Deserializer: fix a crash with global variables and cross-module-optimization

In case of cross-module-optimizations it can happen that a private global variable is changed to public,
but it's declaration is not available in the module file.

Author: eeckstein

lib/Serialization/DeserializeSIL.cpp

@@ -3651,11 +3651,23 @@ SILGlobalVariable *SILDeserializer::readGlobalVar(StringRef Name) {
     return nullptr;
   }
 
+  VarDecl *globalDecl = nullptr;
+  if (dID) {
+    llvm::Expected<Decl *> d = MF->getDeclChecked(dID);
+    if (d) {
+      globalDecl = cast<VarDecl>(d.get());
+    } else {
+      // This can happen with cross-module-optimizations, if the linkage of a
+      // private global variable is changed to public.
+      consumeError(d.takeError());
+    }
+  }
+
   auto Ty = MF->getType(TyID);
   SILGlobalVariable *v = SILGlobalVariable::create(
       SILMod, linkage.value(), SerializedKind_t(serializedKind),
       Name.str(), getSILType(Ty, SILValueCategory::Object, nullptr),
-      std::nullopt, dID ? cast<VarDecl>(MF->getDecl(dID)) : nullptr);
+      std::nullopt, globalDecl);
   v->setLet(IsLet);
   globalVarOrOffset.set(v, true /*isFullyDeserialized*/);
   v->setDeclaration(IsDeclaration);

test/SILOptimizer/Inputs/cross-module/cross-module.swift

@@ -287,6 +287,12 @@ public func callCImplementationOnly<T>(_ t: T) -> Int {
 
 public let globalLet = 529387
 
+private var privateVar = Int.random(in: 0..<100)
+
+public func getRandom() -> Int {
+  return privateVar
+}
+
 public struct StructWithClosure {
   public static let c = { (x: Int) -> Int in return x }
 }

test/SILOptimizer/cross-module-optimization.swift

@@ -165,6 +165,12 @@ func testImplementationOnly() {
   // CHECK-SIL2: } // end sil function '$s4Main22testImplementationOnlyyyF'
 }
 
+@inline(never)
+func testPrivateVar() {
+  // CHECK-OUTPUT: {{[0-9]+}}
+  print(getRandom())
+}
+
 testNestedTypes()
 testClass()
 testError()
@@ -175,4 +181,5 @@ testKeypath()
 testMisc()
 testGlobal()
 testImplementationOnly()
+testPrivateVar()