VS 2019 optimize code in release mode broken?

Question

For me it looks quite strange, and like a bug.
This code in Release mode in Visual Studio 2019 provides infinite loop.

class Program
{
    private static int _a;

    static void Main(string[] args)
    {
        _a = 1;
        while (_a == 1)
        {
            Console.WriteLine(_a);
            _a = 0;
        }
    }
}

volatile or Thread.MemoryBarrier(); (after _a = 0;) solves the issue. Don't think I had such issue with VS2015. Is this correct behavior? What exact part is optimized?

Which .NET Framework or .NET Core SDK version? Which platform when .NET Core? — nkr, Aug 28 '19 at 13:22
I can reproduce this. It certainly looks like a bug - there's no multithreading here, so it shouldn't be necessary to use Volatile or anything like that! (Reproduced with x64, .net 4.8, Visual Studio 2019 version 16.2.3, release build) — Matthew Watson, Aug 28 '19 at 13:30
This should definitely be reported to Microsoft. It's a nice simple repro of a pretty nasty error. — Matthew Watson, Aug 28 '19 at 13:34
works fine as well if you initialize the variable: private static int _a = 0; — Michael Sander, Aug 28 '19 at 13:36
Disabling "Prefer 32-bit" with AnyCPU is also making problems. — nkr, Aug 28 '19 at 13:37
@nkr you are right, disabling "Prefer 32-bit" with Any CPU results in issue. Locally I have x64 windows — Alexey Klipilin, Aug 28 '19 at 13:41
I have reported this bug to Microsoft and linked the bug report back to this thread. See https://developercommunity.visualstudio.com/content/problem/715760/jit-compiler-bug-incorrect-caching-of-loop-variabl.html — Matthew Watson, Aug 28 '19 at 13:45
I was able to reproduce the problem with VS 2017. Changing "Any CPU" to x64 and it's infinite loop with `0`. Works fine with x86 (Windows 10, 64-bit). Bug appears from .Net Framework 4.0 and upwards, e.g. 2.0 doesn't have it. — Sinatr, Aug 29 '19 at 07:37
[I've also reported this on GitHub](https://github.com/dotnet/coreclr/issues/26417) (which seems to be the correct place for the bug report!) — Matthew Watson, Aug 29 '19 at 09:56
I was able to reproduce this issue on vs 2019 community 16.2.4, release — Maytham Fahmi, Sep 06 '19 at 20:10

score 3 · Accepted Answer · answered Sep 06 '19 at 20:02

3

Thank you for reporting this issue.

A fix for it is in the works and will be available shortly.

answered Sep 06 '19 at 20:02

Brian Sullivan

149
3

1

Is this a comment? – Maytham Fahmi Sep 06 '19 at 20:05
@maytham-ɯɐɥʇʎɐɯ It's a response from a contributor to the codebase; implicit in this post is an acknowledgement that the report above is indeed a bug rather than intended behavior. – TylerH Sep 17 '19 at 16:42
Fixed in CoreCLR 3.0 with https://github.com/dotnet/coreclr/pull/26952 – Brian Sullivan Nov 26 '19 at 18:54

VS 2019 optimize code in release mode broken?

1 Answers1