[libcxx-commits] [libcxx] [libc++] Diagnoses insufficiently aligned pointers for std::assume_aligned during constant evaluation (PR #73775)

[Rajveer Singh Bharadwaj via libcxx-commits]

https://github.com/Rajveer100 updated https://github.com/llvm/llvm-project/pull/73775

>From fc732f5d2dd6b69ef6cef874367c1f638bc36ad8 Mon Sep 17 00:00:00 2001
From: Rajveer <rajveer.developer at icloud.com>
Date: Wed, 29 Nov 2023 15:55:25 +0530
Subject: [PATCH] [libc++] Diagnoses insufficiently aligned pointers for
 std::assume_aligned during constant evaluation

Resolves Issue #64078

This is a libc++ enhancement when violating alignment assumption for __builtin_assume_aligned.
---
 libcxx/include/__memory/assume_aligned.h      |  1 +
 .../assume_aligned.const_eval.verify.cpp      | 22 +++++++++++++++++++
 2 files changed, 23 insertions(+)
 create mode 100644 libcxx/test/libcxx/utilities/memory/ptr.align/assume_aligned.const_eval.verify.cpp

diff --git a/libcxx/include/__memory/assume_aligned.h b/libcxx/include/__memory/assume_aligned.h
index bce00bf02a718..c87c3d9db16df 100644
--- a/libcxx/include/__memory/assume_aligned.h
+++ b/libcxx/include/__memory/assume_aligned.h
@@ -28,6 +28,7 @@ _LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp* __ass
     "std::assume_aligned<N>(p) requires N to be a power of two");
 
   if (__libcpp_is_constant_evaluated()) {
+    (void)__builtin_assume_aligned(__ptr, _Np);
     return __ptr;
   } else {
     _LIBCPP_ASSERT_UNCATEGORIZED(reinterpret_cast<uintptr_t>(__ptr) % _Np == 0, "Alignment assumption is violated");
diff --git a/libcxx/test/libcxx/utilities/memory/ptr.align/assume_aligned.const_eval.verify.cpp b/libcxx/test/libcxx/utilities/memory/ptr.align/assume_aligned.const_eval.verify.cpp
new file mode 100644
index 0000000000000..db6c10acd454e
--- /dev/null
+++ b/libcxx/test/libcxx/utilities/memory/ptr.align/assume_aligned.const_eval.verify.cpp
@@ -0,0 +1,22 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <memory>
+
+template <size_t Size>
+constexpr bool test() {
+  char data[1];
+    
+  auto data1 = std::assume_aligned<1>(data); // expected-warning {{unused variable 'data1'}}
+  auto data2 = std::assume_aligned<2>(data); // expected-warning {{unused variable 'data2'}}
+    
+  return true;
+}
+
+static_assert(test<1>()); // expected-error {{static assertion expression is not an integral constant expression}}
+static_assert(test<2>()); // expected-error {{static assertion expression is not an integral constant expression}}