Warnings for in/readonly ref

[benaadams]

Primitive value-types that will be passed by register (byte, short, int, etc) should generate a warning when passed as in as passing by readonly reference in the common case is likely a deoptimization error (dereference vs register)

Reference-types should generate a warning when passed as in as passing by readonly reference in the common case is likely a deoptimization error (double dereference vs dereference)

from: dotnet/roslyn#23327

/cc @mikedn

[jcouv]

Tagging @VSadov

[VSadov]

There are cases where having an in parameter typed as reference type makes sense.
Like Volatile.Read(in object variable)

In general it is indeed not very useful, but there would be a need for a whitelist of few reasonable uses.
I could be a good idea for an analyzer.

[benaadams]

There are cases where having an in parameter typed as reference type makes sense.
Like Volatile.Read(in object variable)

But they could be #pragma'd as valid?

[tannergooding]

I agree this would be a good case for an analyzer.

That being said, for some types whether or not it should be passed as 'in' depends on the underlying architecture and calling convention. So the diagnostics should likely only cover the well-defined cases that are consistent between all targets.

[MaStr11]

This should be considered for the warning waves dotnet/roslyn#1580 and the code fix candidates dotnet/roslyn#23326.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[mavasani]

Transferred to dotnet/runtime for triage

[ghost]

Tagging subscribers to this area: @dotnet/area-system-runtime
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Issue Details

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Primitive value-types that will be passed by register (byte, short, int, etc) should generate a warning when passed as in as passing by readonly reference in the common case is likely a deoptimization error (dereference vs register)

Reference-types should generate a warning when passed as in as passing by readonly reference in the common case is likely a deoptimization error (double dereference vs dereference)

from: dotnet/roslyn#23327

/cc @mikedn

Author:	benaadams
Assignees:	-
Labels:	area-System.Runtime, untriaged
Milestone:	-

[buyaa-n]

So the diagnostics should likely only cover the well-defined cases that are consistent between all targets.

A list of cases that need to be covered would be very helpful for further triaging and implementing the analyzer.

For reference types it is suggested to exclude object type. What cases should be excluded from primitive value-types that will be passed as in parameter?

Please comment any exclusions should be covered