The Jit does not add explicit null checks for zero-offset field addresses

[SingleAccretion]

Reproduction:

Console.WriteLine((nint)Problem(null));

[MethodImpl(MethodImplOptions.NoInlining)]
static int* Problem(StructWithIndex* s)
{
    return &s->Index;
}

struct StructWithIndex
{
    public int Index;
    public int Value;
}

Compile and run.

Expected result: a NullReferenceException is thrown.

Actual result: 0 is printed.

Cause: odd code in fgMorphField that special-cases fields with zero offsets.

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Issue Details

---

Reproduction:

Console.WriteLine((nint)Problem(null));

[MethodImpl(MethodImplOptions.NoInlining)]
static int* Problem(StructWithIndex* s)
{
    return &s->Index;
}

struct StructWithIndex
{
    public int Index;
    public int Value;
}

Compile and run.

Expected result: a NullReferenceException is thrown.

Actual result: 0 is printed.

Cause: odd code in fgMorphField that special-cases fields with zero offsets.

Author:	SingleAccretion
Assignees:	SingleAccretion
Labels:	bug, area-CodeGen-coreclr
Milestone:	8.0.0