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Chapter 5 Cache and Memory Optimizations 117
Software Optimization Guide for AMD64 Processors
25112 Rev. 3.06 September 2005
5.11 Sorting 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
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.
Example
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