[clang] [ObjC] Check entire chain of superclasses to see if class layout can be statically known (PR #81335)

[via cfe-commits]

https://github.com/AtariDreams updated https://github.com/llvm/llvm-project/pull/81335

>From 3287e1ddb572f73ca774e50323b037d50b600e34 Mon Sep 17 00:00:00 2001
From: Rose <83477269+AtariDreams at users.noreply.github.com>
Date: Fri, 9 Feb 2024 17:51:15 -0500
Subject: [PATCH 1/2] [ObjC] Add pre-commit tests [NFC]

---
 .../constant-non-fragile-ivar-offset.m        | 82 +++++++++++++++++++
 1 file changed, 82 insertions(+)

diff --git a/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m b/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
index 788b3220af3067..6806fbf8f435aa 100644
--- a/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
+++ b/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
@@ -1,6 +1,12 @@
 // RUN: %clang_cc1 -triple x86_64-apple-macosx10.14.0 -emit-llvm %s -o - | FileCheck %s
 
 // CHECK: @"OBJC_IVAR_$_StaticLayout.static_layout_ivar" = hidden constant i64 20
+// CHECK: @"OBJC_IVAR_$_MyClass.myIvar" = constant i64 20
+// CHECK: @"OBJC_IVAR_$_MyClass._myProperty" = hidden constant i64 24
+// CHECK: @"OBJC_IVAR_$_AnotherClass.privateId" = constant i64 24
+// CHECK: @"OBJC_IVAR_$_AnotherClass.anotherPrivateId" = hidden constant i64 32
+// CHECK: @"OBJC_IVAR_$_SuperClass.superClassIvar" = constant i64 20
+// CHECK: @"OBJC_IVAR_$_SubClass.subClassIvar" = global i64 24
 // CHECK: @"OBJC_IVAR_$_NotStaticLayout.not_static_layout_ivar" = hidden global i64 12
 
 @interface NSObject {
@@ -20,6 +26,82 @@ -(void)meth {
 }
 @end
 
+// Scenario 1: Ivars declared in the @interface
+ at interface MyClass : NSObject
+{
+    int myIvar; // Declare an ivar
+}
+
+ at property (nonatomic, assign) int myProperty; // Synthesize a property
+
+ at end
+
+ at implementation MyClass 
+
+- (void)exampleMethod {
+    self.myProperty = 42; // Access the property
+    myIvar = 10; // Access the ivar directly
+}
+
+ at end
+
+// Scenario 2: Ivars declared directly in the @implementation
+ at interface AnotherClass : NSObject
+{
+  id privateId;
+}
+
+ at end
+
+ at implementation AnotherClass
+{
+    id anotherPrivateId; // Declare an ivar directly in the implementation
+}
+
+- (void)doSomething {
+   privateId = anotherPrivateId;
+}
+
+ at end
+
+// Scenario 3: Inheritance and Ivars
+ at interface SuperClass : NSObject
+{
+    int superClassIvar;
+}
+ at end
+
+ at implementation SuperClass
+ at end
+
+ at interface SubClass : SuperClass
+{
+    double subClassIvar;
+}
+ at end
+
+ at implementation SubClass
+- (void)exampleMethod {
+    // CHECK: load i64, ptr @"OBJC_IVAR_$SuperClass
+    superClassIvar = 100; // Access superclass ivar
+    subClassIvar = 3.14; // Access subclass ivar
+}
+ at end
+
+// Scenario 4: Custom Getter/Setter Methods
+ at interface CustomPropertyClass : NSObject
+ at property (nonatomic, strong, getter=myCustomGetter, setter=myCustomSetter:) id customProperty;
+ at end
+
+ at implementation CustomPropertyClass
+- (id) myCustomGetter {
+    return 0;
+}
+
+- (void)myCustomSetter:(id)newValue {
+}
+ at end
+
 @interface NotNSObject {
   int these, might, change;
 }

>From 5dfaf1dfbad2ae5fc8eb57bb320348910dcd1fb1 Mon Sep 17 00:00:00 2001
From: Rose <83477269+AtariDreams at users.noreply.github.com>
Date: Fri, 9 Feb 2024 17:47:59 -0500
Subject: [PATCH 2/2] [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.

Perhaps there is another way to figure this out at link-time as well, but that is for another time and another PR.

Fixes: #81369
---
 clang/lib/CodeGen/CGObjCMac.cpp               | 22 ++++++++++++++-----
 .../constant-non-fragile-ivar-offset.m        |  4 ++--
 2 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/clang/lib/CodeGen/CGObjCMac.cpp b/clang/lib/CodeGen/CGObjCMac.cpp
index 27d77e9a8a5511..ceee70a5d79b3e 100644
--- a/clang/lib/CodeGen/CGObjCMac.cpp
+++ b/clang/lib/CodeGen/CGObjCMac.cpp
@@ -1593,12 +1593,22 @@ 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 base class NSObject is a fixed size
+      if (ID->getName() == "NSObject")
+        return true;
+
+      // If we cannot see the @implementation of a class, we cannot assume fixed
+      // offsets
+      if (!ID->getImplementation())
+        return false;
+
+      // Test superclass
+      ID = ID->getSuperClass();
+    }
+    return false;
   }
 
 public:
diff --git a/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m b/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
index 6806fbf8f435aa..058017af6e296a 100644
--- a/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
+++ b/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
@@ -6,7 +6,7 @@
 // CHECK: @"OBJC_IVAR_$_AnotherClass.privateId" = constant i64 24
 // CHECK: @"OBJC_IVAR_$_AnotherClass.anotherPrivateId" = hidden constant i64 32
 // CHECK: @"OBJC_IVAR_$_SuperClass.superClassIvar" = constant i64 20
-// CHECK: @"OBJC_IVAR_$_SubClass.subClassIvar" = global i64 24
+// CHECK: @"OBJC_IVAR_$_SubClass.subClassIvar" = constant i64 24
 // CHECK: @"OBJC_IVAR_$_NotStaticLayout.not_static_layout_ivar" = hidden global i64 12
 
 @interface NSObject {
@@ -82,7 +82,7 @@ @interface SubClass : SuperClass
 
 @implementation SubClass
 - (void)exampleMethod {
-    // CHECK: load i64, ptr @"OBJC_IVAR_$SuperClass
+    // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$SuperClass
     superClassIvar = 100; // Access superclass ivar
     subClassIvar = 3.14; // Access subclass ivar
 }