diff --git a/libc/src/__support/threads/identifier.h b/libc/src/__support/threads/identifier.h
index 5103c11f6eb86..a570abfd5dc73 100644
--- a/libc/src/__support/threads/identifier.h
+++ b/libc/src/__support/threads/identifier.h
@@ -15,6 +15,7 @@
 
 #include "hdr/types/pid_t.h"
 #include "src/__support/OSUtil/syscall.h"
+#include "src/__support/macros/optimization.h"
 #include <sys/syscall.h>
 
 namespace LIBC_NAMESPACE_DECL {