ref: a4fdef8a867263627510ea20e9b4a758549f7c98
dir: /lib/math/sqrt-impl.myr/
use std
use "fpmath"
/* See [Mul+10], sections 5.4 and 8.7 */
pkg math =
pkglocal const sqrt32 : (x : flt32 -> flt32)
pkglocal const sqrt64 : (x : flt64 -> flt64)
;;
extern const fma32 : (x : flt32, y : flt32, z : flt32 -> flt32)
extern const fma64 : (x : flt64, y : flt64, z : flt64 -> flt64)
type fltdesc(@f, @u, @i) = struct
explode : (f : @f -> (bool, @i, @u))
assem : (n : bool, e : @i, s : @u -> @f)
fma : (x : @f, y : @f, z : @f -> @f)
tobits : (f : @f -> @u)
frombits : (u : @u -> @f)
nan : @u
emin : @i
emax : @i
normmask : @u
sgnmask : @u
ab : (@u, @u)[:]
iterlim : int
;;
/*
The starting point of the N-R iteration of 1/sqrt, after significand
has been normalized to [1, 4).
See [KM06] for the construction and notation. Case p = -2. The
dividers (left values) are chosen roughly so that maximal error
of N-R, after 3 iterations, starting with the right value, is
less than an ulp (of the result). If g falls in [a_i, a_{i+1}),
N-R should start with b_{i+1}.
In the flt64 case, we need only one more iteration.
*/
const ab32 : (uint32, uint32)[7] = [
(0x3f800000, 0x3f800000), /* Nothing should ever get normalized to < 1.0 */
(0x3fa66666, 0x3f6f30ae), /* [1.0, 1.3 ) -> 0.9343365431 */
(0x3fd9999a, 0x3f5173ca), /* [1.3, 1.7 ) -> 0.8181730509 */
(0x40100000, 0x3f3691d3), /* [1.7, 2.25) -> 0.713162601 */
(0x40333333, 0x3f215342), /* [2.25, 2.8 ) -> 0.6301766634 */
(0x4059999a, 0x3f118e0e), /* [2.8, 3.4 ) -> 0.5685738325 */
(0x40800000, 0x3f053049), /* [3.4, 4.0 ) -> 0.520268023 */
]
const ab64 : (uint64, uint64)[8] = [
(0x3ff0000000000000, 0x3ff0000000000000), /* < 1.0 */
(0x3ff3333333333333, 0x3fee892ce1608cbc), /* [1.0, 1.2) -> 0.954245033445111356940060431953 */
(0x3ff6666666666666, 0x3fec1513a2184094), /* [1.2, 1.4) -> 0.877572838393478438234751592972 */
(0x3ffc000000000000, 0x3fe9878eb3e9ba20), /* [1.4, 1.75) -> 0.797797538178034670863780775107 */
(0x400199999999999a, 0x3fe6ccb14eeb238d), /* [1.75, 2.2) -> 0.712486890924184046447464879748 */
(0x400599999999999a, 0x3fe47717c17cd34f), /* [2.2, 2.7) -> 0.639537694840876969060161627567 */
(0x400b333333333333, 0x3fe258df212a8e9a), /* [2.7, 3.4) -> 0.573348583963212421465982515656 */
(0x4010000000000000, 0x3fe0a5989f2dc59a), /* [3.4, 4.0) -> 0.520214377304159869552790951275 */
]
const desc32 : fltdesc(flt32, uint32, int32) = [
.explode = std.flt32explode,
.assem = std.flt32assem,
.tobits = std.flt32bits,
.frombits = std.flt32frombits,
.nan = 0x7fc00000,
.emin = -127,
.emax = 128,
.normmask = 1 << 23,
.sgnmask = 1 << 31,
.ab = ab32[0:7],
.iterlim = 3,
]
const desc64 : fltdesc(flt64, uint64, int64) = [
.explode = std.flt64explode,
.assem = std.flt64assem,
.tobits = std.flt64bits,
.frombits = std.flt64frombits,
.nan = 0x7ff8000000000000,
.emin = -1023,
.emax = 1024,
.normmask = 1 << 52,
.sgnmask = 1 << 63,
.ab = ab64[0:8],
.iterlim = 4,
]
const sqrt32 = {x : flt32
-> sqrtgen(x, desc32)
}
const sqrt64 = {x : flt64
-> sqrtgen(x, desc64)
}
generic sqrtgen = {x : @f, d : fltdesc(@f, @u, @i) :: numeric,floating,std.equatable @f, numeric,integral @u, numeric,integral @i
var n : bool, e : @i, s : @u, e2 : @i
(n, e, s) = d.explode(x)
/* Special cases: +/- 0.0, negative, NaN, and +inf */
if e == d.emin && s == 0
-> x
elif n || std.isnan(x)
/* Make sure to return a quiet NaN */
-> d.frombits(d.nan)
elif e == d.emax
-> x
;;
/*
Remove a factor of 2^(even) to normalize significand.
*/
if e == d.emin
e = d.emin + 1
while s & d.normmask == 0
s <<= 1
e--
;;
;;
if e % 2 != 0
e2 = e - 1
e = 1
else
e2 = e
e = 0
;;
var a : @f = d.assem(false, e, s)
var au : @u = d.tobits(a)
/*
We shall perform iterated Newton-Raphson in order to
compute 1/sqrt(g), then multiply by g to obtain sqrt(g).
This is faster than calculating sqrt(g) directly because
it avoids division. (The multiplication by g is built
into Markstein's r, g, n variables.)
*/
var xn : @f = d.frombits(0)
for (ai, beta) : d.ab
if au <= ai
xn = d.frombits(beta)
break
;;
;;
/* split up "x_{n+1} = x_n (3 - ax_n^2)/2" */
var epsn = fma(-1.0 * a, xn * xn, 1.0)
var rn = 0.5 * epsn
var gn = a * xn
var hn = 0.5 * xn
for var j = 0; j < d.iterlim; ++j
rn = fma(-1.0 * gn, hn, 0.5)
gn = fma(gn, rn, gn)
hn = fma(hn, rn, hn)
;;
/*
gn is almost what we want, except that we might want to
adjust by an ulp in one direction or the other. This is
the Tuckerman test.
Exhaustive testing has shown that we need only 3 adjustments
in the flt32 case (and it should be 4 in the flt64 case).
*/
(_, e, s) = d.explode(gn)
e += (e2 / 2)
var r : @f = d.assem(false, e, s)
for var j = 0; j < d.iterlim; ++j
var r_plus_ulp : @f = d.frombits(d.tobits(r) + 1)
var r_minus_ulp : @f = d.frombits(d.tobits(r) - 1)
var delta_1 = fma(r, r_minus_ulp, -1.0 * x)
if d.tobits(delta_1) & d.sgnmask == 0
r = r_minus_ulp
else
var delta_2 = fma(r, r_plus_ulp, -1.0 * x)
if d.tobits(delta_2) & d.sgnmask != 0
r = r_plus_ulp
else
-> r
;;
;;
;;
-> r
}