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Unaligned versus naturally aligned memory access

ANEESH R
aneeshr2020@gmail.com

Unaligned memory access
Unaligned memory accesses occur when you try to read N bytes of data starting from an
address that is not evenly divisible by N (i.e. addr % N !=0). For example, reading 4 bytes of data
form address 0x1004 is fine, but reading 4 bytes of data from address 0x10005 would be an
unaligned memory access.
The above statement may seem a little vague, as memory access can happen in different ways.
The context here is at the machine code level: certain instructions read or write a number of
bytes to or from memory (e.g. movb, movw, movl in x86 assembly). As will become clear, it is
relatively easy to spot C statements which will compile to multiple-byte memory access
instructions, namely when dealing with types such as u16, u32, and u64.
Natural memory access
The rule mentioned above forms what we refer to as natural alignment: When accessing N
bytes of memory, the base memory address must be evenly divisible by N, i.e. addr % N ==0.
When writing code, assume the target architecture has natural alignment requirements. In
reality, only a few architectures require natural alignment on all sizes of memory access.
However, we must consider ALL supported architectures; writing code that satisfies natural
alignment requirements is the easiest way to achieve full portability.
Why unaligned access is bad
The effects of performing an unaligned memory access vary from architecture to architecture. A
summary of the common scenarios is presented below:
•

Some architecture is able to perform unaligned memory access transparently, but there
is usually a significant performance cost.

•

Some architecture raises processor exceptions when unaligned accesses happen. The
exception handler is able to correct the unaligned access, at significant cost to
performance.

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•

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Some architecture raises processor exceptions when unaligned accesses happen, but
the exceptions do not contain enough information for the unaligned access to be
corrected.

•

Some architecture is not capable of unaligned memory access, but silently performs a
different memory access to the one that was requested, resulting in a suitable code bug
that is hard to detect.

It should be obvious form the above that if your code causes unaligned memory accesses to
happen, your code will not work correctly on certain platforms and will causes performance
problem on others.

Code does not causes unaligned access
At first, the concept above may seem a little hard to relate to actual coding practice. After all,
you don’t have a deal of control over memory address of certain variables, etc. Fortunately
things are not too complex, as in most cases, the compiler ensures that things will work for you.
For example, take the following structure:
struct foo {
u16 field1;
u32 field2;
u8 field3;
};
Let us assume that an instance of the above structure resides in memory starting at address
0x10000. With a basic level of understanding, it would not be unreasonable to expect that
accessing field2 would cause an unaligned access. You'd be expecting field2 to be located at
offset 2 bytes into the structure, i.e. address 0x10002, but that address is not evenly divisible by
4 (remember, we're reading a 4 byte value here).
Fortunately, the compiler understands the alignment constraints, so in the above case it would
insert 2 bytes of padding in between field1 and field2. Therefore, for standard structure types
you can always rely on the compiler to pad structures so that accesses to fields are suitably
aligned (assuming you do not cast the field to a type of different length).

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Similarly, you can also rely on the compiler to align variables and function parameters to a
naturally aligned scheme, based on the size of the type of the variable.
At this point, it should be clear that accessing a single byte (u8 or char) will never cause an
unaligned access, because all memory addresses are evenly divisible by one.
On a related topic, with the above considerations in mind you may observe that, you could
reorder the fields in the structure in order to place fields where padding would otherwise be
inserted, and hence reduce the overall resident memory size of structure instances. The
optimal layout of the above example is:
struct foo {
u32 field2;
u16 field1;
u8 field3;
};
For a natural alignment scheme, the compiler would only have to add a single byte of padding
at the end of the structure. This padding is added in order to satisfy alignment constraints for
arrays of these structures.
Another point worth mentioning is the use of __attribute__((packed)) on a structure type. This
GCC-specific attribute tells the compiler never to insert any padding within structures, useful
when you want to use a C “struct” to represent some data that comes in a fixed arrangement
'off the wire'.
you might be inclined to believe that usage of this attribute can easily lead to unaligned
accesses when accessing fields that do not satisfy architectural alignment requirements.
However, again, the compiler is aware of the alignment constraints and will generate extra
instructions to perform the memory access in a way that does not cause unaligned access. Of
course, the extra instructions obviously cause a loss in performance compared to the nonpacked case, so the packed attribute should only be used when avoiding structure padding is of
importance.
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ANEESH R

Code does causes unaligned access
With the above in mind, let's move onto a real life example of a function that can cause an
unaligned memory access. The following function adapted from include/linux/etherdevice.h is
an optimized routine to compare two ethernet MAC addresses for equality.
unsigned int compare_ether_addr(const u8 *addr1, const u8 *addr2)
{
const u16 *a = (const u16 *) addr1;
const u16 *b = (const u16 *) addr2;
return ((a[0] ^ b[0]) | (a[1] ^ b[1]) | (a[2] ^ b[2])) != 0;
}
In the above function, the reference to a[0] causes 2 bytes (16 bits) to be read from memory
starting at address addr1. Think about what would happen if addr1 was an odd address such as
0x10003. (Hint: it'd be an unaligned access.)
Despite the potential unaligned access problems with the above function, it is included in the
kernel anyway but is understood to only work on 16-bit-aligned addresses. It is up to the caller
to ensure this alignment or not use this function at all. This alignment-unsafe function is still
useful as it is a decent optimization for the cases when you can ensure alignment, which is true
almost all of the time in Ethernet networking context. Here is another example of some code
that could cause unaligned accesses:
void myfunc(u8 *data, u32 value)
{
[...]
*((u32 *) data) = cpu_to_le32(value);
[...]
}
This code will cause unaligned accesses every time the data parameter points to an address
that is not evenly divisible by 4. In summary, the 2 main scenarios where you may run into
unaligned access problems involve:
1 Casting variables to types of different lengths.
2 Pointer arithmetic followed by access to at least 2 bytes of data.

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Avoiding unaligned accesses
The easiest way to avoid unaligned access is to use the get_unaligned() and put_unaligned()
macros provided by the <asm/unaligned.h> header file.
Going back to an earlier example of code that potentially causes unaligned access:
void myfunc(u8 *data, u32 value)
{
[...]
*((u32 *) data) = cpu_to_le32(value);
[...]
}
To avoid the unaligned memory access, you would rewrite it as follows:void myfunc(u8 *data, u32 value)
{
[...]
value = cpu_to_le32(value);
put_unaligned(value, (u32 *) data);
[...]
}
The get_unaligned() macro works similarly. Assuming 'data' is a pointer to memory and you
wish to avoid unaligned access, its usage is as follows:
u32 value = get_unaligned((u32 *) data);
These macros work for memory accesses of any length (not just 32 bits as in the examples
above). Be aware that when compared to standard access of aligned memory, using these
macros to access unaligned memory can be costly in-terms of performance.
If use of such macros is not convenient, another option is to use memcpy(),where the source or
destination (or both) are of type u8* or unsigned char*. Due to the byte-wise nature of this
operation, unaligned accesses are avoided.

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