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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: AtariDreams (AtariDreams)

<details>
<summary>Changes</summary>

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

---
Full diff: https://github.com/llvm/llvm-project/pull/85465.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CGObjCMac.cpp (+8-1) 
- (modified) clang/test/CodeGenObjC/constant-non-fragile-ivar-offset.m (+29-1) 


``````````diff
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

``````````

</details>


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