[perf] System.Text.** 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.Threading.Tests.Perf_ThreadStatic.SetThreadStatic	2.17	2.37	9.22
System.Text.Tests.Perf_Encoding.GetChars(size: 16, encName: utf-8)	134.63	172.69	28.27
System.Text.Tests.Perf_Encoding.GetByteCount(size: 512, encName: ascii)	8.81	9.34	6.02
System.Text.Tests.Perf_Encoding.GetByteCount(size: 16, encName: ascii)	8.81	9.36	6.24
System.Text.RegularExpressions.Tests.Perf_Regex_Common.Uri_IsNotMatch(Options: IgnoreCase, Compiled)	367.94	481.19	30.78
System.Text.RegularExpressions.Tests.Perf_Regex_Common.ReplaceWords(Options: Compiled)	12433.17	19087.83	53.52
System.Text.RegularExpressions.Tests.Perf_Regex_Common.OneNodeBacktracking(Options: IgnoreCase, Compiled)	303.51	556.79	83.45
System.Text.RegularExpressions.Tests.Perf_Regex_Common.MatchWord(Options: IgnoreCase, Compiled)	528.17	833.11	57.74
System.Text.RegularExpressions.Tests.Perf_Regex_Common.IP_IsNotMatch(Options: Compiled)	346.94	510	47
System.Text.RegularExpressions.Tests.Perf_Regex_Cache.IsMatch(total: 40000, unique: 1600, cacheSize: 3200)	18834360.75	20212941.8	7.32
System.Text.Json.Tests.Utf8JsonReaderCommentsTests.Utf8JsonReaderCommentParsing(CommentHandling: Allow, SegmentSize: 100, TestCase: LongMultiLine)	41771.44	83590.36	100.11
System.Text.Json.Tests.Perf_Strings.WriteStringsUtf16(Formatted: True, SkipValidation: False, Escaped: AllEscaped)	193792673.31	207213555.8	6.93
System.Text.Json.Tests.Perf_Strings.WriteStringsUtf16(Formatted: False, SkipValidation: False, Escaped: AllEscaped)	192543548.4	213535368.27	10.9
System.Text.Json.Tests.Perf_Get.GetString	27491.32	39333.15	43.07
System.Text.Json.Tests.Perf_Get.GetBoolean	364.4	398.96	9.48
System.Text.Json.Document.Tests.Perf_DocumentParse.Parse(IsDataIndented: True, TestRandomAccess: True, TestCase: BasicJson)	6017.26	6396.89	6.31
System.Text.Json.Document.Tests.Perf_DocumentParse.Parse(IsDataIndented: False, TestRandomAccess: True, TestCase: HelloWorld)	881.58	983.05	11.51

[kotlarmilos]

@eiriktsarpalis @stephentoub We are trying to narrow down changes between May 2nd, 2023 and July 19th, 2023 that may have caused the regressions. If you are aware of any changes that might have potentially caused these regressions, please let us know.

[lewing]

#85558

[steveisok]

There does not appear to be a whole lot of action we can take as this is about .NET 8 preview to preview regressions.