Zero initReg in genSetGSSecurityCookie (#24371)

Fix an issue that is reproduced when 

1) a register is used in the prolog for initializing GSSecurityCookie (i.e. the register contains a random non-zero value) and 
2) the same register holds a must-init GC variable in the next basic block and 
3) the variable is live at the beginning of this basic block. 

The register was not zeroed at the end of the prolog and this was causing segmentation fault during GC.Collect().

2 files changed, 91 insertions, 0 deletions

diff --git a/tests/src/JIT/Regression/JitBlue/GitHub_24278/GitHub_24278.il b/tests/src/JIT/Regression/JitBlue/GitHub_24278/GitHub_24278.il
new file mode 100644
index 0000000000..e20cc03a79
--- /dev/null
+++ b/tests/src/JIT/Regression/JitBlue/GitHub_24278/GitHub_24278.il
@@ -0,0 +1,67 @@
+// 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.
+
+.assembly extern System.Runtime { }
+.assembly GitHub_24278 { }
+.module GitHub_24278.exe
+
+// GitHub 24278: a bug is reproduced when
+// 1) a register is used in the prolog for initializing GSSecurityCookie (i.e. the register contains a random non-zero value) and
+// 2) the same register holds a must-init GC variable in the next basic block and
+// 3) the variable is live at the beginning of this basic block.
+// The register was not zeroed at the end of the prolog and this was causing segmentation fault during GC.Collect().
+//
+// This happens in MainMethod at IL_0013:
+// V_0 is not initialized by IL instructions of one of the path to IL_0013 and must be zeroed in the prolog since it is live at IL_0013.
+
+.class private auto ansi beforefieldinit GitHub_24278.Program
+       extends [System.Runtime]System.Object
+{
+  .method private hidebysig static bool  True() cil managed noinlining
+  {
+    // Code size       2 (0x2)
+    .maxstack  8
+    IL_0000:  ldc.i4.1
+    IL_0001:  ret
+  } // end of method Program::True
+
+  .method private hidebysig static void  Use(int32 arg0) cil managed noinlining
+  {
+    // Code size       6 (0x6)
+    .maxstack  8
+    IL_0000:  call       void [System.Runtime]System.GC::Collect()
+    IL_0005:  ret
+  } // end of method Program::Use
+
+  .method private hidebysig static void  MainMethod() cil managed noinlining
+  {
+    //
+    .maxstack  1
+    .locals init (object V_0)
+    IL_0000:  call       bool GitHub_24278.Program::True()
+    IL_0005:  brtrue.s   IL_000d
+
+    IL_0007:  newobj     instance void [System.Runtime]System.Object::.ctor()
+    IL_000c:  stloc.0
+    IL_000d:  ldc.i4.s   32
+    IL_000f:  conv.u
+    IL_0010:  localloc
+    IL_0012:  ldind.i4
+    IL_0013:  call       void GitHub_24278.Program::Use(int32)
+    IL_0018:  ldloc.0
+    IL_0019:  call       void [System.Runtime]System.GC::KeepAlive(object)
+    IL_001e:  ret
+  } // end of method Program::MainMethod
+
+  .method private hidebysig static int32
+          Main(string[] args) cil managed
+  {
+    .entrypoint
+    // Code size       8 (0x8)
+    .maxstack  8
+    IL_0000:  call       void GitHub_24278.Program::MainMethod()
+    IL_0005:  ldc.i4.s   100
+    IL_0007:  ret
+  } // end of method Program::Main
+}
diff --git a/tests/src/JIT/Regression/JitBlue/GitHub_24278/GitHub_24278.ilproj b/tests/src/JIT/Regression/JitBlue/GitHub_24278/GitHub_24278.ilproj
new file mode 100644
index 0000000000..506f235e2f
--- /dev/null
+++ b/tests/src/JIT/Regression/JitBlue/GitHub_24278/GitHub_24278.ilproj
@@ -0,0 +1,24 @@
+<?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>$(MSBuildProjectName)</AssemblyName>
+    <ProjectGuid>{95DFC527-4DC1-495E-97D7-E94EE1F7140D}</ProjectGuid>
+    <OutputType>Exe</OutputType>
+    <SolutionDir Condition="$(SolutionDir) == '' Or $(SolutionDir) == '*Undefined*'">..\..\</SolutionDir>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' "></PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' "></PropertyGroup>
+  <PropertyGroup>
+    <DebugType>None</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="GitHub_24278.il" />
+  </ItemGroup>
+  <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.targets))\dir.targets" />
+  <PropertyGroup Condition=" '$(MsBuildProjectDirOverride)' != '' ">
+  </PropertyGroup>
+</Project>