A system and associated method for optimizing a write barrier in garbage collection. The method marks a bit in a bitmap corresponding to a slot when the slot content is modified to refer to a new object. The method also logs the address of the slot when the object referred by the slot is updated. The method pops the address of the slot to clean from the log and clears the bit corresponding to the slot in the bitmap and when the object referred to by the slot is not already marked, marking said object and pushing its address to the mark stack. When the slot is located within an active object range, the method puts the address of the slot in a deferred log for a later processing.