Make static [milli/micro/nano]seconds members on Duration inlinable (#73429)

This means that they can't use _Int128 for their implementation, but efficient implementation of these using only 64b arithmetic is pretty straightforward, so that's OK. This allows them to be specialized and mostly optimized away in release builds.

Author: stephentyrone

stdlib/public/core/Duration.swift

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2024 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -108,9 +108,13 @@ extension Duration {
   ///
   /// - Returns: A `Duration` representing a given number of seconds.
   @available(SwiftStdlib 5.7, *)
+  @inlinable
   public static func seconds<T: BinaryInteger>(_ seconds: T) -> Duration {
-    return Duration(_attoseconds:
-      _Int128(seconds).multiplied(by: 1_000_000_000_000_000_000 as UInt64))
+    guard let high = Int64(exactly: seconds >> 64) else { fatalError() }
+    let low = UInt64(truncatingIfNeeded: seconds)
+    var lowScaled = low.multipliedFullWidth(by: 1_000_000_000_000_000_000)
+    var highScaled = high * 1_000_000_000_000_000_000
+    return Duration(_high: highScaled + Int64(lowScaled.high), low: lowScaled.low)
   }
 
   /// Construct a `Duration` given a duration and scale, taking care so that
@@ -148,11 +152,15 @@ extension Duration {
   ///
   /// - Returns: A `Duration` representing a given number of milliseconds.
   @available(SwiftStdlib 5.7, *)
+  @inlinable
   public static func milliseconds<T: BinaryInteger>(
     _ milliseconds: T
   ) -> Duration {
-    return Duration(_attoseconds:
-      _Int128(milliseconds).multiplied(by: 1_000_000_000_000_000 as UInt64))
+    guard let high = Int64(exactly: milliseconds >> 64) else { fatalError() }
+    let low = UInt64(truncatingIfNeeded: milliseconds)
+    var lowScaled = low.multipliedFullWidth(by: 1_000_000_000_000_000)
+    var highScaled = high * 1_000_000_000_000_000
+    return Duration(_high: highScaled + Int64(lowScaled.high), low: lowScaled.low)
   }
 
   /// Construct a `Duration` given a number of seconds milliseconds as a 
@@ -173,11 +181,15 @@ extension Duration {
   ///
   /// - Returns: A `Duration` representing a given number of microseconds.
   @available(SwiftStdlib 5.7, *)
+  @inlinable
   public static func microseconds<T: BinaryInteger>(
     _ microseconds: T
   ) -> Duration {
-    return Duration(_attoseconds:
-      _Int128(microseconds).multiplied(by: 1_000_000_000_000 as UInt64))
+    guard let high = Int64(exactly: microseconds >> 64) else { fatalError() }
+    let low = UInt64(truncatingIfNeeded: microseconds)
+    var lowScaled = low.multipliedFullWidth(by: 1_000_000_000_000)
+    var highScaled = high * 1_000_000_000_000
+    return Duration(_high: highScaled + Int64(lowScaled.high), low: lowScaled.low)
   }
 
   /// Construct a `Duration` given a number of seconds microseconds as a 
@@ -198,11 +210,15 @@ extension Duration {
   ///
   /// - Returns: A `Duration` representing a given number of nanoseconds.
   @available(SwiftStdlib 5.7, *)
+  @inlinable
   public static func nanoseconds<T: BinaryInteger>(
     _ nanoseconds: T
   ) -> Duration {
-    return Duration(_attoseconds:
-      _Int128(nanoseconds).multiplied(by: 1_000_000_000))
+    guard let high = Int64(exactly: nanoseconds >> 64) else { fatalError() }
+    let low = UInt64(truncatingIfNeeded: nanoseconds)
+    var lowScaled = low.multipliedFullWidth(by: 1_000_000_000)
+    var highScaled = high * 1_000_000_000
+    return Duration(_high: highScaled + Int64(lowScaled.high), low: lowScaled.low)
   }
 }