::Memory(dst, disp)) => {
+                dynasm!(self ; lock cmpxchg [Rq(dst as u8) + disp], Rw(src as u8));
+            }
+            (Size::S32, Location::GPR(src), Location::Memory(dst, disp)) => {
+                dynasm!(self ; lock cmpxchg [Rq(dst as u8) + disp], Rd(src as u8));
+            }
+            (Size::S64, Location::GPR(src), Location::Memory(dst, disp)) => {
+                dynasm!(self ; lock cmpxchg [Rq(dst as u8) + disp], Rq(src as u8));
+            }
+            _ => panic!("LOCK CMPXCHG {:?} {:?} {:?}", sz, src, dst),
+        }
+    }
+
     fn emit_btc_gpr_imm8_32(&mut self, src: u8, dst: GPR) {
         dynasm!(self ; btc Rd(dst as u8), BYTE src as i8);
     }