BitOperations arm64 intrinsic for LeadingZeroCount, TrailingZeroCount and Log2 (#34486)

ARM64 intrinsic for:
- LeadingZeroCount
- Log2
- TrailingZeroCount

Author: kunalspathak

src/libraries/System.Private.CoreLib/src/System/Numerics/BitOperations.cs

@@ -4,6 +4,7 @@
 
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Runtime.Intrinsics.Arm;
 using System.Runtime.Intrinsics.X86;
 
 #if SYSTEM_PRIVATE_CORELIB
@@ -61,6 +62,11 @@ public static int LeadingZeroCount(uint value)
                 return (int)Lzcnt.LeadingZeroCount(value);
             }
 
+            if (ArmBase.IsSupported)
+            {
+                return ArmBase.LeadingZeroCount(value);
+            }
+
             // Unguarded fallback contract is 0->31
             if (value == 0)
             {
@@ -85,6 +91,11 @@ public static int LeadingZeroCount(ulong value)
                 return (int)Lzcnt.X64.LeadingZeroCount(value);
             }
 
+            if (ArmBase.Arm64.IsSupported)
+            {
+                return ArmBase.Arm64.LeadingZeroCount(value);
+            }
+
             uint hi = (uint)(value >> 32);
 
             if (hi == 0)
@@ -104,6 +115,12 @@ public static int LeadingZeroCount(ulong value)
         [CLSCompliant(false)]
         public static int Log2(uint value)
         {
+            // Enforce conventional contract 0->0 (Log(0) is undefined)
+            if (value == 0)
+            {
+                return 0;
+            }
+
             // value    lzcnt   actual  expected
             // ..0000   32      0        0 (by convention, guard clause)
             // ..0001   31      31-31    0
@@ -113,16 +130,15 @@ public static int Log2(uint value)
             // 1000..    0      31-0    31
             if (Lzcnt.IsSupported)
             {
-                // Enforce conventional contract 0->0 (Log(0) is undefined)
-                if (value == 0)
-                {
-                    return 0;
-                }
-
                 // LZCNT contract is 0->32
                 return 31 - (int)Lzcnt.LeadingZeroCount(value);
             }
 
+            if (ArmBase.IsSupported)
+            {
+                return 31 - ArmBase.LeadingZeroCount(value);
+            }
+
             // Fallback contract is 0->0
             return Log2SoftwareFallback(value);
         }
@@ -136,18 +152,23 @@ public static int Log2(uint value)
         [CLSCompliant(false)]
         public static int Log2(ulong value)
         {
-            if (Lzcnt.X64.IsSupported)
+            // Enforce conventional contract 0->0 (Log(0) is undefined)
+            if (value == 0)
             {
-                // Enforce conventional contract 0->0 (Log(0) is undefined)
-                if (value == 0)
-                {
-                    return 0;
-                }
+                return 0;
+            }
 
+            if (Lzcnt.X64.IsSupported)
+            {
                 // LZCNT contract is 0->64
                 return 63 - (int)Lzcnt.X64.LeadingZeroCount(value);
             }
 
+            if (ArmBase.Arm64.IsSupported)
+            {
+                return 63 - ArmBase.Arm64.LeadingZeroCount(value);
+            }
+
             uint hi = (uint)(value >> 32);
 
             if (hi == 0)
@@ -275,6 +296,11 @@ public static int TrailingZeroCount(uint value)
                 return (int)Bmi1.TrailingZeroCount(value);
             }
 
+            if (ArmBase.IsSupported)
+            {
+                return ArmBase.LeadingZeroCount(ArmBase.ReverseElementBits(value));
+            }
+
             // Unguarded fallback contract is 0->0
             if (value == 0)
             {
@@ -313,6 +339,10 @@ public static int TrailingZeroCount(ulong value)
                 return (int)Bmi1.X64.TrailingZeroCount(value);
             }
 
+            if (ArmBase.Arm64.IsSupported)
+            {
+                return ArmBase.Arm64.LeadingZeroCount(ArmBase.Arm64.ReverseElementBits(value));
+            }
             uint lo = (uint)value;
 
             if (lo == 0)

src/libraries/System.Utf8String.Experimental/src/System/Runtime/Intrinsics/Intrinsics.Shims.cs

@@ -103,3 +103,19 @@ public abstract class X64
         public static bool TestZ(Vector128<ushort> left, Vector128<ushort> right) => throw new PlatformNotSupportedException();
     }
 }
+
+namespace System.Runtime.Intrinsics.Arm
+{
+    internal abstract class ArmBase
+    {
+        public abstract class Arm64
+        {
+            public const bool IsSupported = false;
+            public static int LeadingZeroCount(ulong value) => throw new PlatformNotSupportedException();
+            public static uint ReverseElementBits(ulong value) => throw new PlatformNotSupportedException();
+        }
+        public const bool IsSupported = false;
+        public static int LeadingZeroCount(uint value) => throw new PlatformNotSupportedException();
+        public static uint ReverseElementBits(uint value) => throw new PlatformNotSupportedException();
+    }
+}