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Data alignment the C++ way

 3 years ago
source link: https://vorbrodt.blog/2019/04/06/data-alignment-the-c-way/
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Data alignment the C++ way

Before modern C++ the only way to align variables or structures on a given byte boundary was to inject padding; to align a struct to 16 bytes you had to do this:

struct Old
int x;
char padding[16 - sizeof(int)];

Not any more! Modern C++ introduced a keyword just for that: alignas (read more about it here). Now you can specify struct’s alignment like this:

struct alignas(16) New
int x;

This can be of great help when dealing with constructive or destructive interference of L1 cache lines. You can also space local variables apart, as well as struct/class members. Here’s a complete example (alignas.cpp):

#include <iostream>
using namespace std;
int main()
struct Old
int x;
char padding[16 - sizeof(int)];
cout << "sizeof(Old): " << sizeof(Old) << endl << endl;
struct alignas(16) New
int x;
cout << "sizeof(New): " << sizeof(New) << endl << endl;
alignas(16) int x{}, y{};
alignas(16) int z{};
ptrdiff_t delta1 = (uint8_t*)&y - (uint8_t*)&x;
ptrdiff_t delta2 = (uint8_t*)&z - (uint8_t*)&y;
cout << "Address of 'x'      : " << &x << endl;
cout << "Address of 'y'      : " << &y << endl;
cout << "Address of 'z'      : " << &z << endl;
cout << "Distance 'x' to 'y' : " << delta1 << endl;
cout << "Distance 'y' to 'z' : " << delta2 << endl << endl;
struct             Empty   {};
struct alignas(64) Empty64 {};
cout << "sizeof(Empty)  : " << sizeof(Empty)   << endl;
cout << "sizeof(Empty64): " << sizeof(Empty64) << endl << endl;
struct Full
alignas(32) char c;
alignas(16) int x, y;
cout << "sizeof(Full): " << sizeof(Full) << endl << endl;

sizeof(Old): 16
sizeof(New): 16
Address of ‘x’      : 0x7ffee4a448c0
Address of ‘y’      : 0x7ffee4a448d0
Address of ‘z’      : 0x7ffee4a448e0
Distance ‘x’ to ‘y’ : 16
Distance ‘y’ to ‘z’ : 16
sizeof(Empty)  : 1
sizeof(Empty64): 64
sizeof(Full): 64

Program output.


report

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