String mapi ~2.5x perf improvement (v2)

[abelbraaksma]

See for details of timings that motivated this improvement: #9390 (comment).

TLDR: using a char [] improves ~2.1x, then 2x unrolling brings this to ~2.5x (edit: no unrolling will be done, the JIT should do that). In other words, if it took 1 second before, it now takes 0.4 seconds.

Dependent on: #9481.

[abelbraaksma]

Hmm, CI errors:

at NUnit.Framework.Assert.AreEqual(Object expected, Object actual, String message) in /home/vsts/work/1/s/tests/FSharp.Core.UnitTests/NUnitFrameworkShims.fs:line 93
at FSharp.Core.UnitTests.Collections.StringModule.MapI() in /home/vsts/work/1/s/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Collections/StringModule.fs:line 85

That's this test:

[<Test>]memberthis.MapI()=lete1= String.mapi (fun i c -> char(int c + i))"foo"
Assert.AreEqual("fpq", e1)lete2= String.mapi (fun i c -> c)null Assert.AreEqual("", e2)

And yes, these were wrong, incorrectly copied & changed the code from the BDN project. I've added more tests since we're now unrolling by % 2, which warranted adding the corners for 0, 1, 2 and I've added a few general ones to ensure the mapping function isn't called when it shouldn't: 4d15697

[abelbraaksma]

Another sourcelink fail?

C:\Users\VssAdministrator.nuget\packages\microsoft.sourcelink.common\1.1.0-beta-20206-02\build\InitializeSourceControlInformation.targets(3,81): error MSB4022: The result "" of evaluating the value "$(_MicrosoftSourceLinkCommonAssemblyFile)" of the "AssemblyFile" attribute in element <UsingTask> is not valid.

Not sure what causes this, doesn't seem related.

[abelbraaksma]

All's green now, newly added tests passed.

[abelbraaksma]

@KevinRansom, the unrolling was removed, as discussed, all requested changes implemented and green. Good to go?

[abelbraaksma]

@cartermp, you merged master into this, I think this is now ready?

[cartermp]

This will require a re-review by @KevinRansom to merge

[abelbraaksma]

/cc @KevinRansom

[abelbraaksma]

Thanks @KevinRansom!

[KevinRansom]

@abelbraaksma , happy to be of service.