fold-const.c

gcc库的原代码,对编程有很大帮助.

     HOST_WIDE_INT lnum_orig, hnum_orig; /* num == numerator == dividend */
     HOST_WIDE_INT lden_orig, hden_orig; /* den == denominator == divisor */
     HOST_WIDE_INT *lquo, *hquo, *lrem, *hrem;
{
  int quo_neg = 0;
  HOST_WIDE_INT num[4 + 1];	/* extra element for scaling.  */
  HOST_WIDE_INT den[4], quo[4];
  register int i, j;
  unsigned HOST_WIDE_INT work;
  register unsigned HOST_WIDE_INT carry = 0;
  HOST_WIDE_INT lnum = lnum_orig;
  HOST_WIDE_INT hnum = hnum_orig;
  HOST_WIDE_INT lden = lden_orig;
  HOST_WIDE_INT hden = hden_orig;
  int overflow = 0;

  if ((hden == 0) && (lden == 0))
    abort ();

  /* calculate quotient sign and convert operands to unsigned.  */
  if (!uns) 
    {
      if (hnum < 0)
	{
	  quo_neg = ~ quo_neg;
	  /* (minimum integer) / (-1) is the only overflow case.  */
	  if (neg_double (lnum, hnum, &lnum, &hnum) && (lden & hden) == -1)
	    overflow = 1;
	}
      if (hden < 0) 
	{
	  quo_neg = ~ quo_neg;
	  neg_double (lden, hden, &lden, &hden);
	}
    }

  if (hnum == 0 && hden == 0)
    {				/* single precision */
      *hquo = *hrem = 0;
      /* This unsigned division rounds toward zero.  */
      *lquo = lnum / (unsigned HOST_WIDE_INT) lden;
      goto finish_up;
    }

  if (hnum == 0)
    {				/* trivial case: dividend < divisor */
      /* hden != 0 already checked.  */
      *hquo = *lquo = 0;
      *hrem = hnum;
      *lrem = lnum;
      goto finish_up;
    }

  bzero ((char *) quo, sizeof quo);

  bzero ((char *) num, sizeof num);	/* to zero 9th element */
  bzero ((char *) den, sizeof den);

  encode (num, lnum, hnum); 
  encode (den, lden, hden);

  /* Special code for when the divisor < BASE.  */
  if (hden == 0 && lden < BASE)
    {
      /* hnum != 0 already checked.  */
      for (i = 4 - 1; i >= 0; i--)
	{
	  work = num[i] + carry * BASE;
	  quo[i] = work / (unsigned HOST_WIDE_INT) lden;
	  carry = work % (unsigned HOST_WIDE_INT) lden;
	}
    }
  else
    {
      /* Full double precision division,
	 with thanks to Don Knuth's "Seminumerical Algorithms".  */
    int num_hi_sig, den_hi_sig;
    unsigned HOST_WIDE_INT quo_est, scale;

    /* Find the highest non-zero divisor digit.  */
    for (i = 4 - 1; ; i--)
      if (den[i] != 0) {
	den_hi_sig = i;
	break;
      }

    /* Insure that the first digit of the divisor is at least BASE/2.
       This is required by the quotient digit estimation algorithm.  */

    scale = BASE / (den[den_hi_sig] + 1);
    if (scale > 1) {		/* scale divisor and dividend */
      carry = 0;
      for (i = 0; i <= 4 - 1; i++) {
	work = (num[i] * scale) + carry;
	num[i] = LOWPART (work);
	carry = HIGHPART (work);
      } num[4] = carry;
      carry = 0;
      for (i = 0; i <= 4 - 1; i++) {
	work = (den[i] * scale) + carry;
	den[i] = LOWPART (work);
	carry = HIGHPART (work);
	if (den[i] != 0) den_hi_sig = i;
      }
    }

    num_hi_sig = 4;

    /* Main loop */
    for (i = num_hi_sig - den_hi_sig - 1; i >= 0; i--) {
      /* guess the next quotient digit, quo_est, by dividing the first
	 two remaining dividend digits by the high order quotient digit.
	 quo_est is never low and is at most 2 high.  */
      unsigned HOST_WIDE_INT tmp;

      num_hi_sig = i + den_hi_sig + 1;
      work = num[num_hi_sig] * BASE + num[num_hi_sig - 1];
      if (num[num_hi_sig] != den[den_hi_sig])
	quo_est = work / den[den_hi_sig];
      else
	quo_est = BASE - 1;

      /* refine quo_est so it's usually correct, and at most one high.   */
      tmp = work - quo_est * den[den_hi_sig];
      if (tmp < BASE
	  && den[den_hi_sig - 1] * quo_est > (tmp * BASE + num[num_hi_sig - 2]))
	quo_est--;

      /* Try QUO_EST as the quotient digit, by multiplying the
         divisor by QUO_EST and subtracting from the remaining dividend.
	 Keep in mind that QUO_EST is the I - 1st digit.  */

      carry = 0;
      for (j = 0; j <= den_hi_sig; j++)
	{
	  work = quo_est * den[j] + carry;
	  carry = HIGHPART (work);
	  work = num[i + j] - LOWPART (work);
	  num[i + j] = LOWPART (work);
	  carry += HIGHPART (work) != 0;
	}

      /* if quo_est was high by one, then num[i] went negative and
	 we need to correct things.  */

      if (num[num_hi_sig] < carry)
	{
	  quo_est--;
	  carry = 0;		/* add divisor back in */
	  for (j = 0; j <= den_hi_sig; j++)
	    {
	      work = num[i + j] + den[j] + carry;
	      carry = HIGHPART (work);
	      num[i + j] = LOWPART (work);
	    }
	  num [num_hi_sig] += carry;
	}

      /* store the quotient digit.  */
      quo[i] = quo_est;
    }
  }

  decode (quo, lquo, hquo);

 finish_up:
  /* if result is negative, make it so.  */
  if (quo_neg)
    neg_double (*lquo, *hquo, lquo, hquo);

  /* compute trial remainder:  rem = num - (quo * den)  */
  mul_double (*lquo, *hquo, lden_orig, hden_orig, lrem, hrem);
  neg_double (*lrem, *hrem, lrem, hrem);
  add_double (lnum_orig, hnum_orig, *lrem, *hrem, lrem, hrem);

  switch (code)
    {
    case TRUNC_DIV_EXPR:
    case TRUNC_MOD_EXPR:	/* round toward zero */
    case EXACT_DIV_EXPR:	/* for this one, it shouldn't matter */
      return overflow;

    case FLOOR_DIV_EXPR:
    case FLOOR_MOD_EXPR:	/* round toward negative infinity */
      if (quo_neg && (*lrem != 0 || *hrem != 0))   /* ratio < 0 && rem != 0 */
	{
	  /* quo = quo - 1;  */
	  add_double (*lquo, *hquo, (HOST_WIDE_INT) -1, (HOST_WIDE_INT)  -1,
		      lquo, hquo);
	}
      else return overflow;
      break;

    case CEIL_DIV_EXPR:
    case CEIL_MOD_EXPR:		/* round toward positive infinity */
      if (!quo_neg && (*lrem != 0 || *hrem != 0))  /* ratio > 0 && rem != 0 */
	{
	  add_double (*lquo, *hquo, (HOST_WIDE_INT) 1, (HOST_WIDE_INT) 0,
		      lquo, hquo);
	}
      else return overflow;
      break;
    
    case ROUND_DIV_EXPR:
    case ROUND_MOD_EXPR:	/* round to closest integer */
      {
	HOST_WIDE_INT labs_rem = *lrem, habs_rem = *hrem;
	HOST_WIDE_INT labs_den = lden, habs_den = hden, ltwice, htwice;

	/* get absolute values */
	if (*hrem < 0) neg_double (*lrem, *hrem, &labs_rem, &habs_rem);
	if (hden < 0) neg_double (lden, hden, &labs_den, &habs_den);

	/* if (2 * abs (lrem) >= abs (lden)) */
	mul_double ((HOST_WIDE_INT) 2, (HOST_WIDE_INT) 0,
		    labs_rem, habs_rem, &ltwice, &htwice);
	if (((unsigned HOST_WIDE_INT) habs_den
	     < (unsigned HOST_WIDE_INT) htwice)
	    || (((unsigned HOST_WIDE_INT) habs_den
		 == (unsigned HOST_WIDE_INT) htwice)
		&& ((HOST_WIDE_INT unsigned) labs_den
		    < (unsigned HOST_WIDE_INT) ltwice)))
	  {
	    if (*hquo < 0)
	      /* quo = quo - 1;  */
	      add_double (*lquo, *hquo,
			  (HOST_WIDE_INT) -1, (HOST_WIDE_INT) -1, lquo, hquo);
	    else
	      /* quo = quo + 1; */
	      add_double (*lquo, *hquo, (HOST_WIDE_INT) 1, (HOST_WIDE_INT) 0,
			  lquo, hquo);
	  }
	else return overflow;
      }
      break;

    default:
      abort ();
    }

  /* compute true remainder:  rem = num - (quo * den)  */
  mul_double (*lquo, *hquo, lden_orig, hden_orig, lrem, hrem);
  neg_double (*lrem, *hrem, lrem, hrem);
  add_double (lnum_orig, hnum_orig, *lrem, *hrem, lrem, hrem);
  return overflow;
}

#ifndef REAL_ARITHMETIC
/* Effectively truncate a real value to represent the nearest possible value
   in a narrower mode.  The result is actually represented in the same data
   type as the argument, but its value is usually different.

   A trap may occur during the FP operations and it is the responsibility
   of the calling function to have a handler established.  */

REAL_VALUE_TYPE
real_value_truncate (mode, arg)
     enum machine_mode mode;
     REAL_VALUE_TYPE arg;
{
  return REAL_VALUE_TRUNCATE (mode, arg);
}

#if TARGET_FLOAT_FORMAT == IEEE_FLOAT_FORMAT

/* Check for infinity in an IEEE double precision number.  */

int
target_isinf (x)
     REAL_VALUE_TYPE x;
{
  /* The IEEE 64-bit double format.  */
  union {
    REAL_VALUE_TYPE d;
    struct {
      unsigned sign      :  1;
      unsigned exponent  : 11;
      unsigned mantissa1 : 20;
      unsigned mantissa2;
    } little_endian;
    struct {
      unsigned mantissa2;
      unsigned mantissa1 : 20;
      unsigned exponent  : 11;
      unsigned sign      :  1;
    } big_endian;    
  } u;

  u.d = dconstm1;
  if (u.big_endian.sign == 1)
    {
      u.d = x;
      return (u.big_endian.exponent == 2047
	      && u.big_endian.mantissa1 == 0
	      && u.big_endian.mantissa2 == 0);
    }
  else
    {
      u.d = x;
      return (u.little_endian.exponent == 2047
	      && u.little_endian.mantissa1 == 0
	      && u.little_endian.mantissa2 == 0);
    }
}

/* Check whether an IEEE double precision number is a NaN.  */

int
target_isnan (x)
     REAL_VALUE_TYPE x;
{
  /* The IEEE 64-bit double format.  */
  union {
    REAL_VALUE_TYPE d;
    struct {
      unsigned sign      :  1;
      unsigned exponent  : 11;
      unsigned mantissa1 : 20;
      unsigned mantissa2;
    } little_endian;
    struct {
      unsigned mantissa2;
      unsigned mantissa1 : 20;
      unsigned exponent  : 11;
      unsigned sign      :  1;
    } big_endian;    
  } u;

  u.d = dconstm1;
  if (u.big_endian.sign == 1)
    {
      u.d = x;
      return (u.big_endian.exponent == 2047
	      && (u.big_endian.mantissa1 != 0
		  || u.big_endian.mantissa2 != 0));
    }
  else
    {
      u.d = x;
      return (u.little_endian.exponent == 2047
	      && (u.little_endian.mantissa1 != 0
		  || u.little_endian.mantissa2 != 0));
    }
}

/* Check for a negative IEEE double precision number.  */

int
target_negative (x)
     REAL_VALUE_TYPE x;
{
  /* The IEEE 64-bit double format.  */
  union {
    REAL_VALUE_TYPE d;
    struct {
      unsigned sign      :  1;
      unsigned exponent  : 11;
      unsigned mantissa1 : 20;
      unsigned mantissa2;
    } little_endian;
    struct {
      unsigned mantissa2;
      unsigned mantissa1 : 20;
      unsigned exponent  : 11;
      unsigned sign      :  1;
    } big_endian;    
  } u;

  u.d = dconstm1;
  if (u.big_endian.sign == 1)
    {
      u.d = x;
      return u.big_endian.sign;
    }
  else
    {
      u.d = x;
      return u.little_endian.sign;
    }
}
#else /* Target not IEEE */

/* Let's assume other float formats don't have infinity.
   (This can be overridden by redefining REAL_VALUE_ISINF.)  */

target_isinf (x)
     REAL_VALUE_TYPE x;
{
  return 0;
}

/* Let's assume other float formats don't have NaNs.
   (This can be overridden by redefining REAL_VALUE_ISNAN.)  */

target_isnan (x)
     REAL_VALUE_TYPE x;
{
  return 0;
}

/* Let's assume other float formats don't have minus zero.
   (This can be overridden by redefining REAL_VALUE_NEGATIVE.)  */

target_negative (x)
     REAL_VALUE_TYPE x;
{
  return x < 0;
}
#endif /* Target not IEEE */
#endif /* no REAL_ARITHMETIC */

/* Split a tree IN into a constant and a variable part
   that could be combined with CODE to make IN.
   CODE must be a commutative arithmetic operation.
   Store the constant part into *CONP and the variable in &VARP.
   Return 1 if this was done; zero means the tree IN did not decompose
   this way.

   If CODE is PLUS_EXPR we also split trees that use MINUS_EXPR.
   Therefore, we must tell the caller whether the variable part
   was subtracted.  We do this by storing 1 or -1 into *VARSIGNP.
   The value stored is the coefficient for the variable term.
   The constant term we return should always be added;
   we negate it if necessary.  */

static int
split_tree (in, code, varp, conp, varsignp)
     tree in;
     enum tree_code code;
     tree *varp, *conp;
     int *varsignp;
{
  register tree outtype = TREE_TYPE (in);
  *varp = 0;
  *conp = 0;

  /* Strip any conversions that don't change the machine mode.  */
  while ((TREE_CODE (in) == NOP_EXPR
	  || TREE_CODE (in) == CONVERT_EXPR)
	 && (TYPE_MODE (TREE_TYPE (in))
	     == TYPE_MODE (TREE_TYPE (TREE_OPERAND (in, 0)))))
    in = TREE_OPERAND (in, 0);

  if (TREE_CODE (in) == code
      || (! FLOAT_TYPE_P (TREE_TYPE (in))
	  /* We can associate addition and subtraction together
	     (even though the C standard doesn't say so)
	     for integers because the value is not affected.
	     For reals, the value might be affected, so we can't.  */
	  && ((code == PLUS_EXPR && TREE_CODE (in) == MINUS_EXPR)
	      || (code == MINUS_EXPR && TREE_CODE (in) == PLUS_EXPR))))
    {
      enum tree_code code = TREE_CODE (TREE_OPERAND (in, 0));
      if (code == INTEGER_CST)
	{