Friday, 14 March 2014

x264 Performance with Different Compilers

Recently I made a post about compiling x264 natively using Visual Studio under Windows. With that post I also made available my git repository that includes all the necessary Visual Studio projects to get x264 up and compiling under Windows. Since the default build chain for x264 uses a gcc based compiler (which is MinGW on Windows) I wondered if there was any performance difference between the generated binaries.

So I set out by building the x264 command line tool using both MSVC from Visual Studio 2013, Intel Compiler XE 2013 SP1 and MinGW 4.8.2. To test I passed the first minute of Big Buck Bunny from the 1080p mp4 file found on the main site (www.bigbuckbunny.org/index.php/download/). I then did a CRF=20.0 encode using the 'Very Slow' pre-set. This may not exactly be the most exhaustive test but I wasn't feeling like waiting around for 10+ hour encodes.

The results can be seen for themselves:

  • MinGW:    3min 5sec
  • MSVC:      3min 5sec
  • ICL:           3min 6sec
  • ICL (O3):   3min 5sec

ICL shows up twice because I did the first one using the default compilation options that MSVC was using. I then did a second build using the higher compilation options that Intel compiler can support (a.k.a I set the /O3 compile option as opposed to the default /O2).

As you can see the difference is negligible (there so little to see here I didn't even bother making a nice table or graph). In fact the only variations was with ICL and that was probably within the error of the test measurement anyway so for all intents and purposes all the final builds performed identically. This is not too surprising as x264 uses a lot of hand tuned assembly in the key bits which there isn't much more that any of the compilers could do with. The result is that the code is optimised as much as possible so there is no room for the compiler to improve it. Therefore the performance is pretty much consistent across all compilers.

Of course Ill be the first to admit that this was a pretty quick and dirty test. Its quite possible that after a 10+ hour encode some larger differences may start to appear. However based on these values it is unlikely that those differences would be very substantial (a couple of minutes here and there over a 10+ hour run).

So if you were expecting (or hoping) for big differences then I'm sorry to disappoint. What can be taken away from this is that it appears that it doesn't matter what compiler you use to build x264 you will still achieve the same performance. So for those who prefer GCC they can continue to MSYS it up, while those who prefer Visual Studio can do so happily as there is no performance downside in doing so.

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