It is slower to use std::string::find with a char literal argument than a string literal #47013
Description
| Bugzilla Link | 47669 |
| Version | 10.0 |
| OS | Linux |
| Reporter | LLVM Bugzilla Contributor |
| CC | @dwblaikie,@mclow |
Extended Description
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