mandelbrot C gcc #2 program

/* The Computer Language Benchmarks Game
 * https://salsa.debian.org/benchmarksgame-team/benchmarksgame/

   contributed by Greg Buchholz
   
   for the debian (AMD) machine...
   compile flags:  -O3 -ffast-math -march=athlon-xp -funroll-loops

   for the gp4 (Intel) machine...
   compile flags:  -O3 -ffast-math -march=pentium4 -funroll-loops
*/

#include<stdio.h>

int main (int argc, char **argv)
{
    int w, h, bit_num = 0;
    char byte_acc = 0;
    int i, iter = 50;
    double x, y, limit = 2.0;
    double Zr, Zi, Cr, Ci, Tr, Ti;
    
    w = h = atoi(argv[1]);

    printf("P4\n%d %d\n",w,h);

    for(y=0;y<h;++y) 
    {
        for(x=0;x<w;++x)
        {
            Zr = Zi = Tr = Ti = 0.0;
            Cr = (2.0*x/w - 1.5); Ci=(2.0*y/h - 1.0);
        
            for (i=0;i<iter && (Tr+Ti <= limit*limit);++i)
            {
                Zi = 2.0*Zr*Zi + Ci;
                Zr = Tr - Ti + Cr;
                Tr = Zr * Zr;
                Ti = Zi * Zi;
            }
       
            byte_acc <<= 1; 
            if(Tr+Ti <= limit*limit) byte_acc |= 0x01;
                
            ++bit_num; 

            if(bit_num == 8)
            {
                putc(byte_acc,stdout);
                byte_acc = 0;
                bit_num = 0;
            }
            else if(x == w-1)
            {
                byte_acc <<= (8-w%8);
                putc(byte_acc,stdout);
                byte_acc = 0;
                bit_num = 0;
            }
        }
    }	
}

    

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


Mon, 04 May 2020 18:46:55 GMT

MAKE:
/usr/bin/gcc -pipe -Wall -O3 -fomit-frame-pointer -march=core2  mandelbrot.gcc-2.c -o mandelbrot.gcc-2.gcc_run 
mandelbrot.gcc-2.c: In function ‘main’:
mandelbrot.gcc-2.c:23:13: warning: implicit declaration of function ‘atoi’ [-Wimplicit-function-declaration]
   23 |     w = h = atoi(argv[1]);
      |             ^~~~
rm mandelbrot.gcc-2.c

4.00s to complete and log all make actions

COMMAND LINE:
./mandelbrot.gcc-2.gcc_run 16000

(BINARY) PROGRAM OUTPUT NOT SHOWN