ARM > Efficient C for ARM > Padding

by David Thomas on —

Padding

Structures often end up containing padding.

  • Required because of target’s data type restrictions.
    • e.g. ARM keeps ints on a 32-bit boundary.

Easy to waste memory if you’re not aware of where padding is inserted.

Solution:

  • Sort elements in the structure by size:
    • Place elements in small-to-large or large-to-small order.
    • This minimises the amount of padding.

The compiler cannot perform this transformation itself as the C standard guarantees that structure members will be laid out in the order that they’re specified.

Example

Consider the structure:

struct
{
  unsigned char type;
  int           product;
  short         delta;
}

It is laid out in memory like this:

Structure layout in memory - before.

But if we sort the structure members by size, largest to smallest:

struct
{
  int           product;
  short         delta;
  unsigned char type;
}

Structure layout in memory - after.

The padding can be removed and the structure reduces by four bytes (a third).