JIT: improve return types in cases with spill temps (#15766)

If the jit sees that an inlinee has multiple return sites or has gc ref locals
it will choose to return the inline result via a temp. The jit was not assigning
a type to that temp and so losing track of some type information.

So, for inlinees returning ref types, initially type the return spill temp with
the declared return type of the method.

When importing we may discover that particular return sites will return more
specific types. If all discovered return sites agree, we can update the return
type for the spill temp to match the consensus improved type.

This can lead to removal of some type checks and also to devirtualization.

Addresses issues discussed in #9908 and #15743.

2 files changed, 139 insertions, 0 deletions

diff --git a/tests/src/JIT/opt/Devirtualization/spilledreturn.cs b/tests/src/JIT/opt/Devirtualization/spilledreturn.cs
new file mode 100644
index 0000000000..ac63806019
--- /dev/null
+++ b/tests/src/JIT/opt/Devirtualization/spilledreturn.cs
@@ -0,0 +1,100 @@
+// 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.
+
+using System;
+using System.Runtime.CompilerServices;
+
+// Examples where methods potentially return multiple types
+// but the jit can prune the set down to one type during
+// importation, which then triggers late devirtualization.
+
+public class Base 
+{
+    public virtual void Foo() { Console.WriteLine("Base:Foo"); }
+    public virtual void Bar() { Console.WriteLine("Base:Bar"); }
+}
+
+public class Derived : Base
+{
+    public override sealed void Foo() { Console.WriteLine("Derived:Foo"); }
+    public override void Bar() { Console.WriteLine("Derived:Bar"); }
+}
+
+public class Derived2 : Base
+{
+    public override sealed void Foo() { Console.WriteLine("Derived2:Foo"); }
+    public override void Bar() { Console.WriteLine("Derived2:Bar"); }
+}
+
+public class Test
+{
+    static bool vague;
+
+    // Constant prop
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static Base M(int x)
+    {
+        if (x > 0)
+        {
+            return new Derived();
+        }
+        else 
+        {
+            return new Derived2();
+        }
+    }
+
+    // All returns agree on type
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static Base N(bool b)
+    {
+        if (b)
+        {
+            Console.WriteLine("b true");
+            return new Derived();
+        }
+        else 
+        {
+            Console.WriteLine("b false");
+            return new Derived();
+        }
+    }
+
+    // Type specialization
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static Base G<T>()
+    {
+        if (typeof(T) == typeof(int))
+        {
+            return new Derived();
+        }
+        else
+        {
+            return new Derived2();
+        }
+    }
+
+    public static int Main(string[] args)
+    {
+        vague = args.Length > 0;
+
+        M(0).Foo();
+        M(0).Bar();
+        M(1).Foo();
+        M(1).Bar();
+
+        N(vague).Foo();
+        N(!vague).Bar();
+
+        G<int>().Foo();
+        G<byte>().Foo();
+        G<string>().Foo();
+
+        return 100;
+    }
+}
+
+
+        
+    
diff --git a/tests/src/JIT/opt/Devirtualization/spilledreturn.csproj b/tests/src/JIT/opt/Devirtualization/spilledreturn.csproj
new file mode 100644
index 0000000000..b3ee960d96
--- /dev/null
+++ b/tests/src/JIT/opt/Devirtualization/spilledreturn.csproj
@@ -0,0 +1,39 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.props))\dir.props" />
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <AssemblyName>$(MSBuildProjectName)</AssemblyName>
+    <SchemaVersion>2.0</SchemaVersion>
+    <ProjectGuid>{95DFC527-4DC1-495E-97D7-E94EE1F7140D}</ProjectGuid>
+    <OutputType>Exe</OutputType>
+    <AppDesignerFolder>Properties</AppDesignerFolder>
+    <FileAlignment>512</FileAlignment>
+    <ProjectTypeGuids>{786C830F-07A1-408B-BD7F-6EE04809D6DB};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
+    <ReferencePath>$(ProgramFiles)\Common Files\microsoft shared\VSTT	.0\UITestExtensionPackages</ReferencePath>
+    <SolutionDir Condition="$(SolutionDir) == '' Or $(SolutionDir) == '*Undefined*'">..\..\</SolutionDir>
+    <NuGetPackageImportStamp>7a9bfb7d</NuGetPackageImportStamp>
+    <CLRTestPriority>1</CLRTestPriority>
+  </PropertyGroup>
+  <!-- Default configurations to help VS understand the configurations -->
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' "></PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' "></PropertyGroup>
+  <ItemGroup>
+    <CodeAnalysisDependentAssemblyPaths Condition=" '$(VS100COMNTOOLS)' != '' " Include="$(VS100COMNTOOLS)..\IDE\PrivateAssemblies">
+      <Visible>False</Visible>
+    </CodeAnalysisDependentAssemblyPaths>
+  </ItemGroup>
+  <PropertyGroup>
+    <DebugType>PdbOnly</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="spilledreturn.cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <Service Include="{82A7F48D-3B50-4B1E-B82E-3ADA8210C358}" />
+  </ItemGroup>
+  <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.targets))\dir.targets" />
+  <PropertyGroup Condition=" '$(MsBuildProjectDirOverride)' != '' "></PropertyGroup>
+</Project>
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