Re: C++ math optimization problem...

[Stefan Strasser <sstrasser at systemhaus-gruppe dot de>]

I think it is a bug, or a "missing feature".
I tried to simpify the testcase below and ended up with a comlete 
different testcase, but it causes the same problem:

it seems to be about FPU registers, if anything causes the compiler to 
store the value to memory, it treats it as it would be volatile.

void otherfunc();

void test(){
  double result=0.0;  //stored in fpu register
  otherfunc();        //"result" saved to memory
  for (int j = 1; j < 100000000; ++j) result += 1.0;  //"result" read 
and written to/from memory each cycle

  std::cerr << result << std::endl;
}

on i386 the inner loops look like this:

without otherfunc():
.L7:
  decl  %eax
  fadd  %st, %st(1)
  jns .L7


with otherfunc():
.L7:
  fldl  -8(%ebp)
  decl  %eax
  fadd  %st(1), %st  //FPU stack ordering problem?
  fstl  -8(%ebp)
  js  .L11
  fstp  %st(0)
  jmp .L7
.L11:


I don't really know what keywords to search for in bugzilla, could 
anyone please look up if this is a known bug?


Mfg,




Benjamin Redelings I schrieb:
> Hi,
>     I have a C++ program that runs slower under 4.0 CVS than 3.4.  So, I 
> am trying to make some test-cases that might help deduce the reason. 
> However, when I reduced this testcase sufficiently, it began behaving 
> badly under BOTH 3.4 and 4.0.... but I guess I should start with the 
> most reduced case first.
>
>     Basically, the code just does a lot of multiplies and adds.  
> However, if I take the main loop outside of an if-block, it goes 5x 
> faster. Also, if I implement an array as 'double*' instead of 
> 'vector<double>' it also goes 5x faster.  Using valarray<double> instead 
> of vector<double> does not give any improvement.
>
> MATH INSIDE IF-BLOCK
> % time ./2h 1
> double addition
> result = 83283300.006041
>
> real    0m0.995s
> user    0m1.000s
> sys     0m0.000s
>
> MATH OUTSIDE IF-BLOCK
> % time ./2i 1
> result = 83283299.999998
>
> real    0m0.218s
> user    0m0.220s
> sys     0m0.000s
>
> Should I submit a PR?  Any help would be appreciated...
>
> -BenRI
>
> ------------ begin testcase -------------
> #include <vector>
>
> const int OUTER = 100000;
> const int INNER = 1000;
>
> using namespace std;
>
> int main(int argn, char *argv[])
> {
>   int s = atoi(argv[1]);
>
>   double result;
>   if (s == 1) {  //remove this condition to get a 5x speedup
>     // initialize d
>     vector<double> d(INNER); //change to double* to get 5x speedup
>     for (int i = 0; i < INNER; i++)
>       d[i] = double(1+i) / INNER;
>
>     // calc result
>     result=0;
>     for (int i = 0; i < OUTER; ++i)
>       for (int j = 1; j < INNER; ++j)
>         result += d[j]*d[j-1] + d[j-1];
>   }
>   else
>     exit(-1);
>
>   printf("result = %f\n",result);
>   return 0;
> }
> ----------- end testcase --------------



--
Stefan Strasser