LLVM Clang produces extremely slow input/output on OS X - c++

Question

Q: Is it possible to improve IO of this code with LLVM Clang under OS X:
test_io.cpp:
#include <iostream>
#include <string>
constexpr int SIZE = 1000*1000;
int main(int argc, const char * argv[]) {
std::ios_base::sync_with_stdio(false);
std::cin.tie(nullptr);
std::string command(argv[1]);
if (command == "gen") {
for (int i = 0; i < SIZE; ++i) {
std::cout << 1000*1000*1000 << " ";
}
} else if (command == "read") {
int x;
for (int i = 0; i < SIZE; ++i) {
std::cin >> x;
}
}
}
Compile:
clang++ -x c++ -lstdc++ -std=c++11 -O2 test_io.cpp -o test_io
Benchmark:
> time ./test_io gen | ./test_io read
real 0m2.961s
user 0m3.675s
sys 0m0.012s
Apart from the sad fact that reading of 10MB file costs 3 seconds, it's much slower than g++ (installed via homebrew):
> gcc-6 -x c++ -lstdc++ -std=c++11 -O2 test_io.cpp -o test_io
> time ./test_io gen | ./test_io read
real 0m0.149s
user 0m0.167s
sys 0m0.040s
My clang version is Apple LLVM version 7.0.0 (clang-700.0.72). clangs installed from homebrew (3.7 and 3.8) also produce slow io. clang installed on Ubuntu (3.8) generates fast io. Apple LLVM version 8.0.0 generates slow io (2 people asked).
I also dtrussed it a bit (sudo dtruss -c "./test_io gen | ./test_io read") and found that clang version makes 2686 write_nocancel syscalls, while gcc version makes 2079 writev syscalls. Which probably points to the root of the problem.

Answer 1

The issue is in libc++ that does not implement sync_with_stdio.
Your command line clang++ -x c++ -lstdc++ -std=c++11 -O2 test_io.cpp -o test_io does not use libstdc++, it will use libc++. To force use libstdc++ you need -stdlib=libstdc++.
Minimal example if you have the input file ready:
int main(int argc, const char * argv[]) {
std::ios_base::sync_with_stdio(false);
int x;
for (int i = 0; i < SIZE; ++i) {
std::cin >> x;
}
}
Timings:
$ clang++ test_io.cpp -o test -O2 -std=c++11
$ time ./test read < input
real 0m2.802s
user 0m2.780s
sys 0m0.015s
$ clang++ test_io.cpp -o test -O2 -std=c++11 -stdlib=libstdc++
clang: warning: libstdc++ is deprecated; move to libc++
$ time ./test read < input
real 0m0.185s
user 0m0.169s
sys 0m0.012s