If a spell does 2d6 damage but half if target saves, do ya roll 1d6 or 2d6?

@ChrisPerkinsDnD Sorry to bother; if a spell does 2d6 damage but half if target saves, do ya roll 1d6 or 2d6 (minus half the result)?

— Ted Althouse (@TedtheRedDM) March 31, 2016

Roll all the dice (2d6 in this case), then halve the total, rounding down. https://t.co/ylmOfGe2hy

— Christopher Perkins (@ChrisPerkinsDnD) March 31, 2016