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Remove checks in UR[M]BP.Iterator.next() #62965

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Jan 12, 2023
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Remove checks in UR[M]BP.Iterator.next() #62965

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17 changes: 15 additions & 2 deletions stdlib/public/core/UnsafeRawBufferPointer.swift.gyb
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Original file line number Diff line number Diff line change
Expand Up @@ -148,8 +148,21 @@ extension UnsafeRawBufferPointer.Iterator: IteratorProtocol, Sequence {
public mutating func next() -> UInt8? {
if _position == _end { return nil }
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This should compile to a single comparison, but doesn't. I'm opening an LLVM bug to see what we can do about that.


let result = _position!.load(as: UInt8.self)
_position! += 1
// We can do an unchecked unwrap here by borrowing invariants from the pointer.
// For a validly constructed buffer pointer, the only way _position can be nil is
// if _end is also nil. We checked that case above. Thus, we can safely do an
// unchecked unwrap here.
//
// Additionally, validly constructed buffer pointers also have an _end that is
// strictly greater than or equal to _position, and so we do not need to do checked
// arithmetic here as we cannot possibly overflow.
//
// We check these invariants in debug builds to defend against invalidly constructed
// pointers.
_debugPrecondition(_position! < _end!)
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Couldn't this be hoisted in to the iterator's initializer, as a check that _position <= _end? That would give us the same assurances, but IMO would be a bit cleaner; we wouldn't need to talk about 'validly constructed buffer pointer's any more, more like 'validly constructed iterators'. Additionally, it may improve performance in debug builds, which is also important.

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There's value in validating invariants outside of the initializer, to detect bugs that corrupt them sooner rather than later. Looking at the codegen, this doesn't cost us much, so it should probably stick around (but I would prefer they be _unsafelyUnwrappedUnchecked)

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@stephentyrone Would you like a != nil _debugPrecondition as well?

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I think I'm fine with it as you have it.

let position = _position._unsafelyUnwrappedUnchecked
let result = position.load(as: UInt8.self)
_position = position + 1
return result
}
}
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