Computer Systems A Programmer's Perspective Second Edition
Many computer systems place restrictions on the allowable addresses for the
primitive data types, requiring that the address for some type of object must be a
multiple of some value K (typically 2, 4, or 8). Such alignment restrictions simplify
the design of the hardware forming the interface between the processor and the
memory system. For example, suppose a processor always fetches 8 bytes from
memory with an address that must be a multiple of 8. If we can guarantee that any
double will be aligned to have its address be a multiple of 8, then the value can
be read or written with a single memory operation.Otherwise, we may need to
perform two memory accesses, since the object might be split across two 8-byte
memory blocks.
Linux follows an alignment policy where 2-byte data types (e.g.,
short) must have an address that is a multiple of 2, while any larger data types
(e.g.,int,int *,float, and double) must have an address that is a multiple of 4.
Note that this requirement means that the least significant bit of the address of
an object of type short must equal zero. Similarly, any object of type int, or any
pointer, must be at an address having the low-order 2 bits equal to zero.
Microsoft Windows imposes a stronger alignment requirement—any primitive object of
K bytes, for K=2, 4, or 8, must have an address that is a multiple of K. In particular,
it requires that the address of a double or a long long be a multiple of 8. This
requirement enhances the memory performance at the expense of some wasted space.
The Linux convention, where 8-byte values are aligned on 4-byte boundaries was probably
good for the i386, back when memory was scarce and memory interfaces were
only 4 bytes wide. With modern processors, Microsoft’s alignment is a better design decision.
Data type long double, for which gcc generates IA32 code allocating 12 bytes (even though
the actual data type requires only 10 bytes) has a 4-byte alignment requirement with
both Windows and Linux.