137 lines
4.9 KiB
C
137 lines
4.9 KiB
C
/* Single-precision floating point square root.
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Copyright (C) 1997 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Library General Public License as
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published by the Free Software Foundation; either version 2 of the
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License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Library General Public License for more details.
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You should have received a copy of the GNU Library General Public
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License along with the GNU C Library; see the file COPYING.LIB. If not,
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write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
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Boston, MA 02111-1307, USA. */
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#include <math.h>
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#include <math_private.h>
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#include <fenv_libc.h>
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#include <inttypes.h>
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static const float almost_half = 0.50000006; /* 0.5 + 2^-24 */
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static const uint32_t a_nan = 0x7fc00000;
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static const uint32_t a_inf = 0x7f800000;
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static const float two48 = 281474976710656.0;
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static const float twom24 = 5.9604644775390625e-8;
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extern const float __t_sqrt[1024];
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/* The method is based on a description in
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Computation of elementary functions on the IBM RISC System/6000 processor,
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P. W. Markstein, IBM J. Res. Develop, 34(1) 1990.
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Basically, it consists of two interleaved Newton-Rhapson approximations,
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one to find the actual square root, and one to find its reciprocal
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without the expense of a division operation. The tricky bit here
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is the use of the POWER/PowerPC multiply-add operation to get the
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required accuracy with high speed.
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The argument reduction works by a combination of table lookup to
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obtain the initial guesses, and some careful modification of the
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generated guesses (which mostly runs on the integer unit, while the
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Newton-Rhapson is running on the FPU). */
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float
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__sqrtf(float x)
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{
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const float inf = *(const float *)&a_inf;
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/* x = f_washf(x); *//* This ensures only one exception for SNaN. */
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if (x > 0)
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{
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if (x != inf)
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{
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/* Variables named starting with 's' exist in the
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argument-reduced space, so that 2 > sx >= 0.5,
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1.41... > sg >= 0.70.., 0.70.. >= sy > 0.35... .
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Variables named ending with 'i' are integer versions of
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floating-point values. */
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float sx; /* The value of which we're trying to find the square
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root. */
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float sg,g; /* Guess of the square root of x. */
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float sd,d; /* Difference between the square of the guess and x. */
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float sy; /* Estimate of 1/2g (overestimated by 1ulp). */
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float sy2; /* 2*sy */
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float e; /* Difference between y*g and 1/2 (note that e==se). */
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float shx; /* == sx * fsg */
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float fsg; /* sg*fsg == g. */
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fenv_t fe; /* Saved floating-point environment (stores rounding
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mode and whether the inexact exception is
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enabled). */
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uint32_t xi, sxi, fsgi;
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const float *t_sqrt;
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GET_FLOAT_WORD (xi, x);
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fe = fegetenv_register ();
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relax_fenv_state ();
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sxi = xi & 0x3fffffff | 0x3f000000;
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SET_FLOAT_WORD (sx, sxi);
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t_sqrt = __t_sqrt + (xi >> 23-8-1 & 0x3fe);
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sg = t_sqrt[0];
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sy = t_sqrt[1];
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/* Here we have three Newton-Rhapson iterations each of a
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division and a square root and the remainder of the
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argument reduction, all interleaved. */
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sd = -(sg*sg - sx);
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fsgi = xi + 0x40000000 >> 1 & 0x7f800000;
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sy2 = sy + sy;
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sg = sy*sd + sg; /* 16-bit approximation to sqrt(sx). */
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e = -(sy*sg - almost_half);
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SET_FLOAT_WORD (fsg, fsgi);
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sd = -(sg*sg - sx);
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sy = sy + e*sy2;
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if ((xi & 0x7f800000) == 0)
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goto denorm;
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shx = sx * fsg;
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sg = sg + sy*sd; /* 32-bit approximation to sqrt(sx),
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but perhaps rounded incorrectly. */
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sy2 = sy + sy;
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g = sg * fsg;
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e = -(sy*sg - almost_half);
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d = -(g*sg - shx);
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sy = sy + e*sy2;
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fesetenv_register (fe);
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return g + sy*d;
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denorm:
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/* For denormalised numbers, we normalise, calculate the
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square root, and return an adjusted result. */
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fesetenv_register (fe);
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return __sqrtf(x * two48) * twom24;
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}
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}
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else if (x < 0)
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{
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#ifdef FE_INVALID_SQRT
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feraiseexcept (FE_INVALID_SQRT);
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/* For some reason, some PowerPC processors don't implement
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FE_INVALID_SQRT. I guess no-one ever thought they'd be
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used for square roots... :-) */
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if (!fetestexcept (FE_INVALID))
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#endif
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feraiseexcept (FE_INVALID);
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#ifndef _IEEE_LIBM
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if (_LIB_VERSION != _IEEE_)
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x = __kernel_standard(x,x,126);
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else
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#endif
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x = *(const float*)&a_nan;
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}
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return f_washf(x);
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}
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weak_alias (__sqrtf, sqrtf)
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/* Strictly, this is wrong, but the only places where _ieee754_sqrt is
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used will not pass in a negative result. */
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strong_alias(__sqrtf,__ieee754_sqrtf)
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