mandelbrot C gcc #8 program

/* The Computer Language Benchmarks Game
 https://salsa.debian.org/benchmarksgame-team/benchmarksgame/
 contributed by Ralph Ganszky
 modified for Swift 3.0 by Daniel Muellenborn
 modified for C & gcc by Dominic Letz

 Compile with: gcc -O3 -mno-fma -march=native -fopenmp main.c
 */

#include <stdio.h>
#include <stdlib.h>

int Iter = 50;

typedef double Vec __attribute__ ((vector_size(16)));
const int Vec__size = sizeof(Vec)/sizeof(double);
typedef unsigned char Byte;

int bt(Vec lhs, double rhs) {
  for (int i = 0; i < Vec__size; i++)
    if (lhs[i] <= rhs)
      return 0;
  return 1;
}

// Calculate mandelbrot set for one Vec into one byte
Byte mand(Vec cr, Vec ci) {
  Vec Zr = {0.0};
  Vec Zi = {0.0};
  Vec Tr = {0.0};
  Vec Ti = {0.0};

  for (int i = 0; i < Iter/5; i++) {
    for (int j = 0; j < 5; j++) {
      Zi = (Zr + Zr) * Zi + ci;
      Zr = Tr - Ti + cr;
      Tr = Zr * Zr;
      Ti = Zi * Zi;
    }
    if (bt(Tr + Ti, 4.0)) {
       return 0;
    }
  }
  Byte byte = 0;
  Vec t = Tr + Ti;
  for (int i = 0; i < Vec__size; i++) {
    byte |= t[i] <= 4.0 ? (0x80 >> i) : 0;
  }
  return byte;
}

// Parse command line arguments
int main(int argc, char* argv[])
{
  int n = (argc > 1) ? atoi(argv[1]) : 200;
  int N = (n + Vec__size - 1) & ~(Vec__size - 1);
  double inv = 2.0 / ((double)n);
  Vec xvals[N/Vec__size];
  Vec yvals[N];
  Byte *rows = malloc(n*N/8);

  for (int i = 0; i < N; i++) {
    xvals[i/Vec__size][i%Vec__size] = ((double)i) * inv - 1.5;
    for (int j = 0; j < Vec__size; j++) {
      yvals[i][j] = ((double)i) * inv - 1.0;
    }
  }

#pragma omp parallel for schedule(guided)
  for (int y = 0; y < n; y++) {
    for (int x = 0; x < N/Vec__size; x+=8/Vec__size) {
      Byte b = 0;
      for (int v = 0; v < 8/Vec__size; v++) {
        b |= mand(xvals[x + v], yvals[y]) >> (v*Vec__size);
      }
      rows[y*N/8+x/(8/Vec__size)] = b;
    }
  }

  FILE* out = (argc == 3) ? fopen(argv[2], "wb") : stdout;
  fprintf(out, "P4\n%u %u\n", n, n);
  fwrite(&rows[0], n*N/8, 1, out);

  if (out != stdout)
  {
      fclose(out);
  }
  free(rows);
  return 0;
}
    

NOTES:
64-bit Ubuntu quad core
gcc (Ubuntu 9.3.0-10ubuntu2) 9.3.0


Mon, 04 May 2020 18:33:31 GMT

MAKE:
/usr/bin/gcc -pipe -Wall -O3 -fomit-frame-pointer -march=core2 -fopenmp mandelbrot.gcc-8.c -o mandelbrot.gcc-8.gcc_run 
rm mandelbrot.gcc-8.c

4.24s to complete and log all make actions

COMMAND LINE:
./mandelbrot.gcc-8.gcc_run 16000

(BINARY) PROGRAM OUTPUT NOT SHOWN