[clang] [ObjC] Expand isClassLayoutKnownStatically to base classes as long as the implementation of it is known (PR #85465)

[via cfe-commits]

https://github.com/AtariDreams created https://github.com/llvm/llvm-project/pull/85465

Only NSObject we can trust the layout of won't change even though we cannot directly see its @implementation

>From 3c66561377e5bfdbb3eefa658da63f68026e5a2e Mon Sep 17 00:00:00 2001
From: Rose <gfunni234 at gmail.com>
Date: Fri, 15 Mar 2024 16:43:10 -0400
Subject: [PATCH] [ObjC] Expand isClassLayoutKnownStatically to base classes as
 long as the implementation of it is known

Only NSObject we can trust the layout of won't change even though we cannot directly see its @implementation
---
 clang/lib/CodeGen/CGObjCMac.cpp               |  9 +++++-
 .../constant-non-fragile-ivar-offset.m        | 30 ++++++++++++++++++-
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/clang/lib/CodeGen/CGObjCMac.cpp b/clang/lib/CodeGen/CGObjCMac.cpp
index e815e097e1fb48..da47ef786cf816 100644
--- a/clang/lib/CodeGen/CGObjCMac.cpp
+++ b/clang/lib/CodeGen/CGObjCMac.cpp
@@ -1595,6 +1595,11 @@ class CGObjCNonFragileABIMac : public CGObjCCommonMac {
   bool isClassLayoutKnownStatically(const ObjCInterfaceDecl *ID) {
     // Test a class by checking its superclasses up to
     // its base class if it has one.
+
+    // Cannot check a null class
+    if (!ID)
+      return false;
+
     for (; ID; ID = ID->getSuperClass()) {
       // The layout of base class NSObject
       // is guaranteed to be statically known
@@ -1606,7 +1611,9 @@ class CGObjCNonFragileABIMac : public CGObjCCommonMac {
       if (!ID->getImplementation())
         return false;
     }
-    return false;
+
+    // We know the layout of all the intermediate classes and superclasses.
+    return true;
   }
 
 public:
diff --git a/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m b/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
index 8d55e6c7d23081..ee4034e4b7f205 100644
--- a/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
+++ b/clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m
@@ -8,6 +8,12 @@
 // CHECK: @"OBJC_IVAR_$_IntermediateClass._intermediateProperty" = hidden constant i64 48
 // CHECK: @"OBJC_IVAR_$_SubClass.subClassIvar" = constant i64 56
 // CHECK: @"OBJC_IVAR_$_SubClass._subClassProperty" = hidden constant i64 64
+
+// CHECK: @"OBJC_IVAR_$_RootClass.these" = constant i64 0
+// CHECK: @"OBJC_IVAR_$_RootClass.dont" = constant i64 4
+// CHECK: @"OBJC_IVAR_$_RootClass.change" = constant i64 4
+// CHECK: @"OBJC_IVAR_$_StillStaticLayout.static_layout_ivar" = hidden global i32 12
+
 // CHECK: @"OBJC_IVAR_$_NotStaticLayout.not_static_layout_ivar" = hidden global i64 12
 
 @interface NSObject {
@@ -120,7 +126,29 @@ -(void)intermediateSubclassVar {
 // CHECK: getelementptr inbounds i8, ptr %1, i64 64
 @end
 
- at interface NotNSObject {
+ __attribute((objc_root_class))  @interface RootClass {
+  int these, dont, change;
+}
+ at end
+
+ at implementation RootClass 
+ at end
+
+ at interface StillStaticLayout : RootClass
+ at end
+
+ at implementation StillStaticLayout {
+  int static_layout_ivar;
+}
+
+// CHECK-LABEL: define internal void @"\01-[StillStaticLayout meth]"
+-(void)meth {
+  static_layout_ivar = 0;
+  // CHECK-NOT: load i64, ptr @"OBJC_IVAR_$StillStaticLayout.static_layout_ivar
+}
+ at end
+
+__attribute((objc_root_class))  @interface NotNSObject {
   int these, might, change;
 }
 @end