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8b09af572b
Since the last operation is destructive, the first argument to the FMA also has to be the first argument to the special-case in order to avoid unnecessary MOVs. Reorder arguments and adjust special-case bounds to facilitate this. Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com>
90 lines
3.4 KiB
C
90 lines
3.4 KiB
C
/* Single-precision vector (AdvSIMD) exp2 function
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Copyright (C) 2023-2024 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 "v_math.h"
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#include "poly_advsimd_f32.h"
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static const struct data
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{
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uint32x4_t off, offset_lower_bound;
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uint16x8_t special_bound;
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uint32x4_t mantissa_mask;
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float32x4_t poly[9];
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} data = {
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/* Coefficients generated using Remez algorithm approximate
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log2(1+r)/r for r in [ -1/3, 1/3 ].
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rel error: 0x1.c4c4b0cp-26. */
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.poly = { V4 (0x1.715476p0f), /* (float)(1 / ln(2)). */
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V4 (-0x1.715458p-1f), V4 (0x1.ec701cp-2f), V4 (-0x1.7171a4p-2f),
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V4 (0x1.27a0b8p-2f), V4 (-0x1.e5143ep-3f), V4 (0x1.9d8ecap-3f),
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V4 (-0x1.c675bp-3f), V4 (0x1.9e495p-3f) },
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/* Lower bound is the smallest positive normal float 0x00800000. For
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optimised register use subnormals are detected after offset has been
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subtracted, so lower bound is 0x0080000 - offset (which wraps around). */
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.offset_lower_bound = V4 (0x00800000 - 0x3f2aaaab),
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.special_bound = V8 (0x7f00), /* top16(asuint32(inf) - 0x00800000). */
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.off = V4 (0x3f2aaaab), /* 0.666667. */
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.mantissa_mask = V4 (0x007fffff),
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};
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static float32x4_t VPCS_ATTR NOINLINE
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special_case (float32x4_t n, uint32x4_t u_off, float32x4_t p, float32x4_t r,
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uint16x4_t cmp, const struct data *d)
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{
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/* Fall back to scalar code. */
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return v_call_f32 (log2f, vreinterpretq_f32_u32 (vaddq_u32 (u_off, d->off)),
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vfmaq_f32 (n, p, r), vmovl_u16 (cmp));
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}
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/* Fast implementation for single precision AdvSIMD log2,
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relies on same argument reduction as AdvSIMD logf.
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Maximum error: 2.48 ULPs
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_ZGVnN4v_log2f(0x1.558174p+0) got 0x1.a9be84p-2
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want 0x1.a9be8p-2. */
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float32x4_t VPCS_ATTR NOINLINE V_NAME_F1 (log2) (float32x4_t x)
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{
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const struct data *d = ptr_barrier (&data);
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/* To avoid having to mov x out of the way, keep u after offset has been
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applied, and recover x by adding the offset back in the special-case
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handler. */
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uint32x4_t u_off = vreinterpretq_u32_f32 (x);
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/* x = 2^n * (1+r), where 2/3 < 1+r < 4/3. */
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u_off = vsubq_u32 (u_off, d->off);
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float32x4_t n = vcvtq_f32_s32 (
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vshrq_n_s32 (vreinterpretq_s32_u32 (u_off), 23)); /* signextend. */
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uint16x4_t special = vcge_u16 (vsubhn_u32 (u_off, d->offset_lower_bound),
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vget_low_u16 (d->special_bound));
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uint32x4_t u = vaddq_u32 (vandq_u32 (u_off, d->mantissa_mask), d->off);
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float32x4_t r = vsubq_f32 (vreinterpretq_f32_u32 (u), v_f32 (1.0f));
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/* y = log2(1+r) + n. */
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float32x4_t r2 = vmulq_f32 (r, r);
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float32x4_t p = v_pw_horner_8_f32 (r, r2, d->poly);
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if (__glibc_unlikely (v_any_u16h (special)))
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return special_case (n, u_off, p, r, special, d);
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return vfmaq_f32 (n, p, r);
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}
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libmvec_hidden_def (V_NAME_F1 (log2))
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HALF_WIDTH_ALIAS_F1 (log2)
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