[SemanticARCOpts] Strip some lexical borrow scopes.

If a value's lifetime is preserved by a different lexical scope, it is
fine to strip a different scope.  Specifically, if it is preserved by
being a guaranteed argument or a nested borrow of an outer lexical
scope, allow a lexical borrow scope to be stripped.

Author: nate-chandler

lib/SILOptimizer/SemanticARC/BorrowScopeOpts.cpp

@@ -24,15 +24,35 @@
 using namespace swift;
 using namespace swift::semanticarc;
 
+/// Whether the provided lexical begin_borrow instruction is redundant.
+///
+/// A begin_borrow [lexical] is redundant if the borrowed value's lifetime is
+/// otherwise guaranteed.  That happens if:
+/// - the value is a guaranteed argument to the function
+/// - the value is itself a begin_borrow [lexical]
+static bool isRedundantLexicalBeginBorrow(BeginBorrowInst *bbi) {
+  assert(bbi->isLexical());
+  auto value = bbi->getOperand();
+  if (auto *outerBBI = dyn_cast<BeginBorrowInst>(value)) {
+    return outerBBI->isLexical();
+  }
+  if (auto *arg = dyn_cast<SILFunctionArgument>(value)) {
+    return arg->getOwnershipKind() == OwnershipKind::Guaranteed;
+  }
+  return false;
+}
+
 bool SemanticARCOptVisitor::visitBeginBorrowInst(BeginBorrowInst *bbi) {
   // Quickly check if we are supposed to perform this transformation.
   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;
+  if (bbi->isLexical()) {
+    if (!isRedundantLexicalBeginBorrow(bbi))
+      return false;
+  }
 
   auto kind = bbi->getOperand().getOwnershipKind();
   SmallVector<EndBorrowInst *, 16> endBorrows;