Handle a restored double Interval at block boundary

During the process of freeing registers that are no longer live at the start of a new block, we may restore a 'previousInterval'. If that is a double (and the freed interval was float), we need to skip the next float.

src/jit/lsra.cpp

@@ -4957,6 +4957,12 @@ void LinearScan::processBlockStartLocations(BasicBlock* currentBlock, bool alloc
                 }
 
 #ifdef _TARGET_ARM_
+                // unassignPhysReg, above, may have restored a 'previousInterval', in which case we need to
+                // get the value of 'physRegRecord->assignedInterval' rather than using 'assignedInterval'.
+                if (physRegRecord->assignedInterval != nullptr)
+                {
+                    assignedInterval = physRegRecord->assignedInterval;
+                }
                 if (assignedInterval->registerType == TYP_DOUBLE)
                 {
                     // Skip next float register, because we already addressed a double register

tests/src/JIT/Regression/JitBlue/DevDiv_541653/DevDiv_541653.cs

@@ -0,0 +1,74 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+//
+
+// The bug captured by this test was a case where:
+// - We have a double register pair that was previous occupied by a double lclVar.
+// - That lclVar becomes dead, but has subsequent references, so it remains as the
+//   previousInterval on the RegRecord.
+// - The first float half is then assigned to another lclVar. It is live across a
+//   loop backedge, so it is live at the end of the loop, but is then released before
+//   the next block is allocated.
+// - At this time, the double lclVar is restored to that RegRecord (as inactive), but
+//   the loop over the lclVars sees only the second half, and asserts because it doesn't
+//   expect to ever encounter an interval in the second half (it should have been skipped).
+
+using System;
+using System.Runtime.CompilerServices;
+
+public class DevDiv_541643
+{
+    public const int Pass = 100;
+    public const int Fail = -1;
+
+    [MethodImplAttribute(MethodImplOptions.NoInlining)]
+    public static float GetFloat(int i)
+    {
+        return (float)i;
+    }
+    [MethodImplAttribute(MethodImplOptions.NoInlining)]
+    public static double GetDouble(int i)
+    {
+        return (double)i;
+    }
+    [MethodImplAttribute(MethodImplOptions.NoInlining)]
+    public static int GetInt(float f)
+    {
+        return (int)f;
+    }
+    [MethodImplAttribute(MethodImplOptions.NoInlining)]
+    public static int GetInt(double d)
+    {
+        return (int)d;
+    }
+    [MethodImplAttribute(MethodImplOptions.NoInlining)]
+    public static int test(int count)
+    {
+        double d = GetDouble(0);
+        // Use d; it will be dead until we redefine it below.
+        int result = (int)d;
+
+        // Now define our float lclVar and use it in a loop.
+        float f = GetFloat(1);
+        for (int i = 0; i < count; i++)
+        {
+            result += GetInt(f);
+        }
+
+        // Finally, redefine d and use it.
+        d = GetDouble(3);
+        for (int i = 0; i < count; i++)
+        {
+            result += GetInt(d);
+        }
+
+        Console.WriteLine("Result: " + result);
+        return result;
+    }
+    public static int Main()
+    {
+        int result = test(10);
+        return Pass;
+    }
+}

tests/src/JIT/Regression/JitBlue/DevDiv_541653/DevDiv_541653.csproj

@@ -0,0 +1,37 @@
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+  <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.props))\dir.props" />
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+    <SolutionDir Condition="$(SolutionDir) == '' Or $(SolutionDir) == '*Undefined*'">..\..\</SolutionDir>
+  </PropertyGroup>
+  <!-- Default configurations to help VS understand the configurations -->
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
+  </PropertyGroup>
+  <ItemGroup>
+    <CodeAnalysisDependentAssemblyPaths Condition=" '$(VS100COMNTOOLS)' != '' " Include="$(VS100COMNTOOLS)..\IDE\PrivateAssemblies">
+      <Visible>False</Visible>
+    </CodeAnalysisDependentAssemblyPaths>
+  </ItemGroup>
+  <PropertyGroup>
+    <DebugType></DebugType>
+    <Optimize>True</Optimize>
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+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <Service Include="{82A7F48D-3B50-4B1E-B82E-3ADA8210C358}" />
+  </ItemGroup>
+  <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.targets))\dir.targets" />
+  <PropertyGroup Condition=" '$(MsBuildProjectDirOverride)' != '' ">
+  </PropertyGroup> 
+</Project>