atomicity of x86 bt/bts/btr/btc?

[Jay K]

gcc-4.5/gcc/config/i386/i386.md:

;; %%% bts, btr, btc, bt.
;; In general these instructions are *slow* when applied to memory,
;; since they enforce atomic operation. When applied to registers,


I haven't found documented confirmation that these instructions are atomic without a lock prefix,
having checked Intel and AMD documentation and random web searching.
They are mentioned as instructions that can be used with lock prefix.


- Jay