torstai 31. joulukuuta 2015

Are exceptions slower than error objects

One stated reason for using error objects/error codes instead of exceptions is that they are faster. The real question, then, are they. I wrote a bunch of C and C++ test applications and put them up on Github.

The tests are simple. They call a function that either does nothing or throws an exception/returns an error object depending on its argument in a tight loop. The implementation is put in a separate file and LTO is not used so the compiler can't inline the code and thus optimize away the checks.

The error objects used mimic what GLib's GError does. That is, it contains a dynamically allocated string describing the error (a dummy string in our case). On all error cases the created error object needs to be deallocated properly.

A sample run on OSX looks like this (output directly from Meson's ninja benchmark command):

  1/6 exceptions, not thrown                  OK      0.00520 s +- 0.00009 s
  2/6 exceptions, thrown                      OK      1.65206 s +- 0.01488 s
  3/6 error object, not created               OK      0.00563 s +- 0.00017 s
  4/6 error object, created                   OK      0.19703 s +- 0.00769 s
  5/6 5/loop, exceptions, no throws           OK      0.00528 s +- 0.00017 s

  6/6 5/loop, error object, no errors         OK      0.00669 s +- 0.00072 s

Here we see that when errors happen exceptions (case 2) are 10x slower than error objects (case 4). We can also see that when errors do not happen, exceptions (case 1) are measurably faster than exception objects (case 3).

Cases 5 and 6 are the same as 1 and 3, respectively, except that they call the function five times in a single loop iteration. This does not change the end result, exceptions are still a bit faster.

I ran this test on a bunch of machines and compilers, such as OSX, Linux, Windows, gcc, clang, vs, Raspberry Pi and so on and the results were consistent. When not thrown, exceptions were never slower and usually were a bit faster than error objects. The only exception was MinGW on Windows XP, where exceptions were noticeably slower even when not thrown. Thrown exceptions were always roughly 10x slower than error objects.

The reason for this is (probably, I haven't looked at the resulting assembly) the fact that the error object code always has a branch operation that slows down the CPU. Exceptions have an optimization for this case which avoids the need for a comparison.

Pulling all that together we can ask whether you should use exceptions in high performance code or not. And the answer is maybe. If you have a case where exceptions are rare, then they might very well be faster. This is very data dependent so if this an issue that really matters to you, run your own benchmarks and make the decision based on measurements rather than wild guessing.

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