C++ Include test: full matrix

Finally I am able to present some include experiment results! Previously I wrote about Code Generator and now I can actually run this tool and get some numbers out of it. I compared VC11.0 (Visual Studio 2012 For Desktop) and GCC 4.7.2 (using MinGW32 4.7.2 and DevCpp).

Basics of experiment

• VC11.0 - Visual Studio 2012 For Desktop, Release Mode, 32bit, no optimizations.
  • I turned build timings on to have detailed build performance.
• GCC 4.7.2 32bit - MinGW 4.7.2 version and run from DevCpp 5.4.1
  • g++.exe -c testHeaders.cpp -o testHeaders.o -ftime-report
• The machine: Core i5, 4 cores, 4GB RAM, Windows 8 64bit
  • compilation will probably take place only on one core only (there will be one translation unit only)

Code structure

• testHeader.cpp includes N header files
• m-th header file includes N-1 other header files (so we have "cross" include)
• each header file has its proper include guard

Test N headers

A header can look like this in this tests:

#ifndef _INCLUDED_HEADER_HEADER_5_H
    #define _INCLUDED_HEADER_HEADER_5_H 
    #include "header_0.h"
    #include "header_1.h"
    #include "header_2.h"
    #include "..."  

Test N headers - additional ifDef

A header can look like this in this tests:

#ifndef _INCLUDED_HEADER_HEADER_5_H
    #define _INCLUDED_HEADER_HEADER_5_H
    #ifndef _INCLUDED_HEADER_HEADER_0_H
        #include "header_0.h"
    #endif
    #ifndef _INCLUDED_HEADER_HEADER_1_H
        #include "header_1.h"
    #endif
    #ifndef _INCLUDED_HEADER_HEADER_2_H
        #include "header_2.h"
    #endif
    #include "..." 

Test N headers - #pragma once

A header can look like this in this tests:

#pragma once
    #ifndef _INCLUDED_HEADER_HEADER_5_H
    #define _INCLUDED_HEADER_HEADER_5_H
    #include "header_0.h"
    #include "header_1.h"
    #include "header_2.h"
    #include "..." 

Conclusion

• The code structure is rather theoretical and does not represent 'common' structures that may appear in projects.
• GCC and VC build code a bit different. GCC linker phase is much longer than in VC.
• GCC uses lots of optimization for header files. There is almost no need to do any 'tricks' with includes. Header guards are enough.
• VC likes header files 'tricks'!
  • There is above 2X to 3X speedup using additional include guards or pragma once
  • Pragma once for such code structure seems to be a bit slower than additional include guards: from 5% to even 10% slower.
• All in all VC11.0 is faster than GCC.

What I have learnt

• VC Release mode is not the same as GCC basic compilation
• Make sure you set up proper experiment base
• Automate as much as possible
• Tool for spreadsheet is a must have-software
• It is valuable to figure out how to break the compiler

Links