[release/6.0] Do not promote struct locals with holes (#62738)

* Make sure the combined field size matches the struct size

* Fix the condition

* Update test and include containHoles condition

Co-authored-by: Kunal Pathak <Kunal.Pathak@microsoft.com>

src/coreclr/jit/lclvars.cpp

@@ -2103,8 +2103,13 @@ bool Compiler::StructPromotionHelper::ShouldPromoteStructVar(unsigned lclNum)
         // multiple registers?
         if (compiler->lvaIsMultiregStruct(varDsc, compiler->info.compIsVarArgs))
         {
-            if ((structPromotionInfo.fieldCnt != 2) &&
-                !((structPromotionInfo.fieldCnt == 1) && varTypeIsSIMD(structPromotionInfo.fields[0].fldType)))
+            if (structPromotionInfo.containsHoles && structPromotionInfo.customLayout)
+            {
+                JITDUMP("Not promoting multi-reg struct local V%02u with holes.\n", lclNum);
+                shouldPromote = false;
+            }
+            else if ((structPromotionInfo.fieldCnt != 2) &&
+                     !((structPromotionInfo.fieldCnt == 1) && varTypeIsSIMD(structPromotionInfo.fields[0].fldType)))
             {
                 JITDUMP("Not promoting multireg struct local V%02u, because lvIsParam is true, #fields != 2 and it's "
                         "not a single SIMD.\n",

src/tests/JIT/Regression/JitBlue/Runtime_62597/Runtime_62597.cs

@@ -0,0 +1,70 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+//
+// Note: In below test case, we were not honoring the fact that the explicit struct size
+//       of struct is 32 bytes while the only 2 fields it has is just 2 bytes. In such case,
+//       we would pass partial struct value.
+using System;
+using System.Reflection.Emit;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using System.Text;
+
+[StructLayout(LayoutKind.Explicit, Size = 32)]
+public readonly unsafe struct SmallString
+{
+    [FieldOffset(0)] private readonly byte _length;
+    [FieldOffset(1)] private readonly byte _firstByte;
+
+    public SmallString(string value)
+    {
+        fixed (char* srcPtr = value)
+        fixed (byte* destPtr = &_firstByte)
+        {
+            Encoding.ASCII.GetBytes(srcPtr, value.Length, destPtr, value.Length);
+        }
+
+        _length = (byte)value.Length;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public byte Dump()
+    {
+        fixed (byte* ptr = &_firstByte)
+        {
+            byte* next = ptr + 1;
+            return *next;
+        }
+    }
+}
+
+public static class Program
+{
+    static int result = 0;
+    public static int Main()
+    {
+        var value = new SmallString("foobar");
+
+        TheTest(value);
+
+        return result;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static void TheTest(SmallString foo)
+    {
+        Execute(foo);
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    public static object Execute(SmallString foo)
+    {
+        byte value = foo.Dump();
+        // 111 corresponds to the ASCII code of 2nd characted of string "foobar" i.e. ASCII value of 'o'.
+        if (value == 111)
+        {
+            result = 100;
+        }
+        return new StringBuilder();
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_62597/Runtime_62597.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <AllowUnsafeBlocks>True</AllowUnsafeBlocks>
+  </PropertyGroup>
+  <PropertyGroup>
+    <DebugType>None</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>