SILGen: Evaluate loaded lvalues in a writeback scope.

emitLValue always has to occur in a writeback scope, even if the lvalue isn't formally accessed until later, because some lvalue productions immediately access their parent lvalue expression (namely optional chaining expressions). Fixes rdar://problem/26642478.

Author: jckarter

lib/SILGen/SILGenExpr.cpp

@@ -803,6 +803,8 @@ RValue RValueEmitter::visitStringLiteralExpr(StringLiteralExpr *E,
 }
 
 RValue RValueEmitter::visitLoadExpr(LoadExpr *E, SGFContext C) {
+  // Any writebacks here are tightly scoped.
+  WritebackScope writeback(SGF);
   LValue lv = SGF.emitLValue(E->getSubExpr(), AccessKind::Read);
   return SGF.emitLoadOfLValue(E, std::move(lv), C);
 }
@@ -3436,13 +3438,15 @@ void SILGenFunction::emitIgnoredExpr(Expr *E) {
   FullExpr scope(Cleanups, CleanupLocation(E));
   if (!E->getType()->isMaterializable()) {
     // Emit the l-value, but don't perform an access.
+    WritebackScope scope(*this);
     emitLValue(E, AccessKind::Read);
     return;
   }
 
   // If this is a load expression, we try hard not to actually do the load
   // (which could materialize a potentially expensive value with cleanups).
   if (auto *LE = dyn_cast<LoadExpr>(E)) {
+    WritebackScope scope(*this);
     LValue lv = emitLValue(LE->getSubExpr(), AccessKind::Read);
     // If the lvalue is purely physical, then it won't have any side effects,
     // and we don't need to drill into it.

lib/SILGen/SILGenLValue.cpp

@@ -1514,6 +1514,11 @@ void LValue::print(raw_ostream &OS) const {
 }
 
 LValue SILGenFunction::emitLValue(Expr *e, AccessKind accessKind) {
+  // Some lvalue nodes (namely BindOptionalExprs) require immediate evaluation
+  // of their subexpression, so we must have a writeback scope open while
+  // building an lvalue.
+  assert(InWritebackScope && "must be in a writeback scope");
+
   LValue r = SILGenLValue(*this).visit(e, accessKind);
   // If the final component has an abstraction change, introduce a
   // reabstraction component.

test/SILGen/optional_chain_addressor.swift

@@ -0,0 +1,6 @@
+// RUN: %target-swift-frontend -emit-silgen -verify -parse-as-library %s
+
+func foo(x: UnsafeMutablePointer<UnsafeMutablePointer<()>?>) {
+  _ = x.pointee?.pointee
+  _ = x.pointee?.dynamicType
+}