What is overalignment of execution regions and input sections?

Question

I came across code similar to the following today and I am curious as to what is actually happening:

#pragma pack(1)
__align(2) static unsigned char multi_array[7][24] = { 0 };
__align(2) static unsigned char another_multi_array[7][24] = { 0 };
#pragma pack()

When searching for a reference to the __align keyword in the Keil compiler, I came across this:

Overalignment of execution regions and input sections There are situations when you want to overalign code and data sections... If you have access to the original source code, you can do this at compile time with the __align(n) keyword...

I do not understand what is meant by "overaligning code and data sections". Can someone help to clarify how this overalignment occurrs?

Answer 1

The compiler will naturally "align" data based on the needs of the system. For example, on a typical 32-bit system, a 32-bit integer should always be a single 4-byte word (as opposed to being partly in one word and partly on the next), so it will always start on a 4-byte-word boundary. (This mostly has to do with the instructions available on the processor. A system is very likely to have an instruction to load a single word from memory into a register, and much less likely to have a single instruction to load an arbitrary sequence of four adjacent bytes into a register.)

The compiler normally does this by introducing gaps in the data; for example, a struct with a char followed by a 32-bit int, on such a system, would require eight bytes: one byte for the char, three bytes of filler so the int is aligned right, and four bytes for the int itself.

To "overalign" the data is to request greater alignment than the compiler would naturally provide. For example, you might request that a 32-bit integer start on an 8-byte boundary, even on a system that uses 4-byte words. (One major reason to do this would be if you're aiming for byte-level interoperability with a system that uses 8-byte words: if you pass structs from one system to the other, you want the same gaps in both systems.)

Answer 2

Overalignment is when the data is aligned to more than its default alignment. For example, a 4-byte int usually has a default alignment of 4 bytes. (meaning the address will be divisible by 4)

The default alignment of a datatype is quite-often (but not always) the size of the datatype.

Overalignment allows you to increase this alignment to something greater than the default.

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As for why you would want to do this:

One reason for this is to be able access the data with a larger datatype (that has a larger alignment).

For example:

char buffer[16];

int *ptr = (int*)&buffer;

ptr[0] = 1;
ptr[1] = 2;

By default, buffer will only be aligned to 1 byte. However, int requires a 4-byte alignment. If buffer isn't aligned to 4 bytes, you will get a misalignment exception. (AFAIK, ARM doesn't allow misaligned memory access... x86/64 usually does, but with performance penalty)

__align() will let you force the alignment higher to make it work:

 __align(4) char buffer[16];

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A similar situation appears when using SIMD instructions. You will be accessing smaller datatype with a large SIMD datatype - which will likely require a larger alignment.