What is a stack and memory alignment technologies?

Stack is a dynamic data structure implementing the LIFO (Last In, First Out) principle. In the context of program execution, the stack is used to store:

• Local variables of functions.
• Function arguments.
• Return addresses after function calls.
• Register states before a function call.

When a function is called, a stack frame is created for that function. When the function finishes, the stack frame is removed.

void function(int a, int b) {
    // a, b - function arguments stored on the stack
    int c = a + b; // c - local variable stored on the stack
    // ...
} // When exiting the function, the stack frame is removed

Memory Alignment

Memory alignment is a requirement that the address of data of certain types be a multiple of a specific number, called the alignment boundary.

Reasons and consequences:

• Performance: Most modern processors read and write data more efficiently when it is aligned. This is related to how cache memory and data buses work. Unaligned data may require multiple read/write operations.
• Atomicity: Some atomic operations require aligned data.
• Architecture requirements: On some architectures, accessing unaligned data results in an error.

Compilers automatically align data, arranging it in memory according to these requirements. This can lead to the appearance of padding between structure members or between objects in an array.

struct MyStruct {
    char c; // 1 byte
    int i;  // 4 bytes
    short s; // 2 bytes
};

Without alignment, MyStruct would occupy 1 + 4 + 2 = 7 bytes. On a 32-bit architecture with 4-byte alignment for int, the layout would be:

Member	Size (bytes)	Offset (bytes)
c	1	0
Padding	3	1-3
i	4	4
s	2	8
Padding	2	10-11

The total size of the structure would be 12 bytes.

In C++11 and later, alignas can be used to specify minimum alignment:

struct alignas(16) AlignedStruct {
    int data[4]; // Will be aligned to a 16-byte boundary
};

The function alignof(type) returns the required alignment for a type.