Performance: g++ Cygwin vs. other compilers (copying char[] to vector)

["Alex Vinokur" <alexvn at connect dot to>]

Hi,

How to explain so considerable difference in performance: g++ Cygwin vs. other compilers in tests below?


Simple C/C++ Perfometer : Copying char[] to vector (Version CS-1.0)
* func_memcpy
* func_copy
* func_copy_with_reserve
* func_transform
* func_ctor

* The whole program at http://thread.gmane.org/gmane.comp.lang.c++.perfometer/58
  contains the following files :
  - copystr.h     // Header file                : http://article.gmane.org/gmane.comp.lang.c++.perfometer/58/
  - info.cpp      // Information about compiler : http://article.gmane.org/gmane.comp.lang.c++.perfometer/59/
  - cps_test.cpp  // Function tested            : http://article.gmane.org/gmane.comp.lang.c++.perfometer/60/
  - cps_main.cpp  // Main program               : http://article.gmane.org/gmane.comp.lang.c++.perfometer/61/

Payload code      : cps_test.cpp.
Housekeeping code : copystr.h, info.cpp, cps_main.cpp.


###### C++ Code Tested : BEGIN ######
// Code from http://article.gmane.org/gmane.comp.lang.c++.perfometer/60


#define STR_LEN 100

char cstr[STR_LEN];
struct char_identity
{
  char operator()(char ch) const { return ch; }
};


// ====== Function tested using Simple C++ Perfometer ======
void func_memcpy()
{
vector<char> vect (STR_LEN);
  memcpy(&vect[0], cstr, STR_LEN);
}

void func_copy()
{
vector<char> vect;
  copy(cstr, cstr + STR_LEN, back_inserter(vect));
}


void func_copy_with_reserve()
{
vector<char> vect;
  vect.reserve(STR_LEN);
  copy(cstr, cstr + STR_LEN, back_inserter(vect));
}


void func_transform()
{
vector<char> vect (STR_LEN);
  transform(cstr, cstr + STR_LEN, vect.begin(), char_identity());
}


void func_ctor()
{
vector<char> vect (cstr, cstr + STR_LEN);
}


// =========================================================


###### C++ Code Tested : END ########




###### Run Log (Fragments) : BEGIN ########
>From http://article.gmane.org/gmane.comp.lang.c++.perfometer/62/



========================================
Simple C/C++ Perfometer : Copying String
Version CS-1.0
========================================

        C/C++ Performance Tests
        =======================
        Using Simple C/C++ Perfometer (Copying char[] to vector), Version CS-1.0
        http://thread.gmane.org/gmane.comp.lang.c++.perfometer/58/  (The whole program)
        http://article.gmane.org/gmane.comp.lang.c++.perfometer/60/ (Functions tested only)




    Environment
    -----------
    Windows 2000 Professional
    Intel(R) Celeron(R) CPU 1.70 GHz


 ####################################
 ### Compilation with no optimization
 ####################################


 ### char[] size           : 100
 ### Number of repetitions : 100000


----------------------
GNU gcc 3.3.1 (CYGWIN)
----------------------

 ### CLOCKS_PER_SEC        : 1000

func_memcpy            :   2937 units (2.937 secs)
func_copy              :  14487 units (14.487 secs)
func_copy_with_reserve :   4098 units (4.098 secs)
func_transform         :   3428 units (3.428 secs)
func_ctor              :   2944 units (2.944 secs)



---------------------
GNU gcc 3.3.1 (MINGW)
---------------------

 ### CLOCKS_PER_SEC        : 1000

func_memcpy            :     74 units (0.074 secs)
func_copy              :   1117 units (1.117 secs)
func_copy_with_reserve :    584 units (0.584 secs)
func_transform         :    303 units (0.303 secs)
func_ctor              :     57 units (0.057 secs)



-------------------------
GNU gcc 3.3.4 (DGGPP 2.3)
-------------------------

 ### CLOCKS_PER_SEC        : 91

func_memcpy            :     11 units (0.121 secs)
func_copy              :    243 units (2.670 secs)
func_copy_with_reserve :    121 units (1.330 secs)
func_transform         :     65 units (0.714 secs)
func_ctor              :     10 units (0.110 secs)



-------------------------------
Microsoft C++ 13.00 (Unmanaged)
-------------------------------

 ### CLOCKS_PER_SEC        : 1000
func_memcpy            :   1465 units (1.465 secs)
func_copy              :  13202 units (13.202 secs)
func_copy_with_reserve :   4609 units (4.609 secs)
func_transform         :   1963 units (1.963 secs)
func_ctor              :   1378 units (1.378 secs)



-------------------------
Digital Mars C/C++ 8.40.2
-------------------------

 ### CLOCKS_PER_SEC        : 1000

func_memcpy            :     63 units (0.063 secs)
func_copy              :    574 units (0.574 secs)
func_copy_with_reserve :    220 units (0.220 secs)
func_transform         :    130 units (0.130 secs)
func_ctor              :     60 units (0.060 secs)



###### Run Log (Fragments) : END ##########


List of the functions sorted according to performance:
* func_ctor  (the fastest)
* func_memcpy
* func_transform
* func_copy_with_reserve
* func_copy





-- 
   Alex Vinokur
     http://mathforum.org/library/view/10978.html
     http://sourceforge.net/users/alexvn





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