Emit debug info for global variables without storage.

As with local variables this is done by emitting a constant 0 location
and a type of size zero.


<rdar://problem/26660971>

Author: adrian-prantl

lib/IRGen/GenDecl.cpp

@@ -1395,7 +1395,7 @@ llvm::GlobalVariable *LinkInfo::createVariable(IRGenModule &IGM,
   if (IGM.DebugInfo && !DbgTy.isNull() && ForDefinition)
     IGM.DebugInfo->emitGlobalVariableDeclaration(
         var, DebugName.empty() ? getName() : DebugName, getName(), DbgTy,
-        DebugLoc);
+        var->hasInternalLinkage(), DebugLoc);
 
   return var;
 }

lib/IRGen/GenInit.cpp

@@ -21,9 +21,11 @@
 #include "swift/SIL/SILGlobalVariable.h"
 #include "llvm/IR/GlobalVariable.h"
 
+#include "DebugTypeInfo.h"
 #include "Explosion.h"
 #include "GenHeap.h"
 #include "GenTuple.h"
+#include "IRGenDebugInfo.h"
 #include "IRGenFunction.h"
 #include "IRGenModule.h"
 #include "FixedTypeInfo.h"
@@ -37,8 +39,17 @@ Address IRGenModule::emitSILGlobalVariable(SILGlobalVariable *var) {
 
   // If the variable is empty in all resilience domains, don't actually emit it;
   // just return undef.
-  if (ti.isKnownEmpty(ResilienceExpansion::Minimal))
+  if (ti.isKnownEmpty(ResilienceExpansion::Minimal)) {
+    if (DebugInfo && var->getDecl()) {
+      auto Zero = llvm::ConstantInt::get(Int64Ty, 0);
+      DebugTypeInfo DbgTy(var->getDecl(), var->getLoweredType().getSwiftType(),
+                          Int8Ty, Size(0), Alignment(1));
+      DebugInfo->emitGlobalVariableDeclaration(
+          Zero, var->getDecl()->getName().str(), "", DbgTy,
+          var->getLinkage() != SILLinkage::Public, SILLocation(var->getDecl()));
+    }
     return ti.getUndefAddress();
+  }
 
   /// Get the global variable.
   Address addr = getAddrOfSILGlobalVariable(var, ti,

lib/IRGen/IRGenDebugInfo.cpp

@@ -983,10 +983,11 @@ void IRGenDebugInfo::emitDbgIntrinsic(llvm::BasicBlock *BB,
 }
 
 
-void IRGenDebugInfo::emitGlobalVariableDeclaration(llvm::GlobalValue *Var,
+void IRGenDebugInfo::emitGlobalVariableDeclaration(llvm::Constant *Var,
                                                    StringRef Name,
                                                    StringRef LinkageName,
                                                    DebugTypeInfo DbgTy,
+                                                   bool IsLocalToUnit,
                                                    Optional<SILLocation> Loc) {
   if (Opts.DebugInfoKind == IRGenDebugInfoKind::LineTables)
     return;
@@ -1004,7 +1005,7 @@ void IRGenDebugInfo::emitGlobalVariableDeclaration(llvm::GlobalValue *Var,
 
   // Emit it as global variable of the current module.
   DBuilder.createGlobalVariable(MainModule, Name, LinkageName, File, L.Line, Ty,
-                                Var->hasInternalLinkage(), Var, nullptr);
+                                IsLocalToUnit, Var, nullptr);
 }
 
 StringRef IRGenDebugInfo::getMangledName(DebugTypeInfo DbgTy) {

lib/IRGen/IRGenDebugInfo.h

@@ -181,9 +181,10 @@ class IRGenDebugInfo {
                         const SILDebugScope *DS);
 
   /// Create debug metadata for a global variable.
-  void emitGlobalVariableDeclaration(llvm::GlobalValue *Storage, StringRef Name,
+  void emitGlobalVariableDeclaration(llvm::Constant *Storage, StringRef Name,
                                      StringRef LinkageName,
                                      DebugTypeInfo DebugType,
+                                     bool IsLocalToUnit,
                                      Optional<SILLocation> Loc);
 
   /// Emit debug metadata for type metadata (for generic types). So meta.

test/DebugInfo/nostorage.swift

@@ -1,6 +1,7 @@
 // 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
+// RUN: cat %t | FileCheck %s --check-prefix=CHECK3
 
 func used<T>(_ t: T) {}
 
@@ -32,3 +33,8 @@ public func app() {
 
   used(at)
 }
+
+public enum empty { case exists }
+public let globalvar = empty.exists
+// CHECK3: !DIGlobalVariable(name: "globalvar", {{.*}}line: [[@LINE-1]],
+// CHECK3-SAME:          isLocal: false, isDefinition: true, variable: i64 0)