GMC_THRESHOLD_BITS
(GMC_THRESHOLD_BITS - ovl->data->gmc_bits);
(GMC_THRESHOLD_BITS - ovl->data->gmc_bits);
#define GMC_THRESHOLD_HIGH ((1 << GMC_THRESHOLD_BITS) / 4)
#define GMC_THRESHOLD_LOW ((1 << GMC_THRESHOLD_BITS) / 8)