From 3aa1b9cdfacc5405b8649929369957836b3829a6 Mon Sep 17 00:00:00 2001
From: maruipu <maruipu@bytedance.com>
Date: Mon, 9 Nov 2020 23:53:42 +0800
Subject: [PATCH] Fix wrong strong layout result because of unaligned instance
 start.

Inspired by:
https://opensource.apple.com/source/objc4/objc4-756.2/runtime/objc-class.mm
void _class_lookUpIvar()
---
 .../Layout/Classes/FBClassStrongLayout.mm     |  3 ++-
 .../FBClassStrongLayoutTests.mm               | 24 +++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/FBRetainCycleDetector/Layout/Classes/FBClassStrongLayout.mm b/FBRetainCycleDetector/Layout/Classes/FBClassStrongLayout.mm
index a1feacf..f377021 100644
--- a/FBRetainCycleDetector/Layout/Classes/FBClassStrongLayout.mm
+++ b/FBRetainCycleDetector/Layout/Classes/FBClassStrongLayout.mm
@@ -146,7 +146,8 @@ static NSUInteger FBGetMinimumIvarIndex(__unsafe_unretained Class aCls) {
   if (count > 0) {
     Ivar ivar = ivars[0];
     ptrdiff_t offset = ivar_getOffset(ivar);
-    minimumIndex = offset / (sizeof(void *));
+    // unaligned bytes at the start of this class's ivars are not represented in the layout bitmap.
+    minimumIndex = (offset + (sizeof(void *) - 1)) / (sizeof(void *));
   }
 
   free(ivars);
diff --git a/FBRetainCycleDetectorTests/FBClassStrongLayoutTests.mm b/FBRetainCycleDetectorTests/FBClassStrongLayoutTests.mm
index 07ac1d5..d2bbe6d 100644
--- a/FBRetainCycleDetectorTests/FBClassStrongLayoutTests.mm
+++ b/FBRetainCycleDetectorTests/FBClassStrongLayoutTests.mm
@@ -13,6 +13,7 @@
 
 #import <FBRetainCycleDetector/FBClassStrongLayout.h>
 #import <FBRetainCycleDetector/FBRetainCycleDetector.h>
+#import <FBRetainCycleDetector/FBObjectReference.h>
 
 @interface _RCDTestEmptyClass : NSObject
 @end
@@ -157,6 +158,22 @@ @implementation _RCDTestClassWithCppStructAndStrongProperty
 }
 @end
 
+@interface _RCDTestClassWithUnalignEndIvar : NSObject {
+  char content[20];
+}
+@end
+@implementation _RCDTestClassWithUnalignEndIvar
+@end
+
+@interface _RCDTestClassWithUnalignEndIvarParentClass : _RCDTestClassWithUnalignEndIvar {
+  int integer;
+  __weak NSObject *object1;
+  NSObject *object2;
+}
+@end
+@implementation _RCDTestClassWithUnalignEndIvarParentClass
+@end
+
 @interface FBClassStrongLayoutTests : XCTestCase
 @end
 
@@ -260,4 +277,11 @@ - (void)testLayoutForClassWithCppStructAndStrongPropertyWillNotCrashAndFetchStro
   XCTAssertEqual([ivars count], 1);
 }
 
+- (void)testLayoutForClassWithParentClassHasUnalignEndIvar
+{
+  NSArray<id<FBObjectReference>> *ivars = FBGetObjectStrongReferences([_RCDTestClassWithUnalignEndIvarParentClass new], nil);
+  XCTAssertEqual([ivars count], 1);
+  XCTAssertEqualObjects([[ivars firstObject] namePath], @[@"object2"]);
+}
+
 @end