Question

Usually data is aligned at power of two addresses depending on its size.

How should I align a struct or class with size of 20 bytes or another non-power-of-two size?

I’m creating a custom stack allocator so I guess that the compiler wont align data for me since I’m working with a continuous block of memory.

Some more context:

I have an Allocator class that uses malloc() to allocate a large amount of data.
Then I use void* allocate(U32 size_of_object) method to return the pointer that where I can store whether objects I need to store.
This way all objects are stored in the same region of memory and it will hopefully fit in the cache reducing cache misses.

Answer 1

Although the compiler (or interpreter) normally allocates individual data items on aligned boundaries, data structures often have members with different alignment requirements. To maintain proper alignment the translator normally inserts additional unnamed data members so that each member is properly aligned. In addition the data structure as a whole may be padded with a final unnamed member. This allows each member of an array of structures to be properly aligned. http://en.wikipedia.org/wiki/Data_structure_alignment#Typical_alignment_of_C_structs_on_x86

This says that the compiler takes care of it for you, 99.9% of the time. As for how to force an object to align a specific way, that is compiler specific, and only works in certain circumstances.

MSVC: http://msdn.microsoft.com/en-us/library/83ythb65.aspx

__declspec(align(20)) 
struct S{ int a, b, c, d; };
//must be less than or equal to 20 bytes

GCC: http://gcc.gnu.org/onlinedocs/gcc-3.4.0/gcc/Type-Attributes.html

struct S{ int a, b, c, d; } 
__attribute__ ((aligned (20)));

I don’t know of a cross-platform way (including macros!) to do this, but there’s probably neat macro somewhere.