Unnecessary copy generated in async code #21139
Description
Compile the following method using Visual Studio 2017 15.3.0 Preview 5.0 and inspect the generated IL code:
static async Task Test() => await Task.CompletedTask;.locals init(
[0] valuetype Program/'<Test>d__6',
[1] valuetype[System.Private.CoreLib] System.Runtime.CompilerServices.AsyncTaskMethodBuilder
)
IL_0000: ldloca.s 0
IL_0002: call valuetype[System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder[System.Private.CoreLib] System.Runtime.CompilerServices.AsyncTaskMethodBuilder::Create()
IL_0007: stfld valuetype[System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program/'<Test>d__6'::'<>t__builder'
IL_000c: ldloca.s 0
IL_000e: ldc.i4.m1
IL_000f: stfld int32 Program/'<Test>d__6'::'<>1__state'
IL_0014: ldloc.0
IL_0015: ldfld valuetype[System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program/'<Test>d__6'::'<>t__builder'
IL_001a: stloc.1
IL_001b: ldloca.s 1
IL_001d: ldloca.s 0
IL_001f: call instance void[System.Private.CoreLib] System.Runtime.CompilerServices.AsyncTaskMethodBuilder::Start<valuetype Program/'<Test>d__6'>(!!0&)
IL_0024: ldloca.s 0
IL_0026: ldflda valuetype[System.Private.CoreLib]System.Runtime.CompilerServices.AsyncTaskMethodBuilder Program/'<Test>d__6'::'<>t__builder'
IL_002b: call instance class [System.Private.CoreLib]System.Threading.Tasks.Task[System.Private.CoreLib] System.Runtime.CompilerServices.AsyncTaskMethodBuilder::get_Task()
Note that a copy of the builder is created and Start is invoked on that and not on the builder in t__builder.
Start doesn't use any members of the builder so this doesn't impact correctness but the generated code is larger, slower and uses more stack space than it should.