Combine two table lookups into one while formatting Guid on Arm64 (#87126)

* Combine two table lookups into one while formatting Guid on Arm64

* Add a comment to explain the combined table look-up and hex indices

Author: SwapnilGaikwad

src/libraries/System.Private.CoreLib/src/System/Guid.cs

@@ -1405,7 +1405,7 @@ private static (Vector128<byte>, Vector128<byte>, Vector128<byte>) FormatGuidVec
 
             if (useDashes)
             {
-                // We divide 16 bytes into 3 x Vector128<byte>:
+                // We divide 32 bytes into 3 x Vector128<byte>:
                 //
                 // ________-____-____-____-____________
                 // xxxxxxxxxxxxxxxx
@@ -1421,14 +1421,29 @@ private static (Vector128<byte>, Vector128<byte>, Vector128<byte>) FormatGuidVec
                     Vector128.Create(0x7060504FF030201, 0xF0E0D0C0B0A0908).AsByte());
 
                 // Vector "z" - we need to merge some elements of hexLow with hexHigh and add 4 dashes.
-                Vector128<byte> mid1 = Vector128.Shuffle(hexLow,
-                    Vector128.Create(0x0D0CFF0B0A0908FF, 0xFFFFFFFFFFFF0F0E).AsByte());
-                Vector128<byte> mid2 = Vector128.Shuffle(hexHigh,
-                    Vector128.Create(0xFFFFFFFFFFFFFFFF, 0xFF03020100FFFFFF).AsByte());
-                Vector128<byte> dashesMask = Vector128.Shuffle(Vector128.CreateScalarUnsafe((byte)'-'),
-                    Vector128.Create(0xFFFF00FFFFFFFF00, 0x00FFFFFFFF00FFFF).AsByte());
-
-                Vector128<byte> vecZ = (mid1 | mid2 | dashesMask);
+                Vector128<byte> vecZ;
+                Vector128<byte> dashesMask = Vector128.Create(0x00002D000000002D, 0x2D000000002D0000).AsByte();
+                if (AdvSimd.Arm64.IsSupported)
+                {
+                    // Arm64 allows shuffling values using a 32-byte wide look-up table consisting of two 128-bit registers.
+                    // Each byte in the second arg represents a value between 0 to 31 that acts as an index in the look-up table.
+                    // Now we can create a "z" vector by selecting 12 values starting from the 9th element (index 0x08) and
+                    // leaving gaps for dashes. Thus, the wider look-up table allows combining two shuffles, as used in the
+                    // generic else-case, into a single instruction on Arm64.
+                    // TODO: Check if the JIT can merge the consecutive table look-ups and avoid the Arm64 specific if-case.
+                    Vector128<byte> mid = AdvSimd.Arm64.VectorTableLookup((hexLow, hexHigh),
+                        Vector128.Create(0x0D0CFF0B0A0908FF, 0xFF13121110FF0F0E).AsByte());
+                    vecZ = (mid | dashesMask);
+                }
+                else
+                {
+                    Vector128<byte> mid1 = Vector128.Shuffle(hexLow,
+                        Vector128.Create(0x0D0CFF0B0A0908FF, 0xFFFFFFFFFFFF0F0E).AsByte());
+                    Vector128<byte> mid2 = Vector128.Shuffle(hexHigh,
+                        Vector128.Create(0xFFFFFFFFFFFFFFFF, 0xFF03020100FFFFFF).AsByte());
+                    vecZ = (mid1 | mid2 | dashesMask);
+                }
+
                 return (vecX, vecY, vecZ);
             }