------- Comment #3 from steven at gcc dot gnu dot org 2006-04-10 20:31 ------- This is what the i386 machine description has to say about BT and friends:
;; %%% bts, btr, btc, bt. ;; In general these instructions are *slow* when applied to memory, ;; since they enforce atomic operation. When applied to registers, ;; it depends on the cpu implementation. They're never faster than ;; the corresponding and/ior/xor operations, so with 32-bit there's ;; no point. But in 64-bit, we can't hold the relevant immediates ;; within the instruction itself, so operating on bits in the high ;; 32-bits of a register becomes easier. ;; ;; These are slow on Nocona, but fast on Athlon64. We do require the use ;; of btrq and btcq for corner cases of post-reload expansion of absdf and ;; negdf respectively, so they can never be disabled entirely. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25671
