CODEWITHSUNDEEP
c++visual-studio-2012c++11mingwbenchmarking
Paul Jurczak
Paul Jurczak

Reputation: 8127

Why std::to_string() runs 16-times slower with MinGW than with VS2012

I was surprised when I benchmarked this function:

int f(int N = 999) {  
  int nMax = 0;

  for (int i = 1; i <= N; ++i)
    for (int j = i; j <= N; ++j) {
      string digits  = to_string(i*j);
      string rDigits = digits;

      reverse(rDigits.begin(), rDigits.end());

      if (digits == rDigits)
        nMax = max(i*j, nMax);
    }

  return nMax;
}

with VS2012, VS2013 (release, /O2) and MinGW 4.8.0, 4.8.1 (-Ofast) on Windows 7 32-bit and 64-bit. I noticed that MinGW built versions run about 13 times slower than VS ones. Is this a problem with implementation of to_string() and reverse()? Any other reasons?

The code I used is here: https://github.com/pauljurczak/Benchmark-2/blob/master/benchmark.cpp

EDIT

I narrowed down the problem to std::to_string() function, which is about 16 times slower with MinGW then with VS. I used this snippet for testing:

int f(int N = 100000) {  
  int len = 0;

  for (int i = 0; i <= N; ++i) 
    len += to_string(i).length();

  return len;
}

When I compile it with g++ 4.7.3 on Ubuntu, performance is about the same as VS2012.

Upvotes: 2

Views: 1161

Answers (2)

tomas
tomas

Reputation: 11

Check for C++ runtime version used by your MINGW build (use dependency walker or something similar). VS2012 supports r-values and perhaps in your case more importantly small string optimization. Using that would eliminate any memory allocation coming from to_string. Memory allocation requires significantly more CPU than finding max or reverse.

Upvotes: 1

Kyle_the_hacker
Kyle_the_hacker

Reputation: 1434

Visual Studio could have reversed the if-condition:

if (digits != rDigits)
    continue;
else
    nMax = max(i*j, nMax);

But that is only a guess...

By the way, I would rather write:

string rDigits(digits.rbegin(), digits.rend());

And you could also take a look at: https://stackoverflow.com/a/17909430/1689664, it might give you some ideas to optimize your algorithm.

Upvotes: 1

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