1. A method for computing a gain control metric used in controlling gain in a wireless repeater, the wireless repeater receiving an input signal at a receiving antenna and transmitting an output signal on the transmitting antenna, the input signal being a sum of a remote signal to be repeated and a feedback signal resulting from a feedback channel between the receiving antenna and a transmitting antenna, the feedback channel thus forming a feedback loop between the transmitting antenna and the receiving antenna, the method comprising: providing a tap-specific gain control metric indicative of a loop gain of the repeater, the tap-specific gain control metric at a given channel tap being computed from a gain control input signal and being a square of a correlation term divided by a normalization term, the correlation term and the normalization term having an integration length of N, the correlation term being indicative of a correlation of the gain control input signal and a delayed version of the gain control input signal and the normalization term being indicative of a power of the gain control input signal that is not delayed, the gain control input signal being taken from any point in the feedback loop of the repeater; providing a first register for storing a stored correlation value, the stored correlation value being the correlation term computed over a previous N samples; providing a second register for storing a stored normalization value, the stored normalization value being the normalization term computed over a previous N samples; receiving a new sample of the gain control input signal; computing an updated correlation value by performing a first multiplication using an obsolete sample of the gain control input signal, the obsolete sample being the (N+1)th previous sample, and a second multiplication using the new sample of the gain control input signal, the updated correlation value being computed by adding the product from the second multiplication and subtracting the product from the first multiplication; computing an updated normalization value by performing a third multiplication using an obsolete sample of the gain control input signal, the obsolete sample being the (N+1)th previous sample, and a fourth multiplication using the new sample of the gain control input signal, the updated normalization value being computed by adding the product from the fourth multiplication and subtracting the product from the third multiplication; and computing the tap-specific gain control metric using the updated correlation value and the updated normalization value.