Merge pull request swiftlang#30064 from gottesmm/pr-f768cd12b2b9ed81d02c7df2f86e47aa07ef8cbe

[stdlib] Mark Unmanaged._withUnsafeGuaranteedRef as _transparent and add a test/comments as requested in the original PR where this landed.

Author: gottesmm

stdlib/public/core/Unmanaged.swift

@@ -203,10 +203,16 @@ public struct Unmanaged<Instance: AnyObject> {
   ///    }
   ///  }
   @inlinable // unsafe-performance
+  @_transparent
   public func _withUnsafeGuaranteedRef<Result>(
     _ body: (Instance) throws -> Result
   ) rethrows -> Result {
     var tmp = self
+    // Builtin.convertUnownedUnsafeToGuaranteed expects to have a base value
+    // that the +0 value depends on. In this case, we are assuming that is done
+    // for us opaquely already. So, the builtin will emit a mark_dependence on a
+    // trivial object. The optimizer knows to eliminate that so we do not have
+    // any overhead from this.
     let fakeBase: Int? = nil
     return try body(Builtin.convertUnownedUnsafeToGuaranteed(fakeBase,
                                                              &tmp._value))

test/stdlib/unmanaged_rc.swift

@@ -0,0 +1,38 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -emit-sil | %FileCheck %s
+// RUN: %target-swift-frontend %s -emit-sil -O | %FileCheck -check-prefix=OPT %s
+
+// Make sure that when we invoke Unmanaged._withUnsafeGuaranteedRef, we do not
+// have any ref count overhead.
+public class Klass {}
+public class KlassContainer {
+    let k = Klass()
+}
+
+@inline(never)
+public func myPrint(_ k: Klass) { print(k) }
+
+// Check the codegen of _withUnsafeGuaranteedRef
+//
+// CHECK-LABEL: sil public_external [transparent] [serialized] @$ss9UnmanagedV24_withUnsafeGuaranteedRefyqd__qd__xKXEKlF : $@convention(method) <Instance where Instance : AnyObject><Result> (@noescape @callee_guaranteed <τ_0_0, τ_0_1 where τ_0_0 : _RefCountedObject> in (@guaranteed τ_0_0) -> (@out τ_0_1, @error Error) for <Instance, Result>, Unmanaged<Instance>) -> (@out Result, @error Error) {
+// CHECK: bb0([[RESULT:%.*]] : $*Result, [[FUNC:%.*]] : $@noescape @callee_guaranteed <τ_0_0, τ_0_1 where τ_0_0 : _RefCountedObject> in (@guaranteed τ_0_0) -> (@out τ_0_1, @error Error) for <Instance, Result>, [[UNMANAGED:%.*]] : $Unmanaged<Instance>):
+// CHECK: [[UNMANAGED_REF:%.*]] = struct_extract [[UNMANAGED]]
+// CHECK: [[REF:%.*]] = unmanaged_to_ref [[UNMANAGED_REF]]
+// CHECK: [[REF_MARK_DEP:%.*]] = mark_dependence [[REF]]
+// CHECK: try_apply {{%.*}}([[RESULT]], [[REF_MARK_DEP]]) : $@noescape @callee_guaranteed <τ_0_0, τ_0_1 where τ_0_0 : _RefCountedObject> in (@guaranteed τ_0_0) -> (@out τ_0_1, @error Error) for <Instance, Result>, normal bb2, error bb1
+// CHECK-NOT: destroy_value
+// CHECK: } // end sil function '$ss9UnmanagedV24_withUnsafeGuaranteedRefyqd__qd__xKXEKlF'
+
+// OPT-LABEL: sil @$s12unmanaged_rc12useUnmanagedyys0D0VyAA14KlassContainerCGF : $@convention(thin) (Unmanaged<KlassContainer>) -> () {
+// OPT: bb0([[UNMANAGED:%.*]] :
+// OPT:   [[UNMANAGED_REF:%.*]] = struct_extract [[UNMANAGED]]
+// OPT:   [[REF:%.*]] = unmanaged_to_ref [[UNMANAGED_REF]]
+// OPT:   [[REF_ELT_ADDR:%.*]] = ref_element_addr [[REF]] : $KlassContainer, #KlassContainer.k
+// OPT:   [[VALUE:%.*]] = load [[REF_ELT_ADDR]]
+// OPT:   apply {{%.*}}([[VALUE]])
+// OPT: } // end sil function '$s12unmanaged_rc12useUnmanagedyys0D0VyAA14KlassContainerCGF'
+public func useUnmanaged(_ u: Unmanaged<KlassContainer>) {
+    u._withUnsafeGuaranteedRef {
+        myPrint($0.k)
+    }
+}