[SILOptimizer] Keep lexical lifetime markers.

Previously, TempRValueElimination would peephole simple alloc_stacks,
even when they were lexical; here, they are left for Mem2Reg to properly
handle.

Previously, SemanticARCOpts would eliminate lexical begin_borrows,
incorrectly allowing the lifetime of the value borrowed by them to be
observably shortened.  Here, those borrow scopes are not eliminated if
they are lexical.

Added an executable test that verifies that a local variable strongly
referencing a delegate object keeps that delegate alive through the call
to an object that weakly references the delegate and calls out to it.

Author: nate-chandler

lib/SILOptimizer/SemanticARC/BorrowScopeOpts.cpp

@@ -29,6 +29,11 @@ bool SemanticARCOptVisitor::visitBeginBorrowInst(BeginBorrowInst *bbi) {
   if (!ctx.shouldPerform(ARCTransformKind::RedundantBorrowScopeElimPeephole))
     return false;
 
+  // Lexical borrow scopes must remain in order to ensure that value lifetimes
+  // are not observably shortened.
+  if (bbi->isLexical())
+    return false;
+
   auto kind = bbi->getOperand().getOwnershipKind();
   SmallVector<EndBorrowInst *, 16> endBorrows;
   for (auto *op : bbi->getUses()) {

lib/SILOptimizer/Transforms/TempRValueElimination.cpp

@@ -495,6 +495,12 @@ void TempRValueOptPass::tryOptimizeCopyIntoTemp(CopyAddrInst *copyInst) {
   if (!tempObj)
     return;
 
+  // If the storage corresponds to a source-level var, do not optimize here.
+  // Mem2Reg will transform the lexical alloc_stack into a lexical begin_borrow
+  // which will ensure that the value's lifetime isn't observably shortened.
+  if (tempObj->isLexical())
+    return;
+
   bool isOSSA = copyInst->getFunction()->hasOwnership();
 
   SILValue copySrc = copyInst->getSrc();
@@ -633,6 +639,13 @@ TempRValueOptPass::tryOptimizeStoreIntoTemp(StoreInst *si) {
     return std::next(si->getIterator());
   }
 
+  // If the storage corresponds to a source-level var, do not optimize here.
+  // Mem2Reg will transform the lexical alloc_stack into a lexical begin_borrow
+  // which will ensure that the value's lifetime isn't observably shortened.
+  if (tempObj->isLexical()) {
+    return std::next(si->getIterator());
+  }
+
   // If our tempObj has a dynamic lifetime (meaning it is conditionally
   // initialized, conditionally taken, etc), we can not convert its uses to SSA
   // while eliminating it simply. So bail.

validation-test/SILOptimizer/lexical-lifetimes.swift

@@ -0,0 +1,54 @@
+// RUN: %target-run-simple-swift -Xfrontend -enable-experimental-lexical-lifetimes -O -Xfrontend -enable-copy-propagation | %FileCheck %s
+
+// =============================================================================
+// = Declarations                                                           {{ =
+// =============================================================================
+
+class C {
+  init(_ d: D) {
+    self.d = d
+  }
+  weak var d: D?
+  func foo(_ string: String) {
+    d?.cWillFoo(self, string)
+  }
+}
+class D {
+  func cWillFoo(_ c: C, _ string: String) {
+    print(#function, string)
+  }
+}
+
+// =============================================================================
+// = Declarations                                                           }} =
+// =============================================================================
+
+// =============================================================================
+// = Tests                                                                  {{ =
+// =============================================================================
+
+func test_localLetKeepsObjectAliveBeyondCallToClassWithWeakReference() {
+  let d = D()
+  let c = C(d)
+  // CHECK: cWillFoo{{.*}} test_localLetKeepsObjectAliveBeyondCallToClassWithWeakReference
+  c.foo(#function)
+}
+
+func test_localVarKeepsObjectAliveBeyondCallToClassWithWeakReference() {
+  var d = D()
+  let c = C(d)
+  // CHECK: cWillFoo{{.*}} test_localVarKeepsObjectAliveBeyondCallToClassWithWeakReference
+  c.foo(#function)
+}
+
+// =============================================================================
+// = Tests                                                                  }} =
+// =============================================================================
+
+func run() {
+  test_localLetKeepsObjectAliveBeyondCallToClassWithWeakReference()
+  test_localVarKeepsObjectAliveBeyondCallToClassWithWeakReference()
+}
+
+run()
+