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/* Double-precision vector (SVE) exp function.
Copyright (C) 2023 Free Software Foundation, Inc.
This file is part of the GNU C Library.
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
<https://www.gnu.org/licenses/>. */
#include "sv_math.h"
static const struct data
{
double poly[4];
double ln2_hi, ln2_lo, inv_ln2, shift, thres;
} data = {
.poly = { /* ulp error: 0.53. */
0x1.fffffffffdbcdp-2, 0x1.555555555444cp-3, 0x1.555573c6a9f7dp-5,
0x1.1111266d28935p-7 },
.ln2_hi = 0x1.62e42fefa3800p-1,
.ln2_lo = 0x1.ef35793c76730p-45,
/* 1/ln2. */
.inv_ln2 = 0x1.71547652b82fep+0,
/* 1.5*2^46+1023. This value is further explained below. */
.shift = 0x1.800000000ffc0p+46,
.thres = 704.0,
};
#define C(i) sv_f64 (d->poly[i])
#define SpecialOffset 0x6000000000000000 /* 0x1p513. */
/* SpecialBias1 + SpecialBias1 = asuint(1.0). */
#define SpecialBias1 0x7000000000000000 /* 0x1p769. */
#define SpecialBias2 0x3010000000000000 /* 0x1p-254. */
/* Update of both special and non-special cases, if any special case is
detected. */
static inline svfloat64_t
special_case (svbool_t pg, svfloat64_t s, svfloat64_t y, svfloat64_t n)
{
/* s=2^n may overflow, break it up into s=s1*s2,
such that exp = s + s*y can be computed as s1*(s2+s2*y)
and s1*s1 overflows only if n>0. */
/* If n<=0 then set b to 0x6, 0 otherwise. */
svbool_t p_sign = svcmple (pg, n, 0.0); /* n <= 0. */
svuint64_t b
= svdup_u64_z (p_sign, SpecialOffset); /* Inactive lanes set to 0. */
/* Set s1 to generate overflow depending on sign of exponent n. */
svfloat64_t s1 = svreinterpret_f64 (
svsubr_x (pg, b, SpecialBias1)); /* 0x70...0 - b. */
/* Offset s to avoid overflow in final result if n is below threshold. */
svfloat64_t s2 = svreinterpret_f64 (
svadd_x (pg, svsub_x (pg, svreinterpret_u64 (s), SpecialBias2),
b)); /* as_u64 (s) - 0x3010...0 + b. */
/* |n| > 1280 => 2^(n) overflows. */
svbool_t p_cmp = svacgt (pg, n, 1280.0);
svfloat64_t r1 = svmul_x (pg, s1, s1);
svfloat64_t r2 = svmla_x (pg, s2, s2, y);
svfloat64_t r0 = svmul_x (pg, r2, s1);
return svsel (p_cmp, r1, r0);
}
/* SVE exp algorithm. Maximum measured error is 1.01ulps:
SV_NAME_D1 (exp)(0x1.4619d7b04da41p+6) got 0x1.885d9acc41da7p+117
want 0x1.885d9acc41da6p+117. */
svfloat64_t SV_NAME_D1 (exp) (svfloat64_t x, const svbool_t pg)
{
const struct data *d = ptr_barrier (&data);
svbool_t special = svacgt (pg, x, d->thres);
/* Use a modifed version of the shift used for flooring, such that x/ln2 is
rounded to a multiple of 2^-6=1/64, shift = 1.5 * 2^52 * 2^-6 = 1.5 *
2^46.
n is not an integer but can be written as n = m + i/64, with i and m
integer, 0 <= i < 64 and m <= n.
Bits 5:0 of z will be null every time x/ln2 reaches a new integer value
(n=m, i=0), and is incremented every time z (or n) is incremented by 1/64.
FEXPA expects i in bits 5:0 of the input so it can be used as index into
FEXPA hardwired table T[i] = 2^(i/64) for i = 0:63, that will in turn
populate the mantissa of the output. Therefore, we use u=asuint(z) as
input to FEXPA.
We add 1023 to the modified shift value in order to set bits 16:6 of u to
1, such that once these bits are moved to the exponent of the output of
FEXPA, we get the exponent of 2^n right, i.e. we get 2^m. */
svfloat64_t z = svmla_x (pg, sv_f64 (d->shift), x, d->inv_ln2);
svuint64_t u = svreinterpret_u64 (z);
svfloat64_t n = svsub_x (pg, z, d->shift);
/* r = x - n * ln2, r is in [-ln2/(2N), ln2/(2N)]. */
svfloat64_t ln2 = svld1rq (svptrue_b64 (), &d->ln2_hi);
svfloat64_t r = svmls_lane (x, n, ln2, 0);
r = svmls_lane (r, n, ln2, 1);
/* y = exp(r) - 1 ~= r + C0 r^2 + C1 r^3 + C2 r^4 + C3 r^5. */
svfloat64_t r2 = svmul_x (pg, r, r);
svfloat64_t p01 = svmla_x (pg, C (0), C (1), r);
svfloat64_t p23 = svmla_x (pg, C (2), C (3), r);
svfloat64_t p04 = svmla_x (pg, p01, p23, r2);
svfloat64_t y = svmla_x (pg, r, p04, r2);
/* s = 2^n, computed using FEXPA. FEXPA does not propagate NaNs, so for
consistent NaN handling we have to manually propagate them. This comes at
significant performance cost. */
svfloat64_t s = svexpa (u);
/* Assemble result as exp(x) = 2^n * exp(r). If |x| > Thresh the
multiplication may overflow, so use special case routine. */
if (__glibc_unlikely (svptest_any (pg, special)))
{
/* FEXPA zeroes the sign bit, however the sign is meaningful to the
special case function so needs to be copied.
e = sign bit of u << 46. */
svuint64_t e = svand_x (pg, svlsl_x (pg, u, 46), 0x8000000000000000);
/* Copy sign to s. */
s = svreinterpret_f64 (svadd_x (pg, e, svreinterpret_u64 (s)));
return special_case (pg, s, y, n);
}
/* No special case. */
return svmla_x (pg, s, s, y);
}
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