(vec & cns) == zero is generating an extra vpand

[PranavSenthilnathan]

Regression: #110413
Repro: EgorBot/runtime-utils#260

The change around here has regressed code gen for AVX2 CPUs without AVX-512: https://github.com/dotnet/runtime/pull/104488/files#diff-6b4906abc01dc4699f348f7c1df72e2f640f240aa31ea67cd47642221b2021f5L1560-L1571

Previous code gen (in GetIndexOfFirstNonAsciiChar_Vector):

G_M000_IG03:                ;; offset=0x002D
       vptest   ymm0, ymmword ptr [rdi]
       jne      SHORT G_M000_IG04
       add      rdi, 32
       cmp      rdi, rcx
       jbe      SHORT G_M000_IG03

Current code gen:

G_M000_IG03:                ;; offset=0x0030
       vpand    ymm1, ymm0, ymmword ptr [rdi]
       vptest   ymm1, ymm1
       jne      SHORT G_M000_IG04
       add      rdi, 32
       cmp      rdi, rcx
; ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ (cmp: 2 ; jcc erratum) 32B boundary ...............................
       jbe      SHORT G_M000_IG03

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[PranavSenthilnathan]

@tannergooding I assigned myself to fix this in my free time in the next few days but feel free to reassign if this needs to be fixed more urgently.

[EgorBo]

@tannergooding I assigned myself to fix this in my free time in the next few days but feel free to reassign if this needs to be fixed more urgently.

Just in case - the issue has to be fixed in JIT rather than C# impl

[EgorBo]

internal static bool VectorContainsNonAsciiChar(Vector256<ushort> utf16Vector)
{
    const ushort asciiMask = ushort.MaxValue - 127; // 0xFF80
    Vector256<ushort> zeroIsAscii = utf16Vector & Vector256.Create(asciiMask);
    return zeroIsAscii != Vector256<ushort>.Zero;
}

AVX512=1:

; Method Benchmarks:VectorContainsNonAsciiChar(System.Runtime.Intrinsics.Vector256`1[ushort]):ubyte (FullOpts)
       vmovups  ymm0, ymmword ptr [rcx]
       vpandd   ymm0, ymm0, dword ptr [reloc @RWD00] {1to8}
       vptest   ymm0, ymm0
       setne    al
       movzx    rax, al
       vzeroupper 
       ret      
RWD00  	dd	FF80FF80h

AVX512=0:

; Method Benchmarks:VectorContainsNonAsciiChar(System.Runtime.Intrinsics.Vector256`1[ushort]):ubyte (FullOpts)
       vmovups  ymm0, ymmword ptr [rcx]
       vptest   ymm0, ymmword ptr [reloc @RWD00]
       setne    al
       movzx    rax, al
       vzeroupper 
       ret      
RWD00  	dq	FF80FF80FF80FF80h, FF80FF80FF80FF80h, FF80FF80FF80FF80h, FF80FF80FF80FF80h
; Total bytes of code: 23

[EgorBo]

I guess the optimization that transforms V1 & VEC_CNS into PTEST gets confused by AVX512's BroadcastScalarToVector256 optimization

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[PranavSenthilnathan]

@EgorBot -intel -profiler -commit main --envvars DOTNET_JitDisasm:Vector DOTNET_EnableAVX512F:0

using System.Text;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

// Actual runner is optional, but if it exists, it has to be like this:
BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

public class Bench
{
    public int size = 512;

    public string encName = "utf-8";
    private Encoding _enc;
    private string _toEncode;
    private char[] _chars;
    [GlobalSetup]
    public void SetupGetBytes()
    {
        _enc = Encoding.GetEncoding(encName);
        _toEncode = CreateString(size);
        _chars = _toEncode.ToCharArray();
    }

    [Benchmark]
    [MemoryRandomization]
    public int GetByteCount() => _enc.GetByteCount(_chars);

    public static string CreateString(int length)
    {
        char[] str = new char[length];

        for (int i = 0; i < str.Length; i++)
        {
            // Add path separator so folders aren't too long.
            if (i % 20 == 0)
            {
                str[i] = Path.DirectorySeparatorChar;
            }
            else
            {
                str[i] = 'a';
            }
        }

        return new string(str);
    }
}