valuetask

[bitfaster]

Use ValueTask for async methods, eliminate Task allocs.

Before:

Method	Runtime	Mean	Error	StdDev	Code Size	Gen 0	Allocated
GetOrAddAsync	.NET 6.0	84.79 ns	1.618 ns	4.059 ns	1,683 B	0.0167	72 B
AtomicGetOrAddAsync	.NET 6.0	77.26 ns	0.587 ns	0.549 ns	1,847 B	0.0167	72 B
GetOrAddAsync	.NET Framework 4.8	172.44 ns	1.518 ns	1.268 ns	12,202 B	0.0184	80 B
AtomicGetOrAddAsync	.NET Framework 4.8	178.61 ns	1.501 ns	1.253 ns	12,202 B	0.0184	80 B

After:

Method	Runtime	Mean	Error	StdDev	Code Size	Allocated
GetOrAddAsync	.NET 6.0	79.65 ns	0.754 ns	0.668 ns	1,935 B	-
AtomicGetOrAddAsync	.NET 6.0	83.76 ns	0.465 ns	0.388 ns	2,099 B	-
GetOrAddAsync	.NET Framework 4.8	173.62 ns	0.915 ns	0.811 ns	12,513 B	-
AtomicGetOrAddAsync	.NET Framework 4.8	183.51 ns	1.019 ns	0.903 ns	12,513 B	-