[clang] [ObjC] Check entire chain of superclasses to see if class has fixed offsets (PR #81335)

[via cfe-commits]

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

>From 18987af24d2f53d00e2ac42998a8f1580e35afdf 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]

---
 .../CodeGenObjC/constant-non-fragile-ivar-offset.m  | 13 +++++++++++++
 1 file changed, 13 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..40a97ba62ec6cf 100644
--- a/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
+++ b/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -triple x86_64-apple-macosx10.14.0 -emit-llvm %s -o - | FileCheck %s
 
 // CHECK: @"OBJC_IVAR_$_StaticLayout.static_layout_ivar" = hidden constant i64 20
+// CHECK: @"OBJC_IVAR_$_StaticLayoutSubClass.static_layout_ivar2" = hidden global i64 24
 // CHECK: @"OBJC_IVAR_$_NotStaticLayout.not_static_layout_ivar" = hidden global i64 12
 
 @interface NSObject {
@@ -20,6 +21,18 @@ -(void)meth {
 }
 @end
 
+ at interface StaticLayoutSubClass : StaticLayout
+ at end
+
+ at implementation StaticLayoutSubClass {
+  int static_layout_ivar2;
+}
+-(void)meth2 {
+  static_layout_ivar2 = 0;
+  // CHECK: load i64, ptr @"OBJC_IVAR_$_StaticLayoutSubClass
+}
+ at end
+
 @interface NotNSObject {
   int these, might, change;
 }

>From 6f5944a577b45d6a9fb65e72c78cb55766c4f753 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
 has fixed offsets

As of now, we only check if a class directly inherits from NSObject to determine if said lass has fixed offsets and can therefore "opt-out" from the non-fragile ABI for ivars.

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.
---
 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 40a97ba62ec6cf..c5bc2be0dfdb84 100644
--- a/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
+++ b/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
@@ -1,7 +1,7 @@
 // RUN: %clang_cc1 -triple x86_64-apple-macosx10.14.0 -emit-llvm %s -o - | FileCheck %s
 
 // CHECK: @"OBJC_IVAR_$_StaticLayout.static_layout_ivar" = hidden constant i64 20
-// CHECK: @"OBJC_IVAR_$_StaticLayoutSubClass.static_layout_ivar2" = hidden global i64 24
+// CHECK: @"OBJC_IVAR_$_StaticLayoutSubClass.static_layout_ivar2" = hidden constant i64 24
 // CHECK: @"OBJC_IVAR_$_NotStaticLayout.not_static_layout_ivar" = hidden global i64 12
 
 @interface NSObject {
@@ -29,7 +29,7 @@ @implementation StaticLayoutSubClass {
 }
 -(void)meth2 {
   static_layout_ivar2 = 0;
-  // CHECK: load i64, ptr @"OBJC_IVAR_$_StaticLayoutSubClass
+  // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$_StaticLayoutSubClass
 }
 @end