libc/src/__support/math/sincos_integer_utils.h

//===-- Trig range reduction and evaluation using integer-only --*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_LIBC_SRC___SUPPORT_MATH_SINCOS_INTEGER_UTILS_H
#define LLVM_LIBC_SRC___SUPPORT_MATH_SINCOS_INTEGER_UTILS_H

#include "src/__support/CPP/bit.h"
#include "src/__support/FPUtil/FPBits.h"
#include "src/__support/FPUtil/PolyEval.h"
#include "src/__support/FPUtil/multiply_add.h"
#include "src/__support/big_int.h"
#include "src/__support/frac128.h"
#include "src/__support/macros/config.h"
#include "src/__support/macros/optimization.h"
#include "src/__support/math_extras.h"

#undef LIBC_TARGET_IS_BIG_ENDIAN
#if !defined(__BYTE_ORDER__) || !defined(__ORDER_LITTLE_ENDIAN__) ||           \
    !defined(__ORDER_BIG_ENDIAN__)
#define LIBC_TARGET_IS_BIG_ENDIAN 0
#else
#define LIBC_TARGET_IS_BIG_ENDIAN (__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)
#endif // /LIBC_TARGET_IS_BIG_ENDIAN

namespace LIBC_NAMESPACE_DECL {

namespace math {

namespace integer_only {

// 1280 + 64 bits of 2/pi, printed using MPFR.
// We also add 8 more bytes to extend to all non-negative exponents.
LIBC_INLINE_VAR constexpr unsigned TWO_OVER_PI_LENGTH = 1280 / 8 + 7;

#if LIBC_TARGET_IS_BIG_ENDIAN
LIBC_INLINE_VAR constexpr uint8_t TWO_OVER_PI[TWO_OVER_PI_LENGTH] = {
    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xA2, 0xF9, 0x83, 0x6E, 0x4E,
    0x44, 0x15, 0x29, 0xFC, 0x27, 0x57, 0xD1, 0xF5, 0x34, 0xDD, 0xC0, 0xDB,
    0x62, 0x95, 0x99, 0x3C, 0x43, 0x90, 0x41, 0xFE, 0x51, 0x63, 0xAB, 0xDE,
    0xBB, 0xC5, 0x61, 0xB7, 0x24, 0x6E, 0x3A, 0x42, 0x4D, 0xD2, 0xE0, 0x06,
    0x49, 0x2E, 0xEA, 0x09, 0xD1, 0x92, 0x1C, 0xFE, 0x1D, 0xEB, 0x1C, 0xB1,
    0x29, 0xA7, 0x3E, 0xE8, 0x82, 0x35, 0xF5, 0x2E, 0xBB, 0x44, 0x84, 0xE9,
    0x9C, 0x70, 0x26, 0xB4, 0x5F, 0x7E, 0x41, 0x39, 0x91, 0xD6, 0x39, 0x83,
    0x53, 0x39, 0xF4, 0x9C, 0x84, 0x5F, 0x8B, 0xBD, 0xF9, 0x28, 0x3B, 0x1F,
    0xF8, 0x97, 0xFF, 0xDE, 0x05, 0x98, 0x0F, 0xEF, 0x2F, 0x11, 0x8B, 0x5A,
    0x0A, 0x6D, 0x1F, 0x6D, 0x36, 0x7E, 0xCF, 0x27, 0xCB, 0x09, 0xB7, 0x4F,
    0x46, 0x3F, 0x66, 0x9E, 0x5F, 0xEA, 0x2D, 0x75, 0x27, 0xBA, 0xC7, 0xEB,
    0xE5, 0xF1, 0x7B, 0x3D, 0x07, 0x39, 0xF7, 0x8A, 0x52, 0x92, 0xEA, 0x6B,
    0xFB, 0x5F, 0xB1, 0x1F, 0x8D, 0x5D, 0x08, 0x56, 0x03, 0x30, 0x46, 0xFC,
    0x7B, 0x6B, 0xAB, 0xF0, 0xCF, 0xBC, 0x20, 0x9A, 0xF4, 0x36, 0x1D,
};
#else  // !LIBC_TARGET_IS_BIG_ENDIAN
LIBC_INLINE_VAR constexpr uint8_t TWO_OVER_PI[TWO_OVER_PI_LENGTH] = {
    0x1D, 0x36, 0xF4, 0x9A, 0x20, 0xBC, 0xCF, 0xF0, 0xAB, 0x6B, 0x7B, 0xFC,
    0x46, 0x30, 0x03, 0x56, 0x08, 0x5D, 0x8D, 0x1F, 0xB1, 0x5F, 0xFB, 0x6B,
    0xEA, 0x92, 0x52, 0x8A, 0xF7, 0x39, 0x07, 0x3D, 0x7B, 0xF1, 0xE5, 0xEB,
    0xC7, 0xBA, 0x27, 0x75, 0x2D, 0xEA, 0x5F, 0x9E, 0x66, 0x3F, 0x46, 0x4F,
    0xB7, 0x09, 0xCB, 0x27, 0xCF, 0x7E, 0x36, 0x6D, 0x1F, 0x6D, 0x0A, 0x5A,
    0x8B, 0x11, 0x2F, 0xEF, 0x0F, 0x98, 0x05, 0xDE, 0xFF, 0x97, 0xF8, 0x1F,
    0x3B, 0x28, 0xF9, 0xBD, 0x8B, 0x5F, 0x84, 0x9C, 0xF4, 0x39, 0x53, 0x83,
    0x39, 0xD6, 0x91, 0x39, 0x41, 0x7E, 0x5F, 0xB4, 0x26, 0x70, 0x9C, 0xE9,
    0x84, 0x44, 0xBB, 0x2E, 0xF5, 0x35, 0x82, 0xE8, 0x3E, 0xA7, 0x29, 0xB1,
    0x1C, 0xEB, 0x1D, 0xFE, 0x1C, 0x92, 0xD1, 0x09, 0xEA, 0x2E, 0x49, 0x06,
    0xE0, 0xD2, 0x4D, 0x42, 0x3A, 0x6E, 0x24, 0xB7, 0x61, 0xC5, 0xBB, 0xDE,
    0xAB, 0x63, 0x51, 0xFE, 0x41, 0x90, 0x43, 0x3C, 0x99, 0x95, 0x62, 0xDB,
    0xC0, 0xDD, 0x34, 0xF5, 0xD1, 0x57, 0x27, 0xFC, 0x29, 0x15, 0x44, 0x4E,
    0x6E, 0x83, 0xF9, 0xA2, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
};
#endif // LIBC_TARGET_IS_BIG_ENDIAN

LIBC_INLINE_VAR constexpr Frac128 PI_OVER_2_M1({0x898c'c517'01b8'39a2,
                                                0x921f'b544'42d1'8469});

// Perform range reduction mod pi/2
//
// Inputs:
//   x_u: explicit mantissa
//   x_e: biased exponent
// Output:
//   k     : round(x * 2/pi) mod 4
//   x_frac: |x - k * pi/2|
// Return:
//   x_frac_is_neg.
LIBC_INLINE bool trig_range_reduction(uint64_t x_u, unsigned x_e, unsigned &k,
                                      Frac128 &x_frac) {
  using FPBits = typename fputil::FPBits<double>;
  bool x_frac_is_neg = false;
  // We do multiplication x * (2/pi)
  // Let T[i] be the i'th byte of 2/pi expansion:
  // Then 2/pi = T[0] * 2^-8 + T[1] * 2^-16 + ...
  //           = sum_i T[i] * 2^(-8(i + 1))
  // To be able to drop all T[j] * 2^(-8(j + 1)) for small j < i, we will want
  //   ulp(x) * lsb(T[i - 1] * 2^(-8 * i)) >= 4 = 2^2  (since 4 * pi/2 = 2*pi)
  // So:
  //   2^(e - 52) * 2^(-8 * i) >= 2^2
  // Or equivalently,
  //   e - 54 - 8*i >= 0.
  // Define:
  //   i = floor( (e - 54)/8 ),
  // and let
  //   s = e - 54 - 8i >= 0.
  // Since we store the mantissa of x, which is 53 bits long in a 64 bit
  // integer, we have some wiggle room to shuffle the lsb of x.
  // By shifting mantissa of x_u to the left by s, the lsb of x_u will be:
  //   2^(e - 52 - s), for which, the product of lsb's is now exactly 4
  //   lsb(x_u) * 2^(-8 * i)) = 4.
  // This will allow us to compute the full product:
  //   x_u * (T[i] * 2^(-8(i + 1)) + ... ) in exact fixed point.
  // From the formula of i, in order for i >= 0, e >= 54.  To support all the
  // exponents e >= 0, we could add ceil(54 / 8) = 7 0x00 bytes and shift the
  // index by 7.
  unsigned e_num = x_e - FPBits::EXP_BIAS + 2; // e - 54 + 7*8
  // With
  //   i = floor( (e - 54) / 8 ),
  // the shifted-by-7 index is:
  //   j = i + 7 = floor( (e - 54) / 8 ) + 7
  // Since the 64-bit integer chunk will be form by T[j] ... T[j + 7],
  // and we store the table in the little-endian form, we will index to the
  // lowest part of the 64-bit integer chunk, which is:
  //   idx = the index of the T[j + 7] part.
  unsigned j = e_num >> 3;
  unsigned idx =
      LIBC_TARGET_IS_BIG_ENDIAN ? j : (TWO_OVER_PI_LENGTH - 1 - j - 7);
  unsigned shift = e_num & 7; // s = e - 54 - 8*i
  x_u <<= shift;              // lsb(x_u) = 2^(e - 52 - s)
  UInt<64> x_u64(x_u);
  // Gather parts
#if LIBC_TARGET_IS_BIG_ENDIAN
  auto get_uint64 = [](const uint8_t *ptr) -> uint64_t {
    return ptr[7] | (uint64_t(ptr[6]) << 8) | (uint64_t(ptr[5]) << 16) |
           (uint64_t(ptr[4]) << 24) | (uint64_t(ptr[3]) << 32) |
           (uint64_t(ptr[2]) << 40) | (uint64_t(ptr[1]) << 48) |
           (uint64_t(ptr[0]) << 56);
  };
#else  // !LIBC_TARGET_IS_BIG_ENDIAN
  auto get_uint64 = [](const uint8_t *ptr) -> uint64_t {
    return ptr[0] | (uint64_t(ptr[1]) << 8) | (uint64_t(ptr[2]) << 16) |
           (uint64_t(ptr[3]) << 24) | (uint64_t(ptr[4]) << 32) |
           (uint64_t(ptr[5]) << 40) | (uint64_t(ptr[6]) << 48) |
           (uint64_t(ptr[7]) << 56);
  };
#endif // LIBC_TARGET_IS_BIG_ENDIAN
  // lsb(c0) = 2^(-8i - 64)
  uint64_t c0 = get_uint64(&TWO_OVER_PI[idx]);
  // lsb(p0) = lsb(x_u) * lsb(c0)
  //         = 2^(e - 52 - s) * 2^(-8i - 64)
  //         = 2^(-62)
  // msb(p0) = 2^(-62 + 63) = 2^1.
  uint64_t p0 = x_u * c0;
  // lsb(c1) = lsb(c0) * 2^-64 = 2^(-8i - 128)
  // lsb(c2) = lsb(c1) * 2^-64 = 2^(-8i - 192)
#if LIBC_TARGET_IS_BIG_ENDIAN
  UInt<64> c1(get_uint64(&TWO_OVER_PI[idx + 8]));
  UInt<64> c2(get_uint64(&TWO_OVER_PI[idx + 16]));
#else  // !LIBC_TARGET_IS_BIG_ENDIAN
  UInt<64> c1(get_uint64(&TWO_OVER_PI[idx - 8]));
  UInt<64> c2(get_uint64(&TWO_OVER_PI[idx - 16]));
#endif // LIBC_TARGET_IS_BIG_ENDIAN
  // lsb(p1) = lsb(x_u) * lsb(c1) = 2^(-62 - 64) = 2^-126
  UInt<128> p1 = x_u64.ful_mul(c1);
  // lsb(p2) = lsb(x_u) * lsb(c2) * 2^64 = 2^-126
  UInt<128> p2(x_u64.quick_mul_hi(c2));
  UInt<128> sum = p1 + p2;
  sum.val[1] += p0;
  // Get the highest 2 bits.
  k = static_cast<unsigned>(sum.val[1] >> 62);
  bool round_bit = sum.val[1] & 0x2000'0000'0000'0000;
  // Shift so that the leading bit is 0.5.
  sum <<= 2;
  x_frac = Frac128(sum.val);
  // Round to nearest k.
  if (round_bit) {
    // Flip the sign.
    x_frac_is_neg = true;
    ++k;
    // Fast approximation of `1 - x_frac` with error = -lsb(x_frac) = -2^-128.
    // Since in 2-complement, -x = ~x + lsb(x).
    x_frac = ~x_frac;
  }

  // Perform multiplication x_frac * pi/2
  x_frac = fputil::multiply_add(x_frac, PI_OVER_2_M1, x_frac);

  return x_frac_is_neg;
}

// 128-bit fixed-point minimax polynomial approximation of sin(x) generated by
// Sollya with:
// > P = fpminimax(sin(x), [|1, 3, 5, 7, 9, 11, 13|], [|1, 128...|],
//                 [0, pi/4], fixed);
// > dirtyinfnorm( (sin(x) - P(x))/sin(x), [0, pi/4]);
// 0x1.17a4...p-58
// Storing absolute values of the coefficients.
LIBC_INLINE_VAR constexpr Frac128 SIN_COEFF[] = {
    Frac128({0x91b3'96a3'd5c5'fd6a, 0x2aaa'aaaa'aaaa'8ff2}), // x^3
    Frac128({0x321f'bc0b'b8ca'f059, 0x0222'2222'221e'eac3}), // x^5
    Frac128({0x36aa'355c'3311'996d, 0x000d'00d0'0cdf'8c9b}), // x^7
    Frac128({0x0556'929e'ad60'7cb2, 0x0000'2e3b'c6ab'd75e}), // x^9
    Frac128({0xa260'c74f'239d'd891, 0x0000'006b'9795'15a2}), // x^11
    Frac128({0x4c97'758e'92ac'214c, 0x0000'0000'aec7'1a39}), // x^13
};
// 128-bit fixed-point minimax polynomial approximation of cos(x) generated by
// Sollya with:
// > P = fpminimax(cos(x), [|0, 2, 4, 6, 8, 10, 12|], [|1, 128...|],
//                 [0, pi/4], fixed);
// > dirtyinfnorm( (cos(x) - P(x))/cos(x), [0, pi/4]);
// 0x1.269f...p-54
// Storing absolute values of the coefficients.
LIBC_INLINE_VAR constexpr Frac128 COS_COEFF[] = {
    Frac128({0x56f6'2e74'b16e'5555, 0x7fff'ffff'fffe'4bfe}), // x^2
    Frac128({0x860a'3e6c'cc50'e0d8, 0x0aaa'aaaa'aa77'5c33}), // x^4
    Frac128({0xa87a'8f81'7440'7dd6, 0x005b'05b0'58fc'6fed}), // x^6
    Frac128({0x84b2'76a3'c971'e7b8, 0x0001'a019'f80a'8ad5}), // x^8
    Frac128({0x0082'310d'4e65'6b1f, 0x0000'049f'7cff'73d2}), // x^10
    Frac128({0xed56'891e'f750'c7a9, 0x0000'0008'dc50'133d}), // x^12
};

// Compute sin(x) with relative errors ~ 2^-54.
LIBC_INLINE double sin_eval(const Frac128 &x_frac, unsigned k, bool is_neg,
                            bool x_frac_is_neg) {
  // cos when k = 1, 3
  bool is_cos = ((k & 1) == 1);
  // flip sign when k = 2, 3
  is_neg = is_neg != ((k & 2) == 2);

  const Frac128 *coeffs = is_cos ? COS_COEFF : SIN_COEFF;

  Frac128 xsq = x_frac * x_frac;
  // Calculating the alternating polynommial
  // p = x^2 * (C[0] - x^2 C[1] + x^4 C[2] - ...)
  Frac128 p = xsq * fputil::altpolyeval(xsq, coeffs[0], coeffs[1], coeffs[2],
                                        coeffs[3], coeffs[4], coeffs[5]);
  // r ~ 1 - p
  Frac128 r = ~p;
  if (!is_cos) {
    // sin(x) = x * r.
    is_neg = (is_neg != x_frac_is_neg);
    r *= x_frac;
  }

  // Worst-case for range reduction > 2^-61, so the top 64-bits should be
  // non-zero for non-zero output.
  if (r.val[1] == 0)
    return 0.0;

  unsigned n = cpp::countl_zero(r.val[1]);
  uint64_t result = r.val[1];
  if (n > 0) {
    result <<= n;
    result |= (r.val[0] >> (64 - n));
  }
  unsigned rounding = ((static_cast<unsigned>(result) & 0x400) > 0);
  result >>= 11;
  result += (static_cast<uint64_t>(1021 - n) << 52) + rounding;
  result |= (static_cast<uint64_t>(is_neg) << 63);

  return cpp::bit_cast<double>(result);
}

} // namespace integer_only

} // namespace math

} // namespace LIBC_NAMESPACE_DECL

#endif // LLVM_LIBC_SRC___SUPPORT_MATH_SINCOS_INTEGER_UTILS_H