[perf] System.Memory.Span.** and System.Memory.ReadOnly.** 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.Net.Tests.Perf_WebUtility.HtmlDecode_Entities	793.41	1068.41	34.66
System.Memory.Span<Int32>.LastIndexOfValue(Size: 4)	10.6	30.11	184.06
System.Memory.Span<Int32>.IndexOfValue(Size: 512)	186.47	610.48	227.39
System.Memory.Span<Int32>.IndexOfValue(Size: 4)	11.31	40.59	258.89
System.Memory.Span<Int32>.BinarySearch(Size: 4)	20.89	21.95	5.07
System.Memory.Span<Char>.SequenceEqual(Size: 4)	23.92	33.69	40.84
System.Memory.Span<Char>.Reverse(Size: 4)	11.19	14.47	29.31
System.Memory.Span<Char>.LastIndexOfValue(Size: 4)	10.6	12.61	18.96
System.Memory.Span<Char>.IndexOfValue(Size: 512)	186.48	340.74	82.72
System.Memory.Span<Char>.IndexOfValue(Size: 33)	20.49	65.79	221.08
System.Memory.Span<Char>.IndexOfAnyFourValues(Size: 4)	28.18	39.38	39.74
System.Memory.Span<Byte>.Reverse(Size: 4)	23.92	27.06	13.13
System.Memory.Span<Byte>.LastIndexOfAnyValues(Size: 4)	12.2	19.55	60.25
System.Memory.Span<Byte>.IndexOfAnyTwoValues(Size: 4)	13.28	20.79	56.55
System.Memory.Span<Byte>.IndexOfAnyFourValues(Size: 4)	29.86	38.39	28.57
System.Memory.ReadOnlySpan.IndexOfString(input: だだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだxだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだ, value: x, comparisonType: Ordinal)	359.55	615.97	71.32
System.Memory.ReadOnlySpan.IndexOfString(input: だだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだすだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだだ, value: す, comparisonType: Ordinal)	46.6	112.47	141.35
System.Memory.ReadOnlySpan.IndexOfString(input: Hello Worldbbbbbbbbbbbbbbcbbbbbbbbbbbbbbbbbbba!, value: y, comparisonType: Ordinal)	44.77	81.37	81.75
System.Memory.ReadOnlySpan.IndexOfString(input: Hello WorldbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbareallyreallylongHello WorldbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbareallyreallylongHello Worldbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbareallyreallylong!xyz, value: ~, comparisonType: Ordinal)	143.47	237.72	65.69
System.Memory.ReadOnlySequence.Slice_Start(Segment: Single)	10.32	12.79	23.93

[kotlarmilos]

@EgorBo @MihaZupan 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.

/cc: @vargaz

[MihaZupan]

@kotlarmilos possibly #88101 or #86655 if the extra code affects Mono codegen somehow.
In case Mono cares about AggressiveOptimization, #85819 removed that from a few of these.