How to program sbits which can be indexed ?

I would like to to use the bit addressable memory range (bdata) with an index variable as in the following example:

#define SETBIT(I) WAIT_4_MUTEX; flags^I=1; FREE_MUTEX;

unsigned char bdata flags;

func () {
unsigned char i;
  for (i=0; i < MAX_CELL; i++) {
    // do interesting stuff
    SETBIT(i);
  }
}

This construction of coarse gives a lval error on the use of the SETBIT macro.

My current fall back are the macro's:

#define SETBIT(I) base |= ((unsigned char) (1 << I))
#define CLEARBIT(I) base &= ~((unsigned char) (1 << I))

But still have the feeling that I am wasting cycles on the shift operators in these two macro's and because this is within a mutex region the timing might become an issue.

Is there a better way to set/reset the i-th bit of a bit addressable variable ?