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Micro Optimization: Elide overflow checking in Data.bytes #1550

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Oct 15, 2025
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Micro Optimization: Elide overflow checking in Data.bytes #1550

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20 changes: 11 additions & 9 deletions Sources/FoundationEssentials/Data/Data.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -207,7 +207,7 @@ internal final class __DataStorage : @unchecked Sendable {

@inlinable // This is @inlinable as trivially computable.
var mutableBytes: UnsafeMutableRawPointer? {
return _bytes?.advanced(by: -_offset)
return _bytes?.advanced(by: _offset &* -1) // _offset is guaranteed to be non-negative, so it can never overflow when negating
}

@inlinable
Expand Down Expand Up @@ -955,7 +955,8 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
@inlinable // This is @inlinable as trivially computable.
var count: Int {
get {
return Int(slice.upperBound - slice.lowerBound)
// The upper bound is guaranteed to be greater than or equal to the lower bound, and the lower bound must be non-negative so subtraction can never overflow
return Int(slice.upperBound &- slice.lowerBound)
}
set(newValue) {
assert(newValue < HalfInt.max)
Expand Down Expand Up @@ -1112,7 +1113,8 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect

@inlinable @inline(__always) // This is @inlinable as trivially computable.
var count: Int {
return range.upperBound - range.lowerBound
// The upper bound is guaranteed to be greater than or equal to the lower bound, and the lower bound must be non-negative so subtraction can never overflow
return range.upperBound &- range.lowerBound
}

@inlinable @inline(__always) // This is @inlinable as a trivial initializer.
Expand Down Expand Up @@ -2214,10 +2216,10 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
switch _representation {
case .empty:
buffer = UnsafeRawBufferPointer(start: nil, count: 0)
case .inline:
case .inline(let inline):
buffer = unsafe UnsafeRawBufferPointer(
start: UnsafeRawPointer(Builtin.addressOfBorrow(self)),
count: _representation.count
count: inline.count
)
case .large(let slice):
buffer = unsafe UnsafeRawBufferPointer(
Expand Down Expand Up @@ -2252,10 +2254,10 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
switch _representation {
case .empty:
buffer = UnsafeMutableRawBufferPointer(start: nil, count: 0)
case .inline:
case .inline(let inline):
buffer = unsafe UnsafeMutableRawBufferPointer(
start: UnsafeMutableRawPointer(Builtin.addressOfBorrow(self)),
count: _representation.count
count: inline.count
)
case .large(var slice):
// Clear _representation during the unique check to avoid double counting the reference, and assign the mutated slice back to _representation afterwards
Expand Down Expand Up @@ -2293,10 +2295,10 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
switch _representation {
case .empty:
buffer = UnsafeMutableRawBufferPointer(start: nil, count: 0)
case .inline:
case .inline(let inline):
buffer = unsafe UnsafeMutableRawBufferPointer(
start: UnsafeMutableRawPointer(Builtin.addressOfBorrow(self)),
count: _representation.count
count: inline.count
)
case .large(var slice):
// Clear _representation during the unique check to avoid double counting the reference, and assign the mutated slice back to _representation afterwards
Expand Down