This makes the compiler less likely to reload the content from %gs. The 'P' modifier drops all synteax prefixes and 'n' constraint treats input as a known at compilation time immediate integer. Example reloading victim was spinlock_enter. Stolen from: OpenBSD Reported by: jtl Reviewed by: kib Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D17615