[release/9.0-staging] [mono] Switch generic instance cache back to GHashTable; improve ginst hash function

[github-actions]

Backport of #113287 to release/9.0-staging

/cc @kg

Customer Impact

• Customer reported
• Found internally

Multiple customers have reported seeing significantly slower iOS/MacCatalyst AOT compilation times in .NET 9 compared to .NET 8. We found the slowdown was specific to ARM macs and was due to ARM specific inefficiencies with a hashtable we introduced early on in the .NET 9 cycle.

AOT build times on Apple hardware regressed significantly in .NET 9 and 10, i.e. from ~50 seconds to ~250 seconds.

This change will revert to the hashtable container used for the generic instance cache in .NET 8.0 to address a performance regression introduced by changing to a different container in 9. Also improves the hash function used for the cache (the existing one was suboptimal.)

Regression

• Yes
• No

Can be traced back to #102039

Testing

Manual benchmarks were performed by @steveisok and @kotlarmilos on Apple hardware. I also performed measurements on Ampere ARM64 and AMD x64 hardware to verify that the performance characteristics of the alternative container used here are acceptable.

Running the test @rolfbjarne used in #110406 (comment) shows we get back to the expected range:

.NET 8
real     0m32.759s
user    0m29.893s
sys      0m1.876s

.NET 9
real       0m36.223s
user      0m33.228s
sys        0m1.929s

Risk

Low. We used this container in .NET 8 without issue. May cause a slight startup hit for the WASM target specifically since that motivated the original change, but profiling data showed that at most this container accounted for 3% of CPU time in the startup profile we were targeting, so any regression should be in the neighborhood of 5ms or less.

IMPORTANT: If this backport is for a servicing release, please verify that:

• The PR target branch is release/X.0-staging, not release/X.0.

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

[jeffschwMSFT]

lgtm. please get a code review. we will take for consideration in 9.0.x

[carlossanlop]

@kg @vitek-karas @jeffschwMSFT Today is code complete for the April Release. If you want this fix included in this release, please get it approved by Tactics and merge it before 4pm PT.

[steveisok]

Approved by tactics over email