[clang] [Clang] Convert __builtin_dynamic_object_size into a calculation (PR #80256)

[Bill Wendling via cfe-commits]

================
@@ -1051,6 +1052,145 @@ CodeGenFunction::emitFlexibleArrayMemberSize(const Expr *E, unsigned Type,
   return Builder.CreateSelect(Cmp, Res, ConstantInt::get(ResType, 0, IsSigned));
 }
 
+namespace {
+
+/// \p StructBaseExpr returns the base \p Expr with a structure or union type.
+struct StructBaseExpr : public ConstStmtVisitor<StructBaseExpr, const Expr *> {
+  StructBaseExpr() = default;
+
+  //===--------------------------------------------------------------------===//
+  //                            Visitor Methods
+  //===--------------------------------------------------------------------===//
+
+  const Expr *VisitStmt(const Stmt *S) { return nullptr; }
+
+  const Expr *Visit(const Expr *E) {
+    QualType Ty = E->getType();
+    if (Ty->isStructureType() || Ty->isUnionType())
+      return E;
+
+    return ConstStmtVisitor<StructBaseExpr, const Expr *>::Visit(E);
+  }
+
+  const Expr *VisitDeclRefExpr(const DeclRefExpr *E) { return E; }
+
+  const Expr *VisitMemberExpr(const MemberExpr *E) {
+    return Visit(E->getBase());
+  }
+  const Expr *VisitArraySubscriptExpr(const ArraySubscriptExpr *E) {
+    return Visit(E->getBase());
+  }
+  const Expr *VisitCastExpr(const CastExpr *E) {
+    return Visit(E->getSubExpr());
+  }
+  const Expr *VisitParenExpr(const ParenExpr *E) {
+    return Visit(E->getSubExpr());
+  }
+  const Expr *VisitUnaryAddrOf(const clang::UnaryOperator *E) {
+    return Visit(E->getSubExpr());
+  }
+  const Expr *VisitUnaryDeref(const clang::UnaryOperator *E) {
+    return Visit(E->getSubExpr());
+  }
----------------
bwendling wrote:

The Visitor is only to get the structure that the member resides in. The thing about `__builtin_dynamic_object_size(p, 0)` is that we're not trying to calculate what `p` points to, but the size of the struct from p to the end. It's counter-intuitive, but it's not (currently) supported to ask `__bdos` the size of what `p` points to is.

https://github.com/llvm/llvm-project/pull/80256