c語言中x*x和pow(x,2)哪個計算更快一點？

The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.

The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
&. */

#include &
#include &

CFLOAT
M_DECL_FUNC (__cpow) (CFLOAT x, CFLOAT c)
{
return M_SUF (__cexp) (c * M_SUF (__clog) (x));
}

declare_mgen_alias (__cpow, cpow)

{
SET_RESTORE_ROUND (FE_TONEAREST);

/* Avoid internal underflow for tiny y. The exact value of y does
not matter if |y| &<= 2**-64. */ if (fabs (y) &< 0x1p-64) y = y &< 0 ? -0x1p-64 : 0x1p-64; z = log1 (x, aa, error); /* x^y =e^(y log (X)) */ t = y * CN; y1 = t - (t - y); y2 = y - y1; t = z * CN; a1 = t - (t - z); a2 = (z - a1) + aa; a = y1 * a1; aa = y2 * a1 + y * a2; a1 = a + aa; a2 = (a - a1) + aa; error = error * fabs (y); t = __exp1 (a1, a2, 1.9e16 * error); /* return -10 or 0 if wasn"t computed exactly */ retval = (t &> 0) ? t : power1 (x, y);
}

if (isinf (retval))
retval = huge * huge;
else if (retval == 0)
retval = tiny * tiny;
else
math_check_force_underflow_nonneg (retval);
return retval;
}

if (x == 0)
{
if (((v.i[HIGH_HALF] 0x7fffffff) == 0x7ff00000 v.i[LOW_HALF] != 0)
|| (v.i[HIGH_HALF] 0x7fffffff) &> 0x7ff00000) /* NaN */
return y + y;
if (fabs (y) &> 1.0e20)
return (y &> 0) ? 0 : 1.0 / 0.0;
k = checkint (y);
if (k == -1)
return y &< 0 ? 1.0 / x : x; else return y &< 0 ? 1.0 / 0.0 : 0.0; /* return 0 */ } qx = u.i[HIGH_HALF] 0x7fffffff; /* no sign */ qy = v.i[HIGH_HALF] 0x7fffffff; /* no sign */ if (qx &>= 0x7ff00000 (qx &> 0x7ff00000 || u.i[LOW_HALF] != 0)) /* NaN */
return x + y;
if (qy &>= 0x7ff00000 (qy &> 0x7ff00000 || v.i[LOW_HALF] != 0)) /* NaN */
return x == 1.0 !issignaling (y) ? 1.0 : y + y;

/* if x&<0 */ if (u.i[HIGH_HALF] &< 0) { k = checkint (y); if (k == 0) { if (qy == 0x7ff00000) { if (x == -1.0) return 1.0; else if (x &> -1.0)
return v.i[HIGH_HALF] &< 0 ? INF.x : 0.0; else return v.i[HIGH_HALF] &< 0 ? 0.0 : INF.x; } else if (qx == 0x7ff00000) return y &< 0 ? 0.0 : INF.x; return (x - x) / (x - x); /* y not integer and x&<0 */ } else if (qx == 0x7ff00000) { if (k &< 0) return y &< 0 ? nZERO.x : nINF.x; else return y &< 0 ? 0.0 : INF.x; } /* if y even or odd */ if (k == 1) return __ieee754_pow (-x, y); else { double retval; { SET_RESTORE_ROUND (FE_TONEAREST); retval = -__ieee754_pow (-x, y); } if (isinf (retval)) retval = -huge * huge; else if (retval == 0) retval = -tiny * tiny; return retval; } } /* x&>0 */

if (qx == 0x7ff00000) /* x= 2^-0x3ff */
return y &> 0 ? x : 0;