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[stdlib] ContiguousArray & ArraySlice: Stop swapping self in wUMBP #38898

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Aug 17, 2021
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[stdlib] ContiguousArray & ArraySlice: Stop swapping self in wUMBP #38898

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3 changes: 2 additions & 1 deletion stdlib/public/core/Array.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -1604,7 +1604,7 @@ extension Array {
_makeMutableAndUnique()
let count = _buffer.mutableCount

// Create an UnsafeBufferPointer over work that we can pass to body
// Create an UnsafeBufferPointer that we can pass to body
let pointer = _buffer.mutableFirstElementAddress
var inoutBufferPointer = UnsafeMutableBufferPointer(
start: pointer, count: count)
Expand All @@ -1615,6 +1615,7 @@ extension Array {
inoutBufferPointer.count == count,
"Array withUnsafeMutableBufferPointer: replacing the buffer is not allowed")
_endMutation()
_fixLifetime(self)
}

// Invoke the body.
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20 changes: 3 additions & 17 deletions stdlib/public/core/ArraySlice.swift
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Expand Up @@ -1239,32 +1239,18 @@ extension ArraySlice {
// Ensure unique storage
_makeMutableAndUnique()

// Ensure that body can't invalidate the storage or its bounds by
// moving self into a temporary working array.
// NOTE: The stack promotion optimization that keys of the
// "array.withUnsafeMutableBufferPointer" semantics annotation relies on the
// array buffer not being able to escape in the closure. It can do this
// because we swap the array buffer in self with an empty buffer here. Any
// escape via the address of self in the closure will therefore escape the
// empty array.

var work = ArraySlice()
(work, self) = (self, work)

// Create an UnsafeBufferPointer over work that we can pass to body
let pointer = work._buffer.firstElementAddress
// Create an UnsafeBufferPointer that we can pass to body
let pointer = _buffer.firstElementAddress
var inoutBufferPointer = UnsafeMutableBufferPointer(
start: pointer, count: count)

// Put the working array back before returning.
defer {
_precondition(
inoutBufferPointer.baseAddress == pointer &&
inoutBufferPointer.count == count,
"ArraySlice withUnsafeMutableBufferPointer: replacing the buffer is not allowed")

(work, self) = (self, work)
_endMutation()
_fixLifetime(self)
}

// Invoke the body.
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20 changes: 3 additions & 17 deletions stdlib/public/core/ContiguousArray.swift
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Expand Up @@ -1176,32 +1176,18 @@ extension ContiguousArray {
_makeMutableAndUnique()
let count = _buffer.mutableCount

// Ensure that body can't invalidate the storage or its bounds by
// moving self into a temporary working array.
// NOTE: The stack promotion optimization that keys of the
// "array.withUnsafeMutableBufferPointer" semantics annotation relies on the
// array buffer not being able to escape in the closure. It can do this
// because we swap the array buffer in self with an empty buffer here. Any
// escape via the address of self in the closure will therefore escape the
// empty array.

var work = ContiguousArray()
(work, self) = (self, work)

// Create an UnsafeBufferPointer over work that we can pass to body
let pointer = work._buffer.mutableFirstElementAddress
// Create an UnsafeBufferPointer that we can pass to body
let pointer = _buffer.mutableFirstElementAddress
var inoutBufferPointer = UnsafeMutableBufferPointer(
start: pointer, count: count)

// Put the working array back before returning.
defer {
_precondition(
inoutBufferPointer.baseAddress == pointer &&
inoutBufferPointer.count == count,
"ContiguousArray withUnsafeMutableBufferPointer: replacing the buffer is not allowed")

(work, self) = (self, work)
_endMutation()
_fixLifetime(self)
}

// Invoke the body.
Expand Down