Emit debug info for storage-less variables as constant 0 instead of undef.

LLVM drops debug intrinsics for undef values in SelectionDAG and FastISel.
Until this is fixed, emit a constant 0 to force the variable to be emitted.

<rdar://problem/26429250>

Author: adrian-prantl

lib/IRGen/IRGenDebugInfo.cpp

@@ -949,8 +949,9 @@ void IRGenDebugInfo::emitVariableDeclaration(
 
   // Emit locationless intrinsic for variables that were optimized away.
   if (Storage.size() == 0) {
-    auto *undef = llvm::UndefValue::get(DbgTy.StorageType);
-    emitDbgIntrinsic(BB, undef, Var, DBuilder.createExpression(), Line,
+    auto Zero =
+        llvm::ConstantInt::get(llvm::Type::getInt64Ty(M.getContext()), 0);
+    emitDbgIntrinsic(BB, Zero, Var, DBuilder.createExpression(), Line,
                      Loc.Column, Scope, DS);
   }
 }

test/DebugInfo/letclause.swift

@@ -6,7 +6,8 @@ func peek() -> Symbol? { return Symbol() }
 
 func foo() {
 // CHECK: define {{.*}}foo
-// CHECK: call void @llvm.dbg.declare(metadata %V9letclause6Symbol undef, metadata ![[S:.*]], metadata !{{[0-9]+}})
+// CHECK: call void @llvm.dbg.value(metadata i{{.*}} 0,
+// CHECK-SAME:                        metadata ![[S:.*]], metadata !{{[0-9]+}})
 // CHECK: ![[S]] = !DILocalVariable(name: "s"
 // CHECK-SAME:                      line: [[@LINE+1]],
   while let s = peek() {

test/DebugInfo/nostorage.swift

@@ -1,28 +1,31 @@
-// RUN: %target-swift-frontend %s -emit-ir -g -o - | FileCheck %s
+// RUN: %target-swift-frontend %s -emit-ir -g -o %t
+// RUN: cat %t | FileCheck %s --check-prefix=CHECK1
+// RUN: cat %t | FileCheck %s --check-prefix=CHECK2
 
-func markUsed<T>(_ t: T) {}
+func used<T>(_ t: T) {}
 
-class AClass {
-  func f () -> Int64 { return 1 }
+public class Foo {
+    func foo() {
+      { [weak self] in
+      // CHECK1: call void @llvm.dbg.value(metadata i{{.*}} 0,
+      // CHECK1-SAME:                      metadata ![[TYPE:.*]], metadata
+      // CHECK1: ![[TYPE]] = !DILocalVariable(name: "type",
+      // CHECK1-SAME:                         line: [[@LINE+1]],
+            let type = self.dynamicType
+            used(type)
+        }()
+    }
 }
 
-class AnotherClass : AClass {
-  override func f() -> Int64 { return 2 }
-}
-
-struct AStruct {
-  func f() -> Int64 { return 3 }
-}
+struct AStruct {}
 
-// CHECK: define hidden void @_TF9nostorage3appFT_T_()
-func app() {
-  var ac: AClass = AnotherClass()
+// CHECK2: define{{.*}}app
+public func app() {
   // No members? No storage! Emitted as a constant 0, because.
-  // CHECK: call void @llvm.dbg.value(metadata {{.*}}, i64 0, metadata ![[AT:.*]], metadata !{{[0-9]+}}), !dbg
-  // CHECK: ![[AT]] = !DILocalVariable(name: "at",
-  // CHECK-SAME:                       line: [[@LINE+1]]
+  // CHECK2: call void @llvm.dbg.value(metadata i{{.*}} 0,
+  // CHECK2-SAME:                      metadata ![[AT:.*]], metadata
+  // CHECK2: ![[AT]] = !DILocalVariable(name: "at",{{.*}}line: [[@LINE+1]]
   var at = AStruct()
-  markUsed("\(ac.f()) \(at.f())")
+  
+  used(at)
 }
-
-app()

test/DebugInfo/self-nostorage.swift

@@ -3,8 +3,8 @@
 public struct S {
   func f() {
     // CHECK: define {{.*}}_TFV4main1S1ffT_T_
-    // CHECK: call void @llvm.dbg.declare(metadata %V4main1S undef,
-    // CHECK-SAME:                        metadata ![[SELF:[0-9]+]]
+    // CHECK: call void @llvm.dbg.value(metadata i{{.*}} 0,
+    // CHECK-SAME:                      metadata ![[SELF:[0-9]+]]
     // CHECK: ![[SELF]] = !DILocalVariable(name: "self", arg: 1,
   }
 }