From d089482467d4099dcd5030a18de8d7222a923424 Mon Sep 17 00:00:00 2001
From: KRM7 <70973547+KRM7@users.noreply.github.com>
Date: Mon, 5 Feb 2024 22:46:40 +0100
Subject: [PATCH] fix object layout

---
 src/small_vector.hpp  | 33 +++++++++++++++++++++------------
 test/small_vector.cpp | 18 ++++++++++++++++++
 2 files changed, 39 insertions(+), 12 deletions(-)

diff --git a/src/small_vector.hpp b/src/small_vector.hpp
index a4358e0..abae0e9 100644
--- a/src/small_vector.hpp
+++ b/src/small_vector.hpp
@@ -18,12 +18,10 @@
 #include <cstddef>
 #include <cassert>
 
-#if defined(_MSC_VER) && !defined(__clang__)
+#if defined(_MSC_VER)
 #   define SV_NO_UNIQUE_ADDRESS [[msvc::no_unique_address]]
-#elif __has_cpp_attribute(no_unique_address)
-#   define SV_NO_UNIQUE_ADDRESS [[no_unique_address]]
 #else
-#   define SV_NO_UNIQUE_ADDRESS
+#   define SV_NO_UNIQUE_ADDRESS [[no_unique_address]]
 #endif
 
 namespace detail
@@ -320,7 +318,7 @@ namespace detail
 
     inline constexpr std::size_t cache_line_size = 64;
 
-    template<typename T, std::size_t Size, std::size_t Align = cache_line_size>
+    template<typename T, std::size_t Size>
     struct small_vector_buffer
     {
     public:
@@ -335,15 +333,25 @@ namespace detail
 
         constexpr std::size_t size() const noexcept { return Size; }
     private:
-        inline constexpr static std::size_t align_req = std::max(alignof(T), Align);
-
         union
         {
             unsigned char dummy_ = {};
-            alignas(align_req) std::array<T, Size> data_;
+            std::array<T, Size> data_;
         };
     };
 
+    template<typename T>
+    struct small_vector_buffer<T, 0>
+    {
+        constexpr T* begin() noexcept { return nullptr; }
+        constexpr const T* begin() const noexcept { return nullptr; }
+
+        constexpr T* end() noexcept { return nullptr; }
+        constexpr const T* end() const noexcept { return nullptr; }
+
+        constexpr std::size_t size() const noexcept { return 0; }
+    };
+
 
     template<typename T>
     struct default_small_size
@@ -896,15 +904,16 @@ class small_vector
     }
 
 private:
+    static constexpr double growth_factor = 1.618;
+    static constexpr std::size_t alignment = !!Size * std::max(alignof(T), detail::cache_line_size);
 
-    detail::small_vector_buffer<T, Size> buffer_;
+    alignas(alignment)
+    SV_NO_UNIQUE_ADDRESS detail::small_vector_buffer<T, Size> buffer_;
     pointer first_      = nullptr;
     pointer last_       = nullptr;
     pointer last_alloc_ = nullptr;
     SV_NO_UNIQUE_ADDRESS allocator_type alloc_;
 
-    static inline constexpr double growth_factor_ = 1.618;
-
 
     constexpr void allocate_n(size_type count)
     {
@@ -988,7 +997,7 @@ class small_vector
 
     constexpr size_type next_capacity(size_type min_capacity = 0) const noexcept
     {
-        return std::max(min_capacity, size_type(growth_factor_ * capacity()) + 1);
+        return std::max(min_capacity, size_type(growth_factor * capacity()) + 1);
     }
 
 }; // class small_vector
diff --git a/test/small_vector.cpp b/test/small_vector.cpp
index fbfaecc..8815c11 100644
--- a/test/small_vector.cpp
+++ b/test/small_vector.cpp
@@ -84,6 +84,24 @@ constexpr auto equal_to(T rhs)
 }
 
 
+    //-----------------------------------//
+    //           OBJECT LAYOUT           //
+    //-----------------------------------//
+
+
+TEST_CASE("small_vector_size", "[object_layout][!mayfail]") // fails under clang-cl because of no support for no_unique_address
+{
+    STATIC_REQUIRE(std::is_standard_layout_v<small_vector<int>>);
+    STATIC_REQUIRE(std::is_standard_layout_v<small_vector<int, 0>>);
+
+    CHECK(sizeof(small_vector<int>) == detail::cache_line_size);
+    CHECK(alignof(small_vector<int>) == detail::cache_line_size);
+    
+    CHECK(sizeof(small_vector<int, 0>) == 3 * sizeof(int*));
+    CHECK(alignof(small_vector<int, 0>) == alignof(int*));
+}
+
+
     //-----------------------------------//
     //            CONSTRUCTORS           //
     //-----------------------------------//