Is explicit alignment necessary?

Question

After some readings, I understand that compiler has done the padding for structs or classes such that each member can be accessed on its natural aligned boundary. So under what circumstance is it necessary for coders to make explicit alignment to achieve better performance? My question arises from here:

Intel 64 and IA-32 Architechtures Optimization Reference Manual:

For best performance, align data as follows:
Align 8-bit data at any address.
Align 16-bit data to be contained within an aligned 4-byte word.
Align 32-bit data so that its base address is a multiple of four.
Align 64-bit data so that its base address is a multiple of eight.
Align 80-bit data so that its base address is a multiple of sixteen.
Align 128-bit data so that its base address is a multiple of sixteen.

So suppose I have a struct:

struct A
{
    int a;
    int b;
    int c;
}
// size = 12;
// aligned on boundary of: 4

By creating an array of type A, even if I do nothing, it is properly aligned. Then what's the point to follow the guide and make the alignment stronger?

Is it because of cache line split? Assuming the cache line is 64 bytes. With the 6th access of object in the array, the byte starts from 61 to 72, which slows down the program??

BTW, is there a macro in standard library that tells me the alignment requirement based on the running machine by returning a value of std::size_t?

Answer 1

Let me answer your question directly: No, there is no need to explicitly align data in C++ for performance.

Any decent compiler will properly align the data for underlying system.

The problem would come (variation on above) if you had:

 struct 
 {
     int w ;
     char x ;
     int y ;
     char z ;
 } 

This illustrates the two common structure alignment problems.

(1) It is likely a compiler would insert (2) 3 alignment bytes after both x and z. If there is no padding after x, y is unaligned. If there is no padding after z, w and x will be unaligned in arrays.

The instructions are you are reading in the manual are targeted towards assembly language programmers and compiler writers.

When data is unaligned, on some systems (not Intel) it causes an exception and on others it take multiple processor cycles to fetch and write the data.