Do you still need to zero-initialize stackalloc using Span<T>.Clear()?

[samuel-lucas6]

From this blog post back in 2020:

DON’T: Assume stack allocations are zero initialized
Most normal uses of stackalloc result in zero-initialized data. This behavior is however not guaranteed, and can change depending if the application is built for Debug or Release, and other contents of the method. Therefore, don’t assume that any of the elements in a stackalloced Span are initialized to something by default.

The stackalloc documentation also says:

The content of the newly allocated memory is undefined. You should initialize it before the use. For example, you can use the Span.Clear method that sets all the items to the default value of type T.

However, I haven't come across a case where it hasn't been zero-initialized, and I thought that example (Debug vs Release) was fixed. Is this still necessary in .NET 6 and the upcoming 7?

[PathogenDavid]

As you've read, the only guarantee you get is that there is no guarantee. You don't necessarily need to use Span.Clear, but you need to ensure you don't read data which wasn't previously written.

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In practice though, locals (including stack allocations) will always be initialized to zero unless you use SkipLocalsInit, which is why you aren't ever seeing it as non-zero. (I don't think it has ever actually differed between debug and release builds.)

However if you're relying on this, you're still relying on unspecified behavior. You're also creating a ticking time bomb if anyone were to apply SkipLocalsInit project-wide. (Which is not uncommon in performance-minded code bases.)

If you do want to rely on the stackalloc being initialized to zero and you don't want to pay the cost of double-initializing it: You should explicitly clear it and apply SkipLocalsInit to your method.