[stdlib] internal UnsafeRawBufferPointer tweaks

Author: glessard

stdlib/public/core/Span/OutputRawSpan.swift

@@ -340,15 +340,15 @@ extension OutputRawSpan {
     ) throws(E) -> R
   ) throws(E) -> R {
     guard let start = unsafe _pointer, capacity > 0 else {
-      let buffer = unsafe UnsafeMutableRawBufferPointer(start: nil, count: 0)
+      let buffer = UnsafeMutableRawBufferPointer(_empty: ())
       var initializedCount = 0
       defer {
         _precondition(initializedCount == 0, "OutputRawSpan capacity overflow")
       }
       return unsafe try body(buffer, &initializedCount)
     }
     let buffer = unsafe UnsafeMutableRawBufferPointer(
-      start: start, count: capacity
+      _uncheckedStart: start, count: capacity
     )
     var initializedCount = _count
     defer {

stdlib/public/core/UnsafeRawBufferPointer.swift.gyb

@@ -100,6 +100,17 @@ public struct Unsafe${Mutable}RawBufferPointer {
   @usableFromInline
   internal let _position, _end: Unsafe${Mutable}RawPointer?
 
+  // This works around _debugPrecondition() impacting the performance of
+  // optimized code. (rdar://72246338)
+  @_alwaysEmitIntoClient
+  internal init(
+    @_nonEphemeral _uncheckedStart start: Unsafe${Mutable}RawPointer?,
+    count: Int
+  ) {
+    unsafe _position = start
+    unsafe _end = start.map { unsafe $0 + _assumeNonNegative(count) }
+  }
+
   /// Creates a buffer over the specified number of contiguous bytes starting
   /// at the given pointer.
   ///
@@ -116,9 +127,14 @@ public struct Unsafe${Mutable}RawBufferPointer {
     _debugPrecondition(count >= 0, "${Self} with negative count")
     _debugPrecondition(unsafe count == 0 || start != nil,
       "${Self} has a nil start and nonzero count")
+    unsafe self.init(_uncheckedStart: start, count: count)
+  }
 
-    unsafe _position = start
-    unsafe _end = start.map { unsafe $0 + _assumeNonNegative(count) }
+  @safe
+  @_alwaysEmitIntoClient
+  public init(_empty: ()) {
+    unsafe _position = nil
+    unsafe _end = nil
   }
 }