Alphabet said on Tuesday it will spend $75 billion on its AI buildout this year, 29% more than Wall Street expected, while posting cloud revenue that missed analysts' target.The tech giant's shares were down 9% in extended trading.

Alphabet's fourth-quarter earnings day is here, with numbers expected to be released after the markets close Tuesday. This will be followed by an earnings call at roughly 4:30 p.m. ET. The stock has been riding high over the past month as investors grew bullish over Google's AI investment.

CEO Sundar Pichai defended the dramatic increase on a conference call with analysts, who are raising new questions about capital spending.