new arena implementation using std::align()

src/internal/mana_arena.h

@@ -13,6 +13,11 @@
 namespace mesytec::mnode::mana
 {
 
+template <typename T> inline bool is_aligned(const T *ptr, size_t alignment = alignof(T))
+{
+    return (((uintptr_t)ptr) % alignment) == 0;
+}
+
 class Arena
 {
   public:
@@ -26,16 +31,11 @@ class Arena
     Arena(Arena &&other) = default;
     Arena &operator=(Arena &&other) = default;
 
-    // pushes at least the required amount of memory onto the arena.
+    // Pushes at least the required amount of memory onto the arena. The memory is aligned to
-    // 'required' is rounded up to the nearest multiple of 'default_align_to' to make
+    // 'default_align_to' and zeroed
-    // subsequent allocations aligned.
     u8 *push_size(size_t required)
     {
-        size_t padded = round_up(required, default_align_to);
+        if (!segment_)
-        size_t pad_waste = padded - required;
-        required = padded;
-
-        if (!segment_ || segment_->free() < required)
         {
             if (segment_count() < max_segments())
             {
@@ -52,12 +52,20 @@ class Arena
 
         assert(segment_ && segment_->free() >= required);
 
-        auto result = segment_->cur();
+        void *ptr = segment_->cur();
-        std::memset(result, 0, required);
+        size_t space = segment_->free();
-        segment_->used += required;
+        if (!std::align(default_align_to, required, ptr, space))
-        ++allocations_;
+            throw std::bad_alloc();
+
+        assert(is_aligned(ptr, default_align_to));
+
+        size_t pad_waste = segment_->free() - space;
+        segment_->used += required + pad_waste;
         pad_waste_ += pad_waste;
-        return result;
+        ++allocations_;
+
+        std::memset(ptr, 0, required);
+        return reinterpret_cast<u8 *>(ptr);
     }
 
     template <typename T> T *push_t(size_t count = 1)