Clang builds a working 2.6.36 Kernel

> compiles 100 times faster or runs 100 times faster?

Run time. The following code runs much faster when compiled clang++ (0.003s) than when compiled using g++ (1.593s):

#include <iostream>
#include <boost/thread.hpp>

#define len 1000000000L

static void f(unsigned long a, unsigned long b, unsigned long *va)
{
    for (*va = 0; a < b; a++)
        *va += a;
}
    
int main()
{
    unsigned long va = 0;
    boost::thread a(f, 0l,  2* len, &va);
    a.join();
    std::cout << va  << std::endl;
    return 0;
}

for (*va = 0; a < b; a++)
    *va += a;

*va = (1 + b)*(b/2) - b

*va = (1 + b)*(b/2) - b

*va = (b)*(b-1)/2 - (a)*(a-1)/2

   -funroll-loops
       Unroll loops whose number of iterations can be determined at
       compile time or upon entry to the loop.  -funroll-loops implies
       -frerun-cse-after-loop, -fweb and -frename-registers.  It also
       turns on complete loop peeling (i.e. complete removal of loops with
       small constant number of iterations).  This option makes code
       larger, and may or may not make it run faster.