The test found that StrAppend is slower than the concatenation operation

[pwr617]

I am a newcomer to the Abseil library. I learned from the official documentation（https://abseil.io/docs/cpp/guides/strings） that StrAppend is more efficient than concatenation operations. I attempted to use Google Benchmark to test the performance of both, but I arrived at a contrary conclusion. I would like to inquire if there might be an issue with my usage or testing methodology.

The code is as follows:

#include <string>
#include <benchmark/benchmark.h>
#include "absl/strings/str_cat.h"

static void BM_StringCon(benchmark::State& state) {
  std::string s1 = "A string";
  std::string another = " and another string";
  for (auto _ : state) {
    s1 += " and some other string" + another;
  }
}
// Register the function as a benchmark
BENCHMARK(BM_StringCon);

// Define another benchmark
static void BM_StringCat(benchmark::State& state) {
  std::string s1 = "A string";
  std::string another = " and another string";
  for (auto _ : state) {
    absl::StrAppend(&s1, " and some other string", another);
  }
}
BENCHMARK(BM_StringCat);

BENCHMARK_MAIN();

google benchmark:

2024-04-20T18:56:39+08:00
Running ./str_append
Run on (48 X 2500.59 MHz CPU s)
CPU Caches:
  L1 Data 32 KiB (x24)
  L1 Instruction 32 KiB (x24)
  L2 Unified 256 KiB (x24)
  L3 Unified 30720 KiB (x2)
Load Average: 0.30, 0.19, 0.21
-------------------------------------------------------
Benchmark             Time             CPU   Iterations
-------------------------------------------------------
BM_StringCon        156 ns          156 ns      5314965
BM_StringCat        185 ns          185 ns      3924664

Compile with g++ (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0, Abseil using the latest version

Thanks for any help!