[libcxx-commits] [libcxx] [libc++] Fix arithmetic underflow in internal capacity evaluation for vector<bool> (PR #120577)

[Peng Liu via libcxx-commits]

https://github.com/winner245 updated https://github.com/llvm/llvm-project/pull/120577

>From e1b3a681d3e2535bb2b5c90a66de9c5b4c8c2206 Mon Sep 17 00:00:00 2001
From: Peng Liu <winner245 at hotmail.com>
Date: Thu, 19 Dec 2024 08:36:10 -0500
Subject: [PATCH] Fix erroneous internal capacity evaluation

---
 libcxx/include/__vector/vector_bool.h                        | 2 +-
 .../test/std/containers/sequences/vector.bool/flip.pass.cpp  | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/libcxx/include/__vector/vector_bool.h b/libcxx/include/__vector/vector_bool.h
index 8658745b8a8f9e..ba0a14123583a3 100644
--- a/libcxx/include/__vector/vector_bool.h
+++ b/libcxx/include/__vector/vector_bool.h
@@ -115,7 +115,7 @@ class _LIBCPP_TEMPLATE_VIS vector<bool, _Allocator> {
   }
   _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 static size_type
   __external_cap_to_internal(size_type __n) _NOEXCEPT {
-    return (__n - 1) / __bits_per_word + 1;
+    return (__n + __bits_per_word - 1) / __bits_per_word;
   }
 
 public:
diff --git a/libcxx/test/std/containers/sequences/vector.bool/flip.pass.cpp b/libcxx/test/std/containers/sequences/vector.bool/flip.pass.cpp
index f8f575cdc0e219..fb0de061bd04c3 100644
--- a/libcxx/test/std/containers/sequences/vector.bool/flip.pass.cpp
+++ b/libcxx/test/std/containers/sequences/vector.bool/flip.pass.cpp
@@ -32,6 +32,11 @@ TEST_CONSTEXPR_CXX20 void test_vector_flip(std::size_t n, Allocator a) {
 }
 
 TEST_CONSTEXPR_CXX20 bool tests() {
+  // Test empty vectors
+  test_vector_flip(0, std::allocator<bool>());
+  test_vector_flip(0, min_allocator<bool>());
+  test_vector_flip(0, test_allocator<bool>(5));
+
   // Test small vectors with different allocators
   test_vector_flip(3, std::allocator<bool>());
   test_vector_flip(3, min_allocator<bool>());