We maintain four equity compensation plans that provide for the issuance of shares of our common stock to our officers and other employees, directors, and consultants: the 2010 Stock Plan (the “2010 Plan”), the 2013 Equity Incentive Plan (the “2013 Plan”), the 2019 Equity Incentive Plan (the “2019 Plan”), and the 2019 Employee Stock Purchase Plan (the “ESPP”), which have all been approved by stockholders. Following our initial public offering (“IPO”) in May 2019, we have only issued awards under the 2019 Plan and the ESPP, and no additional awards will be granted under the 2010 and 2013 Plans. These plans provide for the issuance of incentive stock options (“ISOs”), nonqualified stock options (“NSOs”), stock appreciation rights (“SARs”), restricted stock awards, restricted stock units (“RSUs”), performance-based awards, and other awards (that are based in whole or in part by reference to our common stock).

A summary of stock option and SAR activity for the six months ended June 30, 2024 is as follows (in millions, except share amounts which are reflected in thousands, per share amounts, and years):


	SARs Outstanding Number of SARs	Options Outstanding Number of Shares	Weighted-Average Exercise Price Per Share		Weighted-Average Remaining Contractual Life (in years)	Aggregate Intrinsic Value
As of December 31, 2023	123	12,641	$	20.03	2.79	$	535
Granted	—	227	$	81.03
Exercised	(71)	(5,206)	$	17.77
Canceled and forfeited	—	(520)	$	33.08
As of June 30, 2024	52	7,142	$	22.47	3.09	$	363
Vested and expected to vest as of June 30, 2024	52	6,067	$	19.00	2.60	$	328
Exercisable as of June 30, 2024	52	6,067	$	19.00	2.60	$	328


	Number of Shares	Weighted-Average Grant-Date Fair Value per Share
Unvested and outstanding as of December 31, 2023	90,827	$	34.49
Granted	24,198	$	75.55
Vested	(23,702)	$	37.08
Canceled and forfeited	(5,864)	$	37.97
Unvested and outstanding as of June 30, 2024	85,459	$	45.34

Stock-based compensation expense is allocated based on the cost center to which the award holder belongs. The following table summarizes total stock-based compensation expense by function (in millions):


	Three Months Ended June 30,				Six Months Ended June 30,
	2023		2024		2023		2024
Operations and support	$	45	$	54	$	83	$	121
Sales and marketing	26		24		50		45
Research and development	317		277		607		576
General and administrative	116		100		234		197
Total	$	504	$	455	$	974	$	939

As of June 30, 2024, there was $3.8 billion of unamortized compensation costs related to all unvested awards. The unamortized compensation costs are expected to be recognized over a weighted-average period of approximately 2.81 years.

The tax benefits recognized in the condensed consolidated statements of operations for stock-based compensation arrangements were not material during the three and six months ended June 30, 2023 and 2024.

In February 2024, our board of directors authorized the repurchase of up to $7.0 billion in shares of our outstanding common stock (the “Share Repurchase Program”). The timing, manner, price and amount of any repurchases are determined by the discretion of management, depending on market conditions and other factors. Repurchases may be made through open market purchases and accelerated share repurchases. The exact number of shares to be repurchased by us, if any, is not guaranteed. Depending on market conditions and other factors, these repurchases may be commenced or suspended at any time or periodically without prior notice.

During the three months ended June 30, 2024, we repurchased and subsequently retired 4.8 million shares of common stock for $326 million, including broker commissions and fees. As of June 30, 2024, we had $6.7 billion available to repurchase shares pursuant to the Share Repurchase Program.

The Inflation Reduction Act imposed a nondeductible 1% excise tax on the net value of certain stock repurchases. During the three months ended June 30, 2024, the excise tax on net share repurchases was not material.