llvm-project

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/*
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 * Single-precision log function.
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 *
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 * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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 * See https://llvm.org/LICENSE.txt for license information.
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 * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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 */
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#include <math.h>
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#include <stdint.h>
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#include "math_config.h"
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/*
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LOGF_TABLE_BITS = 4
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LOGF_POLY_ORDER = 4
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ULP error: 0.818 (nearest rounding.)
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Relative error: 1.957 * 2^-26 (before rounding.)
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*/
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#define T __logf_data.tab
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#define A __logf_data.poly
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#define Ln2 __logf_data.ln2
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#define N (1 << LOGF_TABLE_BITS)
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#define OFF 0x3f330000
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float
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logf (float x)
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{
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  /* double_t for better performance on targets with FLT_EVAL_METHOD==2.  */
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  double_t z, r, r2, y, y0, invc, logc;
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  uint32_t ix, iz, tmp;
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  int k, i;
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  ix = asuint (x);
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#if WANT_ROUNDING
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  /* Fix sign of zero with downward rounding when x==1.  */
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  if (unlikely (ix == 0x3f800000))
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    return 0;
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#endif
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  if (unlikely (ix - 0x00800000 >= 0x7f800000 - 0x00800000))
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    {
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      /* x < 0x1p-126 or inf or nan.  */
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      if (ix * 2 == 0)
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	return __math_divzerof (1);
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      if (ix == 0x7f800000) /* log(inf) == inf.  */
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	return x;
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      if ((ix & 0x80000000) || ix * 2 >= 0xff000000)
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	return __math_invalidf (x);
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      /* x is subnormal, normalize it.  */
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      ix = asuint (x * 0x1p23f);
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      ix -= 23 << 23;
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    }
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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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  tmp = ix - OFF;
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  i = (tmp >> (23 - LOGF_TABLE_BITS)) % N;
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  k = (int32_t) tmp >> 23; /* arithmetic shift */
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  iz = ix - (tmp & 0x1ff << 23);
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  invc = T[i].invc;
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  logc = T[i].logc;
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  z = (double_t) asfloat (iz);
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  /* log(x) = log1p(z/c-1) + log(c) + k*Ln2 */
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  r = z * invc - 1;
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  y0 = logc + (double_t) k * Ln2;
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  /* Pipelined polynomial evaluation to approximate log1p(r).  */
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  r2 = r * r;
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  y = A[1] * r + A[2];
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  y = A[0] * r2 + y;
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  y = y * r2 + (y0 + r);
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  return eval_as_float (y);
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}
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#if USE_GLIBC_ABI
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strong_alias (logf, __logf_finite)
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hidden_alias (logf, __ieee754_logf)
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#endif
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