[JIT] Do not force creation of a new IG if the current IG has no instructions (dotnet#89876)

* Added 78891 test

* Fix name

* Fix test

* Feedback

* 78891 test is process isolated to use the environment variables. When enabling GC in the emitter, if the current IG has no instructions, do not force a new IG.

* Remove extend flag

* Quick cleanup

* Quick cleanup

* Update Runtime_78891.csproj

* Feedback

* Fix test

* Fix test

* Reset flags

* Fix test

* Fixing test again...

* Only set the NOGCINTERRUPT flag accordingly

Author: TIHan

src/coreclr/jit/emit.cpp

@@ -1654,7 +1654,22 @@ void* emitter::emitAllocAnyInstr(size_t sz, emitAttr opsz)
     if ((emitCurIGfreeNext + fullSize >= emitCurIGfreeEndp) || emitForceNewIG ||
         (emitCurIGinsCnt >= (EMIT_MAX_IG_INS_COUNT - 1)))
     {
-        emitNxtIG(true);
+        // If the current IG has instructions, then we need to create a new one.
+        if (emitCurIGnonEmpty())
+        {
+            emitNxtIG(true);
+        }
+        else
+        {
+            if (emitNoGCIG)
+            {
+                emitCurIG->igFlags |= IGF_NOGCINTERRUPT;
+            }
+            else
+            {
+                emitCurIG->igFlags &= ~IGF_NOGCINTERRUPT;
+            }
+        }
     }
 
     /* Grab the space for the instruction */

src/tests/JIT/Regression/JitBlue/Runtime_78891/Runtime_78891.cs

@@ -0,0 +1,59 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Runtime.CompilerServices;
+using Xunit;
+
+// This test is to ensure that an assertion does not occur in the JIT.
+public class Runtime_78891
+{
+    class C0
+    {
+        public long F1;
+    }
+
+    struct S5
+    {
+        public bool F1;
+        public int F2;
+        public C0 F4;
+        public short F5;
+        public ulong F6;
+        public uint F7;
+    }
+
+    static S5 s_48;
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    static void Consume(short x)
+    {
+
+    }
+
+    static void M59(S5 arg0, S5 arg1)
+    {
+        try
+        {
+            arg0 = arg1;
+            arg0 = arg1;
+            short var3 = arg1.F5;
+            Consume(var3);
+        }
+        finally
+        {
+            if (s_48.F4.F1 > arg0.F7)
+            {
+                arg0.F1 |= false;
+            }
+        }
+    }
+
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        var vr2 = new S5();
+        var vr3 = new S5();
+        Assert.Throws<NullReferenceException>(() => M59(vr2, vr3));
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_78891/Runtime_78891.csproj

@@ -0,0 +1,17 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <!-- Needed for CLRTestEnvironmentVariable -->
+    <RequiresProcessIsolation>true</RequiresProcessIsolation>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+  <ItemGroup>
+    <CLRTestEnvironmentVariable Include="DOTNET_JitDoVNBasedDeadStoreRemoval" Value="0" />
+    <CLRTestEnvironmentVariable Include="DOTNET_JitEnableEarlyLivenessRange" Value="0" />
+    <CLRTestEnvironmentVariable Include="DOTNET_JitEnablePhysicalPromotion" Value="0" />
+    <CLRTestEnvironmentVariable Include="DOTNET_EnableAVX2" Value="0" />
+    <CLRTestEnvironmentVariable Include="DOTNET_TieredCompilation" Value="0" />
+  </ItemGroup>
+</Project>