Increase default alignment to 2x word size, make DeletedBufferChain allocations more efficient

NB. NeverFreeMemory no longer performs alignment.
This fixes the Bullet crash on Win64.  Need to check Win32.

dtool/src/dtoolbase/deletedBufferChain.cxx

@@ -39,7 +39,7 @@ allocate(size_t size, TypeHandle type_handle) {
   assert(size <= _buffer_size);
 
   // Determine how much space to allocate.
-  const size_t alloc_size = _buffer_size + flag_reserved_bytes;
+  const size_t alloc_size = _buffer_size + flag_reserved_bytes + MemoryHook::get_memory_alignment() - 1;
 
   ObjectNode *obj;
 
@@ -69,7 +69,10 @@ allocate(size_t size, TypeHandle type_handle) {
   // If we get here, the deleted_chain is empty; we have to allocate a new
   // object from the system pool.
 
-  obj = (ObjectNode *)NeverFreeMemory::alloc(alloc_size);
+  // Allocate memory, and make sure the object starts at the proper alignment.
+  void *mem = NeverFreeMemory::alloc(alloc_size);
+  intptr_t pad = ((intptr_t)flag_reserved_bytes - (intptr_t)mem) % MemoryHook::get_memory_alignment();
+  obj = (ObjectNode *)((uintptr_t)mem + pad);
 
 #ifdef USE_DELETEDCHAINFLAG
   obj->_flag = DCF_alive;
@@ -77,6 +80,10 @@ allocate(size_t size, TypeHandle type_handle) {
 
   void *ptr = node_to_buffer(obj);
 
+#ifdef _DEBUG
+  assert(((uintptr_t)ptr % MemoryHook::get_memory_alignment()) == 0);
+#endif
+
 #ifdef DO_MEMORY_USAGE
   type_handle.inc_memory_usage(TypeHandle::MC_deleted_chain_active, alloc_size);
 #endif  // DO_MEMORY_USAGE

dtool/src/dtoolbase/deletedBufferChain.h

@@ -95,12 +95,8 @@ private:
   // Without DELETEDCHAINFLAG, we don't even store the _flag member at all.
   static const size_t flag_reserved_bytes = 0;
 
-#elif defined(LINMATH_ALIGN)
-  // With SSE2 alignment, we need all 16 bytes to preserve alignment.
-  static const size_t flag_reserved_bytes = 16;
-
 #else
-  // Otherwise, we only need enough space for the Integer itself.
+  // Otherwise, we need space for the integer.
   static const size_t flag_reserved_bytes = sizeof(AtomicAdjust::Integer);
 #endif  // USE_DELETEDCHAINFLAG
 

dtool/src/dtoolbase/memoryHook.I

@@ -45,8 +45,9 @@ get_memory_alignment() {
   // don't strictly have to align *everything*, but it's just easier to do so.
   const size_t alignment_size = 16;
 #else
-  // Otherwise, use word alignment.
-  const size_t alignment_size = sizeof(void *);
+  // Otherwise, align to two words.  This seems to be pretty standard to the
+  // point where some code may rely on this being the case.
+  const size_t alignment_size = sizeof(void *) * 2;
 #endif
   return alignment_size;
 }
@@ -72,8 +73,9 @@ get_header_reserved_bytes() {
   static const size_t header_reserved_bytes = sizeof(size_t) + sizeof(size_t);
 
 #else
-  // If we're not aligning, we just need space for the word itself.
-  static const size_t header_reserved_bytes = sizeof(size_t);
+  // Virtually all allocators align to two words, so we make sure we preserve
+  // that alignment for the benefit of anyone who relies upon that.
+  static const size_t header_reserved_bytes = sizeof(void *) * 2;
 #endif
 
   return header_reserved_bytes;

dtool/src/dtoolbase/neverFreeMemory.I

@@ -14,6 +14,8 @@
 /**
  * Returns a pointer to a newly-allocated block of memory of the indicated
  * size.
+ *
+ * Please note that the resulting pointer is not aligned to any boundary.
  */
 INLINE void *NeverFreeMemory::
 alloc(size_t size) {

dtool/src/dtoolbase/neverFreeMemory.cxx

@@ -39,13 +39,9 @@ void *NeverFreeMemory::
 ns_alloc(size_t size) {
   _lock.acquire();
 
-  // We always allocate integer multiples of this many bytes, to guarantee
-  // this minimum alignment.
-  static const size_t alignment_size = MemoryHook::get_memory_alignment();
-
-  // Round up to the next alignment_size.
-  size = ((size + alignment_size - 1) / alignment_size) * alignment_size;
-
+  //NB: we no longer do alignment here.  The only class that uses this is
+  // DeletedBufferChain, and we can do the alignment potentially more
+  // efficiently there since we don't end up overallocating as much.
   _total_used += size;
 
   // Look for a page that has sufficient space remaining.