SemanticChecks: Avoid repeated type resolution of [ordered]

[IISResetMe]

PR Summary

Avoid unnecessary attempts to resolve [ordered]'s underlying type (which isn't there) in post-parse checks

PR Context

As highlighted in #17308, parsing the cast expression [ordered]@{} is exactly as slow as parsing [NonExistingType]@{}!

Most of this overhead is incurred during post-parse analysis conducted by the SemanticChecks visitor, when repeated attempts are made to resolve the underlying runtime type represented by [ordered]. Since [ordered] is a special attribute and not really a type literal, the type resolution fails, which in itself is a relatively costly (and non-cacheable) operation.

This PR makes no functional changes to the flow of analysis, but reduces calls to TypeName.GetReflectionType() in situations where we've already assess the target type name is [ordered].

This alone cuts the wallclock time for parsing [ordered]@{} compared to [nonexisting]@{} by 45-50%:

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[iSazonov]

If I understand correctly the intention was to do optimization for all non-resolvable type names:

To the 2nd question, the same performance degradation would happen to another non-resolvable type name in the script, so it would be nice to resolve this perf issue in the root -- not looking up a non-resolvable type name multiple times during parse would provide a significant boost to performance.

[IISResetMe]

I must have misunderstood @daxian-dbw

[daxian-dbw]

@IISResetMe No, you are not 😄 Resolving the root issue will of course be the ultimate goal, but that will obviously require more time and yet still turn out to result in behavior changes. Also, it's possible that no one will pick up that work (or even the work to make ordered a real type accelerator because that also involves changing parser and compiler) for a long time.

The short term fix you discovered will largely mitigate the immediate problem right away with very minimal changes. I think it's reasonable to take it now.

[IISResetMe]

Gotcha, thanks @daxian-dbw!

There's a couple other mitigation ideas that came up while discussing the issue earlier today:

• Early exit from TypeName.GetReflectionType() with a special heuristic for [ordered], just like we already have in parser and compiler anyways
  • Fixes the [ordered] bug
• Refactoring SemanticChecks to aggressively cache type lookups (should be safe, semantic analysis is a self-contained phase)
  • This will benefit all type resolution failure cases, not just [ordered]

I think we should be able to implement both short-term with minimal change in behavior (beyond improving performance of course). I'll play around with it and see if it works :)

[daxian-dbw]

Please add a few tests about [ordered]:

1. define a Ordered type by Add-Type
2. check that [ordered]@{a = 1; b= 2} still returns OrderedDictionary.
3. check that [ordered]<other-expr> still fails with a parsing error.
4. check that <somevealue> -is [ordered] and <somevalue> -as [ordered] continue to treat [ordered] as a normal type name, which resolves to the type we defined.

[daxian-dbw]

There's a couple other mitigation ideas that came up while discussing the issue earlier today:

Please use a separate PR for further optimization, especially if the target is to solve the perf issue regarding non-resolvable type in general.

If the ideas are to further optimize how [ordered] is handled in parser with its current semantics, then I suggest you think about optimization for non-resolvable types in general in the parsing phase, also, think about making [ordered] a true type accelerator to OrderedDictionary (basically address all comments from the Engine WG 😄).

But but, just want to call out, don't feel obligated to address the root issues, and it's not urgent at all!

[daxian-dbw]

Looks good! Please add some tests based on the comment I left above. They are helpful to make sure this quick fix doesn't break the current behavior.

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[ghost]

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