Ensure DecomposeRotate correctly orders parameters. (dotnet#86066)

* Add a regression test for dotnet#86207

* Ensure RotateRight and RotateLeft decomposition orders parameters correctly

* Add explicit tests covering the other two DecomposeRotate paths

* Ensure other code paths that create GT_RSH_LO nodes aren't broken

src/coreclr/jit/codegenxarch.cpp

@@ -4842,17 +4842,22 @@ void CodeGen::genCodeForShiftLong(GenTree* tree)
 
     unsigned int count = (unsigned int)shiftBy->AsIntConCommon()->IconValue();
 
-    regNumber regResult = (oper == GT_LSH_HI) ? regHi : regLo;
-
-    inst_Mov(targetType, tree->GetRegNum(), regResult, /* canSkip */ true);
-
     if (oper == GT_LSH_HI)
     {
+        regNumber tgtReg = tree->GetRegNum();
+        assert(regLo != tgtReg);
+
+        inst_Mov(targetType, tgtReg, regHi, /* canSkip */ true);
         inst_RV_RV_IV(ins, emitTypeSize(targetType), tree->GetRegNum(), regLo, count);
     }
     else
     {
         assert(oper == GT_RSH_LO);
+
+        regNumber tgtReg = tree->GetRegNum();
+        assert(regHi != tgtReg);
+
+        inst_Mov(targetType, tgtReg, regLo, /* canSkip */ true);
         inst_RV_RV_IV(ins, emitTypeSize(targetType), tree->GetRegNum(), regHi, count);
     }
 

src/coreclr/jit/decomposelongs.cpp

@@ -1399,12 +1399,12 @@ GenTree* DecomposeLongs::DecomposeRotate(LIR::Use& use)
     // For longs, we need to change rols into two GT_LSH_HIs and rors into two GT_RSH_LOs
     // so we will get:
     //
-    // shld lo, hi, rotateAmount
+    // shld lo, hiCopy, rotateAmount
     // shld hi, loCopy, rotateAmount
     //
     // or:
     //
-    // shrd lo, hi, rotateAmount
+    // shrd lo, hiCopy, rotateAmount
     // shrd hi, loCopy, rotateAmount
 
     if (oper == GT_ROL)
@@ -1475,6 +1475,11 @@ GenTree* DecomposeLongs::DecomposeRotate(LIR::Use& use)
             hiOp1 = RepresentOpAsLocalVar(hiOp1, gtLong, &gtLong->AsOp()->gtOp2);
         }
 
+        if (oper == GT_RSH_LO)
+        {
+            // lsra/codegen expects these operands in the opposite order
+            std::swap(loOp1, hiOp1);
+        }
         Range().Remove(gtLong);
 
         unsigned loOp1LclNum = loOp1->AsLclVarCommon()->GetLclNum();

src/tests/JIT/Regression/JitBlue/Runtime_86027/Runtime_86207.cs

@@ -0,0 +1,87 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Runtime.CompilerServices;
+using Xunit;
+
+public class Runtime_86027
+{
+    [Fact]
+    public static int TestRotateLeft1()
+    {
+        long[] firstOp = new long[] { 7822136809956075968, 1 };
+
+        if (long.RotateLeft(firstOp[0], 1) != -2802470453797399680)
+        {
+            return 0;
+        }
+
+        return 100;
+    }
+
+    [Fact]
+    public static int TestRotateLeft32()
+    {
+        long[] firstOp = new long[] { 7822136809956075968, 1 };
+
+        if (long.RotateLeft(firstOp[0], 32) != 3886722753596608508)
+        {
+            return 0;
+        }
+
+        return 100;
+    }
+
+    [Fact]
+    public static int TestRotateLeft33()
+    {
+        long[] firstOp = new long[] { 7822136809956075968, 1 };
+
+        if (long.RotateLeft(firstOp[0], 33) != 7773445507193217016)
+        {
+            return 0;
+        }
+
+        return 100;
+    }
+
+    [Fact]
+    public static int TestRotateRight1()
+    {
+        long[] firstOp = new long[] { 7822136809956075968, 1 };
+
+        if (long.RotateRight(firstOp[0], 1) != 3911068404978037984)
+        {
+            return 0;
+        }
+
+        return 100;
+    }
+
+    [Fact]
+    public static int TestRotateRight32()
+    {
+        long[] firstOp = new long[] { 7822136809956075968, 1 };
+
+        if (long.RotateRight(firstOp[0], 32) != 3886722753596608508)
+        {
+            return 0;
+        }
+
+        return 100;
+    }
+
+    [Fact]
+    public static int TestRotateRight33()
+    {
+        long[] firstOp = new long[] { 7822136809956075968, 1 };
+
+        if (long.RotateRight(firstOp[0], 33) != 1943361376798304254)
+        {
+            return 0;
+        }
+
+        return 100;
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_86027/Runtime_86207.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+    <DebugType>None</DebugType>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>