Update borrowed from for guaranteed results of borrow accessors

Author: meg-gupta

SwiftCompilerSources/Sources/SIL/Utilities/BorrowUtils.swift

@@ -535,6 +535,9 @@ public final class EnclosingValueIterator : IteratorProtocol {
           // Recurse through guaranteed forwarding non-phi instructions.
           let ops = forwardingInst.forwardedOperands
           worklist.pushIfNotVisited(contentsOf: ops.lazy.map { $0.value })
+        } else if value.isGuaranteedApplyResult {
+          let selfArgument = (value as! ApplyInst).arguments.last!
+          worklist.pushIfNotVisited(selfArgument)
         } else {
           fatalError("cannot get borrow introducers for unknown guaranteed value")
         }
@@ -617,6 +620,19 @@ extension Value {
     }
     return self
   }
+
+  public var isGuaranteedApplyResult: Bool {
+    guard let definingInstruction = self.definingInstruction else {
+      return false
+    }
+    guard let apply = definingInstruction as? ApplyInst else {
+      return false
+    }
+    guard apply.singleDirectResult != nil else {
+      return false
+    }
+    return apply.functionConvention.results[0].convention == .guaranteed
+  }
 }
 
 extension Phi {

test/SILOptimizer/borrowed_from_updater.sil

@@ -300,3 +300,47 @@ bb2(%5 : @guaranteed $C):
   return %0 : $C
 }
 
+class Klass {}
+
+public struct Wrapper {
+  @_hasStorage var _k: Klass { get set }
+  var k: Klass
+}
+
+public struct GenWrapper<T> {
+  @_hasStorage var _prop: T { get set }
+  public var prop: T
+}
+
+// CHECK-LABEL: sil [ossa] @borrow_loadable_prop : $@convention(method) (@guaranteed Wrapper) -> @guaranteed Klass {
+sil [ossa] @borrow_loadable_prop : $@convention(method) (@guaranteed Wrapper) -> @guaranteed Klass {
+bb0(%0 : @guaranteed $Wrapper):
+  %2 = struct_extract %0, #Wrapper._k
+  return %2
+}
+
+// CHECK-LABEL: sil [ossa] @test_borrow_accessor :
+// CHECK: bb3([[PHI:%.*]] : @guaranteed $Klass):
+// CHECK: [[B:%.*]] = borrowed [[PHI]] : $Klass from (%1 : $Wrapper)
+// CHECK-LABEL: } // end sil function 'test_borrow_accessor'
+sil [ossa] @test_borrow_accessor : $@convention(thin) (@owned Wrapper) -> () {
+bb0(%0 : @owned $Wrapper):
+  %1 = begin_borrow %0
+  %2 = function_ref @borrow_loadable_prop : $@convention(method) (@guaranteed Wrapper) -> @guaranteed Klass
+  cond_br undef, bb1, bb2
+
+bb1:
+  %3 = apply %2(%1) : $@convention(method) (@guaranteed Wrapper) -> @guaranteed Klass
+  br bb3(%3)
+
+bb2:
+  %5 = apply %2(%1) : $@convention(method) (@guaranteed Wrapper) -> @guaranteed Klass
+  br bb3(%5)
+
+bb3(%7 : @guaranteed $Klass):
+  end_borrow %1
+  destroy_value %0
+  %t = tuple ()
+  return %t
+}
+