Use an explicit unsigned comparison for span index checks (#83150)

LLVM ought to be able to do this transformation for us, but it currently fails to do so. We can code around it easily enough. #83172 has a better long-term fix.

Author: stephentyrone

stdlib/public/core/Span/MutableSpan.swift

@@ -290,12 +290,18 @@ extension MutableSpan where Element: ~Copyable {
   @_alwaysEmitIntoClient
   public subscript(_ position: Index) -> Element {
     unsafeAddress {
-      _precondition(indices.contains(position), "index out of bounds")
+      _precondition(
+        UInt(bitPattern: position) < UInt(bitPattern: _count),
+        "Index out of bounds"
+      )
       return unsafe UnsafePointer(_unsafeAddressOfElement(unchecked: position))
     }
     @lifetime(self: copy self)
     unsafeMutableAddress {
-      _precondition(indices.contains(position), "index out of bounds")
+      _precondition(
+        UInt(bitPattern: position) < UInt(bitPattern: _count),
+        "Index out of bounds"
+      )
        return unsafe _unsafeAddressOfElement(unchecked: position)
     }
   }

stdlib/public/core/Span/Span.swift

@@ -419,7 +419,10 @@ extension Span where Element: ~Copyable {
   @inline(__always)
   @_alwaysEmitIntoClient
   internal func _checkIndex(_ position: Index) {
-    _precondition(indices.contains(position), "Index out of bounds")
+    _precondition(
+      UInt(bitPattern: position) < UInt(bitPattern: _count),
+      "Index out of bounds"
+    )
   }
 
   /// Accesses the element at the specified position in the `Span`.

test/stdlib/Span/BoundsCheckOptimization.swift

@@ -0,0 +1,62 @@
+//===--- BoundsCheckOptimization.swift ------------------------*- swift -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 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
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+// RUN: %target-swift-frontend -primary-file %s -O -emit-assembly | %FileCheck %s --check-prefix CHECK --check-prefix CHECK-%target-cpu
+// REQUIRES: swift_stdlib_no_asserts
+
+import Swift
+
+func read(index: Int, span: Span<UInt8>) -> UInt8 {
+  span[index]
+}
+// CHECK: s23BoundsCheckOptimization4read5index4spans5UInt8VSi_s4SpanVyAFGtF:
+
+// CHECK-arm64-NOT: tbnz
+// CHECK-arm64: cmp
+// all unsigned comparison spellings?
+// CHECK-arm64-NEXT: b.{{hs|hi|ls|lo|cc|cs}}
+// CHECK-arm64-NEXT: ldrb
+// CHECK-arm64-NEXT: ret
+
+// CHECK-x86_64-NOT: test
+// CHECK-x86_64: cmp
+// all unsigned comparison spellings?
+// CHECK-x86_64-NEXT: j{{a|ae|b|be|c|na|nae|nb|nbe|nc}}
+// CHECK-x86_64-NEXT: movzb
+// x86_64 might have a frame pointer operation before ret
+
+func write(value: UInt8, index: Int, span: inout MutableSpan<UInt8>) {
+  span[index] = value
+}
+// CHECK: s23BoundsCheckOptimization5write5value5index4spanys5UInt8V_Sis11MutableSpanVyAGGztF:
+
+// CHECK-arm64-NOT: tbnz
+// CHECK-arm64: cmp
+// all unsigned comparison spellings?
+// CHECK-arm64-NEXT: b.{{hs|hi|ls|lo|cc|cs}}
+// no second compare
+// CHECK-arm64-NOT: cmp
+// no test after compare
+// CHECK-arm64-NOT: tbnz
+// CHECK-arm64: strb
+// CHECK-arm64-NEXT: ret
+
+// CHECK-x86_64-NOT: test
+// CHECK-x86_64: cmp
+// all unsigned comparison spellings?
+// CHECK-x86_64-NEXT: j{{a|ae|b|be|c|na|nae|nb|nbe|nc}}
+// no second compare
+// CHECK-x86_64-NOT: cmp
+// no test after compare
+// CHECK-x86_64-NOT: test
+// CHECK-x86_64: movb
+// x86_64 might have a frame pointer operation before ret