Performance difference between accessing local and class member variables - c++

Question

I have the following code in a class member function:
int state = 0;
int code = static_cast<int>(letter_[i]);
if (isalnum(code)) {
state = testTable[state][0];
} else if (isspace(code)) {
state = testTable[state][2];
} else if (code == OPEN_TAG) {
state = testTable[state][3];
} else if (code == CLOSE_TAG) {
state = testTable[state][4];
} else {
state = testTable[state][1];
}
switch (state) {
case 1: // alphanumeric symbol was read
buffer[j] = letter_[i];
++j;
break;
case 2: // delimeter was read
j = 0;
// buffer.clear();
break;
}
However, if state is a class member variable rather than local, the performance drops considerably (~ 5 times). I was reading about differences in accessing local variables and class members, but texts usually say that it affects performance very slightly.
If it helps: I am using MinGW GCC compiler with -O3 option.

Answer 1

I could not reproduce your observation, testing on x86_64 with both, VS10 and g++. The local variant is slightly faster, probably due to what Alan Stokes described in his comment, but at most ~10%. You should check your timing, try to rule out any other problems and best would be the reduce all your code to a very simple test-cast which still shows this behavior.
I think my test-case resembles your scenario quite good, at least like you described it:
#include <iostream>
#include <boost/timer.hpp>
const int max_iter = 1<<31;
const int start_value = 65535;
struct UseMember
{
int member;
void foo()
{
for(int i=0; i<max_iter; ++i)
{
if(member%2)
member = 3*member+1;
else
member = member>>1;
}
std::cout << "Value=" << member << std::endl;
}
};
struct UseLocal
{
void foo()
{
int local = start_value;
for(int i=0; i<max_iter; ++i)
{
if((local%2)!=0) /* odd */
local = 3*local+1;
else /* even */
local = local>>1;
}
std::cout << "Value=" << local << std::endl;
}
};
int main(int argc, char* argv[])
{
/* First, test using member */
std::cout << "** Member Access" << std::endl;
{
UseMember bar;
bar.member = start_value;
boost::timer T;
bar.foo();
double e = T.elapsed();
std::cout << "Time taken: " << e << "s" << std::endl;
}
/* Then, test using local */
std::cout << "** Local Access" << std::endl;
{
UseLocal bar;
boost::timer T;
bar.foo();
double e = T.elapsed();
std::cout << "Time taken: " << e << "s" << std::endl;
}
return 0;
}