(C++) LocalVersusGlobal

March 3, 2018 · View on GitHub

 

 

 

 

 

(C++) LocalVersusGlobal

 

A simple benchmark that tests the speed of local versus member variables versus global variables.

 

The following tests are performed:

 

 

Avoid using global data [1-4].

 

 

 

 

 

Technical facts

 

Application type(s)

Operating system(s) or programming environment(s)

IDE(s):

Project type:

C++ standard:

Compiler(s):

Libraries used:

  • STL STL: GNU ISO C++ Library, version 4.7.2

 

 

 

 

 

Qt project file: CppLocalVersusGlobal.pro

 

TEMPLATE = app CONFIG += console CONFIG -= qt QMAKE_CXXFLAGS += -Wextra -Werror SOURCES += main.cpp

 

 

 

 

 

main.cpp

 

//Benchmark to compare the speed of local versus global variables //From http://www.richelbilderbeek.nl/CppLocalVersusGlobal.htm #include <iostream> #include <boost/timer.hpp> int g_a = 0; int g_b = 0; int g_c = 0; int g_d = 0; int g_e = 0; int g_f = 0; int g_g = 0; int g_h = 0; int g_i = 0; int g_j = 0; int g_k = 0; int g_l = 0; int g_m = 0; int g_n = 0; int g_o = 0; int g_p = 0; int g_q = 0; int g_r = 0; int g_s = 0; int g_t = 0; int g_u = 0; int g_v = 0; int g_w = 0; int g_x = 0; int g_y = 0; int g_z = 0; int g_count = 0; double TestGlobals() {   boost::timer t;   for (g_a=0; g_a!=32; ++g_a)   for (g_b=0; g_b!=g_a; ++g_b)   for (g_c=0; g_c!=g_b; ++g_c)   for (g_d=0; g_d!=g_c; ++g_d)   for (g_e=0; g_e!=g_d; ++g_e)   for (g_f=0; g_f!=g_e; ++g_f)   for (g_g=0; g_g!=g_f; ++g_g)   for (g_h=0; g_h!=g_g; ++g_h)   for (g_i=0; g_i!=g_h; ++g_i)   for (g_j=0; g_j!=g_i; ++g_j)   for (g_k=0; g_k!=g_j; ++g_k)   for (g_l=0; g_l!=g_k; ++g_l)   for (g_m=0; g_m!=g_l; ++g_m)   for (g_n=0; g_n!=g_m; ++g_n)   for (g_o=0; g_o!=g_n; ++g_o)   for (g_p=0; g_p!=g_o; ++g_p)   for (g_q=0; g_q!=g_p; ++g_q)   for (g_r=0; g_r!=g_q; ++g_r)   for (g_s=0; g_s!=g_r; ++g_s)   for (g_t=0; g_t!=g_s; ++g_t)   for (g_u=0; g_u!=g_t; ++g_u)   for (g_v=0; g_v!=g_u; ++g_v)   for (g_w=0; g_w!=g_v; ++g_w)   for (g_x=0; g_x!=g_w; ++g_x)   for (g_y=0; g_y!=g_x; ++g_y)   for (g_z=0; g_z!=g_y; ++g_z)   ++g_count;   const double time = t.elapsed();   std::cout << g_count << std::endl;   return time; } struct MembersOnly {   MembersOnly() : m_count(0) {}   double Test()   {     boost::timer t;     for (m_a=0; m_a!=32; ++m_a)     for (m_b=0; m_b!=m_a; ++m_b)     for (m_c=0; m_c!=m_b; ++m_c)     for (m_d=0; m_d!=m_c; ++m_d)     for (m_e=0; m_e!=m_d; ++m_e)     for (m_f=0; m_f!=m_e; ++m_f)     for (m_g=0; m_g!=m_f; ++m_g)     for (m_h=0; m_h!=m_g; ++m_h)     for (m_i=0; m_i!=m_h; ++m_i)     for (m_j=0; m_j!=m_i; ++m_j)     for (m_k=0; m_k!=m_j; ++m_k)     for (m_l=0; m_l!=m_k; ++m_l)     for (m_m=0; m_m!=m_l; ++m_m)     for (m_n=0; m_n!=m_m; ++m_n)     for (m_o=0; m_o!=m_n; ++m_o)     for (m_p=0; m_p!=m_o; ++m_p)     for (m_q=0; m_q!=m_p; ++m_q)     for (m_r=0; m_r!=m_q; ++m_r)     for (m_s=0; m_s!=m_r; ++m_s)     for (m_t=0; m_t!=m_s; ++m_t)     for (m_u=0; m_u!=m_t; ++m_u)     for (m_v=0; m_v!=m_u; ++m_v)     for (m_w=0; m_w!=m_v; ++m_w)     for (m_x=0; m_x!=m_w; ++m_x)     for (m_y=0; m_y!=m_x; ++m_y)     for (m_z=0; m_z!=m_y; ++m_z)     ++m_count;     const double time = t.elapsed();     std::cout << m_count << std::endl;     return time;   }   private:   int m_a;   int m_b;   int m_c;   int m_d;   int m_e;   int m_f;   int m_g;   int m_h;   int m_i;   int m_j;   int m_k;   int m_l;   int m_m;   int m_n;   int m_o;   int m_p;   int m_q;   int m_r;   int m_s;   int m_t;   int m_u;   int m_v;   int m_w;   int m_x;   int m_y;   int m_z;   int m_count; }; struct MembersToLocal {   MembersToLocal() : m_count(0) {}   double Test()   {     int member_count = 0;     int a = m_a;     int b = m_b;     int c = m_c;     int d = m_d;     int e = m_e;     int f = m_f;     int g = m_g;     int h = m_h;     int i = m_i;     int j = m_j;     int k = m_k;     int l = m_l;     int m = m_m;     int n = m_n;     int o = m_o;     int p = m_p;     int q = m_q;     int r = m_r;     int s = m_s;     int t = m_t;     int u = m_u;     int v = m_v;     int w = m_w;     int x = m_x;     int y = m_y;     int z = m_z;     boost::timer timer;     for (int a=0; a!=32; ++a)     for (int b=0; b!=a; ++b)     for (int c=0; c!=b; ++c)     for (int d=0; d!=c; ++d)     for (int e=0; e!=d; ++e)     for (int f=0; f!=e; ++f)     for (int g=0; g!=f; ++g)     for (int h=0; h!=g; ++h)     for (int i=0; i!=h; ++i)     for (int j=0; j!=i; ++j)     for (int k=0; k!=j; ++k)     for (int l=0; l!=k; ++l)     for (int m=0; m!=l; ++m)     for (int n=0; n!=m; ++n)     for (int o=0; o!=n; ++o)     for (int p=0; p!=o; ++p)     for (int q=0; q!=p; ++q)     for (int r=0; r!=q; ++r)     for (int s=0; s!=r; ++s)     for (int t=0; t!=s; ++t)     for (int u=0; u!=t; ++u)     for (int v=0; v!=u; ++v)     for (int w=0; w!=v; ++w)     for (int x=0; x!=w; ++x)     for (int y=0; y!=x; ++y)     for (int z=0; z!=y; ++z)     ++member_count;     m_a = a;     m_b = b;     m_c = c;     m_d = d;     m_e = e;     m_f = f;     m_g = g;     m_h = h;     m_i = i;     m_j = j;     m_k = k;     m_l = l;     m_m = m;     m_n = n;     m_o = o;     m_p = p;     m_q = q;     m_r = r;     m_s = s;     m_t = t;     m_u = u;     m_v = v;     m_w = w;     m_x = x;     m_y = y;     m_z = z;     m_count = member_count;     const double time = timer.elapsed();     std::cout << m_count << std::endl;     return time;   }   private:   int m_a;   int m_b;   int m_c;   int m_d;   int m_e;   int m_f;   int m_g;   int m_h;   int m_i;   int m_j;   int m_k;   int m_l;   int m_m;   int m_n;   int m_o;   int m_p;   int m_q;   int m_r;   int m_s;   int m_t;   int m_u;   int m_v;   int m_w;   int m_x;   int m_y;   int m_z;   int m_count; }; double TestLocals() {   int local_count = 0;   boost::timer t;   for (int a=0; a!=32; ++a)   for (int b=0; b!=a; ++b)   for (int c=0; c!=b; ++c)   for (int d=0; d!=c; ++d)   for (int e=0; e!=d; ++e)   for (int f=0; f!=e; ++f)   for (int g=0; g!=f; ++g)   for (int h=0; h!=g; ++h)   for (int i=0; i!=h; ++i)   for (int j=0; j!=i; ++j)   for (int k=0; k!=j; ++k)   for (int l=0; l!=k; ++l)   for (int m=0; m!=l; ++m)   for (int n=0; n!=m; ++n)   for (int o=0; o!=n; ++o)   for (int p=0; p!=o; ++p)   for (int q=0; q!=p; ++q)   for (int r=0; r!=q; ++r)   for (int s=0; s!=r; ++s)   for (int t=0; t!=s; ++t)   for (int u=0; u!=t; ++u)   for (int v=0; v!=u; ++v)   for (int w=0; w!=v; ++w)   for (int x=0; x!=w; ++x)   for (int y=0; y!=x; ++y)   for (int z=0; z!=y; ++z)   ++local_count;   const double time = t.elapsed();   std::cout << local_count << std::endl;   return time; } int main() {   const double t_global = TestGlobals();   const double t_local  = TestLocals();   MembersOnly m1;   MembersToLocal m2;   const double t_m1 = m1.Test();   const double t_m2 = m2.Test();   std::cout     << "Globals: " << t_global << " seconds.\n"     << "Members only: " << t_m1 << " seconds.\n"     << "Members with local variables: " << t_m2 << " seconds.\n"     << "Locals : " << t_local  << " seconds.\n"; }

 

 

 

 

 

Results

 

On a laptop (built in around 2011) with Lubuntu:

 

906192 906192 906192 906192 Globals: 7.84 seconds. Members only: 5.9 seconds. Members with local variables: 5.78 seconds. Locals : 5.8 seconds.

 

 

 

 

 

References

 

  • Andrei Alexandrescu. Modern C++ Design. 2001. ISBN: 0201704315. Item 10: 'Minimize global and shared data'.
  • Andrei Alexandrescu. Modern C++ Design. 2001. ISBN: 0201704315. Item 18: 'Declare variables as locally as possible'.
  • Stephen C. Dewhurst. C++ Gotchas. 2003. ISBN: 0-321-12518-5. Gotcha #3: 'Avoid global variables'.
  • C++ FAQ Lite: 'The names of global variables should start with //' and 'Instead of using a global variable, you should seriously consider if there are ways to limit the variable's visibility and/or lifetime'.