Optimizing atof()

I was annoyed that atof() was making my program run 6 times slower. So, I wrote a new, slightly faster one. Still not fast enough for my taste. I tried playing around with mmx, but don't have good results to show, yet. I'll add benchmarks soon. The program will use one integer multiplication, addition and subtraction per digit, and one float multiplication for the whole number.

double my_atof (const char *p)
{
   static unsigned char to_do[256] ;
   static double my_exp[512*2] ;
  
  int i;

 
  if( to_do[0] != DONE ) {
    // Initialize tables for interpreting characters and for exponents.
    for( i=0; i<255; i++) to_do[i] = 255 ;
    to_do['0']=0 ;
    for(i='1'; i<='9'; i++) to_do[i] = DIGIT ;
    
    
    to_do['-'] = MINUS ;
    to_do['.'] = POINT ;
    to_do['e'] = EXPONENT ; 
    to_do['E'] = EXPONENT ;
    to_do[' '] = SPACE ;
    to_do['\t'] = SPACE ;
    to_do[0] = DONE ;
    my_exp[512] = 1.0;
    for( i=1; i<511; i++)
      my_exp[i+512] = my_exp[i-1+512] * 10.0 ;
    for( i=1; i<511; i++)
      my_exp[512-i] = my_exp[512-i+1] * 0.1 ;
#define MY_EXP(x) (my_exp[((x)+512) & 1023])
  }
  
  int neg=FALSE , div=0, val = 0, eval=0 ;


  // The function works in 5 stages:
  // 1. jump over whitespace, till first char is found - either digit, sign or dot.
  // 2. Read digits till first non-digit - either '.' or eE
  // 3. Read digits after decimal point
  // 4. Read exponent digit or sign
  // 5. Read rest of exponent.
  while( TRUE ) {

    switch( to_do[ *p ] ) {
    case DIGIT: val = *p-'0' ; goto end_loop1 ;
    case SPACE: 
      break ;
    case MINUS:
      neg = TRUE ;
      goto end_loop1 ;
    case PLUS:
      goto end_loop1 ;
    case POINT:
      goto found_dot ;
    case 0: goto end_loop1 ;
    default:
      return 0.0 ;
    }
    p++ ;
  }


  end_loop1:
  p++ ;
  while( TRUE ) {
    switch( to_do[ *p ] ) {
    case DIGIT: val= val*10+(*p)-'0'; break ;
    case 0: val *= 10 ;break ;
    case POINT:
      goto found_dot;
    case EXPONENT:
      goto found_exp;
    default:
      if( neg ) val = -val ;
      return (double)( val) ;
    }
     p++ ; 
  }

  found_dot:
  p++ ;

  while( TRUE ) {
    {
      switch( to_do[ *p ] ) {
      case DIGIT: val= val*10+(*p)-'0';div--; break ;
      case 0: val *= 10 ;div--;break ;
      case EXPONENT:
 goto found_exp;
      default:
 if( neg ) val = -val;
 return MY_EXP(div) * val ;
      }
      p++ ;
    }
  }

  found_exp:
  p++ ;
  if( neg ) val = -val ;
  neg=FALSE ;
  while( TRUE ) {
    switch( to_do[ *p ] ) {
    case DIGIT: eval=(*p)-'0';goto end_loop2 ;
    case 0: goto end_loop2 ;
    case SPACE: 
      return MY_EXP(div) * val ;
    case MINUS:
      neg  = TRUE ; //fallthrough
    case PLUS:
      goto end_loop2 ;
    default:
      return MY_EXP(div) * val ;
    }
    p++ ;
  }

  end_loop2:
  p++ ;
  while( TRUE ) {
    switch( to_do[ *p ] ) {
    case DIGIT: eval= eval*10+(*p)-'0'; break ;
    case 0: eval *= 10 ;break ;
    default:
      goto done ;
    }
    p++ ; 
  } 
  done:

  if( neg ) 
    eval = div - eval ;
  else
    eval = div + eval ;
  return  MY_EXP( eval ) * val ;

}