Rare case of not properly rounding up thread stack size on Windows

[AronParker]

The following code states that stack_size rounds up to the next 64 kB:

rust/library/std/src/sys/windows/thread.rs

Lines 31 to 33 in 97cde9f

	// Round up to the next 64 kB because that's what the NT kernel does,
	// might as well make it explicit.
	let stack_size = (stack + 0xfffe) & (!0xfffe);

However, if stack is any of 0xffff * x + 1 (where x is a non-negative number) it will not properly round up. Additionally, the least significant byte is retained, I doubt that is intended either. Am I missing something? Otherwise I believe it's meant to be 0xffff instead of 0xfffe to properly round up.

[ChrisDenton]

To be honest, I think this line may as well be removed. As the comment says, the OS will do the rounding up in any case. And as the MSDN docs say:

The operating system rounds up the specified size to the nearest multiple of the system's allocation granularity (typically 64 KB).

So 64 KB isn't technically guaranteed. I'm not sure there's anything to be gained by doing the rounding up ourselves.

[ChrisDenton]

@rustbot label +O-windows +T-libs

[0xPoe]

@rustbot claim