Optimize Enum.GetUnderlyingType with a known type to a constant

[MichalPetryka]

Description

Enum.GetUnderlyingType currently always does a runtime lookup even when the type is always known, for example due to being a generic parameter. This makes fast generic specialization on enums impossible, unless you specialize on their size, which in turn makes you loose information about the sign.
Having this optimized would make #70167 much more useful since currently you have no fast way to check if the conversion is safe.

Configuration

Core CLR 6.0.322.12309 on amd64 on SharpLab

Regression?

I don't think so, but haven't checked other runtimes.

Data

SharpLab

category:cq
theme:basic-cq
skill-level:intermediate
cost:medium
impact:small

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Issue Details

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Description

Enum.GetUnderlyingType currently always does a runtime lookup even when the type is always known, for example due to being a generic parameter. This makes fast generic specialization on enums impossible, unless you specialize on their size, which in turn makes you loose information about the sign.
Having this optimized would make #70167 much more useful since currently you have no fast way to check if the conversion is safe.

Configuration

Core CLR 6.0.322.12309 on amd64 on SharpLab

Regression?

I don't think so, but haven't checked other runtimes.

Data

SharpLab

Author:	MichalPetryka
Assignees:	-
Labels:	tenet-performance, area-CodeGen-coreclr
Milestone:	-

[EgorBo]

I wasn't able to find any cases dotnet/runtime code base could benefit from such an intrinsic, do you have any real-world examples?

[MichalPetryka]

The ValueToString method in PR #71590 would be an example place to use it, on SharpLab with .Net 6 it results in terrible codegen.

[MichalPetryka]

Fixed by #71685.