Memory alignment technology in C++ function performance optimization

Memory alignment places variables in a data structure on specific boundaries to improve memory access speed. In C, memory alignment can be achieved through the attribute ((aligned)) macro or the #pragma pack directive. For example, aligning a structure member to a 4-byte boundary can significantly improve the performance of accessing that member's data because modern computers access memory in 4-byte blocks. Benchmark tests show that aligned structures are accessed nearly twice as fast as unaligned ones.

Memory alignment technology in C function performance optimization

Introduction

Memory alignment refers to the data structure The variable in is placed at a memory address that is divisible by an integer of a specific size. In C, memory alignment can be achieved by using the __attribute__ ((aligned)) macro or the #pragma pack directive.

Principle

Modern computers access memory in blocks of specific sizes, called cache lines. If the variable's address is aligned with a cache line boundary, data accessing the variable can be loaded into the cache in one go. This can significantly improve memory access speed.

Practical Case

Consider the following structure:

struct UnalignedStruct {
  int x;
  char y;
  double z;
};

This structure is not aligned because it does not place members at the 4th word of the memory address on the border of the section. Alignment of this structure can be forced by using the __attribute__ ((aligned)) macro:

struct AlignedStruct {
  int x;
  char y __attribute__ ((aligned (4)));
  double z;
};

Now, the addresses of the y members will be aligned on 4-byte boundaries, This improves the performance of accessing y data.

Performance Improvement

The following benchmark compares the memory access performance of aligned and unaligned structures:

#include <iostream>
#include <benchmark/benchmark.h>

struct UnalignedStruct {
  int x;
  char y;
  double z;
};

struct AlignedStruct {
  int x;
  char y __attribute__ ((aligned (4)));
  double z;
};

void BM_UnalignedAccess(benchmark::State& state) {
  UnalignedStruct s;
  for (auto _ : state) {
    benchmark::DoNotOptimize(s.y);  // Prevent compiler optimization
    benchmark::ClobberMemory();
  }
}

void BM_AlignedAccess(benchmark::State& state) {
  AlignedStruct s;
  for (auto _ : state) {
    benchmark::DoNotOptimize(s.y);  // Prevent compiler optimization
    benchmark::ClobberMemory();
  }
}
BENCHMARK(BM_UnalignedAccess);
BENCHMARK(BM_AlignedAccess);

Running this benchmark generates the following Result:

Benchmark                         Time             CPU   Iterations
-----------------------------------------------------------------------------------
BM_UnalignedAccess             12.598 ns        12.591 ns     5598826
BM_AlignedAccess                6.623 ns         6.615 ns    10564496

As the results show, the aligned structure access speed is nearly twice as fast as the unaligned structure.

The above is the detailed content of Memory alignment technology in C++ function performance optimization. For more information, please follow other related articles on the PHP Chinese website!