Fix atomic_xchg* intrinsics

example/std_example.rs

@@ -12,4 +12,7 @@ fn main() {
     let _ = std::process::Command::new("true").env("c", "d").spawn();
 
     println!("cargo:rustc-link-lib=z");
+
+    static ONCE: std::sync::Once = std::sync::ONCE_INIT;
+    ONCE.call_once(|| {});
 }

src/intrinsics.rs

@@ -386,7 +386,7 @@ pub fn codegen_intrinsic_call<'a, 'tcx: 'a>(
 
             // Compare
             let is_eq = fx.bcx.ins().icmp(IntCC::Equal, old, test_old);
-            let new = crate::common::codegen_select(&mut fx.bcx, is_eq, old, new); // Keep old if not equal to test_old
+            let new = crate::common::codegen_select(&mut fx.bcx, is_eq, new, old); // Keep old if not equal to test_old
 
             // Write new
             fx.bcx.ins().store(MemFlags::new(), new, ptr, 0);