AMD 250 Manuel D’Utilisation

25112

Rev. 3.06

September 2005

Sort and pad C and C++ structures to achieve natural alignment.

This optimization applies to:

•

32-bit software

•

64-bit software

By sorting and padding structures at the source-code level, if the first member of a structure is 
naturally aligned, then all other members are naturally aligned as well. This allows, for example, 
arrays of structures to be perfectly aligned.

To sort and pad a C or C++ structure, follow these steps:

1. Sort the structure members according to their type sizes, declaring members with larger type sizes 

ahead of members with smaller type sizes.

2. Pad the structure so the size of the structure is a multiple of the largest member’s type size.

Consider the following structure declaration in a C function:

struct {

   char a[5];

   long k;

   double x;

} baz;

Instead of allocating the members in the order in which they are declared, allocate them from lower to 
higher addresses in the following order and add padding:

x, k, a[4], a[3], a[2], a[1], a[0], 

pad_byte6,..., pad_byte0