diff --git a/src/string.hh b/src/string.hh
index 6b66bce47..a6a267cbc 100644
--- a/src/string.hh
+++ b/src/string.hh
@@ -164,6 +164,7 @@ public:
     ZeroTerminatedString zstr() const { return {begin(), end()}; }
 
 private:
+    [[gnu::optimize(3)]] // this is recursive for constexpr reason
     static constexpr ByteCount strlen(const char* s)
     {
         return *s == 0 ? 0 : strlen(s+1) + 1;