Avoid inserting end_cow_mutation_addr in some common cases

Author: meg-gupta

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
   }
 

test/SILOptimizer/mutable_span_bounds_check_tests.swift

@@ -175,13 +175,9 @@ public protocol P {
    mutating func mutate(_ other: Self)
 }
 
-// TODO: Ensure end_cow_mutation_addr is not inserted within the loop
-// This happens because MutableSpan's subscript is implemented with unsafe addressors which introduce mark_dependence [unresolved].
-// This triggers lifetime-dependence-scope-fixup which unnecessarily inserts end_cow_mutation_addr due to the conservative analysis of `mayHaveMutableSpan` on generic types.
-// TODO: Improve `mayHaveMutableSpan` to avoid this. 
 // CHECK-LABEL: sil @$s31mutable_span_bounds_check_tests0a1_B7_doubleyys11MutableSpanVyxGzAA1PRzlF :
 // CHECK-SIL: bb3({{.*}}):
-// TODO-CHECK-NOT: end_cow_mutation
+// CHECK-NOT: end_cow_mutation
 // CHECK-LABEL: } // end sil function '$s31mutable_span_bounds_check_tests0a1_B7_doubleyys11MutableSpanVyxGzAA1PRzlF'
 public func mutable_span_double<T: P>(_ ms: inout MutableSpan<T>) {
   for i in ms.indices {
@@ -225,6 +221,17 @@ extension Int : P {
 public func specializedCaller(_ array: inout Array<Int>) {
   var mut = array.mutableSpan
   mutable_span_double_bitwisecopyable(&mut)
+  mutable_span_double(&mut)
+}
+
+// CHECK-LABEL: sil @$s31mutable_span_bounds_check_tests07output_B9_init_allyys10OutputSpanVyxGz_xtAA1PRzlF :
+// CHECK-SIL: bb3({{.*}}):
+// CHECK-NOT: end_cow_mutation
+// CHECK-LABEL: } // end sil function '$s31mutable_span_bounds_check_tests07output_B9_init_allyys10OutputSpanVyxGz_xtAA1PRzlF'
+public func output_span_init_all<T: P>(_ os: inout OutputSpan<T>, _ value: T) {
+  for i in os.indices {
+    os[i] = value
+  }
 }
 
 // CHECK-LABEL: sil @$s31mutable_span_bounds_check_tests07output_B25_init_all_bitwisecopyableyys10OutputSpanVyxGz_xts15BitwiseCopyableRzAA1PRzlF :