[Clang] Disable use of the counted_by attribute for whole struct pointers (#112636)

The whole struct is specificed in the __bdos. The calculation of the
whole size of the structure can be done in two ways:

    1) sizeof(struct S) + count * sizeof(typeof(fam))
    2) offsetof(struct S, fam) + count * sizeof(typeof(fam))

The first will add any remaining whitespace that might exist after
allocation while the second method is more precise, but not quite
expected from programmers. See [1] for a discussion of the topic.

GCC isn't (currently) able to calculate __bdos on a pointer to the whole
structure. Therefore, because of the above issue, we'll choose to match
what GCC does for consistency's sake.

[1] https://lore.kernel.org/lkml/ZvV6X5FPBBW7CO1f@archlinux/

Co-authored-by: Eli Friedman <[email protected]>

Author: bwendling

clang/lib/CodeGen/CGBuiltin.cpp

@@ -1013,6 +1013,24 @@ CodeGenFunction::emitFlexibleArrayMemberSize(const Expr *E, unsigned Type,
     // Can't find the field referenced by the "counted_by" attribute.
     return nullptr;
 
+  if (isa<DeclRefExpr>(Base))
+    // The whole struct is specificed in the __bdos. The calculation of the
+    // whole size of the structure can be done in two ways:
+    //
+    //     1) sizeof(struct S) + count * sizeof(typeof(fam))
+    //     2) offsetof(struct S, fam) + count * sizeof(typeof(fam))
+    //
+    // The first will add additional padding after the end of the array,
+    // allocation while the second method is more precise, but not quite
+    // expected from programmers. See
+    // https://lore.kernel.org/lkml/ZvV6X5FPBBW7CO1f@archlinux/ for a
+    // discussion of the topic.
+    //
+    // GCC isn't (currently) able to calculate __bdos on a pointer to the whole
+    // structure. Therefore, because of the above issue, we'll choose to match
+    // what GCC does for consistency's sake.
+    return nullptr;
+
   // Build a load of the counted_by field.
   bool IsSigned = CountedByFD->getType()->isSignedIntegerType();
   Value *CountedByInst = EmitLoadOfCountedByField(Base, FAMDecl, CountedByFD);
@@ -1043,32 +1061,9 @@ CodeGenFunction::emitFlexibleArrayMemberSize(const Expr *E, unsigned Type,
   CharUnits Size = Ctx.getTypeSizeInChars(ArrayTy->getElementType());
   llvm::Constant *ElemSize =
       llvm::ConstantInt::get(ResType, Size.getQuantity(), IsSigned);
-  Value *FAMSize =
+  Value *Res =
       Builder.CreateMul(CountedByInst, ElemSize, "", !IsSigned, IsSigned);
-  FAMSize = Builder.CreateIntCast(FAMSize, ResType, IsSigned);
-  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);
-  }
+  Res = Builder.CreateIntCast(Res, ResType, IsSigned);
 
   // A negative \p IdxInst or \p CountedByInst means that the index lands
   // outside of the flexible array member. If that's the case, we want to