JIT: treat throwing trees as GC safe points

src/coreclr/jit/compiler.hpp

@@ -3319,6 +3319,12 @@ inline bool Compiler::IsPotentialGCSafePoint(GenTree* tree) const
         }
     }
 
+    // A tree that may throw can call a throw helper, which is a gc safe point.
+    if ((tree->gtFlags & GTF_EXCEPT) != 0)
+    {
+        return true;
+    }
+
     // TYP_STRUCT-typed stores might be converted into calls (with gc safe points) in Lower.
     // This is quite a conservative fix as it's hard to prove Lower won't do it at this point.
     if (tree->OperIsLocalStore())

src/tests/JIT/Regression_ro_2/Runtime_135966.cs

@@ -0,0 +1,107 @@
+// 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;
+
+// Each victim throws before its GC ref local is first stored, so a GC while the throw
+// helper runs or the exception is dispatched must not see DirtyStack's garbage in it.
+public class Runtime_135966
+{
+    private class Holder
+    {
+        public object Value = new object();
+    }
+
+    [InlineArray(32)]
+    private struct Garbage
+    {
+        private nint _element;
+    }
+
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        int[] array = new int[1];
+
+        for (int i = 0; i < 10; i++)
+        {
+            DirtyStack();
+
+            bool caught = false;
+            try
+            {
+                NullCheckVictim(null);
+            }
+            catch (NullReferenceException) when (Collect())
+            {
+                caught = true;
+            }
+
+            Assert.True(caught);
+
+            DirtyStack();
+
+            caught = false;
+            try
+            {
+                BoundsCheckVictim(array, 5);
+            }
+            catch (IndexOutOfRangeException) when (Collect())
+            {
+                caught = true;
+            }
+
+            Assert.True(caught);
+        }
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void DirtyStack()
+    {
+        Garbage garbage = default;
+        Span<nint> span = garbage;
+        span.Fill(0x1000);
+        Consume(span);
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void Consume(Span<nint> span)
+    {
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static object NullCheckVictim(Holder holder)
+    {
+        object value = holder.Value;
+        KeepAlive(value);
+        return value;
+    }
+
+    // The local is address-exposed, so it is an untracked GC slot on every target.
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static object BoundsCheckVictim(int[] array, int index)
+    {
+        int element = array[index];
+        object value = array;
+        KeepAlive(ref value);
+        return element == 0 ? value : null;
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void KeepAlive(object value)
+    {
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void KeepAlive(ref object value)
+    {
+    }
+
+    private static bool Collect()
+    {
+        GC.Collect();
+        return true;
+    }
+}