[perf] System.IO.** performance regressions in Mono AOT x64 microbenchmarks from Preview 5 to Preview 7

[kotlarmilos]

Description

The following performance regressions occurred between the Preview 4 snapshot (May 2nd, 2023) and Preview 7 snapshot (July 19th, 2023).

The configuration used for the benchmarks is Mono AOT with LLVM on x64 ubuntu 18.04.

Name	Baseline Value	Compare Value	% Difference
System.IO.Tests.StringReaderReadLineTests.ReadLine(LineLengthRange: [ 9, 32])	59315.82	85012.82	43.32
System.IO.Tests.StreamReaderReadToEndTests.ReadToEnd(LineLengthRange: [ 1, 8])	302363619.79	393413008.92	30.11
System.IO.Tests.StreamReaderReadLineTests.ReadLine(LineLengthRange: [ 33, 128])	120659.04	148821.05	23.34
System.IO.Tests.Perf_StreamWriter.WritePartialCharArray(writeLength: 100)	7040317380.69	7436111839.2	5.62
System.IO.Tests.Perf_Path.GetTempPath	475.3	528.5	11.19
System.IO.Tests.Perf_Path.GetFullPathForLegacyLength	892.08	988.29	10.78
System.IO.Tests.Perf_Path.GetFileNameWithoutExtension	72.17	130.12	80.3
System.IO.Tests.Perf_FileStream.Read_NoBuffering(fileSize: 1048576, userBufferSize: 16384, options: None)	89284.96	96396	7.96
System.IO.Tests.Perf_FileInfo.ctor_str	260.26	287.06	10.3
System.IO.Tests.Perf_File.ReadAllLines	49709.46	61925.36	24.57
System.IO.Tests.Perf_Directory.EnumerateFiles	6526091.4	7960634.37	21.98
System.IO.Tests.BinaryWriterTests.WriteUInt64	17.46	18.49	5.9
System.IO.Tests.BinaryWriterTests.WriteSingle	16.59	17.65	6.39

[kotlarmilos]

@MihaZupan Some of them may have regressed in https://github.com/dotnet/runtime/pull/78895.

[MihaZupan]

That change only affected the CreateStackTraceString method, which I doubt is used in any of these benchmarks.

[steveisok]

Since this is about .NET 8 preview regressions, there's not a whole lot of action we can take. Closing.