Return s_defaultResultTask from Task.FromResult in more cases

[stephentoub]

Today we return a cached task from Task.FromResult when the TResult is a primitive and its value is 0. But it needn't be constrained to only being a primitive; what matters is that we need to be able to efficiently compare its bit pattern to 0. And we can do that easily now by using Unsafe.SizeOf to check if the size is the same as a primitive, Unsafe casting to that primitive, and then comparing against 0.

[Benchmark]
public async Task<TimeSpan> Cached() => TimeSpan.Zero;

Method	Toolchain	Mean	Ratio	Allocated	Alloc Ratio
Cached	\main\corerun.exe	19.54 ns	1.00	72 B	1.00
Cached	\pr\corerun.exe	12.47 ns	0.64	-	0.00

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Issue Details

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Today we return a cached task from Task.FromResult when the TResult is a primitive and its value is 0. But it needn't be constrained to only being a primitive; what matters is that we need to be able to efficiently compare its bit pattern to 0. And we can do that easily now by using Unsafe.SizeOf to check if the size is the same as a primitive, Unsafe casting to that primitive, and then comparing against 0.

[Benchmark]
public async Task<TimeSpan> Cached() => TimeSpan.Zero;

Method	Toolchain	Mean	Ratio	Allocated	Alloc Ratio
Cached	\main\corerun.exe	19.54 ns	1.00	72 B	1.00
Cached	\pr\corerun.exe	12.47 ns	0.64	-	0.00

Author:	stephentoub
Assignees:	stephentoub
Labels:	area-System.Threading.Tasks
Milestone:	-