: co jsem v rychlosti našel u Agner's Fog Pentium Optimazition :
BT, BTC, BTR, and BTS change the carry flag but leave the other flags unchanged. This causes a false dependence on the previous value of the flags and costs an extra uop. Use TEST, AND, OR or XOR instead of these instructions.
18.5 Bit test (all processors)
BT, BTC, BTR, and BTS instructions should preferably be replaced by instructions like TEST, AND, OR, XOR, or shifts on P1, PMMX and P4. On PPro, P2 and P3, bit tests with a memory operand should be avoided.