On Mon, Jan 11, 2010 at 8:14 AM, Wen Li <wenli...@gmail.com> wrote: > On x86 processors, I think bitwise operators should be optimized for > individual bit accesses, which should be converted to BT instructions. > > For example: > > x |= 1<<y; compiles to BTS instead of SHL/OR > x ^= 1<<y; compiles to BTC instead of SHL/XOR > x &= ~(1<<y); compiles to BTR instead of SHL/NOT/AND > if (x&(1<<y))... compiles to BT instead of SHL/AND > > Especially when y is not known at compile time, this will provide a > significant performance increase.
I think this issue is known in Bugzilla as http://gcc.gnu.org/PR25671. Ciao! Steven
