[clang] [CodeGen][CGObjCGNU] Fix assertion failure for negative ivar offsets. (PR #215753)

[via cfe-commits]

Hendrik =?utf-8?q?Hübner?= <hhuebner at MacBookPro.lan>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/215753 at github.com>


llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-codegen

Author: Hendrik Hübner  (HendrikHuebner)

<details>
<summary>Changes</summary>

Ivars can have negative offsets, for example if an ivar from a derived class is placed inside the super classes padding.
Currently, the offset is computed as an `uint64_t`, which results in an integer wraparound for negative offsets.
The wrapped offset is then written to a global value to be parsed by libobjc2. When libobjc2 reads the value as a truncated signed `int`, it is again implicitly converted correct negative number. However, with assertions enabled we hit an assertion failure in LLVM when creating the global value. See https://godbolt.org/z/Yb7cvWTqP.

---
Full diff: https://github.com/llvm/llvm-project/pull/215753.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CGObjCGNU.cpp (+7-5) 
- (modified) clang/test/CodeGenObjC/gnustep2-ivar-offset.m (+19) 


``````````diff
diff --git a/clang/lib/CodeGen/CGObjCGNU.cpp b/clang/lib/CodeGen/CGObjCGNU.cpp
index 32a1afe310629..24e7a2e8f7ebb 100644
--- a/clang/lib/CodeGen/CGObjCGNU.cpp
+++ b/clang/lib/CodeGen/CGObjCGNU.cpp
@@ -1885,8 +1885,9 @@ class CGObjCGNUstep2 : public CGObjCGNUstep {
         ivarBuilder.add(MakeConstantString(TypeStr));
         // int *offset;
         uint64_t BaseOffset = ComputeIvarBaseOffset(CGM, OID, IVD);
-        uint64_t Offset = BaseOffset - superInstanceSize;
-        llvm::Constant *OffsetValue = llvm::ConstantInt::get(IntTy, Offset);
+        int64_t Offset = static_cast<int64_t>(BaseOffset) - superInstanceSize;
+        llvm::Constant *OffsetValue =
+            llvm::ConstantInt::getSigned(IntTy, Offset);
         std::string OffsetName = GetIVarOffsetVariableName(classDecl, IVD);
         llvm::GlobalVariable *OffsetVar = TheModule.getGlobalVariable(OffsetName);
         if (OffsetVar)
@@ -3804,11 +3805,12 @@ void CGObjCGNU::GenerateClass(const ObjCImplementationDecl *OID) {
             Context.getTypeSize(IVD->getType())));
       // Get the offset
       uint64_t BaseOffset = ComputeIvarBaseOffset(CGM, OID, IVD);
-      uint64_t Offset = BaseOffset;
+      int64_t Offset = static_cast<int64_t>(BaseOffset);
       if (CGM.getLangOpts().ObjCRuntime.isNonFragile()) {
-        Offset = BaseOffset - superInstanceSize;
+        Offset = static_cast<int64_t>(BaseOffset) - superInstanceSize;
       }
-      llvm::Constant *OffsetValue = llvm::ConstantInt::get(IntTy, Offset);
+      llvm::Constant *OffsetValue =
+          llvm::ConstantInt::getSigned(IntTy, Offset);
       // Create the direct offset value
       std::string OffsetName = "__objc_ivar_offset_value_" + ClassName +"." +
           IVD->getNameAsString();
diff --git a/clang/test/CodeGenObjC/gnustep2-ivar-offset.m b/clang/test/CodeGenObjC/gnustep2-ivar-offset.m
index dd133ba04e307..c238edcf79380 100644
--- a/clang/test/CodeGenObjC/gnustep2-ivar-offset.m
+++ b/clang/test/CodeGenObjC/gnustep2-ivar-offset.m
@@ -28,3 +28,22 @@ @implementation ANObject @end
 // CHECK: @.objc_ivar_list = private global { i32, i64, [4 x { ptr, ptr, ptr, i32, i32 }] } { i32 4, i64 32,
 // Check that we emit 1 as the size of _Bool, not 0.
 // CHECK-SAME:  @__objc_ivar_offset_ANObject.boolIvar.B, i32 1, i32 4
+
+
+// The derived class reuses tail padding in the base class.
+// Its ivar is at offset 12, while the superclass instance size is 16, so the relative offset is -4.
+ at interface Base {
+  long long a;
+  char b;
+}
+ at end
+
+ at interface Derived : Base {
+  int c;
+}
+ at end
+
+ at implementation Base @end
+ at implementation Derived @end
+
+// CHECK: @__objc_ivar_offset_Derived.c.i = global i32 -4

``````````

</details>


https://github.com/llvm/llvm-project/pull/215753