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5a4b6f8e4b
* Transpose table layout for improved memory access
* Use half-vector special comparisons for AdvSIMD
* Improve register use near special-case branches
- Due to the presence of a function call, return value would get
mov-d out of x0 in order to facilitate PCS. By moving the final
computation after the branch this can be avoided
Also change SVE routines to use overloaded intrinsics for readability.
77 lines
3.0 KiB
C
77 lines
3.0 KiB
C
/* Double-precision vector (SVE) log function.
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Copyright (C) 2023 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 Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the 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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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include "sv_math.h"
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#define P(i) sv_f64 (__v_log_data.poly[i])
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#define N (1 << V_LOG_TABLE_BITS)
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#define Off (0x3fe6900900000000)
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#define MaxTop (0x7ff)
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#define MinTop (0x001)
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#define ThreshTop (0x7fe) /* MaxTop - MinTop. */
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static svfloat64_t NOINLINE
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special_case (svfloat64_t x, svfloat64_t y, svbool_t cmp)
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{
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return sv_call_f64 (log, x, y, cmp);
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}
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/* SVE port of AdvSIMD log algorithm.
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Maximum measured error is 2.17 ulp:
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SV_NAME_D1 (log)(0x1.a6129884398a3p+0) got 0x1.ffffff1cca043p-2
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want 0x1.ffffff1cca045p-2. */
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svfloat64_t SV_NAME_D1 (log) (svfloat64_t x, const svbool_t pg)
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{
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svuint64_t ix = svreinterpret_u64 (x);
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svuint64_t top = svlsr_x (pg, ix, 52);
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svbool_t cmp = svcmpge (pg, svsub_x (pg, top, MinTop), sv_u64 (ThreshTop));
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/* x = 2^k z; where z is in range [Off,2*Off) and exact.
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The range is split into N subintervals.
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The ith subinterval contains z and c is near its center. */
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svuint64_t tmp = svsub_x (pg, ix, Off);
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/* Calculate table index = (tmp >> (52 - V_LOG_TABLE_BITS)) % N.
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The actual value of i is double this due to table layout. */
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svuint64_t i
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= svand_x (pg, svlsr_x (pg, tmp, (51 - V_LOG_TABLE_BITS)), (N - 1) << 1);
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svint64_t k
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= svasr_x (pg, svreinterpret_s64 (tmp), 52); /* Arithmetic shift. */
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svuint64_t iz = svsub_x (pg, ix, svand_x (pg, tmp, 0xfffULL << 52));
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svfloat64_t z = svreinterpret_f64 (iz);
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/* Lookup in 2 global lists (length N). */
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svfloat64_t invc = svld1_gather_index (pg, &__v_log_data.table[0].invc, i);
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svfloat64_t logc = svld1_gather_index (pg, &__v_log_data.table[0].logc, i);
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/* log(x) = log1p(z/c-1) + log(c) + k*Ln2. */
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svfloat64_t r = svmad_x (pg, invc, z, -1);
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svfloat64_t kd = svcvt_f64_x (pg, k);
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/* hi = r + log(c) + k*Ln2. */
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svfloat64_t hi = svmla_x (pg, svadd_x (pg, logc, r), kd, __v_log_data.ln2);
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/* y = r2*(A0 + r*A1 + r2*(A2 + r*A3 + r2*A4)) + hi. */
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svfloat64_t r2 = svmul_x (pg, r, r);
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svfloat64_t y = svmla_x (pg, P (2), r, P (3));
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svfloat64_t p = svmla_x (pg, P (0), r, P (1));
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y = svmla_x (pg, y, r2, P (4));
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y = svmla_x (pg, p, r2, y);
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if (__glibc_unlikely (svptest_any (pg, cmp)))
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return special_case (x, svmla_x (svnot_z (pg, cmp), hi, r2, y), cmp);
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return svmla_x (pg, hi, r2, y);
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}
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