From 5e88f38152b7c4463b08a4f16fc347536306fa7e Mon Sep 17 00:00:00 2001
From: Meghana Gupta <meghana_gupta@apple.com>
Date: Thu, 13 Nov 2025 02:24:48 -0800
Subject: [PATCH 1/3] Avoid inserting end_cow_mutation_addr in some common
 cases

---
 .../LifetimeDependenceScopeFixup.swift        | 21 ++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LifetimeDependenceScopeFixup.swift b/SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LifetimeDependenceScopeFixup.swift
index f90815bbee2d9..cf0d20752caab 100644
--- a/SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LifetimeDependenceScopeFixup.swift
+++ b/SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LifetimeDependenceScopeFixup.swift
@@ -176,6 +176,24 @@ private extension Type {
     }
     return false
   }
+
+  // Returns true if a type maybe Array/ArraySlice/ContiguousArray which are optimized COW types.
+  // The standard library introduces builtins begin_cow_mutation/end_cow_mutation for such types which are then used to optimize uniqueness checks.
+  func mayHaveOptimizedCOWType(in function: Function) -> Bool {
+    // Trivial types cannot be Array/ArraySlice/ContiguousArray.
+    if isTrivial(in: function) {
+      return false
+    }
+    // Builtin types do not contain Array/ArraySlice/ContiguousArray.
+    if isBuiltinType {
+      return false
+    }
+    // ~Copyable types cannot contain Array/ArraySlice/ContiguousArray.
+    if isMoveOnly {
+      return false
+    }
+    return true
+  }
 }
 
 /// Insert end_cow_mutation_addr for lifetime dependent values that maybe of type MutableSpan and depend on a mutable address.
@@ -203,7 +221,8 @@ private func createEndCOWMutationIfNeeded(lifetimeDep: LifetimeDependence, _ con
       return
   }
 
-  guard lifetimeDep.dependentValue.type.mayHaveMutableSpan(in: lifetimeDep.dependentValue.parentFunction, context) else {
+  guard lifetimeDep.dependentValue.type.mayHaveMutableSpan(in: lifetimeDep.dependentValue.parentFunction, context) &&
+    lifetimeDep.parentValue.type.mayHaveOptimizedCOWType(in: lifetimeDep.dependentValue.parentFunction) else {
     return
   }
 

From 1dc03fea2509e134756cd450372e0fc0b655aceb Mon Sep 17 00:00:00 2001
From: Meghana Gupta <meghana_gupta@apple.com>
Date: Thu, 13 Nov 2025 10:26:57 -0800
Subject: [PATCH 2/3] Add early exit for Escapable and copyable types from
 mayHaveMutableSpan

---
 .../FunctionPasses/LifetimeDependenceScopeFixup.swift         | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LifetimeDependenceScopeFixup.swift b/SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LifetimeDependenceScopeFixup.swift
index cf0d20752caab..3eea101ed0989 100644
--- a/SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LifetimeDependenceScopeFixup.swift
+++ b/SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LifetimeDependenceScopeFixup.swift
@@ -142,6 +142,10 @@ let lifetimeDependenceScopeFixupPass = FunctionPass(
 
 private extension Type {
   func mayHaveMutableSpan(in function: Function, _ context: FunctionPassContext) -> Bool {
+    // Escapable and Copyable types cannot have MutableSpan
+    if isEscapable || !isMoveOnly {
+      return false
+    }
     if hasArchetype {
       return true
     }

From 404173de831393cdcad41ef359d463232392093d Mon Sep 17 00:00:00 2001
From: Meghana Gupta <meghana_gupta@apple.com>
Date: Thu, 13 Nov 2025 10:39:08 -0800
Subject: [PATCH 3/3] [NFC] Add MutableSpan bounds check test

---
 ...table_span_stdlib_bounds_check_tests.swift | 31 +++++++++++++++++++
 1 file changed, 31 insertions(+)
 create mode 100644 test/SILOptimizer/mutable_span_stdlib_bounds_check_tests.swift

diff --git a/test/SILOptimizer/mutable_span_stdlib_bounds_check_tests.swift b/test/SILOptimizer/mutable_span_stdlib_bounds_check_tests.swift
new file mode 100644
index 0000000000000..79d974787cf55
--- /dev/null
+++ b/test/SILOptimizer/mutable_span_stdlib_bounds_check_tests.swift
@@ -0,0 +1,31 @@
+// RUN: %target-swift-frontend -O -emit-sil %s -disable-availability-checking | %FileCheck %s --check-prefix=CHECK-SIL 
+
+public protocol P {
+   mutating func mutate(_ other: Self)
+}
+
+// CHECK-SIL-LABEL: sil @$s38mutable_span_stdlib_bounds_check_tests0a1_B7_doubleyys11MutableSpanVyxGzAA1PRzlF :
+// CHECK-SIL: bb3({{.*}}):
+// CHECK-SIL-NOT: end_cow_mutation
+// CHECK-SIL-NOT: cond_fail "index out of bounds"
+// CHECK-SIL-LABEL: } // end sil function '$s38mutable_span_stdlib_bounds_check_tests0a1_B7_doubleyys11MutableSpanVyxGzAA1PRzlF'
+public func mutable_span_double<T: P>(_ ms: inout MutableSpan<T>) {
+  for i in ms.indices {
+    ms[i].mutate(ms[i])
+  }
+}
+
+extension Int : P {
+  public mutating func mutate(_ other: Int) {
+    self += other
+  }
+}
+
+// CHECK-SIL-LABEL: sil @$s38mutable_span_stdlib_bounds_check_tests17specializedCalleryySaySiGzF : $@convention(thin) (@inout Array<Int>) -> () {
+// CHECK-SIL-NOT: cond_fail "index out of bounds"
+// CHECK-SIL-LABEL: } // end sil function '$s38mutable_span_stdlib_bounds_check_tests17specializedCalleryySaySiGzF'
+public func specializedCaller(_ array: inout Array<Int>) {
+  var mut = array.mutableSpan
+  mutable_span_double(&mut)
+}
+