/** @file | |
Compute the logrithm of x. | |
Copyright (c) 2010 - 2011, Intel Corporation. All rights reserved.<BR> | |
This program and the accompanying materials are licensed and made available under | |
the terms and conditions of the BSD License that accompanies this distribution. | |
The full text of the license may be found at | |
http://opensource.org/licenses/bsd-license. | |
THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS, | |
WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED. | |
* ==================================================== | |
* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. | |
* | |
* Developed at SunPro, a Sun Microsystems, Inc. business. | |
* Permission to use, copy, modify, and distribute this | |
* software is freely granted, provided that this notice | |
* is preserved. | |
* ==================================================== | |
e_sqrt.c 5.1 93/09/24 | |
NetBSD: e_sqrt.c,v 1.12 2002/05/26 22:01:52 wiz Exp | |
**/ | |
#include <LibConfig.h> | |
#include <sys/EfiCdefs.h> | |
#include <errno.h> | |
#include "math.h" | |
#include "math_private.h" | |
#if defined(_MSC_VER) /* Handle Microsoft VC++ compiler specifics. */ | |
// potential divide by 0 -- near line 129, (x-x)/(x-x) is on purpose | |
#pragma warning ( disable : 4723 ) | |
#endif | |
/* __ieee754_sqrt(x) | |
* Return correctly rounded sqrt. | |
* ------------------------------------------ | |
* | Use the hardware sqrt if you have one | | |
* ------------------------------------------ | |
* Method: | |
* Bit by bit method using integer arithmetic. (Slow, but portable) | |
* 1. Normalization | |
* Scale x to y in [1,4) with even powers of 2: | |
* find an integer k such that 1 <= (y=x*2^(2k)) < 4, then | |
* sqrt(x) = 2^k * sqrt(y) | |
* 2. Bit by bit computation | |
* Let q = sqrt(y) truncated to i bit after binary point (q = 1), | |
* i 0 | |
* i+1 2 | |
* s = 2*q , and y = 2 * ( y - q ). (1) | |
* i i i i | |
* | |
* To compute q from q , one checks whether | |
* i+1 i | |
* | |
* -(i+1) 2 | |
* (q + 2 ) <= y. (2) | |
* i | |
* -(i+1) | |
* If (2) is false, then q = q ; otherwise q = q + 2 . | |
* i+1 i i+1 i | |
* | |
* With some algebric manipulation, it is not difficult to see | |
* that (2) is equivalent to | |
* -(i+1) | |
* s + 2 <= y (3) | |
* i i | |
* | |
* The advantage of (3) is that s and y can be computed by | |
* i i | |
* the following recurrence formula: | |
* if (3) is false | |
* | |
* s = s , y = y ; (4) | |
* i+1 i i+1 i | |
* | |
* otherwise, | |
* -i -(i+1) | |
* s = s + 2 , y = y - s - 2 (5) | |
* i+1 i i+1 i i | |
* | |
* One may easily use induction to prove (4) and (5). | |
* Note. Since the left hand side of (3) contain only i+2 bits, | |
* it does not necessary to do a full (53-bit) comparison | |
* in (3). | |
* 3. Final rounding | |
* After generating the 53 bits result, we compute one more bit. | |
* Together with the remainder, we can decide whether the | |
* result is exact, bigger than 1/2ulp, or less than 1/2ulp | |
* (it will never equal to 1/2ulp). | |
* The rounding mode can be detected by checking whether | |
* huge + tiny is equal to huge, and whether huge - tiny is | |
* equal to huge for some floating point number "huge" and "tiny". | |
* | |
* Special cases: | |
* sqrt(+-0) = +-0 ... exact | |
* sqrt(inf) = inf | |
* sqrt(-ve) = NaN ... with invalid signal | |
* sqrt(NaN) = NaN ... with invalid signal for signaling NaN | |
* | |
* Other methods : see the appended file at the end of the program below. | |
*--------------- | |
*/ | |
static const double one = 1.0, tiny=1.0e-300; | |
double | |
__ieee754_sqrt(double x) | |
{ | |
double z; | |
int32_t sign = (int)0x80000000; | |
int32_t ix0,s0,q,m,t,i; | |
u_int32_t r,t1,s1,ix1,q1; | |
EXTRACT_WORDS(ix0,ix1,x); | |
/* take care of Inf and NaN */ | |
if((ix0&0x7ff00000)==0x7ff00000) { | |
return x*x+x; /* sqrt(NaN)=NaN, sqrt(+inf)=+inf | |
sqrt(-inf)=sNaN */ | |
} | |
/* take care of zero */ | |
if(ix0<=0) { | |
if(((ix0&(~sign))|ix1)==0) return x;/* sqrt(+-0) = +-0 */ | |
else if(ix0<0) { | |
errno = EDOM; | |
return (x-x)/(x-x); /* sqrt(-ve) = sNaN */ | |
} | |
} | |
/* normalize x */ | |
m = (ix0>>20); | |
if(m==0) { /* subnormal x */ | |
while(ix0==0) { | |
m -= 21; | |
ix0 |= (ix1>>11); ix1 <<= 21; | |
} | |
for(i=0;(ix0&0x00100000)==0;i++) ix0<<=1; | |
m -= i-1; | |
ix0 |= (ix1>>(32-i)); | |
ix1 <<= i; | |
} | |
m -= 1023; /* unbias exponent */ | |
ix0 = (ix0&0x000fffff)|0x00100000; | |
if(m&1){ /* odd m, double x to make it even */ | |
ix0 += ix0 + ((ix1&sign)>>31); | |
ix1 += ix1; | |
} | |
m >>= 1; /* m = [m/2] */ | |
/* generate sqrt(x) bit by bit */ | |
ix0 += ix0 + ((ix1&sign)>>31); | |
ix1 += ix1; | |
q = q1 = s0 = s1 = 0; /* [q,q1] = sqrt(x) */ | |
r = 0x00200000; /* r = moving bit from right to left */ | |
while(r!=0) { | |
t = s0+r; | |
if(t<=ix0) { | |
s0 = t+r; | |
ix0 -= t; | |
q += r; | |
} | |
ix0 += ix0 + ((ix1&sign)>>31); | |
ix1 += ix1; | |
r>>=1; | |
} | |
r = sign; | |
while(r!=0) { | |
t1 = s1+r; | |
t = s0; | |
if((t<ix0)||((t==ix0)&&(t1<=ix1))) { | |
s1 = t1+r; | |
if(((t1&sign)==(u_int32_t)sign)&&(s1&sign)==0) s0 += 1; | |
ix0 -= t; | |
if (ix1 < t1) ix0 -= 1; | |
ix1 -= t1; | |
q1 += r; | |
} | |
ix0 += ix0 + ((ix1&sign)>>31); | |
ix1 += ix1; | |
r>>=1; | |
} | |
/* use floating add to find out rounding direction */ | |
if((ix0|ix1)!=0) { | |
z = one-tiny; /* trigger inexact flag */ | |
if (z>=one) { | |
z = one+tiny; | |
if (q1==(u_int32_t)0xffffffff) { q1=0; q += 1;} | |
else if (z>one) { | |
if (q1==(u_int32_t)0xfffffffe) q+=1; | |
q1+=2; | |
} else | |
q1 += (q1&1); | |
} | |
} | |
ix0 = (q>>1)+0x3fe00000; | |
ix1 = q1>>1; | |
if ((q&1)==1) ix1 |= sign; | |
ix0 += (m <<20); | |
INSERT_WORDS(z,ix0,ix1); | |
return z; | |
} | |
/* | |
Other methods (use floating-point arithmetic) | |
------------- | |
(This is a copy of a drafted paper by Prof W. Kahan | |
and K.C. Ng, written in May, 1986) | |
Two algorithms are given here to implement sqrt(x) | |
(IEEE double precision arithmetic) in software. | |
Both supply sqrt(x) correctly rounded. The first algorithm (in | |
Section A) uses newton iterations and involves four divisions. | |
The second one uses reciproot iterations to avoid division, but | |
requires more multiplications. Both algorithms need the ability | |
to chop results of arithmetic operations instead of round them, | |
and the INEXACT flag to indicate when an arithmetic operation | |
is executed exactly with no roundoff error, all part of the | |
standard (IEEE 754-1985). The ability to perform shift, add, | |
subtract and logical AND operations upon 32-bit words is needed | |
too, though not part of the standard. | |
A. sqrt(x) by Newton Iteration | |
(1) Initial approximation | |
Let x0 and x1 be the leading and the trailing 32-bit words of | |
a floating point number x (in IEEE double format) respectively | |
1 11 52 ...widths | |
------------------------------------------------------ | |
x: |s| e | f | | |
------------------------------------------------------ | |
msb lsb msb lsb ...order | |
------------------------ ------------------------ | |
x0: |s| e | f1 | x1: | f2 | | |
------------------------ ------------------------ | |
By performing shifts and subtracts on x0 and x1 (both regarded | |
as integers), we obtain an 8-bit approximation of sqrt(x) as | |
follows. | |
k := (x0>>1) + 0x1ff80000; | |
y0 := k - T1[31&(k>>15)]. ... y ~ sqrt(x) to 8 bits | |
Here k is a 32-bit integer and T1[] is an integer array containing | |
correction terms. Now magically the floating value of y (y's | |
leading 32-bit word is y0, the value of its trailing word is 0) | |
approximates sqrt(x) to almost 8-bit. | |
Value of T1: | |
static int T1[32]= { | |
0, 1024, 3062, 5746, 9193, 13348, 18162, 23592, | |
29598, 36145, 43202, 50740, 58733, 67158, 75992, 85215, | |
83599, 71378, 60428, 50647, 41945, 34246, 27478, 21581, | |
16499, 12183, 8588, 5674, 3403, 1742, 661, 130,}; | |
(2) Iterative refinement | |
Apply Heron's rule three times to y, we have y approximates | |
sqrt(x) to within 1 ulp (Unit in the Last Place): | |
y := (y+x/y)/2 ... almost 17 sig. bits | |
y := (y+x/y)/2 ... almost 35 sig. bits | |
y := y-(y-x/y)/2 ... within 1 ulp | |
Remark 1. | |
Another way to improve y to within 1 ulp is: | |
y := (y+x/y) ... almost 17 sig. bits to 2*sqrt(x) | |
y := y - 0x00100006 ... almost 18 sig. bits to sqrt(x) | |
2 | |
(x-y )*y | |
y := y + 2* ---------- ...within 1 ulp | |
2 | |
3y + x | |
This formula has one division fewer than the one above; however, | |
it requires more multiplications and additions. Also x must be | |
scaled in advance to avoid spurious overflow in evaluating the | |
expression 3y*y+x. Hence it is not recommended uless division | |
is slow. If division is very slow, then one should use the | |
reciproot algorithm given in section B. | |
(3) Final adjustment | |
By twiddling y's last bit it is possible to force y to be | |
correctly rounded according to the prevailing rounding mode | |
as follows. Let r and i be copies of the rounding mode and | |
inexact flag before entering the square root program. Also we | |
use the expression y+-ulp for the next representable floating | |
numbers (up and down) of y. Note that y+-ulp = either fixed | |
point y+-1, or multiply y by nextafter(1,+-inf) in chopped | |
mode. | |
I := FALSE; ... reset INEXACT flag I | |
R := RZ; ... set rounding mode to round-toward-zero | |
z := x/y; ... chopped quotient, possibly inexact | |
If(not I) then { ... if the quotient is exact | |
if(z=y) { | |
I := i; ... restore inexact flag | |
R := r; ... restore rounded mode | |
return sqrt(x):=y. | |
} else { | |
z := z - ulp; ... special rounding | |
} | |
} | |
i := TRUE; ... sqrt(x) is inexact | |
If (r=RN) then z=z+ulp ... rounded-to-nearest | |
If (r=RP) then { ... round-toward-+inf | |
y = y+ulp; z=z+ulp; | |
} | |
y := y+z; ... chopped sum | |
y0:=y0-0x00100000; ... y := y/2 is correctly rounded. | |
I := i; ... restore inexact flag | |
R := r; ... restore rounded mode | |
return sqrt(x):=y. | |
(4) Special cases | |
Square root of +inf, +-0, or NaN is itself; | |
Square root of a negative number is NaN with invalid signal. | |
B. sqrt(x) by Reciproot Iteration | |
(1) Initial approximation | |
Let x0 and x1 be the leading and the trailing 32-bit words of | |
a floating point number x (in IEEE double format) respectively | |
(see section A). By performing shifs and subtracts on x0 and y0, | |
we obtain a 7.8-bit approximation of 1/sqrt(x) as follows. | |
k := 0x5fe80000 - (x0>>1); | |
y0:= k - T2[63&(k>>14)]. ... y ~ 1/sqrt(x) to 7.8 bits | |
Here k is a 32-bit integer and T2[] is an integer array | |
containing correction terms. Now magically the floating | |
value of y (y's leading 32-bit word is y0, the value of | |
its trailing word y1 is set to zero) approximates 1/sqrt(x) | |
to almost 7.8-bit. | |
Value of T2: | |
static int T2[64]= { | |
0x1500, 0x2ef8, 0x4d67, 0x6b02, 0x87be, 0xa395, 0xbe7a, 0xd866, | |
0xf14a, 0x1091b,0x11fcd,0x13552,0x14999,0x15c98,0x16e34,0x17e5f, | |
0x18d03,0x19a01,0x1a545,0x1ae8a,0x1b5c4,0x1bb01,0x1bfde,0x1c28d, | |
0x1c2de,0x1c0db,0x1ba73,0x1b11c,0x1a4b5,0x1953d,0x18266,0x16be0, | |
0x1683e,0x179d8,0x18a4d,0x19992,0x1a789,0x1b445,0x1bf61,0x1c989, | |
0x1d16d,0x1d77b,0x1dddf,0x1e2ad,0x1e5bf,0x1e6e8,0x1e654,0x1e3cd, | |
0x1df2a,0x1d635,0x1cb16,0x1be2c,0x1ae4e,0x19bde,0x1868e,0x16e2e, | |
0x1527f,0x1334a,0x11051,0xe951, 0xbe01, 0x8e0d, 0x5924, 0x1edd,}; | |
(2) Iterative refinement | |
Apply Reciproot iteration three times to y and multiply the | |
result by x to get an approximation z that matches sqrt(x) | |
to about 1 ulp. To be exact, we will have | |
-1ulp < sqrt(x)-z<1.0625ulp. | |
... set rounding mode to Round-to-nearest | |
y := y*(1.5-0.5*x*y*y) ... almost 15 sig. bits to 1/sqrt(x) | |
y := y*((1.5-2^-30)+0.5*x*y*y)... about 29 sig. bits to 1/sqrt(x) | |
... special arrangement for better accuracy | |
z := x*y ... 29 bits to sqrt(x), with z*y<1 | |
z := z + 0.5*z*(1-z*y) ... about 1 ulp to sqrt(x) | |
Remark 2. The constant 1.5-2^-30 is chosen to bias the error so that | |
(a) the term z*y in the final iteration is always less than 1; | |
(b) the error in the final result is biased upward so that | |
-1 ulp < sqrt(x) - z < 1.0625 ulp | |
instead of |sqrt(x)-z|<1.03125ulp. | |
(3) Final adjustment | |
By twiddling y's last bit it is possible to force y to be | |
correctly rounded according to the prevailing rounding mode | |
as follows. Let r and i be copies of the rounding mode and | |
inexact flag before entering the square root program. Also we | |
use the expression y+-ulp for the next representable floating | |
numbers (up and down) of y. Note that y+-ulp = either fixed | |
point y+-1, or multiply y by nextafter(1,+-inf) in chopped | |
mode. | |
R := RZ; ... set rounding mode to round-toward-zero | |
switch(r) { | |
case RN: ... round-to-nearest | |
if(x<= z*(z-ulp)...chopped) z = z - ulp; else | |
if(x<= z*(z+ulp)...chopped) z = z; else z = z+ulp; | |
break; | |
case RZ:case RM: ... round-to-zero or round-to--inf | |
R:=RP; ... reset rounding mod to round-to-+inf | |
if(x<z*z ... rounded up) z = z - ulp; else | |
if(x>=(z+ulp)*(z+ulp) ...rounded up) z = z+ulp; | |
break; | |
case RP: ... round-to-+inf | |
if(x>(z+ulp)*(z+ulp)...chopped) z = z+2*ulp; else | |
if(x>z*z ...chopped) z = z+ulp; | |
break; | |
} | |
Remark 3. The above comparisons can be done in fixed point. For | |
example, to compare x and w=z*z chopped, it suffices to compare | |
x1 and w1 (the trailing parts of x and w), regarding them as | |
two's complement integers. | |
...Is z an exact square root? | |
To determine whether z is an exact square root of x, let z1 be the | |
trailing part of z, and also let x0 and x1 be the leading and | |
trailing parts of x. | |
If ((z1&0x03ffffff)!=0) ... not exact if trailing 26 bits of z!=0 | |
I := 1; ... Raise Inexact flag: z is not exact | |
else { | |
j := 1 - [(x0>>20)&1] ... j = logb(x) mod 2 | |
k := z1 >> 26; ... get z's 25-th and 26-th | |
fraction bits | |
I := i or (k&j) or ((k&(j+j+1))!=(x1&3)); | |
} | |
R:= r ... restore rounded mode | |
return sqrt(x):=z. | |
If multiplication is cheaper than the foregoing red tape, the | |
Inexact flag can be evaluated by | |
I := i; | |
I := (z*z!=x) or I. | |
Note that z*z can overwrite I; this value must be sensed if it is | |
True. | |
Remark 4. If z*z = x exactly, then bit 25 to bit 0 of z1 must be | |
zero. | |
-------------------- | |
z1: | f2 | | |
-------------------- | |
bit 31 bit 0 | |
Further more, bit 27 and 26 of z1, bit 0 and 1 of x1, and the odd | |
or even of logb(x) have the following relations: | |
------------------------------------------------- | |
bit 27,26 of z1 bit 1,0 of x1 logb(x) | |
------------------------------------------------- | |
00 00 odd and even | |
01 01 even | |
10 10 odd | |
10 00 even | |
11 01 even | |
------------------------------------------------- | |
(4) Special cases (see (4) of Section A). | |
*/ | |