BUG: Sum involving tempStore has a COW type of bug

[kechan]

I ran into a problem while working with sum(...) in a separate module that depend on this one. I found that the base address returned by tempStore is different than what I expect.

extension FloatVector {
  @usableFromInline var storeKey:String { get { return "FloatVector\(Element.self)" } }
  @usableFromInline var tempStore:Self { get {
    if let r = Thread.current.threadDictionary[storeKey] as? Self {
      if (r.count == count) { return r }
    }
    return Thread.setToTLS(Self(count), storeKey)
  }}

I think the line:

if let r = Thread.current.threadDictionary[storeKey] as? Self {

may be at fault. Since self is Array which is a value type, the "if let" may actually create a copy. My prelim debug seemed to confirm that r.p has a different address than whats created previously. And if your downstream logic work by pointer, you may be on an entirely different Array than the cache you expect.

NB: The tempStore will work 1st time you invoke it, but if the dictionary is not clear, the next time when it gets reused, this issue will rear its head.

Apparent simplest fix:

extension FloatVector {
  @usableFromInline var storeKey:String { get { return "FloatVector\(Element.self)" } }
  @usableFromInline var tempStore:Self { get {
    if let r = Thread.current.threadDictionary[storeKey] as? Self {
        if (r.count == count) {
//            return r
            return Thread.current.threadDictionary[storeKey] as! Self    // avoid if let, and COW
        }
    }
    return Thread.setToTLS(Self(count), storeKey)
  }}