[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 2cf00f6ddf7be0e51fdb1a27668f36666c4c92a8 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        | 56 +++++++++++++++++++
 1 file changed, 56 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..44e5af28cdbe02 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_$_StaticLayout.static_layout_ivar" = hidden constant i64 20
+// CHECK: @"OBJC_IVAR_$_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_$_SubClass.subClassIvar" = global i64 56
 // CHECK: @"OBJC_IVAR_$_NotStaticLayout.not_static_layout_ivar" = hidden global i64 12
 
 @interface NSObject {
@@ -20,6 +26,56 @@ -(void)meth {
 }
 @end
 
+// Ivars declared in the @interface
+ at interface SuperClass : NSObject
+ at property (nonatomic, assign) int superClassProperty;
+ at end
+
+ at implementation SuperClass {
+  int superClassIvar; // Declare an ivar
+}
+- (void)superClassMethod {
+    _superClassProperty = 42;
+    superClassIvar = 10;
+    // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$_SuperClass
+}
+ at end
+
+// Inheritance and Ivars
+ at interface IntermediateClass : SuperClass
+{
+    double intermediateClassIvar;
+
+    @protected
+    int intermediateClassIvar2;
+}
+ at property (nonatomic, strong) SuperClass *intermediateProperty;
+ at end
+
+ at implementation IntermediateClass
+ at synthesize intermediateProperty = _intermediateProperty;
+- (void)intermediateClassMethod {
+    intermediateClassIvar = 3.14;
+    _intermediateProperty = 0;
+    // CHECK: load i64, ptr @"OBJC_IVAR_$_IntermediateClass
+}
+ at end
+
+ at interface SubClass : IntermediateClass
+{
+    double subClassIvar;
+}
+ at end
+
+ at implementation SubClass
+- (void)subclassVar {
+    intermediateClassIvar = 3.14;
+    subClassIvar = 6.28;
+    // CHECK: load i64, ptr @"OBJC_IVAR_$_IntermediateClass
+    // CHECK: load i64, ptr @"OBJC_IVAR_$_SubClass
+}
+ at end
+
 @interface NotNSObject {
   int these, might, change;
 }

>From 25d1bb9d4997768577dde1a7e243d2d268b342c2 Mon Sep 17 00:00:00 2001
From: Rose <83477269+AtariDreams at users.noreply.github.com>
Date: Tue, 13 Feb 2024 14:52:49 -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.
---
 clang/lib/CodeGen/CGObjCMac.cpp               | 22 ++++++++++++++-----
 .../constant-non-fragile-ivar-offset.m        | 14 ++++++------
 2 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/clang/lib/CodeGen/CGObjCMac.cpp b/clang/lib/CodeGen/CGObjCMac.cpp
index 27d77e9a8a5511..b3d6c3ba314cc2 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 statically
+      // know the class layout
+      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 44e5af28cdbe02..294ce7ec50fe16 100644
--- a/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
+++ b/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
@@ -3,10 +3,10 @@
 // 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_$_SubClass.subClassIvar" = global i64 56
+// CHECK: @"OBJC_IVAR_$_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_$_SubClass.subClassIvar" = constant i64 56
 // CHECK: @"OBJC_IVAR_$_NotStaticLayout.not_static_layout_ivar" = hidden global i64 12
 
 @interface NSObject {
@@ -57,7 +57,7 @@ @implementation IntermediateClass
 - (void)intermediateClassMethod {
     intermediateClassIvar = 3.14;
     _intermediateProperty = 0;
-    // CHECK: load i64, ptr @"OBJC_IVAR_$_IntermediateClass
+    // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$_IntermediateClass
 }
 @end
 
@@ -71,8 +71,8 @@ @implementation SubClass
 - (void)subclassVar {
     intermediateClassIvar = 3.14;
     subClassIvar = 6.28;
-    // CHECK: load i64, ptr @"OBJC_IVAR_$_IntermediateClass
-    // CHECK: load i64, ptr @"OBJC_IVAR_$_SubClass
+    // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$_IntermediateClass
+    // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$_SubClass
 }
 @end