ReadOnlySpanTests.TestNoMatchValuesLargerLastIndexOfAny_Byte_ManyByte

[pavelsavara]

Test generates random byte array and then assert aborts the runtime on assert.
This is debug build of browser/wasm interpreter. We don't run that combination on CI.
It happened on my devbox machine twice.

I suspect that IndexOfAnyAsciiSearcher.CanUseUniqueLowNibbleSearch doesn't have same meaning as Ascii.IsValid

  info: Starting:    System.Memory.Tests.dll
  warn: [MONO] Process terminated. Assertion failed.
  warn: [MONO] Ascii.IsValid(values) || (AdvSimd.IsSupported && TUniqueLowNibble.Value)
     at System.Diagnostics.DebugProvider.Fail(String message, String detailMessage) in D:\runtime\src\libraries\System.Private.CoreLib\src\System\Diagnostics\DebugProvider.cs:line 22
     at System.Diagnostics.Debug.Fail(String message, String detailMessage) in D:\runtime\src\libraries\System.Private.CoreLib\src\System\Diagnostics\Debug.cs:line 134
     at System.Diagnostics.Debug.Assert(Boolean condition, String message, String detailMessage) in D:\runtime\src\libraries\System.Private.CoreLib\src\System\Diagnostics\Debug.cs:line 98
     at System.Diagnostics.Debug.Assert(Boolean condition, String message) in D:\runtime\src\libraries\System.Private.CoreLib\src\System\Diagnostics\Debug.cs:line 87
     at System.Buffers.AsciiByteSearchValues`1[[System.Buffers.SearchValues.TrueConst, System.Private.CoreLib, Version=10.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]]..ctor(ReadOnlySpan`1 values) in D:\runtime\src\libraries\Syst
  em.Private.CoreLib\src\System\SearchValues\AsciiByteSearchValues.cs:line 23
     at System.Buffers.SearchValues.Create(ReadOnlySpan`1 values) in D:\runtime\src\libraries\System.Private.CoreLib\src\System\SearchValues\SearchValues.cs:line 50
     at System.SpanTests.SpanTests.LastIndexOfAny(Span`1 span, ReadOnlySpan`1 values) in D:\runtime\src\libraries\System.Memory\tests\Span\LastIndexOfAny.byte.cs:line 545
     at System.SpanTests.SpanTests.TestNoMatchValuesLargerLastIndexOfAny_Byte_ManyByte() in D:\runtime\src\libraries\System.Memory\tests\Span\LastIndexOfAny.byte.cs:line 465

runtime/src/libraries/System.Private.CoreLib/src/System/SearchValues/SearchValues.cs

Lines 46 to 51 in 167d862

	// Depending on the hardware, UniqueLowNibble can be faster than even range or 2 values.
	// It's currently consistently faster than 4/5 values on all tested platforms (Arm, Avx2, Avx512).
	if (values.Length >= 4 && IndexOfAnyAsciiSearcher.CanUseUniqueLowNibbleSearch(values, maxInclusive))
	{
	return new AsciiByteSearchValues<TrueConst>(values);
	}

runtime/src/libraries/System.Private.CoreLib/src/System/SearchValues/IndexOfAnyAsciiSearcher.cs

Line 98 in 167d862

public static bool CanUseUniqueLowNibbleSearch<T>(ReadOnlySpan<T> values, int maxInclusive)

runtime/src/libraries/System.Private.CoreLib/src/System/SearchValues/AsciiByteSearchValues.cs

Line 23 in 167d862

Debug.Assert(Ascii.IsValid(values) || (AdvSimd.IsSupported && TUniqueLowNibble.Value));

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[pavelsavara]

cc @MihaZupan

[MihaZupan]

The same logic as for ARM applies to PackedSimd in this case.

This assert is wrong

runtime/src/libraries/System.Private.CoreLib/src/System/SearchValues/AsciiByteSearchValues.cs

Lines 21 to 23 in 167d862

	// Despite the name being Ascii, this type may be used with non-ASCII values on ARM.
	// See IndexOfAnyAsciiSearcher.CanUseUniqueLowNibbleSearch.
	Debug.Assert(Ascii.IsValid(values) || (AdvSimd.IsSupported && TUniqueLowNibble.Value));

It should be AdvSimd.IsSupported || PackedSimd.IsSupported or IndexOfAnyAsciiSearcher.IsVectorizationSupported && !Ssse3.IsSupported.

This corresponds to this check

runtime/src/libraries/System.Private.CoreLib/src/System/SearchValues/IndexOfAnyAsciiSearcher.cs

Lines 108 to 113 in 167d862

	if (Ssse3.IsSupported && maxInclusive > 127)
	{
	// We could support values higher than 127 if we did the "& 0xF" before calling into Shuffle in IndexOfAnyLookupCore.
	// We currently optimize for the common case of ASCII characters instead, saving an instruction there.
	return false;
	}

[pavelsavara]

on wasm we have

• IndexOfAnyAsciiSearcher.IsVectorizationSupported:True
• PackedSimd.IsSupported:True
• Ssse3.IsSupported:False
• AdvSimd.IsSupported:False