AMD 250 manual Sorting and Padding C and C++ Structures

25112 Rev. 3.06 September 2005 Software Optimization Guide for AMD64 Processors

2.18Sorting and Padding C and C++ Structures

Optimization

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

Application

This optimization applies to:

•32-bit software

•64-bit software

Rationale

In order to achieve better alignment for structures, many compilers have options that allow padding of structures to make their sizes multiples of words, doublewords, or quadwords. In addition, to improve the alignment of structure members, some compilers might allocate structure elements in an order that differs from the order in which they are declared. However, some compilers might not offer any of these features, or their implementations might not work properly in all situations.

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.

Sorting and Padding C and C++ Structures

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.

Examples

Avoid structure declarations in which the members are not declared in order of their type sizes and the size of the structure is not a multiple of the size of the largest member’s type:

struct {
char a[5];	\\ Smallest type size (1 byte * 5)
long k;	\\	4 bytes in this example
double x;	\\	Largest type size (8 bytes)
} baz;

Chapter 2 C and C++ Source-Level Optimizations 39