Fix unaligned memory access errors in TraceEvent

[swift-kim]

This fixes unaligned read errors (SIGBUS) caused by unmanaged pointer operations on ARM platforms. The Marshal.Read*() operations are portable and have little performance impacts.

[sharwell]

The JIT output seems very different (reference):

public unsafe double M1(IntPtr pointer, int offset) {
    return *((double*)((byte*)pointer.ToPointer() + offset));
}

C.M1(IntPtr, Int32)
    L0000: vzeroupper
    L0003: movsxd rax, r8d
    L0006: vmovsd xmm0, qword [rdx+rax]
    L000b: ret

public unsafe double M2(IntPtr pointer, int offset) {
    var val = Marshal.ReadInt64(pointer, offset);
    return *(double*)&val;
}

C.M2(IntPtr, Int32)
    L0000: sub rsp, 0x28
    L0004: vzeroupper
    L0007: mov rcx, rdx
    L000a: mov edx, r8d
    L000d: call System.Runtime.InteropServices.Marshal.ReadInt64(IntPtr, Int32)
    L0012: mov [rsp+0x20], rax
    L0017: vmovsd xmm0, qword [rsp+0x20]
    L001d: add rsp, 0x28
    L0021: ret

[sharwell]

This option seems better:

public unsafe double M3(IntPtr pointer, int offset) {
    return Unsafe.ReadUnaligned<double>(IntPtr.Add(pointer, offset).ToPointer());
}

C.M3(IntPtr, Int32)
    L0000: vzeroupper
    L0003: movsxd rax, r8d
    L0006: add rax, rdx
    L0009: vmovsd xmm0, qword [rax]
    L000d: ret

[sharwell]

We should use Unsafe.ReadUnaligned<T> to avoid the overhead of Marshal.Read*.

[swift-kim]

@sharwell Thanks for the review and introducing a good tool.

• I didn't actually consider Unsafe.* because it required an additional package dependency. If you don't mind, I'll add it.

• I replaced IntPtr.Add(pointer, offset).ToPointer() → (byte*)pointer.ToPointer() + offset for consistency with other methods. They are equivalent according to the IL compiler you referenced.

[sharwell]

@brianrob @adamsitnik Please review this added dependency. IMO it is a clear win.

[sharwell]

Looks good to me. Still needs approval from @brianrob .

[swift-kim]

For reminder: @sharwell @brianrob, could you take a look?

[brianrob]

@swift-kim, thanks for contributing this. This looks OK to me - just want to confirm that you've confirmed that the issue is fixed with the latest version of the PR, since you switched APIs?

[swift-kim]

@brianrob Yes, I've confirmed dotnet-counters.dll now works in our production system. I'll report if I ever find any other issue. Thank you.

[brianrob]

Thanks @swift-kim!