C++ 31 Aug 2011 22:27:06

C++ Include Speed

A performance benchmark of which include guard method is faster. Test times the compilation of a main.cpp that includes 10000 files 3 times each.

The tested methods are:

  • #pragma once followed by #ifndef
  • #ifndef followed by #pragma once
  • Only #pragma once
  • Only #ifndef
  • External #ifndef

Sources for the test is at inc.tar.gz, but it’s just 5×10000 files.

The compilers are Microsoft Visual C++ 2010, GNU g++ 4.6.1, and LLVM clang++ 2.9.

Windows: MSVC++ 2010

  • Compiler: MSVC++ 2010, cl.exe /O2
  • Arch: Windows 7 64 bit, 1.60GHz Core i7 Q720, 8 GiB RAM
VC++ 2010Seconds
Pragma + ifndef3.687
Ifndef + pragma3.744
Pragma only3.597
Ifndef only5.153

Linux: GNU g++ 4.6.1

  • Compiler: GNU g++ 4.6.1 -O3
  • Arch: VirtualBox on the Windows machine, VT-x, Arch Linux x86_64, kernel 3.0-ARCH, 1 GiB RAM
g++ 4.6.1Seconds
Pragma + ifndef2.007
Ifndef + pragma1.804
Pragma only2.005
Ifndef only0.309

Linux: LLVM clang++ 2.9

  • Compiler: clang++ 2.9 -O3
  • Arch: VirtualBox on the Windows machine, VT-x, Arch Linux x86_64, kernel 3.0-ARCH, 1 GiB RAM
clang++ 2.9Seconds
Pragma + ifndef0.580
Ifndef + pragma0.578
Pragma only0.532
Ifndef only0.568

5 Responses to “C++ Include Speed”

  1. on 01 Sep 2011 at 20:35:25 1.Nick Porcino said …

    Is it possible that file caching or other VM behaviors by VirtualBox is skewing the results from gcc and clang?

  2. on 01 Sep 2011 at 20:51:59 2.Tino Didriksen said …

    I thought of that, so I tested it on a real machine and got same relative results. And I did run every compilation 5-7 times to make sure the files were well cached.

    Anyway, the conclusion is that it doesn’t matter. If the difference for 10000 almost empty files is a second or two more, then your real world project won’t care whatsoever which include method you use.

  3. on 27 Jun 2014 at 18:33:18 3.John Doe said …

    Two things:

    1. If you tested on an actual machine, why not edit the post with those times instead?

    2. In the Windows you’re running, what else are you running? Anti-virus, hooks, explorer extensions, etc? Can you feasibly run in a fresh W7 with a fresh VC++2010?

  4. on 27 Jun 2014 at 21:42:17 4.Foster Brereton said …

    The question I am interested in getting answered is why is MSVC almost 10x slower in the #ifndef case than clang, and what can be done to mitigate that cost?

  5. on 28 Jun 2014 at 10:30:22 5.Tino Didriksen said …

    Since I got the same relative results, it won’t matter if I put those numbers in. They’ll have the same factors to the other numbers.

    It doesn’t matter what the Windows machine was running, as I redid the tests several times and got the same numbers. Also, since the VBox was running on the Windows machine, any host slowdown would also affect the guests – especially I/O slowdowns. Since there is no seen I/O slowdown, and since tests are run multiple times to cache files, one can conclude that raw I/O throughput is not the issue. But, no active AV.

    And again, test is for 3x 10000 files. For a single file, MSVC’s slowest case would thus be somewhere between 0.0005153 and 0.0001718 seconds – also known as 0.5 and 0.2 milliseconds.

    There is no cost to mitigate. The conclusion is that include speed is super fast in ALL compilers.

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