BLI: use no_unique_address attribute

Even though the `no_unique_address` attribute has only been standardized
in C++20, compilers seem to support it with C++17 already. This attribute
allows reducing the memory footprint of structs which have empty types as
data members (usually that is an allocator or inline buffer in Blender).
Previously, one had to use the empty base optimization to achieve the same
effect, which requires a lot of boilerplate code.

The types that benefit from this the most are `Vector` and `Array`, which
usually become 8 bytes smaller. All types which use these core data structures
get smaller as well of course.

Differential Revision: https://developer.blender.org/D14993

10 files changed, 41 insertions, 23 deletions

diff --git a/source/blender/blenlib/BLI_array.hh b/source/blender/blenlib/BLI_array.hh
index 91dfc81ae27..813277d9968 100644
--- a/source/blender/blenlib/BLI_array.hh
+++ b/source/blender/blenlib/BLI_array.hh
@@ -64,10 +64,10 @@ class Array {
   int64_t size_;
 
   /** Used for allocations when the inline buffer is too small. */
-  Allocator allocator_;
+  BLI_NO_UNIQUE_ADDRESS Allocator allocator_;
 
   /** A placeholder buffer that will remain uninitialized until it is used. */
-  TypedBuffer<T, InlineBufferCapacity> inline_buffer_;
+  BLI_NO_UNIQUE_ADDRESS TypedBuffer<T, InlineBufferCapacity> inline_buffer_;
 
  public:
   /**
diff --git a/source/blender/blenlib/BLI_generic_array.hh b/source/blender/blenlib/BLI_generic_array.hh
index e1b6b29874a..4b917434264 100644
--- a/source/blender/blenlib/BLI_generic_array.hh
+++ b/source/blender/blenlib/BLI_generic_array.hh
@@ -33,7 +33,7 @@ class GArray {
   void *data_ = nullptr;
   int64_t size_ = 0;
 
-  Allocator allocator_;
+  BLI_NO_UNIQUE_ADDRESS Allocator allocator_;
 
  public:
   /**
diff --git a/source/blender/blenlib/BLI_linear_allocator.hh b/source/blender/blenlib/BLI_linear_allocator.hh
index 6532c59a846..deb6ea3b5fd 100644
--- a/source/blender/blenlib/BLI_linear_allocator.hh
+++ b/source/blender/blenlib/BLI_linear_allocator.hh
@@ -18,7 +18,7 @@ namespace blender {
 
 template<typename Allocator = GuardedAllocator> class LinearAllocator : NonCopyable, NonMovable {
  private:
-  Allocator allocator_;
+  BLI_NO_UNIQUE_ADDRESS Allocator allocator_;
   Vector<void *> owned_buffers_;
   Vector<Span<char>> unused_borrowed_buffers_;
 
diff --git a/source/blender/blenlib/BLI_map.hh b/source/blender/blenlib/BLI_map.hh
index d76aa46502d..55233676ed8 100644
--- a/source/blender/blenlib/BLI_map.hh
+++ b/source/blender/blenlib/BLI_map.hh
@@ -130,10 +130,10 @@ class Map {
   uint64_t slot_mask_;
 
   /** This is called to hash incoming keys. */
-  Hash hash_;
+  BLI_NO_UNIQUE_ADDRESS Hash hash_;
 
   /** This is called to check equality of two keys. */
-  IsEqual is_equal_;
+  BLI_NO_UNIQUE_ADDRESS IsEqual is_equal_;
 
   /** The max load factor is 1/2 = 50% by default. */
 #define LOAD_FACTOR 1, 2
diff --git a/source/blender/blenlib/BLI_memory_utils.hh b/source/blender/blenlib/BLI_memory_utils.hh
index d7c41ae88a8..940542c9f1d 100644
--- a/source/blender/blenlib/BLI_memory_utils.hh
+++ b/source/blender/blenlib/BLI_memory_utils.hh
@@ -317,30 +317,36 @@ template<typename T> using destruct_ptr = std::unique_ptr<T, DestructValueAtAddr
  * An `AlignedBuffer` is a byte array with at least the given size and alignment. The buffer will
  * not be initialized by the default constructor.
  */
-template<size_t Size, size_t Alignment> class alignas(Alignment) AlignedBuffer {
- private:
-  /* Don't create an empty array. This causes problems with some compilers. */
-  char buffer_[(Size > 0) ? Size : 1];
+template<size_t Size, size_t Alignment> class AlignedBuffer {
+  struct Empty {
+  };
+  struct alignas(Alignment) Sized {
+    /* Don't create an empty array. This causes problems with some compilers. */
+    std::byte buffer_[Size > 0 ? Size : 1];
+  };
+
+  using BufferType = std::conditional_t<Size == 0, Empty, Sized>;
+  BLI_NO_UNIQUE_ADDRESS BufferType buffer_;
 
  public:
   operator void *()
   {
-    return buffer_;
+    return this;
   }
 
   operator const void *() const
   {
-    return buffer_;
+    return this;
   }
 
   void *ptr()
   {
-    return buffer_;
+    return this;
   }
 
   const void *ptr() const
   {
-    return buffer_;
+    return this;
   }
 };
 
@@ -351,7 +357,7 @@ template<size_t Size, size_t Alignment> class alignas(Alignment) AlignedBuffer {
  */
 template<typename T, int64_t Size = 1> class TypedBuffer {
  private:
-  AlignedBuffer<sizeof(T) * (size_t)Size, alignof(T)> buffer_;
+  BLI_NO_UNIQUE_ADDRESS AlignedBuffer<sizeof(T) * (size_t)Size, alignof(T)> buffer_;
 
  public:
   operator T *()
diff --git a/source/blender/blenlib/BLI_set.hh b/source/blender/blenlib/BLI_set.hh
index 99acae43571..62de4b79e41 100644
--- a/source/blender/blenlib/BLI_set.hh
+++ b/source/blender/blenlib/BLI_set.hh
@@ -136,10 +136,10 @@ class Set {
   uint64_t slot_mask_;
 
   /** This is called to hash incoming keys. */
-  Hash hash_;
+  BLI_NO_UNIQUE_ADDRESS Hash hash_;
 
   /** This is called to check equality of two keys. */
-  IsEqual is_equal_;
+  BLI_NO_UNIQUE_ADDRESS IsEqual is_equal_;
 
   /** The max load factor is 1/2 = 50% by default. */
 #define LOAD_FACTOR 1, 2
diff --git a/source/blender/blenlib/BLI_stack.hh b/source/blender/blenlib/BLI_stack.hh
index a06515a7781..ed123f43a6b 100644
--- a/source/blender/blenlib/BLI_stack.hh
+++ b/source/blender/blenlib/BLI_stack.hh
@@ -96,7 +96,7 @@ class Stack {
   int64_t size_;
 
   /** The buffer used to implement small object optimization. */
-  TypedBuffer<T, InlineBufferCapacity> inline_buffer_;
+  BLI_NO_UNIQUE_ADDRESS TypedBuffer<T, InlineBufferCapacity> inline_buffer_;
 
   /**
    * A chunk referencing the inline buffer. This is always the bottom-most chunk.
@@ -105,7 +105,7 @@ class Stack {
   Chunk inline_chunk_;
 
   /** Used for allocations when the inline buffer is not large enough. */
-  Allocator allocator_;
+  BLI_NO_UNIQUE_ADDRESS Allocator allocator_;
 
  public:
   /**
diff --git a/source/blender/blenlib/BLI_utildefines.h b/source/blender/blenlib/BLI_utildefines.h
index b8407a5453f..7f9470a9111 100644
--- a/source/blender/blenlib/BLI_utildefines.h
+++ b/source/blender/blenlib/BLI_utildefines.h
@@ -843,6 +843,18 @@ extern bool BLI_memory_is_zero(const void *arr, size_t arr_size);
  */
 #define BLI_ENABLE_IF(condition) typename std::enable_if_t<(condition)> * = nullptr
 
+#if defined(_MSC_VER)
+#  define BLI_NO_UNIQUE_ADDRESS [[msvc::no_unique_address]]
+#elif defined(__has_cpp_attribute)
+#  if __has_cpp_attribute(no_unique_address)
+#    define BLI_NO_UNIQUE_ADDRESS [[no_unique_address]]
+#  else
+#    define BLI_NO_UNIQUE_ADDRESS
+#  endif
+#else
+#  define BLI_NO_UNIQUE_ADDRESS [[no_unique_address]]
+#endif
+
 /** \} */
 
 #ifdef __cplusplus
diff --git a/source/blender/blenlib/BLI_vector.hh b/source/blender/blenlib/BLI_vector.hh
index acf47f67168..c23d846d277 100644
--- a/source/blender/blenlib/BLI_vector.hh
+++ b/source/blender/blenlib/BLI_vector.hh
@@ -84,10 +84,10 @@ class Vector {
   T *capacity_end_;
 
   /** Used for allocations when the inline buffer is too small. */
-  Allocator allocator_;
+  BLI_NO_UNIQUE_ADDRESS Allocator allocator_;
 
   /** A placeholder buffer that will remain uninitialized until it is used. */
-  TypedBuffer<T, InlineBufferCapacity> inline_buffer_;
+  BLI_NO_UNIQUE_ADDRESS TypedBuffer<T, InlineBufferCapacity> inline_buffer_;
 
   /**
    * Store the size of the vector explicitly in debug builds. Otherwise you'd always have to call
diff --git a/source/blender/blenlib/BLI_vector_set.hh b/source/blender/blenlib/BLI_vector_set.hh
index 4ae1bf9000d..b0a3696f245 100644
--- a/source/blender/blenlib/BLI_vector_set.hh
+++ b/source/blender/blenlib/BLI_vector_set.hh
@@ -117,10 +117,10 @@ class VectorSet {
   uint64_t slot_mask_;
 
   /** This is called to hash incoming keys. */
-  Hash hash_;
+  BLI_NO_UNIQUE_ADDRESS Hash hash_;
 
   /** This is called to check equality of two keys. */
-  IsEqual is_equal_;
+  BLI_NO_UNIQUE_ADDRESS IsEqual is_equal_;
 
   /** The max load factor is 1/2 = 50% by default. */
 #define LOAD_FACTOR 1, 2