[ObjC] Check entire chain of superclasses to see if class layout can be statically known

As of now, we only check if a class directly inherits from NSObject to determine if said class has fixed offsets and can therefore have its memory layout statically known.

However, if an NSObject subclass has fixed offsets, then so must the subclasses of that subclass, so this allows us to optimize instances of subclasses of subclasses that inherit from NSObject and so on.

To determine this, we need to find that the compiler can see the implementation of each intermediate class, as that means it is statically linked.

Author: AZero13

clang/lib/CodeGen/CGObjCMac.cpp

@@ -1593,12 +1593,23 @@ class CGObjCNonFragileABIMac : public CGObjCCommonMac {
   }
 
   bool isClassLayoutKnownStatically(const ObjCInterfaceDecl *ID) {
-    // NSObject is a fixed size. If we can see the @implementation of a class
-    // which inherits from NSObject then we know that all it's offsets also must
-    // be fixed. FIXME: Can we do this if see a chain of super classes with
-    // implementations leading to NSObject?
-    return ID->getImplementation() && ID->getSuperClass() &&
-           ID->getSuperClass()->getName() == "NSObject";
+    // Test a class by checking its superclasses up to
+    // its base class if it has one.
+    while (ID) {
+      // The layout of base class NSObject
+      // is guaranteed to be statically known
+      if (ID->getName() == "NSObject")
+        return true;
+
+      // If we cannot see the @implementation of a class,
+      // we cannot statically know the class layout.
+      if (!ID->getImplementation())
+        return false;
+
+      // Test superclass
+      ID = ID->getSuperClass();
+    }
+    return false;
   }
 
 public:

clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m

@@ -3,11 +3,11 @@
 // CHECK: @"OBJC_IVAR_$_StaticLayout.static_layout_ivar" = hidden constant i64 20
 // CHECK: @"OBJC_IVAR_$_SuperClass.superClassIvar" = hidden constant i64 20
 // CHECK: @"OBJC_IVAR_$_SuperClass._superClassProperty" = hidden constant i64 24
-// CHECK: @"OBJC_IVAR_$_IntermediateClass.intermediateClassIvar" = global i64 32
-// CHECK: @"OBJC_IVAR_$_IntermediateClass.intermediateClassIvar2" = global i64 40
-// CHECK: @"OBJC_IVAR_$_IntermediateClass._intermediateProperty" = hidden global i64 48
-// CHECK: @"OBJC_IVAR_$_SubClass.subClassIvar" = global i64 56
-// CHECK: @"OBJC_IVAR_$_SubClass._subClassProperty" = hidden global i64 64
+// CHECK: @"OBJC_IVAR_$_IntermediateClass.intermediateClassIvar" = constant i64 32
+// CHECK: @"OBJC_IVAR_$_IntermediateClass.intermediateClassIvar2" = constant i64 40
+// CHECK: @"OBJC_IVAR_$_IntermediateClass._intermediateProperty" = hidden constant i64 48
+// CHECK: @"OBJC_IVAR_$_SubClass.subClassIvar" = constant i64 56
+// CHECK: @"OBJC_IVAR_$_SubClass._subClassProperty" = hidden constant i64 64
 // CHECK: @"OBJC_IVAR_$_NotStaticLayout.not_static_layout_ivar" = hidden global i64 12
 
 @interface NSObject {
@@ -21,13 +21,15 @@ @interface StaticLayout : NSObject
 @implementation StaticLayout {
   int static_layout_ivar;
 }
+
+// CHECK-LABEL: define internal void @"\01-[StaticLayout meth]"
 -(void)meth {
   static_layout_ivar = 0;
   // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$_StaticLayout
+  // CHECK: getelementptr inbounds i8, ptr %0, i64 20
 }
 @end
 
-// Ivars declared in the @interface
 @interface SuperClass : NSObject
 @property (nonatomic, assign) int superClassProperty;
 @end
@@ -40,21 +42,19 @@ @implementation SuperClass {
 - (void)superClassMethod {
     _superClassProperty = 42;
     superClassIvar = 10;
-    // CHECK: load i64, ptr @"OBJC_IVAR_$_SuperClass
+    // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$_SuperClass
     // CHECK: getelementptr inbounds i8, ptr %1, i64 20
 }
 
-// implicitly synthesized method here
+// Implicitly synthesized method here
 // CHECK-LABEL: define internal i32 @"\01-[SuperClass superClassProperty]"
 // CHECK: getelementptr inbounds i8, ptr %0, i64 24
 
 // CHECK-LABEL: define internal void @"\01-[SuperClass setSuperClassProperty:]"
 // CHECK: getelementptr inbounds i8, ptr %1, i64 24
 @end
 
-// Inheritance and Ivars
-@interface IntermediateClass : SuperClass
-{
+@interface IntermediateClass : SuperClass {
     double intermediateClassIvar;
 
     @protected
@@ -65,10 +65,12 @@ @interface IntermediateClass : SuperClass
 
 @implementation IntermediateClass
 @synthesize intermediateProperty = _intermediateProperty;
+
+// CHECK-LABEL: define internal void @"\01-[IntermediateClass intermediateClassMethod]"
 - (void)intermediateClassMethod {
     intermediateClassIvar = 3.14;
-    // CHECK: load i64, ptr @"OBJC_IVAR_$_IntermediateClass
-    // CHECK: getelementptr inbounds i8, ptr %0, i64 %ivar
+    // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$_IntermediateClass
+    // CHECK: getelementptr inbounds i8, ptr %0, i64 32
 }
 
 // CHECK-LABEL: define internal void @"\01-[IntermediateClass intermediateClassPropertyMethod]"
@@ -81,39 +83,41 @@ - (void)intermediateClassPropertyMethod {
 // CHECK-LABEL: define internal void @"\01-[IntermediateClass intermediateClassPropertyMethodDirect]"
 - (void)intermediateClassPropertyMethodDirect {
     _intermediateProperty = 0;
-    // CHECK: load i64, ptr @"OBJC_IVAR_$_IntermediateClass._intermediateProperty"
+    // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$_IntermediateClass._intermediateProperty"
+    // CHECK: getelementptr inbounds i8, ptr %0, i64 48
 }
 @end
 
-@interface SubClass : IntermediateClass
-{
+@interface SubClass : IntermediateClass {
     double subClassIvar;
 }
 @property (nonatomic, assign) SubClass *subClassProperty;
 @end
 
 @implementation SubClass
+
 // CHECK-LABEL: define internal void @"\01-[SubClass subclassVar]"
 - (void)subclassVar {
-    
     subClassIvar = 6.28;
-    // CHECK: load i64, ptr @"OBJC_IVAR_$_SubClass
-    // CHECK: getelementptr inbounds i8, ptr %0, i64 %ivar
+    // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$_SubClass
+    // CHECK: getelementptr inbounds i8, ptr %0, i64 56
 }
 
 // CHECK-LABEL: define internal void @"\01-[SubClass intermediateSubclassVar]"
 -(void)intermediateSubclassVar {
     intermediateClassIvar = 3.14;
-    // CHECK: load i64, ptr @"OBJC_IVAR_$_IntermediateClass
-    // CHECK: getelementptr inbounds i8, ptr %0, i64 %ivar
+    // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$_IntermediateClass
+    // CHECK: getelementptr inbounds i8, ptr %0, i64 32
 }
 
-// implicit synthesized method here:
+// Implicit synthesized method here:
 // CHECK-LABEL: define internal ptr @"\01-[SubClass subClassProperty]"
-// CHECK: load i64, ptr @"OBJC_IVAR_$_SubClass._subClassProperty"
+// CHECK-NOT: load i64, ptr @"OBJC_IVAR_$_SubClass._subClassProperty"
+// CHECK: getelementptr inbounds i8, ptr %0, i64 64
 
 // CHECK-LABEL: define internal void @"\01-[SubClass setSubClassProperty:]"
-// CHECK: load i64, ptr @"OBJC_IVAR_$_SubClass._subClassProperty"
+// CHECK-NOT: load i64, ptr @"OBJC_IVAR_$_SubClass._subClassProperty"
+// CHECK: getelementptr inbounds i8, ptr %1, i64 64
 @end
 
 @interface NotNSObject {
@@ -127,6 +131,8 @@ @interface NotStaticLayout : NotNSObject
 @implementation NotStaticLayout {
   int not_static_layout_ivar;
 }
+
+// CHECK-LABEL: define internal void @"\01-[NotStaticLayout meth]"
 -(void)meth {
   not_static_layout_ivar = 0;
   // CHECK: load i64, ptr @"OBJC_IVAR_$_NotStaticLayout.not_static_layout_ivar