Cache GenericTypeDefinition to improve perf

[lateapexearlyspeed]

Fix #73860

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Issue Details

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Fix #73860

Author:	lateapexearlyspeed
Assignees:	-
Labels:	area-System.Reflection
Milestone:	-

[jkotas]

I have pushed some changes:

• Switch to dedicated field for the generic type definition. There was a non-trivial risk for activator cache and generic type definition cache to collide, and regress overall scenario performance a lot.
• Miscellaneous performance fixes (details are in the commit descriptions). Performance numbers with these changes:
  • typeof(List<int>).GetGenericTypeDefinition() is 10x faster when the cache is hit
  • typeof(List<>).GetGenericTypeDefinition() is 3x faster when the cache is hit
  • Reflection per-type cache stores an extra pointer (grows from 152 bytes to 160 bytes on 64-bit)

[jkotas]

@lateapexearlyspeed Thank you for getting this started!