Attribute to enforce padding after a variable?

Question

Is there an attribute to enforce padding after a variable?

I have a volatile (non-cached) variable declared as such:

volatile int foo __attribute__((aligned(CACHE_LINE_SIZE));

I would like to prevent other variables from being allocated to the same cache line, to avoid coherency issues. I can add a padding variable after foo, or set the __attribute__((aligned(CACHE_LINE_SIZE)) to the following variable in the same compilation unit. However I was wondering whether there is a cleaner way to do this, such as adding an attribute to variable foo itself to enforce padding.

Answer 1

Some options are introduced in this blog article: Aligning data with cache lines.

I would suggest to use the C++11 alignas:

The simplest version

using my_padded_type = struct alignas(CACHE_LINE_SIZE) { int value;} ;

Padding bits are added automatically. You can verify it by checking sizeof(my_padded_type). See details below.

Other options:

alignas(CACHE_LINE_SIZE) int foo1, foo2;
alignas(CACHE_LINE_SIZE) union { int value; char size[CACHE_LINE_SIZE]; } foo;

std::cout  << "&foo1: " << &foo1 << '\t'
           << "&foo2: " << &foo2 << '\n';
// &foo1: 0x7ffd833ebe00 &foo2: 0x7ffd833ebe40

or

struct alignas(CACHE_LINE_SIZE) my_padded_type {
    int foo;  //  size: 4 
    char p;   //  do not need to specify length here. this line can be omitted entirely
    // auto padding so that sizeof(my_padded_type) == CACHE_LINE_SIZE
}; 
my_padding_type foo;
std::cout<< sizeof(my_padding_type) <<"\n"; // equals CACHE_LINE_SIZE

The padding is done automatically by the compiler, see the example here.