Agner's CPU blog

From INTEL CPU manual, it says "nonaligned data accesses will seriously impact the performance of the processor". Then I do a test in order to prove it. But the result is that aligned and nonaligned data accesses have the same performance. Why??? Could someone help? My code is shown below:
#include <iostream>
#include <stdint.h>
#include <time.h>
#include <chrono>
#include <string.h>
using namespace std;

static inline int64_t get_time_ns()
{
std::chrono::nanoseconds a = std::chrono::high_resolution_clock::now().time_since_epoch();
return a.count();
}
int main(int argc, char** argv)
{
if (argc < 2) {
cout << "Usage：./test [01234567]" << endl;
cout << "0 - aligned, 1-7 - nonaligned offset" << endl;
return 0;
}
uint64_t offset = atoi(argv[1]);
cout << "offset = " << offset << endl;
const uint64_t BUFFER_SIZE = 800000000;
uint8_t* data_ptr = new uint8_t[BUFFER_SIZE];
if (data_ptr == nullptr) {
cout << "apply for memory failed" << endl;
return 0;
}
memset(data_ptr, 0, sizeof(uint8_t) * BUFFER_SIZE);
const uint64_t LOOP_CNT = 300;
cout << "start" << endl;
auto start = get_time_ns();
for (uint64_t i = 0; i < LOOP_CNT; ++i) {
for (uint64_t j = offset; j <= BUFFER_SIZE - 8; j+= 8) { // align:offset = 0 nonalign: offset=1-7
auto tmp = *(uint64_t*)&data_ptr[j]; // read from memory
++tmp;
*(uint64_t*)&data_ptr[j] = tmp; // write to memory
}
}
auto end = get_time_ns();
cout << "time elapse " << end - start << "ns" << endl;
return 0;
}

RESULT:
offset = 0
start
time elapse 28063563503ns
offset = 1
start
time elapse 28950243291ns
offset = 2
start
time elapse 29374504232ns
offset = 3
start
time elapse 28680777833ns
offset = 4
start
time elapse 28962080206ns
offset = 5
start
time elapse 28364894777ns
offset = 6
start
time elapse 28779795011ns
offset = 7
start
time elapse 29268488768ns

Modern high-end microprocessors are able to handle unaligned access quite efficiently in most cases.
In your case, the caches, prefetchers, and in/out buffers are handling the alignment issue smoothly while the bottleneck that determines the execution time is the exchange of data between caches and main memory.

The Intel document you are referring to is somewhat misleading or not up to date.