Remove unnecessary spills from BufferedStream.ReadAsync

[stephentoub]

Saves a field in each state machine.

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Issue Details

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Saves a field in each state machine.

Author:	stephentoub
Assignees:	-
Labels:	area-System.IO
Milestone:	-

[adamsitnik]

LGTM, however I wish our compiler could optimize that for us.

[adamsitnik]

the failures are unrelated, I am merging

[stephentoub]

I wish our compiler could optimize that for us.

For the most part this is by design: to maintain order of operations, if you have A() + await B() the compiler has to evaluate A() first, at which point it has to store the result somewhere across the await. In this constrained case, where A() is actually a local whose address is never exposed, isn't passed by ref, isn't captured by a closure, etc., it could probably avoid it, and for that I opened dotnet/roslyn#54629. But it's also possible the rewards from such a constrained optimization are limited (e.g. these were the only two occurrences in product code of + await in all of dotnet/runtime), and there may be reasons why even for this case the compiler might face correctness issues.

[adamsitnik]

@stephentoub thanks!