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math: new log2
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from /~https://github.com/ARM-software/optimized-routines,
commit 04884bd04eac4b251da4026900010ea7d8850edc

code size change: +2458 bytes (+1524 bytes with fma).
benchmark on x86_64 before, after, speedup:

-Os:
  log2 rthruput:  16.08 ns/call 10.49 ns/call 1.53x
   log2 latency:  44.54 ns/call 25.55 ns/call 1.74x
-O3:
  log2 rthruput:  15.92 ns/call 10.11 ns/call 1.58x
   log2 latency:  44.66 ns/call 26.16 ns/call 1.71x
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@richfelker
Szabolcs Nagy authored and richfelker committed Apr 18, 2019
1 parent 236cd05 commit 2a3210c
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212 changes: 106 additions & 106 deletions src/math/log2.c
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Original file line number Diff line number Diff line change
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/* origin: FreeBSD /usr/src/lib/msun/src/e_log2.c */
/*
* ====================================================
* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
* Double-precision log2(x) function.
*
* Developed at SunSoft, a Sun Microsystems, Inc. business.
* Permission to use, copy, modify, and distribute this
* software is freely granted, provided that this notice
* is preserved.
* ====================================================
*/
/*
* Return the base 2 logarithm of x. See log.c for most comments.
*
* Reduce x to 2^k (1+f) and calculate r = log(1+f) - f + f*f/2
* as in log.c, then combine and scale in extra precision:
* log2(x) = (f - f*f/2 + r)/log(2) + k
* Copyright (c) 2018, Arm Limited.
* SPDX-License-Identifier: MIT
*/

#include <math.h>
#include <stdint.h>
#include "libm.h"
#include "log2_data.h"

static const double
ivln2hi = 1.44269504072144627571e+00, /* 0x3ff71547, 0x65200000 */
ivln2lo = 1.67517131648865118353e-10, /* 0x3de705fc, 0x2eefa200 */
Lg1 = 6.666666666666735130e-01, /* 3FE55555 55555593 */
Lg2 = 3.999999999940941908e-01, /* 3FD99999 9997FA04 */
Lg3 = 2.857142874366239149e-01, /* 3FD24924 94229359 */
Lg4 = 2.222219843214978396e-01, /* 3FCC71C5 1D8E78AF */
Lg5 = 1.818357216161805012e-01, /* 3FC74664 96CB03DE */
Lg6 = 1.531383769920937332e-01, /* 3FC39A09 D078C69F */
Lg7 = 1.479819860511658591e-01; /* 3FC2F112 DF3E5244 */
#define T __log2_data.tab
#define T2 __log2_data.tab2
#define B __log2_data.poly1
#define A __log2_data.poly
#define InvLn2hi __log2_data.invln2hi
#define InvLn2lo __log2_data.invln2lo
#define N (1 << LOG2_TABLE_BITS)
#define OFF 0x3fe6000000000000

double log2(double x)
/* Top 16 bits of a double. */
static inline uint32_t top16(double x)
{
union {double f; uint64_t i;} u = {x};
double_t hfsq,f,s,z,R,w,t1,t2,y,hi,lo,val_hi,val_lo;
uint32_t hx;
int k;

hx = u.i>>32;
k = 0;
if (hx < 0x00100000 || hx>>31) {
if (u.i<<1 == 0)
return -1/(x*x); /* log(+-0)=-inf */
if (hx>>31)
return (x-x)/0.0; /* log(-#) = NaN */
/* subnormal number, scale x up */
k -= 54;
x *= 0x1p54;
u.f = x;
hx = u.i>>32;
} else if (hx >= 0x7ff00000) {
return x;
} else if (hx == 0x3ff00000 && u.i<<32 == 0)
return 0;

/* reduce x into [sqrt(2)/2, sqrt(2)] */
hx += 0x3ff00000 - 0x3fe6a09e;
k += (int)(hx>>20) - 0x3ff;
hx = (hx&0x000fffff) + 0x3fe6a09e;
u.i = (uint64_t)hx<<32 | (u.i&0xffffffff);
x = u.f;
return asuint64(x) >> 48;
}

f = x - 1.0;
hfsq = 0.5*f*f;
s = f/(2.0+f);
z = s*s;
w = z*z;
t1 = w*(Lg2+w*(Lg4+w*Lg6));
t2 = z*(Lg1+w*(Lg3+w*(Lg5+w*Lg7)));
R = t2 + t1;
double log2(double x)
{
double_t z, r, r2, r4, y, invc, logc, kd, hi, lo, t1, t2, t3, p;
uint64_t ix, iz, tmp;
uint32_t top;
int k, i;

/*
* f-hfsq must (for args near 1) be evaluated in extra precision
* to avoid a large cancellation when x is near sqrt(2) or 1/sqrt(2).
* This is fairly efficient since f-hfsq only depends on f, so can
* be evaluated in parallel with R. Not combining hfsq with R also
* keeps R small (though not as small as a true `lo' term would be),
* so that extra precision is not needed for terms involving R.
*
* Compiler bugs involving extra precision used to break Dekker's
* theorem for spitting f-hfsq as hi+lo, unless double_t was used
* or the multi-precision calculations were avoided when double_t
* has extra precision. These problems are now automatically
* avoided as a side effect of the optimization of combining the
* Dekker splitting step with the clear-low-bits step.
*
* y must (for args near sqrt(2) and 1/sqrt(2)) be added in extra
* precision to avoid a very large cancellation when x is very near
* these values. Unlike the above cancellations, this problem is
* specific to base 2. It is strange that adding +-1 is so much
* harder than adding +-ln2 or +-log10_2.
*
* This uses Dekker's theorem to normalize y+val_hi, so the
* compiler bugs are back in some configurations, sigh. And I
* don't want to used double_t to avoid them, since that gives a
* pessimization and the support for avoiding the pessimization
* is not yet available.
*
* The multi-precision calculations for the multiplications are
* routine.
*/
ix = asuint64(x);
top = top16(x);
#define LO asuint64(1.0 - 0x1.5b51p-5)
#define HI asuint64(1.0 + 0x1.6ab2p-5)
if (predict_false(ix - LO < HI - LO)) {
/* Handle close to 1.0 inputs separately. */
/* Fix sign of zero with downward rounding when x==1. */
if (WANT_ROUNDING && predict_false(ix == asuint64(1.0)))
return 0;
r = x - 1.0;
#if __FP_FAST_FMA
hi = r * InvLn2hi;
lo = r * InvLn2lo + __builtin_fma(r, InvLn2hi, -hi);
#else
double_t rhi, rlo;
rhi = asdouble(asuint64(r) & -1ULL << 32);
rlo = r - rhi;
hi = rhi * InvLn2hi;
lo = rlo * InvLn2hi + r * InvLn2lo;
#endif
r2 = r * r; /* rounding error: 0x1p-62. */
r4 = r2 * r2;
/* Worst-case error is less than 0.54 ULP (0.55 ULP without fma). */
p = r2 * (B[0] + r * B[1]);
y = hi + p;
lo += hi - y + p;
lo += r4 * (B[2] + r * B[3] + r2 * (B[4] + r * B[5]) +
r4 * (B[6] + r * B[7] + r2 * (B[8] + r * B[9])));
y += lo;
return eval_as_double(y);
}
if (predict_false(top - 0x0010 >= 0x7ff0 - 0x0010)) {
/* x < 0x1p-1022 or inf or nan. */
if (ix * 2 == 0)
return __math_divzero(1);
if (ix == asuint64(INFINITY)) /* log(inf) == inf. */
return x;
if ((top & 0x8000) || (top & 0x7ff0) == 0x7ff0)
return __math_invalid(x);
/* x is subnormal, normalize it. */
ix = asuint64(x * 0x1p52);
ix -= 52ULL << 52;
}

/* hi+lo = f - hfsq + s*(hfsq+R) ~ log(1+f) */
hi = f - hfsq;
u.f = hi;
u.i &= (uint64_t)-1<<32;
hi = u.f;
lo = f - hi - hfsq + s*(hfsq+R);
/* x = 2^k z; where z is in range [OFF,2*OFF) and exact.
The range is split into N subintervals.
The ith subinterval contains z and c is near its center. */
tmp = ix - OFF;
i = (tmp >> (52 - LOG2_TABLE_BITS)) % N;
k = (int64_t)tmp >> 52; /* arithmetic shift */
iz = ix - (tmp & 0xfffULL << 52);
invc = T[i].invc;
logc = T[i].logc;
z = asdouble(iz);
kd = (double_t)k;

val_hi = hi*ivln2hi;
val_lo = (lo+hi)*ivln2lo + lo*ivln2hi;
/* log2(x) = log2(z/c) + log2(c) + k. */
/* r ~= z/c - 1, |r| < 1/(2*N). */
#if __FP_FAST_FMA
/* rounding error: 0x1p-55/N. */
r = __builtin_fma(z, invc, -1.0);
t1 = r * InvLn2hi;
t2 = r * InvLn2lo + __builtin_fma(r, InvLn2hi, -t1);
#else
double_t rhi, rlo;
/* rounding error: 0x1p-55/N + 0x1p-65. */
r = (z - T2[i].chi - T2[i].clo) * invc;
rhi = asdouble(asuint64(r) & -1ULL << 32);
rlo = r - rhi;
t1 = rhi * InvLn2hi;
t2 = rlo * InvLn2hi + r * InvLn2lo;
#endif

/* spadd(val_hi, val_lo, y), except for not using double_t: */
y = k;
w = y + val_hi;
val_lo += (y - w) + val_hi;
val_hi = w;
/* hi + lo = r/ln2 + log2(c) + k. */
t3 = kd + logc;
hi = t3 + t1;
lo = t3 - hi + t1 + t2;

return val_lo + val_hi;
/* log2(r+1) = r/ln2 + r^2*poly(r). */
/* Evaluation is optimized assuming superscalar pipelined execution. */
r2 = r * r; /* rounding error: 0x1p-54/N^2. */
r4 = r2 * r2;
/* Worst-case error if |y| > 0x1p-4: 0.547 ULP (0.550 ULP without fma).
~ 0.5 + 2/N/ln2 + abs-poly-error*0x1p56 ULP (+ 0.003 ULP without fma). */
p = A[0] + r * A[1] + r2 * (A[2] + r * A[3]) + r4 * (A[4] + r * A[5]);
y = lo + r2 * p + hi;
return eval_as_double(y);
}
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