libc/src/math/exp2f.cpp

   1 //===-- Single-precision 2^x function -------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8
   9 #include "exp_utils.h"
  10 #include "math_utils.h"
  11
  12 #include "include/math.h"
  13 #include "src/__support/common.h"
  14
  15 #include <stdint.h>
  16
  17 #define T exp2f_data.tab
  18 #define C exp2f_data.poly
  19 #define SHIFT exp2f_data.shift_scaled
  20
  21 namespace __llvm_libc {
  22
  23 float LLVM_LIBC_ENTRYPOINT(exp2f)(float x) {
  24   uint32_t abstop;
  25   uint64_t ki, t;
  26   // double_t for better performance on targets with FLT_EVAL_METHOD==2.
  27   double_t kd, xd, z, r, r2, y, s;
  28
  29   xd = static_cast<double_t>(x);
  30   abstop = top12_bits(x) & 0x7ff;
  31   if (unlikely(abstop >= top12_bits(128.0f))) {
  32     // |x| >= 128 or x is nan.
  33     if (as_uint32_bits(x) == as_uint32_bits(-INFINITY))
  34       return 0.0f;
  35     if (abstop >= top12_bits(INFINITY))
  36       return x + x;
  37     if (x > 0.0f)
  38       return overflow<float>(0);
  39     if (x <= -150.0f)
  40       return underflow<float>(0);
  41     if (x < -149.0f)
  42       return may_underflow<float>(0);
  43   }
  44
  45   // x = k/N + r with r in [-1/(2N), 1/(2N)] and int k.
  46   kd = static_cast<double>(xd + SHIFT);
  47   ki = as_uint64_bits(kd);
  48   kd -= SHIFT; // k/N for int k.
  49   r = xd - kd;
  50
  51   // exp2(x) = 2^(k/N) * 2^r ~= s * (C0*r^3 + C1*r^2 + C2*r + 1)
  52   t = T[ki % N];
  53   t += ki << (52 - EXP2F_TABLE_BITS);
  54   s = as_double(t);
  55   z = C[0] * r + C[1];
  56   r2 = r * r;
  57   y = C[2] * r + 1;
  58   y = z * r2 + y;
  59   y = y * s;
  60   return static_cast<float>(y);
  61 }
  62
  63 } // namespace __llvm_libc