[CodeGenObjC] When available, emit a direct call to objc_alloc_init(cls) instead of [objc_alloc(cls) init]

[erik.pilkington]

Summary

This provides a code size win on the caller side, since the init message send is done in the runtime function.

rdar://44987038

Thanks!
Erik

[rjmccall]

clang/lib/CodeGen/CGObjC.cpp

459

You might need to check for a super message send, which can be a class message send in a class method.

Also, I think getInterface() can return null here, although it might need to be contrived to fit the other requirements. Class<SomeProtocol>, where that protocol declares +alloc? If there isn't a way to just emit the receiver pointer of a normal message send from an ObjCMessageExpr, that's really too bad.

[gparker42]

clang/lib/CodeGen/CGObjC.cpp

459

Should this be done inside tryGenerateSpecializedMessageSend, or from the same place that calls tryGenerateSpecializedMessageSend? That position already rejects [super …] call sites, and might make it more obvious how the objc_alloc and objc_alloc_init optimizations interact. (Currently it takes some digging to verify that the objc_alloc_init generator does not need to look for the pattern [objc_alloc(cls) init]).

[gparker42]

clang/lib/CodeGen/CGObjC.cpp

459

(Never mind, that super check wouldn't help catch [[super alloc] init]because it's checking init's receiver.)

[erik.pilkington]

Address review comments.

[erik.pilkington]

clang/lib/CodeGen/CGObjC.cpp

459

You might need to check for a super message send, which can be a class message send in a class method.

Ah, good catch. New patch only does this for class message sends.

475–483

Also, I think getInterface() can return null here, although it might need to be contrived to fit the other requirements. Class<SomeProtocol>, where that protocol declares +alloc? If there isn't a way to just emit the receiver pointer of a normal message send from an ObjCMessageExpr, that's really too bad.

Are you sure? It looks like we unconditionally dereference getInterface() here, and we would take this path if not for the new check above.

[rjmccall]

clang/lib/CodeGen/CGObjC.cpp

475–483

It looks like ObjCMessageExpr::Class is only for messages to type names, and Sema rejects types in that case that aren't just an ObjC class, so you're right, I misunderstood.

clang/include/clang/Basic/ObjCRuntime.h

bool shouldUseRuntimeFunctionsForAlloc() const {
     case GNUstep:
       return false;
     case ObjFW:
       return false;
     }
     llvm_unreachable("bad kind");
   }
 
+  /// Does this runtime provide the objc_alloc_init entrypoint? This can apply
+  /// the same optimization as objc_alloc, but also sends an -init message,
+  /// reducing code size on the caller.
+  bool shouldUseRuntimeFunctionForCombinedAllocInit() const {
+    switch (getKind()) {
+    case MacOSX:
+      return getVersion() >= VersionTuple(10, 14, 4);
+    case iOS:
+      return getVersion() >= VersionTuple(12, 2);
+    case WatchOS:
+      return getVersion() >= VersionTuple(5, 2);
+    default:
+      return false;
+    }
+  }
+
   /// Does this runtime supports optimized setter entrypoints?
   bool hasOptimizedSetter() const {
     switch (getKind()) {
       case MacOSX:
         return getVersion() >= VersionTuple(10, 8);
       case iOS:
         return (getVersion() >= VersionTuple(6));
       case WatchOS:

clang/lib/CodeGen/CGObjC.cpp

     break;
 
   default:
     break;
   }
   return None;
 }
 
+/// Instead of '[[MyClass alloc] init]', try to generate
+/// 'objc_alloc_init(MyClass)'. This provides a code size improvement on the
+/// caller side, as well as the optimized objc_alloc.
+static Optional<llvm::Value *>
+tryEmitSpecializedAllocInit(CodeGenFunction &CGF, const ObjCMessageExpr *OME) {
+  auto &Runtime = CGF.getLangOpts().ObjCRuntime;
+  if (!Runtime.shouldUseRuntimeFunctionForCombinedAllocInit())
+    return None;
+
+  // Match the exact pattern '[[MyClass alloc] init]'.
+  Selector Sel = OME->getSelector();
+  if (!OME->isInstanceMessage() || !OME->getType()->isObjCObjectPointerType() ||
+      !Sel.isUnarySelector() || Sel.getNameForSlot(0) != "init")
+    return None;
+
+  // Okay, this is '[receiver init]', check if 'receiver' is '[cls alloc]'.
+  auto *SubOME =
+      dyn_cast<ObjCMessageExpr>(OME->getInstanceReceiver()->IgnoreParens());
+  if (!SubOME)
+    return None;
+  Selector SubSel = SubOME->getSelector();
+  if (SubOME->getReceiverKind() != ObjCMessageExpr::Class ||
+      !SubOME->getType()->isObjCObjectPointerType() ||
+      !SubSel.isUnarySelector() || SubSel.getNameForSlot(0) != "alloc")
+    return None;
+
+  QualType ReceiverType = SubOME->getClassReceiver();
+  const ObjCObjectType *ObjTy = ReceiverType->getAs<ObjCObjectType>();
+  const ObjCInterfaceDecl *ID = ObjTy->getInterface();
+  assert(ID && "null interface should be impossible here");
+  llvm::Value *Receiver = CGF.CGM.getObjCRuntime().GetClass(CGF, ID);
+  return CGF.EmitObjCAllocInit(Receiver, CGF.ConvertType(OME->getType()));
+}
+
 RValue CodeGenFunction::EmitObjCMessageExpr(const ObjCMessageExpr *E,
                                             ReturnValueSlot Return) {
   // Only the lookup mechanism and first two arguments of the method
   // implementation vary between runtimes.  We can get the receiver and
   // arguments in generic code.
 
   bool isDelegateInit = E->isDelegateInitCall();
 
   const ObjCMethodDecl *method = E->getMethodDecl();
 
   // If the method is -retain, and the receiver's being loaded from
   // a __weak variable, peephole the entire operation to objc_loadWeakRetained.
   if (method && E->getReceiverKind() == ObjCMessageExpr::Instance &&
       method->getMethodFamily() == OMF_retain) {
     if (auto lvalueExpr = findWeakLValue(E->getInstanceReceiver())) {
       LValue lvalue = EmitLValue(lvalueExpr);
       llvm::Value *result = EmitARCLoadWeakRetained(lvalue.getAddress());
       return AdjustObjCObjectType(*this, E->getType(), RValue::get(result));
     }
   }
 
+  if (Optional<llvm::Value *> Val = tryEmitSpecializedAllocInit(*this, E))
+    return AdjustObjCObjectType(*this, E->getType(), RValue::get(*Val));
+
   // We don't retain the receiver in delegate init calls, and this is
   // safe because the receiver value is always loaded from 'self',
   // which we zero out.  We don't want to Block_copy block receivers,
   // though.
   bool retainSelf =
     (!isDelegateInit &&
      CGM.getLangOpts().ObjCAutoRefCount &&
      method &&

       TryEmitResult ter = tryEmitARCRetainScalarExpr(*this,
                                                    E->getInstanceReceiver());
       Receiver = ter.getPointer();
       if (ter.getInt()) retainSelf = false;
     } else
       Receiver = EmitScalarExpr(E->getInstanceReceiver());
     break;
 
   case ObjCMessageExpr::Class: {
     ReceiverType = E->getClassReceiver();
     const ObjCObjectType *ObjTy = ReceiverType->getAs<ObjCObjectType>();
     assert(ObjTy && "Invalid Objective-C class message send");
     OID = ObjTy->getInterface();
     assert(OID && "Invalid Objective-C class message send");
     Receiver = Runtime.GetClass(*this, OID);
     isClassMessage = true;
     break;
   }
 
   case ObjCMessageExpr::SuperInstance:
     ReceiverType = E->getSuperType();
     Receiver = LoadObjCSelf();
     isSuperMessage = true;
     break;
 

 ///   call i8* \@objc_allocWithZone(i8* %value)
 llvm::Value *CodeGenFunction::EmitObjCAllocWithZone(llvm::Value *value,
                                                     llvm::Type *resultType) {
   return emitObjCValueOperation(*this, value, resultType,
                                 CGM.getObjCEntrypoints().objc_allocWithZone,
                                 "objc_allocWithZone", /*MayThrow=*/true);
 }
 
+llvm::Value *CodeGenFunction::EmitObjCAllocInit(llvm::Value *value,
+                                                llvm::Type *resultType) {
+  return emitObjCValueOperation(*this, value, resultType,
+                                CGM.getObjCEntrypoints().objc_alloc_init,
+                                "objc_alloc_init", /*MayThrow=*/true);
+}
+
 /// Produce the code to do a primitive release.
 /// [tmp drain];
 void CodeGenFunction::EmitObjCMRRAutoreleasePoolPop(llvm::Value *Arg) {
   IdentifierInfo *II = &CGM.getContext().Idents.get("drain");
   Selector DrainSel = getContext().Selectors.getSelector(0, &II);
   CallArgList Args;
   CGM.getObjCRuntime().GenerateMessageSend(*this, ReturnValueSlot(),
                               getContext().VoidTy, DrainSel, Arg, Args);

clang/lib/CodeGen/CodeGenFunction.h

   EmitARCStoreStrong(const BinaryOperator *e, bool ignored);
   std::pair<LValue,llvm::Value*>
   EmitARCStoreUnsafeUnretained(const BinaryOperator *e, bool ignored);
 
   llvm::Value *EmitObjCAlloc(llvm::Value *value,
                              llvm::Type *returnType);
   llvm::Value *EmitObjCAllocWithZone(llvm::Value *value,
                                      llvm::Type *returnType);
+  llvm::Value *EmitObjCAllocInit(llvm::Value *value, llvm::Type *resultType);
+
   llvm::Value *EmitObjCThrowOperand(const Expr *expr);
   llvm::Value *EmitObjCConsumeObject(QualType T, llvm::Value *Ptr);
   llvm::Value *EmitObjCExtendObjectLifetime(QualType T, llvm::Value *Ptr);
 
   llvm::Value *EmitARCExtendBlockObject(const Expr *expr);
   llvm::Value *EmitARCReclaimReturnedObject(const Expr *e,
                                             bool allowUnsafeClaim);
   llvm::Value *EmitARCRetainScalarExpr(const Expr *expr);

clang/lib/CodeGen/CodeGenModule.h

struct ObjCEntrypoints {
   ObjCEntrypoints() { memset(this, 0, sizeof(*this)); }
 
   /// void objc_alloc(id);
   llvm::FunctionCallee objc_alloc;
 
   /// void objc_allocWithZone(id);
   llvm::FunctionCallee objc_allocWithZone;
 
+  /// void objc_alloc_init(id);
+  llvm::FunctionCallee objc_alloc_init;
+
   /// void objc_autoreleasePoolPop(void*);
   llvm::FunctionCallee objc_autoreleasePoolPop;
 
   /// void objc_autoreleasePoolPop(void*);
   /// Note this method is used when we are using exception handling
   llvm::FunctionCallee objc_autoreleasePoolPopInvoke;
 
   /// void *objc_autoreleasePoolPush(void);

clang/test/CodeGenObjC/objc-alloc-init.m

+// RUN: %clang_cc1 %s -fobjc-runtime=macosx-10.14.4 -emit-llvm -O0 -o - | FileCheck %s --check-prefix=OPTIMIZED --check-prefix=EITHER
+// RUN: %clang_cc1 %s -fobjc-runtime=macosx-10.14.3 -emit-llvm -O0 -o - | FileCheck %s --check-prefix=NOT_OPTIMIZED --check-prefix=EITHER
+// RUN: %clang_cc1 %s -fobjc-runtime=ios-12.2 -emit-llvm -O0 -o - | FileCheck %s --check-prefix=OPTIMIZED --check-prefix=EITHER
+// RUN: %clang_cc1 %s -fobjc-runtime=ios-12.1 -emit-llvm -O0 -o - | FileCheck %s --check-prefix=NOT_OPTIMIZED --check-prefix=EITHER
+
+@interface X
++(X *)alloc;
+-(X *)init;
+@end
+
+void f() {
+  [[X alloc] init];
+  // OPTIMIZED: call i8* @objc_alloc_init(
+  // NOT_OPTIMIZED: call i8* @objc_alloc(
+}
+
+@interface Y : X
++(void)meth;
+@end
+
+@implementation Y
++(void)meth {
+  [[self alloc] init];
+  // EITHER-NOT: call i8* @objc_alloc
+  // EITHER: call {{.*}} @objc_msgSend
+  // EITHER: call {{.*}} @objc_msgSend
+}
+@end