This repository is for demonstrating a .Net 4.6 tail call issue in RyuJIT when running on x64. We initially noticed the issue with MiniProfiler in Stack Overflow when testing locally (a Visual Studio 2015/.Net 4.6 environment for all developers). The issue does not present on our production tier (which is still .Net 4.5.2, though it is built with Roslyn 1.0.0-rc2 compilers).

A full blog post on how we discovered the issue and severity is here Please, take a few minutes and read. The impact of this subtle bug is, in our opinion, critical.

Let's start off with ways to fix this up front. There are a few options.

• If you haven't installed .Net 4.6, don't. Hold off until this serious issue is resolved.
• If you have or must install .Net 4.6, there are 2 workaround options:
• Disable RyuJIT, via a registry key:
  Set-ItemProperty -Path HKLM:\Software\Microsoft\.NETFramework -Name useLegacyJit -Type DWord -Value 1
• Disable RyuJIT, via an environmental variable:
  COMPLUS_useLegacyJit=1

• This must be run in IIS, we cannot reproduce the bug while in visual studio. Though attaching a debugger is feasible, using the build-in VS hosting does not reproduce.
• StackRedis performance optimizations must be enabled, they are by default when you pull. The DEBUG solution builds a RELEASE StackRedis with optimizations.

While there is a lot of code here to repro, the actual problem area is narrowly constrained. Here's the affected area in this repro case:

public void Set<T>(string key, T value, int? durationSecs, bool sliding, bool broadcastRemoveFromCache = false)
{
    LocalCache.OnLogDuration(key, durationSecs, "LocalCache.Set");
    SetWithPriority<T>(key, value, durationSecs, sliding, System.Web.Caching.CacheItemPriority.Default);
}

public void SetWithPriority<T>(string key, T value, int? durationSecs, bool isSliding, System.Web.Caching.CacheItemPriority priority)
{
    LocalCache.OnLogDuration(key, durationSecs, "LocalCache.SetWithPriority");
    key = KeyInContext(key);

    RawSet(key, value, durationSecs, isSliding, priority);
}

private void RawSet(string cacheKey, object value, int? durationSecs, bool isSliding, System.Web.Caching.CacheItemPriority priority)
{
    LocalCache.OnLogDuration(cacheKey, durationSecs, "RawSet");
    var absolute = !isSliding && durationSecs.HasValue ? DateTime.UtcNow.AddSeconds(durationSecs.Value) : Cache.NoAbsoluteExpiration;
    var sliding = isSliding && durationSecs.HasValue ? TimeSpan.FromSeconds(durationSecs.Value) : Cache.NoSlidingExpiration;

    HttpRuntime.Cache.Insert(cacheKey, value, null, absolute, sliding, priority, Removed);
    var evt = Added;
    if(evt != null) evt(cacheKey, value, absolute, sliding, priority, durationSecs, isSliding);

The LocalCache.OnLogDuration event handlers (added for this test) are logging the value of durationSeconds as it is passed down the stack. Each of these 3 handlers corresponds to the same-name column in the output of the homepage:

The bug manifests at the last method call here. While SetWithPriority<T> has the correct value for durationSeconds, RawSet does not. Its value is...well, we don't know where it's coming from.

For example, what you would expect in a call stack is this:

• cache.Set<Guid>(key, guid, durationSeconds: 60);
• SetWithPriority<T>(key, value, 60, false, CacheItemPriority.Default);
• RawSet(key, value, 60, false, CacheItemPriority.Default);

What actually happens is the last method call does not get the 60 (or any matching value) passed in from above. Instead it gets some other value as seen in the screenshot above. It appears to be a consistent shared value per test run, but we cannot readily explain what's getting substituted in here. The tail call is rigged up incorrectly creating what are seemingly random value substitutions here. In practice it is more restricted on affected scope but no less severe. Take for example a method moving money, or recording the prescription to give a patient and it's easy to see the very severe impact this very, very subtle bug can have.

We have reproduced this on the following platforms:

• Windows 8.1 with VS 2015 RTM, .Net 4.6, IIS 8.5
• Windows 10 with VS 2015 RTM, .Net 4.6, IIS 10
• (we have not deployed to our Server OS installs given the severity, yet)