Vectorize String.Equals for OrdinalIgnoreCase (#77947)

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/libraries/System.Private.CoreLib/src/System/Globalization/Ordinal.cs

@@ -5,6 +5,7 @@
 using System.Text.Unicode;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Runtime.Intrinsics;
 
 namespace System.Globalization
 {
@@ -75,7 +76,62 @@ internal static int CompareStringIgnoreCaseNonAscii(ref char strA, int lengthA,
             return OrdinalCasing.CompareStringIgnoreCase(ref strA, lengthA, ref strB, lengthB);
         }
 
+        private static bool EqualsIgnoreCase_Vector128(ref char charA, ref char charB, int length)
+        {
+            Debug.Assert(length >= Vector128<ushort>.Count);
+            Debug.Assert(Vector128.IsHardwareAccelerated);
+
+            nuint lengthU = (nuint)length;
+            nuint lengthToExamine = lengthU - (nuint)Vector128<ushort>.Count;
+            nuint i = 0;
+            Vector128<ushort> vec1;
+            Vector128<ushort> vec2;
+            do
+            {
+                vec1 = Vector128.LoadUnsafe(ref Unsafe.As<char, ushort>(ref charA), i);
+                vec2 = Vector128.LoadUnsafe(ref Unsafe.As<char, ushort>(ref charB), i);
+
+                if (!Utf16Utility.AllCharsInVector128AreAscii(vec1 | vec2))
+                {
+                    goto NON_ASCII;
+                }
+
+                if (!Utf16Utility.Vector128OrdinalIgnoreCaseAscii(vec1, vec2))
+                {
+                    return false;
+                }
+
+                i += (nuint)Vector128<ushort>.Count;
+            } while (i <= lengthToExamine);
+
+            // Use scalar path for trailing elements
+            return i == lengthU || EqualsIgnoreCase(ref Unsafe.Add(ref charA, i), ref Unsafe.Add(ref charB, i), (int)(lengthU - i));
+
+        NON_ASCII:
+            if (Utf16Utility.AllCharsInVector128AreAscii(vec1) || Utf16Utility.AllCharsInVector128AreAscii(vec2))
+            {
+                // No need to use the fallback if one of the inputs is full-ASCII
+                return false;
+            }
+
+            // Fallback for Non-ASCII inputs
+            return CompareStringIgnoreCase(
+                ref Unsafe.Add(ref charA, i), (int)(lengthU - i),
+                ref Unsafe.Add(ref charB, i), (int)(lengthU - i)) == 0;
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal static bool EqualsIgnoreCase(ref char charA, ref char charB, int length)
+        {
+            if (!Vector128.IsHardwareAccelerated || length < Vector128<ushort>.Count)
+            {
+                return EqualsIgnoreCase_Scalar(ref charA, ref charB, length);
+            }
+
+            return EqualsIgnoreCase_Vector128(ref charA, ref charB, length);
+        }
+
+        internal static bool EqualsIgnoreCase_Scalar(ref char charA, ref char charB, int length)
         {
             IntPtr byteOffset = IntPtr.Zero;
 

src/libraries/System.Private.CoreLib/src/System/Text/Unicode/Utf16Utility.cs

@@ -3,6 +3,7 @@
 
 using System.Runtime.CompilerServices;
 using System.Diagnostics;
+using System.Runtime.Intrinsics;
 
 namespace System.Text.Unicode
 {
@@ -217,5 +218,43 @@ internal static bool UInt64OrdinalIgnoreCaseAscii(ulong valueA, ulong valueB)
             indicator |= 0xFF7F_FF7F_FF7F_FF7Ful;
             return (differentBits & indicator) == 0;
         }
+
+        /// <summary>
+        /// Returns true iff the Vector128 represents 8 ASCII UTF-16 characters in machine endianness.
+        /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static bool AllCharsInVector128AreAscii(Vector128<ushort> vec)
+        {
+            return (vec & Vector128.Create(unchecked((ushort)~0x007F))) == Vector128<ushort>.Zero;
+        }
+
+        /// <summary>
+        /// Given two Vector128 that represent 8 ASCII UTF-16 characters each, returns true iff
+        /// the two inputs are equal using an ordinal case-insensitive comparison.
+        /// </summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static bool Vector128OrdinalIgnoreCaseAscii(Vector128<ushort> vec1, Vector128<ushort> vec2)
+        {
+            // ASSUMPTION: Caller has validated that input values are ASCII.
+
+            // the 0x80 bit of each word of 'lowerIndicator' will be set iff the word has value >= 'A'
+            Vector128<sbyte> lowIndicator1 = Vector128.Create((sbyte)(0x80 - 'A')) + vec1.AsSByte();
+            Vector128<sbyte> lowIndicator2 = Vector128.Create((sbyte)(0x80 - 'A')) + vec2.AsSByte();
+
+            // the 0x80 bit of each word of 'combinedIndicator' will be set iff the word has value >= 'A' and <= 'Z'
+            Vector128<sbyte> combIndicator1 =
+                Vector128.LessThan(Vector128.Create(unchecked((sbyte)(('Z' - 'A') - 0x80))), lowIndicator1);
+            Vector128<sbyte> combIndicator2 =
+                Vector128.LessThan(Vector128.Create(unchecked((sbyte)(('Z' - 'A') - 0x80))), lowIndicator2);
+
+            // Convert both vectors to lower case by adding 0x20 bit for all [A-Z][a-z] characters
+            Vector128<sbyte> lcVec1 =
+                Vector128.AndNot(Vector128.Create((sbyte)0x20), combIndicator1) + vec1.AsSByte();
+            Vector128<sbyte> lcVec2 =
+                Vector128.AndNot(Vector128.Create((sbyte)0x20), combIndicator2) + vec2.AsSByte();
+
+            // Compare two lowercased vectors
+            return (lcVec1 ^ lcVec2) == Vector128<sbyte>.Zero;
+        }
     }
 }