UInt128 improve division by 64 bit on x64 (#99747)

Daniel-Svensson · gfoidl · tannergooding · web-flow · commit c43fc8966036 · 2024-10-30T10:01:42.000-07:00
* Add fast path for UInt128 division on x64 when divisor is as most 64bits

* Update src/libraries/System.Private.CoreLib/src/System/UInt128.cs

Co-authored-by: Günther Foidl &lt;gue@korporal.at&gt;

* Supress warning CA2252 // This API requires opting into preview features

* Check upper bits for 0 and do normal divion for x64 (faster when it is known that upper is zero such as division by constant)

* Change supression to SYSLIB5004 after DivRem was changes to [Experimental]

---------

Co-authored-by: Günther Foidl &lt;gue@korporal.at&gt;
Co-authored-by: Tanner Gooding &lt;tagoo@outlook.com&gt;
diff --git a/src/libraries/System.Private.CoreLib/src/System/UInt128.cs b/src/libraries/System.Private.CoreLib/src/System/UInt128.cs
@@ -8,6 +8,7 @@
 using System.Numerics;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Runtime.Intrinsics.X86;
 
 namespace System
 {
@@ -1103,6 +1104,20 @@ public static UInt128 Log2(UInt128 value)
                     // left and right are both uint64
                     return left._lower / right._lower;
                 }
+                else if (X86Base.X64.IsSupported)
+                {
+                    ulong highRes = 0ul;
+                    ulong remainder = left._upper;
+
+#pragma warning disable SYSLIB5004 // DivRem is marked as [Experimental], partly because it does not get optmized by the JIT for constant inputs
+                    if (remainder >= right._lower)
+                    {
+                        (highRes, remainder) = X86Base.X64.DivRem(left._upper, 0, right._lower);
+                    }
+
+                    return new UInt128(highRes, X86Base.X64.DivRem(left._lower, remainder, right._lower).Quotient);
+#pragma warning restore SYSLIB5004 // DivRem is marked as [Experimental]
+                }
             }
 
             if (right >= left)
