IRGen: restrict generation of read-only static objects to Array buffers

Currently only arrays can be put into a read-only data section.
"Regular" classes have dynamically initialized metadata, which needs to be stored into the isa field at runtime.

Author: eeckstein

lib/IRGen/GenConstant.cpp

@@ -432,7 +432,7 @@ llvm::Constant *irgen::emitConstantObject(IRGenModule &IGM, ObjectInst *OI,
   // Construct the object header.
   llvm::StructType *ObjectHeaderTy = cast<llvm::StructType>(sTy->getElementType(0));
 
-  if (IGM.canMakeStaticObjectsReadOnly()) {
+  if (IGM.canMakeStaticObjectReadOnly(OI->getType())) {
     if (!IGM.swiftImmortalRefCount) {
       auto *var = new llvm::GlobalVariable(IGM.Module, IGM.Int8Ty,
                                         /*constant*/ true, llvm::GlobalValue::ExternalLinkage,

lib/IRGen/GenDecl.cpp

@@ -2804,7 +2804,7 @@ Address IRGenModule::getAddrOfSILGlobalVariable(SILGlobalVariable *var,
     if (initVal) {
       gvar->setInitializer(initVal);
       if (var->isLet() ||
-          (var->isInitializedObject() && canMakeStaticObjectsReadOnly())) {
+          (var->isInitializedObject() && canMakeStaticObjectReadOnly(var->getLoweredType()))) {
         gvar->setConstant(true);
       }
     } else {
@@ -2814,7 +2814,7 @@ Address IRGenModule::getAddrOfSILGlobalVariable(SILGlobalVariable *var,
   }
 
   llvm::Constant *addr = gvar;
-  if (var->isInitializedObject() && !canMakeStaticObjectsReadOnly()) {
+  if (var->isInitializedObject() && !canMakeStaticObjectReadOnly(var->getLoweredType())) {
     // Project out the object from the container.
     llvm::Constant *Indices[2] = {
       llvm::ConstantExpr::getIntegerValue(Int32Ty, APInt(32, 0)),
@@ -2842,7 +2842,7 @@ llvm::Constant *IRGenModule::getGlobalInitValue(SILGlobalVariable *var,
     StructLayout *layout = StaticObjectLayouts[var].get();
     ObjectInst *oi = cast<ObjectInst>(var->getStaticInitializerValue());
     llvm::Constant *initVal = emitConstantObject(*this, oi, layout);
-    if (!canMakeStaticObjectsReadOnly()) {
+    if (!canMakeStaticObjectReadOnly(var->getLoweredType())) {
       // A statically initialized object must be placed into a container struct
       // because the swift_initStaticObject needs a swift_once_t at offset -1:
       //     struct Container {

lib/IRGen/IRGenModule.cpp

@@ -2023,7 +2023,7 @@ bool IRGenModule::canUseObjCSymbolicReferences() {
       context.getObjCSymbolicReferencesAvailability());
 }
 
-bool IRGenModule::canMakeStaticObjectsReadOnly() {
+bool IRGenModule::canMakeStaticObjectReadOnly(SILType objectType) {
   if (getOptions().DisableReadonlyStaticObjects)
     return false;
 
@@ -2032,6 +2032,16 @@ bool IRGenModule::canMakeStaticObjectsReadOnly() {
   if (!Triple.isOSDarwin())
     return false;
 
+  auto *clDecl = objectType.getClassOrBoundGenericClass();
+  if (!clDecl)
+    return false;
+
+  // Currently only arrays can be put into a read-only data section.
+  // "Regular" classes have dynamically initialized metadata, which needs to be
+  // stored into the isa field at runtime.
+  if (clDecl->getNameStr() != "_ContiguousArrayStorage")
+    return false;
+
   if (!getAvailabilityContext().isContainedIn(Context.getStaticReadOnlyArraysAvailability()))
     return false;
 

lib/IRGen/IRGenModule.h

@@ -925,7 +925,7 @@ class IRGenModule {
 
   bool shouldPrespecializeGenericMetadata();
 
-  bool canMakeStaticObjectsReadOnly();
+  bool canMakeStaticObjectReadOnly(SILType objectType);
 
   ClassDecl *getStaticArrayStorageDecl();
 

lib/IRGen/IRGenSIL.cpp

@@ -3097,7 +3097,7 @@ void IRGenSILFunction::visitGlobalValueInst(GlobalValueInst *i) {
                                                 NotForDefinition).getAddress();
   // We don't need to initialize the global object if it's never used for
   // something which can access the object header.
-  if (!i->isBare() && !IGM.canMakeStaticObjectsReadOnly()) {
+  if (!i->isBare() && !IGM.canMakeStaticObjectReadOnly(var->getLoweredType())) {
     auto ClassType = loweredTy.getASTType();
     llvm::Value *Metadata =
       emitClassHeapMetadataRef(*this, ClassType, MetadataValueType::TypeMetadata,

lib/IRGen/Linking.cpp

@@ -97,7 +97,7 @@ LinkEntity LinkEntity::forSILGlobalVariable(SILGlobalVariable *G,
   LinkEntity entity;
   entity.Pointer = G;
   entity.SecondaryPointer = nullptr;
-  auto kind = (G->isInitializedObject() && IGM.canMakeStaticObjectsReadOnly() ?
+  auto kind = (G->isInitializedObject() && IGM.canMakeStaticObjectReadOnly(G->getLoweredType()) ?
                 Kind::ReadOnlyGlobalObject : Kind::SILGlobalVariable);
   entity.Data = unsigned(kind) << KindShift;
   return entity;