JIT: fix regression when returning struct with no fields on SysV

src/jit/compiler.cpp

@@ -992,8 +992,9 @@ var_types Compiler::getReturnTypeForStruct(CORINFO_CLASS_HANDLE clsHnd,
                                            structPassingKind*   wbReturnStruct /* = nullptr */,
                                            unsigned             structSize /* = 0 */)
 {
-    var_types         useType           = TYP_UNKNOWN;
-    structPassingKind howToReturnStruct = SPK_Unknown; // We must change this before we return
+    var_types         useType             = TYP_UNKNOWN;
+    structPassingKind howToReturnStruct   = SPK_Unknown; // We must change this before we return
+    bool              canReturnInRegister = true;
 
     assert(clsHnd != NO_CLASS_HANDLE);
 
@@ -1013,6 +1014,7 @@ var_types Compiler::getReturnTypeForStruct(CORINFO_CLASS_HANDLE clsHnd,
     if (structDesc.eightByteCount == 1)
     {
         assert(structSize <= sizeof(double));
+        assert(structDesc.passedInRegisters);
 
         if (structDesc.eightByteClassifications[0] == SystemVClassificationTypeSSE)
         {
@@ -1022,6 +1024,11 @@ var_types Compiler::getReturnTypeForStruct(CORINFO_CLASS_HANDLE clsHnd,
             howToReturnStruct = SPK_PrimitiveType;
         }
     }
+    else
+    {
+        // Return classification is not always size based...
+        canReturnInRegister = structDesc.passedInRegisters;
+    }
 
 #endif // UNIX_AMD64_ABI
 
@@ -1031,7 +1038,7 @@ var_types Compiler::getReturnTypeForStruct(CORINFO_CLASS_HANDLE clsHnd,
     // The largest primitive type is 8 bytes (TYP_DOUBLE)
     // so we can skip calling getPrimitiveTypeForStruct when we
     // have a struct that is larger than that.
-    if ((useType == TYP_UNKNOWN) && (structSize <= sizeof(double)))
+    if (canReturnInRegister && (useType == TYP_UNKNOWN) && (structSize <= sizeof(double)))
     {
         // We set the "primitive" useType based upon the structSize
         // and also examine the clsHnd to see if it is an HFA of count one

tests/src/JIT/Regression/JitBlue/GitHub_18522/GitHub_18522_8.cs

@@ -0,0 +1,65 @@
+// 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;
+
+// Based on
+// Original generated by Fuzzlyn on 2018-06-20 00:58:58
+// Seed: 11049252875418439527
+// Reduced from 97.5 KiB to 0.5 KiB
+// Debug: Outputs -1
+// Release: Outputs -65536
+
+// Similar to other variants but using an empty (~ 1 byte) struct
+// (which oddly enough currently is returned by ref)
+
+struct S0
+{
+}
+
+struct S1
+{
+    public S0 F3;
+    public sbyte F4;
+    public short F0;
+    public S1(sbyte f4): this()
+    {
+        F4 = f4;
+    }
+}
+
+public class GitHub_18522_8
+{
+    static S1 s_6;
+    static S1[] s_13 = new S1[]{new S1(-1)};
+    public static int Main()
+    {
+        // When generating code for the x64 SysV ABI, the jit was
+        // incorrectly typing the return type from M16, and so
+        // inadvertently overwriting the F4 field of s_13[0] on return
+        // from the call.
+        // 
+        // Here we make sure we properly handle the failed inline case.
+        s_13[0].F3 = M16();
+        return s_13[0].F4 == -1 ? 100 : 0;
+    }
+
+    static S0 M16()
+    {
+        // This bit of code is intended to allow M16 to be an
+        // inline candidate that ultimately does not get inlined.
+        short x = 0;
+        for (int i = 0; i < 10; i++)
+        {
+            for (int j = 0; j < 10; j++)
+            {
+                x++;
+            }
+        }
+        s_6.F0 = x;
+
+        return s_6.F3;
+    }
+}

tests/src/JIT/Regression/JitBlue/GitHub_18522/GitHub_18522_8.csproj

@@ -0,0 +1,34 @@
+<?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>$(MSBuildProjectName)</AssemblyName>
+    <SchemaVersion>2.0</SchemaVersion>
+    <ProjectGuid>{95DFC527-4DC1-495E-97D7-E94EE1F7140D}</ProjectGuid>
+    <OutputType>Exe</OutputType>
+    <ProjectTypeGuids>{786C830F-07A1-408B-BD7F-6EE04809D6DB};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
+    <SolutionDir Condition="$(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="$(VS100COMNTOOLS)..\IDE\PrivateAssemblies">
+      <Visible>False</Visible>
+    </CodeAnalysisDependentAssemblyPaths>
+  </ItemGroup>
+  <PropertyGroup>
+    <DebugType></DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(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>