.NET 4.6 Optimizer Bug Causes Methods To Get Wrong Parameters

tobiasly writes: A serious bug in the just-released .NET 4.6 runtime causes the JIT compiler to generate incorrectly-optimized code which results in methods getting called with different parameters than what were passed in. Nick Craver of Stack Exchange has an excellent write-up of the technical details and temporary workarounds; Microsoft has acknowledged the problem and submitted an as-yet unreleased patch.

This problem is compounded by Microsoft's policy of replacing the existing .NET runtime, as opposed to the side-by-side runtimes which were possible until .NET 2.0. This means that even if your project targets .NET 4.5, it will get the 4.6 runtime if it was installed on that machine. Since it's not possible to install the just-released Visual Studio 2015 without .NET 4.6, this means developers must make the difficult choice between using the latest tools or risking crippling bugs such as this one.

Comments

Re:Who cares?

[gstoddart]

Why is the story of Slashdot being sold not on SLASHDOT!?!?!?

Well, ignoring the rest of your comment, this is actually worth highlighting.

The Company acquired Slashdot Media in 2012 both to provide the Dice business with broader reach into Slashdot's user community base and to extend the Dice business outside North America by engaging with SourceForge's significant international technology user community. The Company, however, has not successfully leveraged the Slashdot user base to further Dice's digital recruitment business; and with the acquisition of The IT Job Board and success of Open Web, the anticipated value to the Company of the SourceForge traffic outside North America has not materialized. The Company now plans to divest the business, as it does not fit within the Company's strategic initiatives and believes the Slashdot Media business will have the opportunity to improve its financial performance under different ownership.

Good riddance, dice.

Sorry we couldn't help you leverage your synergies.

Actually, we're not sorry at all.

The bigger issue

[RingDev]

This bug is in the JIT optimizer of the 4.6 framework. For apps you are developing, it's absolutely no problem, you just go into the compiler settings and uncheck the 'optimize' setting.

The problem though, is that the 4.6 framework is an in-place replacement for the 4.5 framework, which was an in-place replacement for the 4.0 framework. And the JIT optimizer is on by default. So if you install the 4.6 framework, it could potentially introduce this bug into any application developed targeting the 4.0, 4.5, or 4.6 framework that is already distributed.

Luckily, it appears as though the issue is a combination of a nullable int that has a bug in the boxing/unboxing of it's operator when calling the .hasValue method. So the actual number of places where this will actually pop up is hopefully quite limited.

That said, MS better get this patch deployed ASAP. Or if you are in a critical hurry, the correction has already been committed to the .Net Git repo, so you can brave a build from that.

-Rick