[clang] [Clang] Fix __builtin_dynamic_object_size off by 4 (PR #111015)

[Jan Hendrik Farr via cfe-commits]

Cydox wrote:

So, we would actually get gcc's behavior with this patch:

```
diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index c864714182e0..21ffe7b46a6e 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -1049,25 +1049,7 @@ CodeGenFunction::emitFlexibleArrayMemberSize(const Expr *E, unsigned Type,
   Value *Res = FAMSize;
 
   if (isa<DeclRefExpr>(Base)) {
-    // The whole struct is specificed in the __bdos.
-    const ASTRecordLayout &Layout = Ctx.getASTRecordLayout(OuterRD);
-
-    // Get the offset of the FAM.
-    llvm::Constant *FAMOffset = ConstantInt::get(ResType, Offset, IsSigned);
-    Value *OffsetAndFAMSize =
-        Builder.CreateAdd(FAMOffset, Res, "", !IsSigned, IsSigned);
-
-    // Get the full size of the struct.
-    llvm::Constant *SizeofStruct =
-        ConstantInt::get(ResType, Layout.getSize().getQuantity(), IsSigned);
-
-    // max(sizeof(struct s),
-    //     offsetof(struct s, array) + p->count * sizeof(*p->array))
-    Res = IsSigned
-              ? Builder.CreateBinaryIntrinsic(llvm::Intrinsic::smax,
-                                              OffsetAndFAMSize, SizeofStruct)
-              : Builder.CreateBinaryIntrinsic(llvm::Intrinsic::umax,
-                                              OffsetAndFAMSize, SizeofStruct);
+    return nullptr;
   }
 
   // A negative \p IdxInst or \p CountedByInst means that the index lands
```

https://github.com/llvm/llvm-project/pull/111015