[libcxx-dev] Is clang-tidy performance-faster-string-find option wrong for not optimized std::string?

[Pierre TALLOTTE via libcxx-dev]

Hi,

There is clang-tidy option performance-faster-string-find that detects
the use of the std::basic_string::find method (and related ones) with a
single character string literal as argument. According to it, the use of
a character literal is more efficient.

However, I performed a benchmark and noticed it is the case only for
small string (when the small string optimization is used).

Here is my code:

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

static void BM_string_literal(benchmark::State& state)
{
    std::string s;

    for (int i = 0; i < state.range(0); i++)
        s += 'a';

    s += 'b';

    benchmark::DoNotOptimize(s.data());
    benchmark::ClobberMemory();
    size_t pos;

    for (auto _ : state)
    {
        benchmark::DoNotOptimize(pos = s.find("b")); // "b" is a string
literal, it should be longer
        benchmark::ClobberMemory();
    }
}

BENCHMARK(BM_string_literal)->RangeMultiplier(2)->Range(8, 8<<10);;

static void BM_char_literal(benchmark::State& state)
{
    std::string s;

    for (int i = 0; i < state.range(0); i++)
        s += 'a';

    s += 'b';

    benchmark::DoNotOptimize(s.data());
    benchmark::ClobberMemory();
    size_t pos;

    for (auto _ : state)
    {
        benchmark::DoNotOptimize(pos = s.find('b')); // 'b' is a char
literal, it should be faster
        benchmark::ClobberMemory();
    }
}
BENCHMARK(BM_char_literal)->RangeMultiplier(2)->Range(8, 8<<10);;

BENCHMARK_MAIN();

According to clang-tidy, I should prefer the code in BM_char_literal
which is faster. However, the results of the benchmark are the following:

[BM_string_literal vs. BM_char_literal]/8                  
-0.0760         -0.0760             9             8            
9             8
[BM_string_literal vs. BM_char_literal]/16                 
-0.0757         -0.0767             9             8            
9             8
[BM_string_literal vs. BM_char_literal]/32                 
+0.3812         +0.3809             4             5            
4             5
[BM_string_literal vs. BM_char_literal]/64                 
+0.1609         +0.1602             4             5            
4             5
[BM_string_literal vs. BM_char_literal]/128                
+0.1946         +0.1944             4             5            
4             5
[BM_string_literal vs. BM_char_literal]/256                
+0.1616         +0.1623             6             6            
6             6
[BM_string_literal vs. BM_char_literal]/512                
+0.2225         +0.2211             7             9            
7             9
[BM_string_literal vs. BM_char_literal]/1024               
+0.1052         +0.1051            11            12           
11            12
[BM_string_literal vs. BM_char_literal]/2048               
+0.0789         +0.0781            18            20           
18            20
[BM_string_literal vs. BM_char_literal]/4096               
+0.0349         +0.0348            31            32           
31            32
[BM_string_literal vs. BM_char_literal]/8192               
+0.0053         +0.0042            56            57           
56            57

We can see it is faster using a string_literal when the std::string is
at least 32 characters long (I can reproduce these results again and
again, it is not a variance issue).

Is clang-tidy wrong or is there a bug in libc++? Or is my benchmark
wrong somewhere?

To reproduce my case, here are the commands I used (on a debian-stable):

apt-get -y install clang libc++-dev libc++abi-dev git cmake python
python-pip
git clone https://github.com/google/benchmark.git
git clone https://github.com/google/googletest.git benchmark/googletest
pushd benchmark
cmake -E make_directory "build"
cmake -E chdir "build" cmake -DCMAKE_C_COMPILER=clang
-DCMAKE_CXX_COMPILER=clang++ -DCMAKE_BUILD_TYPE=Release
-DCMAKE_CXX_FLAGS="-stdlib=libc++" -DBENCHMARK_DOWNLOAD_DEPENDENCIES=ON ../
cmake --build "build" --config Release --target install
popd
pip install scipy
clang++ -stdlib=libc++ -O3 bench.cpp -lbenchmark -lpthread -o bench
./benchmark/tools/compare.py filters ./bench BM_string_literal
BM_char_literal

Thanks.