Re: Array optimizing problem in C++?

Jon Harrop <>
Sun, 23 Mar 2008 15:40:44 +0000
<> wrote:

If you are going for blinding speed be sure to use proper optimization
flags; otherwise you are only calculating benchmarks for poorly
optimized code, which, like most of your benchmarks, is meaningless:

$ g++ -O0 smooth.cpp
9884.867 ms

$ g++ -O3 smooth.cpp
8791.123 ms

$ g++ -O3 -mfpmath=sse -msse3 -march=prescott -ffast-math smooth.cpp
1207.944 ms

GCC's -ffast-math option breaks semantics, so it is not a valid
optimization. I would not use it in a comparison against Java unless you
know that accuracy is not required for the given task.

$ g++ -O3 -mfpmath=sse -msse3 -march=prescott -ffast-math -funroll-
loops smooth.cpp
1084.385 ms

This is your code slightly modified to use QueryPerformanceCounter for
timing (using Windows). Most of the speed up is a result of -ffast-

Based on the quality and rigor of the other tests I've seen you do,
I'd guess that you missed a few optimization flags on the Java side as

I have altered all implementations of this benchmark to perform progressive
blurring from one array to the other and back rather than overwriting the
destination array repeatedly. This slightly improved performance in all

On my Athlon 64 X2 4400+ (2.2Ghz) running AMD64 Debian Linux, I get
best-of-three results:

g++: 4.080 g++ -O3 bench.cpp -o bench; time ./bench
g++2: 4.080 g++ -O3 bench2.cpp -o bench2; time ./bench2
ocaml: 4.048 ocamlopt -o bench; time ./bench
java: 4.016 javac; time java Arrays

On the same hardware running 32-bit Windows XP Pro, I get:

F#: 3.970 fsc -O3

This shows an insignificant difference in performance which, if nothing
else, is interesting because the second C++ program uses bounds checking. I
was also surprised to see OCaml do so well on this benchmark because it
makes no attempt to hoist bounds checks. F# on .NET typically beats all
open source compilers on Linux on this kind of benchmark (float array
intensive). C# presumably would too.

The bench2 program is the following, more idiomatic, C++ version that uses
bounds checking:

#include <vector>
#include <iostream>
#include <cstdlib>
#include <ctime>

void smooth (std::vector<double> &dest, std::vector<double> &src)
  for (int i = 1; i < src.size() - 1; i++) - 1) = ( - 1) + + + 1)) / 3 ;

void fill (std::vector<double> &src)

  for (int i = 0; i < src.size(); ++ i ) = rand();

int main()
  const int len = 50000;

  std::vector<double> src_array(len);
  std::vector<double> dest_array(len);


  clock_t start=clock();

  for (int i = 0; i < 5000; i++)
    smooth(dest_array, src_array);
    smooth(src_array, dest_array);

  clock_t endt=clock();

  std::cout <<"Time smooth(): " <<
    double(endt-start)/CLOCKS_PER_SEC * 1000 << " ms\n";

  return 0;

Here is my OCaml equivalent:

let smooth dst src =
  let n = Array.length dst in
  for i=1 to n-2 do
    dst.(i-1) <- (src.(i-1) +. src.(i) +. src.(i+1)) /. 3.

let () =
  let n = 50000 in
  let src = Array.init n (fun _ -> Random.float 1.) in
  let dst = Array.create n 0. in
  let t = Sys.time() in
  for i=1 to 5000 do
    smooth dst src;
    smooth src dst;
  Printf.printf "Time smooth(): %fs\n" (Sys.time() -. t)

Dr Jon D Harrop, Flying Frog Consultancy Ltd.

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