[clang] [clang][bytecode] Fix tail padding with __builtin_object_size (PR #209771)

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/209771

This fixes the new libcxx/atomics/builtin_clear_padding.pass.cpp

>From 136b4c2660df42e9feec21c071acf3c976607822 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Wed, 15 Jul 2026 16:24:00 +0200
Subject: [PATCH] [clang][bytecode] Fix tail padding with __builtin_object_size

This fixes the new libcxx/atomics/builtin_clear_padding.pass.cpp
---
 clang/lib/AST/ByteCode/InterpBuiltin.cpp        | 6 ++----
 clang/test/AST/ByteCode/builtin-object-size.cpp | 8 ++++++++
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/clang/lib/AST/ByteCode/InterpBuiltin.cpp b/clang/lib/AST/ByteCode/InterpBuiltin.cpp
index 70717720c592d..a539fb26abc08 100644
--- a/clang/lib/AST/ByteCode/InterpBuiltin.cpp
+++ b/clang/lib/AST/ByteCode/InterpBuiltin.cpp
@@ -2299,11 +2299,9 @@ static bool interp__builtin_memchr(InterpState &S, CodePtr OpPC,
 
 static std::optional<unsigned> computeFullDescSize(const ASTContext &ASTCtx,
                                                    const Descriptor *Desc) {
-  if (Desc->isPrimitive())
+  if (Desc->isPrimitive() || Desc->isArray())
     return ASTCtx.getTypeSizeInChars(Desc->getType()).getQuantity();
-  if (Desc->isArray())
-    return ASTCtx.getTypeSizeInChars(Desc->getElemQualType()).getQuantity() *
-           Desc->getNumElems();
+
   if (Desc->isRecord()) {
     // Can't use Descriptor::getType() as that may return a pointer type. Look
     // at the decl directly.
diff --git a/clang/test/AST/ByteCode/builtin-object-size.cpp b/clang/test/AST/ByteCode/builtin-object-size.cpp
index e4433ea700ccb..295b6b4717740 100644
--- a/clang/test/AST/ByteCode/builtin-object-size.cpp
+++ b/clang/test/AST/ByteCode/builtin-object-size.cpp
@@ -55,3 +55,11 @@ namespace InvalidBase {
   constexpr size_t bos_dtor = __builtin_object_size(&(T&)(T&&)invalid_base_2(), 0); // expected-error {{must be initialized by a constant expression}} \
                                                                                     // expected-note {{non-literal type 'T'}}
 }
+
+namespace Padding {
+  typedef long double LongDouble3Vec __attribute__((ext_vector_type(3)));
+  void foo() {
+      LongDouble3Vec v1, v2;
+      static_assert(__builtin_object_size(&v1, 0) == 64);
+  }
+}