Padding and Alignment

SIMD and Memory Alignment

How does alignment work with SIMD instructions?

Abstract art representing computer programming

SIMD (Single Instruction Multiple Data) operations often have strict alignment requirements for optimal performance. Let's explore this with practical examples:

Basic SIMD Alignment

In the following example, we force 16-byte alignment:

1#include <iostream>
2
3// Force 16-byte alignment for SIMD
4struct alignas(16) SimdVector {
5  float X, Y, Z, W;// 4x4 bytes = 16 bytes
6};
7
8int main() {
9  SimdVector Vec;
10  std::cout << "Vector alignment: "
11    << alignof(SimdVector) << " bytes\n"
12    << "Vector address: "
13    << &Vec << "\n"
14    << "Address divisible by 16? "
15    << (reinterpret_cast<std::uintptr_t>(&Vec)
16      % 16 == 0 ? "Yes" : "No") << "\n";
17}

1Vector alignment: 16 bytes
2Vector address: 000000346EBCF9F0
3Address divisible by 16? Yes

Array Alignment for SIMD

Here’s an example that forces 32-byte alignment for a struct that includes an array:

1#include <iostream>
2#include <memory>
3
4struct alignas(32) SimdData {
5  float Values[8];// 8x4 = 32 bytes
6};
7
8int main() {
9// Aligned allocation
10  std::unique_ptr<SimdData> AlignedData{
11    new SimdData()};
12
13// Check alignment
14  auto Address = reinterpret_cast<std::uintptr_t>(
15    AlignedData.get());
16
17  std::cout << "Is 32-byte aligned? "
18    << (Address % 32 == 0 ? "Yes" : "No");
19}