| //===-- Double-precision 2^x function -------------------------------------===// |
| // |
| // 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 |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "src/math/exp2.h" |
| #include "common_constants.h" // Lookup tables EXP2_MID1 and EXP_M2. |
| #include "explogxf.h" // ziv_test_denorm. |
| #include "src/__support/CPP/bit.h" |
| #include "src/__support/CPP/optional.h" |
| #include "src/__support/FPUtil/FEnvImpl.h" |
| #include "src/__support/FPUtil/FPBits.h" |
| #include "src/__support/FPUtil/PolyEval.h" |
| #include "src/__support/FPUtil/double_double.h" |
| #include "src/__support/FPUtil/dyadic_float.h" |
| #include "src/__support/FPUtil/multiply_add.h" |
| #include "src/__support/FPUtil/nearest_integer.h" |
| #include "src/__support/FPUtil/rounding_mode.h" |
| #include "src/__support/FPUtil/triple_double.h" |
| #include "src/__support/common.h" |
| #include "src/__support/macros/optimization.h" // LIBC_UNLIKELY |
| |
| #include <errno.h> |
| |
| namespace LIBC_NAMESPACE { |
| |
| using fputil::DoubleDouble; |
| using fputil::TripleDouble; |
| using Float128 = typename fputil::DyadicFloat<128>; |
| |
| // Error bounds: |
| // Errors when using double precision. |
| #ifdef LIBC_TARGET_CPU_HAS_FMA |
| constexpr double ERR_D = 0x1.0p-63; |
| #else |
| constexpr double ERR_D = 0x1.8p-63; |
| #endif // LIBC_TARGET_CPU_HAS_FMA |
| |
| // Errors when using double-double precision. |
| constexpr double ERR_DD = 0x1.0p-100; |
| |
| namespace { |
| |
| // Polynomial approximations with double precision. Generated by Sollya with: |
| // > P = fpminimax((2^x - 1)/x, 3, [|D...|], [-2^-13 - 2^-30, 2^-13 + 2^-30]); |
| // > P; |
| // Error bounds: |
| // | output - (2^dx - 1) / dx | < 1.5 * 2^-52. |
| LIBC_INLINE double poly_approx_d(double dx) { |
| // dx^2 |
| double dx2 = dx * dx; |
| double c0 = |
| fputil::multiply_add(dx, 0x1.ebfbdff82c58ep-3, 0x1.62e42fefa39efp-1); |
| double c1 = |
| fputil::multiply_add(dx, 0x1.3b2aba7a95a89p-7, 0x1.c6b08e8fc0c0ep-5); |
| double p = fputil::multiply_add(dx2, c1, c0); |
| return p; |
| } |
| |
| // Polynomial approximation with double-double precision. Generated by Solya |
| // with: |
| // > P = fpminimax((2^x - 1)/x, 5, [|DD...|], [-2^-13 - 2^-30, 2^-13 + 2^-30]); |
| // Error bounds: |
| // | output - 2^(dx) | < 2^-101 |
| DoubleDouble poly_approx_dd(const DoubleDouble &dx) { |
| // Taylor polynomial. |
| constexpr DoubleDouble COEFFS[] = { |
| {0, 0x1p0}, |
| {0x1.abc9e3b39824p-56, 0x1.62e42fefa39efp-1}, |
| {-0x1.5e43a53e4527bp-57, 0x1.ebfbdff82c58fp-3}, |
| {-0x1.d37963a9444eep-59, 0x1.c6b08d704a0cp-5}, |
| {0x1.4eda1a81133dap-62, 0x1.3b2ab6fba4e77p-7}, |
| {-0x1.c53fd1ba85d14p-64, 0x1.5d87fe7a265a5p-10}, |
| {0x1.d89250b013eb8p-70, 0x1.430912f86cb8ep-13}, |
| }; |
| |
| DoubleDouble p = fputil::polyeval(dx, COEFFS[0], COEFFS[1], COEFFS[2], |
| COEFFS[3], COEFFS[4], COEFFS[5], COEFFS[6]); |
| return p; |
| } |
| |
| // Polynomial approximation with 128-bit precision: |
| // Return exp(dx) ~ 1 + a0 * dx + a1 * dx^2 + ... + a6 * dx^7 |
| // For |dx| < 2^-13 + 2^-30: |
| // | output - exp(dx) | < 2^-126. |
| Float128 poly_approx_f128(const Float128 &dx) { |
| using MType = typename Float128::MantissaType; |
| |
| constexpr Float128 COEFFS_128[]{ |
| {false, -127, MType({0, 0x8000000000000000})}, // 1.0 |
| {false, -128, MType({0xc9e3b39803f2f6af, 0xb17217f7d1cf79ab})}, |
| {false, -128, MType({0xde2d60dd9c9a1d9f, 0x3d7f7bff058b1d50})}, |
| {false, -132, MType({0x9d3b15d9e7fb6897, 0xe35846b82505fc59})}, |
| {false, -134, MType({0x184462f6bcd2b9e7, 0x9d955b7dd273b94e})}, |
| {false, -137, MType({0x39ea1bb964c51a89, 0xaec3ff3c53398883})}, |
| {false, -138, MType({0x842c53418fa8ae61, 0x2861225f345c396a})}, |
| {false, -144, MType({0x7abeb5abd5ad2079, 0xffe5fe2d109a319d})}, |
| }; |
| |
| Float128 p = fputil::polyeval(dx, COEFFS_128[0], COEFFS_128[1], COEFFS_128[2], |
| COEFFS_128[3], COEFFS_128[4], COEFFS_128[5], |
| COEFFS_128[6], COEFFS_128[7]); |
| return p; |
| } |
| |
| // Compute 2^(x) using 128-bit precision. |
| // TODO(lntue): investigate triple-double precision implementation for this |
| // step. |
| Float128 exp2_f128(double x, int hi, int idx1, int idx2) { |
| Float128 dx = Float128(x); |
| |
| // TODO: Skip recalculating exp_mid1 and exp_mid2. |
| Float128 exp_mid1 = |
| fputil::quick_add(Float128(EXP2_MID1[idx1].hi), |
| fputil::quick_add(Float128(EXP2_MID1[idx1].mid), |
| Float128(EXP2_MID1[idx1].lo))); |
| |
| Float128 exp_mid2 = |
| fputil::quick_add(Float128(EXP2_MID2[idx2].hi), |
| fputil::quick_add(Float128(EXP2_MID2[idx2].mid), |
| Float128(EXP2_MID2[idx2].lo))); |
| |
| Float128 exp_mid = fputil::quick_mul(exp_mid1, exp_mid2); |
| |
| Float128 p = poly_approx_f128(dx); |
| |
| Float128 r = fputil::quick_mul(exp_mid, p); |
| |
| r.exponent += hi; |
| |
| return r; |
| } |
| |
| // Compute 2^x with double-double precision. |
| DoubleDouble exp2_double_double(double x, const DoubleDouble &exp_mid) { |
| DoubleDouble dx({0, x}); |
| |
| // Degree-6 polynomial approximation in double-double precision. |
| // | p - 2^x | < 2^-103. |
| DoubleDouble p = poly_approx_dd(dx); |
| |
| // Error bounds: 2^-102. |
| DoubleDouble r = fputil::quick_mult(exp_mid, p); |
| |
| return r; |
| } |
| |
| // When output is denormal. |
| double exp2_denorm(double x) { |
| // Range reduction. |
| int k = |
| static_cast<int>(cpp::bit_cast<uint64_t>(x + 0x1.8000'0000'4p21) >> 19); |
| double kd = static_cast<double>(k); |
| |
| uint32_t idx1 = (k >> 6) & 0x3f; |
| uint32_t idx2 = k & 0x3f; |
| |
| int hi = k >> 12; |
| |
| DoubleDouble exp_mid1{EXP2_MID1[idx1].mid, EXP2_MID1[idx1].hi}; |
| DoubleDouble exp_mid2{EXP2_MID2[idx2].mid, EXP2_MID2[idx2].hi}; |
| DoubleDouble exp_mid = fputil::quick_mult(exp_mid1, exp_mid2); |
| |
| // |dx| < 2^-13 + 2^-30. |
| double dx = fputil::multiply_add(kd, -0x1.0p-12, x); // exact |
| |
| double mid_lo = dx * exp_mid.hi; |
| |
| // Approximate (2^dx - 1)/dx ~ 1 + a0*dx + a1*dx^2 + a2*dx^3 + a3*dx^4. |
| double p = poly_approx_d(dx); |
| |
| double lo = fputil::multiply_add(p, mid_lo, exp_mid.lo); |
| |
| if (auto r = ziv_test_denorm(hi, exp_mid.hi, lo, ERR_D); |
| LIBC_LIKELY(r.has_value())) |
| return r.value(); |
| |
| // Use double-double |
| DoubleDouble r_dd = exp2_double_double(dx, exp_mid); |
| |
| if (auto r = ziv_test_denorm(hi, r_dd.hi, r_dd.lo, ERR_DD); |
| LIBC_LIKELY(r.has_value())) |
| return r.value(); |
| |
| // Use 128-bit precision |
| Float128 r_f128 = exp2_f128(dx, hi, idx1, idx2); |
| |
| return static_cast<double>(r_f128); |
| } |
| |
| // Check for exceptional cases when: |
| // * log2(1 - 2^-54) < x < log2(1 + 2^-53) |
| // * x >= 1024 |
| // * x <= -1022 |
| // * x is inf or nan |
| double set_exceptional(double x) { |
| using FPBits = typename fputil::FPBits<double>; |
| FPBits xbits(x); |
| |
| uint64_t x_u = xbits.uintval(); |
| uint64_t x_abs = xbits.abs().uintval(); |
| |
| // |x| < log2(1 + 2^-53) |
| if (x_abs <= 0x3ca71547652b82fd) { |
| // 2^(x) ~ 1 + x/2 |
| return fputil::multiply_add(x, 0.5, 1.0); |
| } |
| |
| // x <= -1022 || x >= 1024 or inf/nan. |
| if (x_u > 0xc08ff00000000000) { |
| // x <= -1075 or -inf/nan |
| if (x_u >= 0xc090cc0000000000) { |
| // exp(-Inf) = 0 |
| if (xbits.is_inf()) |
| return 0.0; |
| |
| // exp(nan) = nan |
| if (xbits.is_nan()) |
| return x; |
| |
| if (fputil::quick_get_round() == FE_UPWARD) |
| return FPBits::min_denormal(); |
| fputil::set_errno_if_required(ERANGE); |
| fputil::raise_except_if_required(FE_UNDERFLOW); |
| return 0.0; |
| } |
| |
| return exp2_denorm(x); |
| } |
| |
| // x >= 1024 or +inf/nan |
| // x is finite |
| if (x_u < 0x7ff0'0000'0000'0000ULL) { |
| int rounding = fputil::quick_get_round(); |
| if (rounding == FE_DOWNWARD || rounding == FE_TOWARDZERO) |
| return FPBits::max_normal(); |
| |
| fputil::set_errno_if_required(ERANGE); |
| fputil::raise_except_if_required(FE_OVERFLOW); |
| } |
| // x is +inf or nan |
| return x + static_cast<double>(FPBits::inf()); |
| } |
| |
| } // namespace |
| |
| LLVM_LIBC_FUNCTION(double, exp2, (double x)) { |
| using FPBits = typename fputil::FPBits<double>; |
| FPBits xbits(x); |
| |
| uint64_t x_u = xbits.uintval(); |
| |
| // x < -1022 or x >= 1024 or log2(1 - 2^-54) < x < log2(1 + 2^-53). |
| if (LIBC_UNLIKELY(x_u > 0xc08ff00000000000 || |
| (x_u <= 0xbc971547652b82fe && x_u >= 0x4090000000000000) || |
| x_u <= 0x3ca71547652b82fd)) { |
| return set_exceptional(x); |
| } |
| |
| // Now -1075 < x <= log2(1 - 2^-54) or log2(1 + 2^-53) < x < 1024 |
| |
| // Range reduction: |
| // Let x = (hi + mid1 + mid2) + lo |
| // in which: |
| // hi is an integer |
| // mid1 * 2^6 is an integer |
| // mid2 * 2^12 is an integer |
| // then: |
| // 2^(x) = 2^hi * 2^(mid1) * 2^(mid2) * 2^(lo). |
| // With this formula: |
| // - multiplying by 2^hi is exact and cheap, simply by adding the exponent |
| // field. |
| // - 2^(mid1) and 2^(mid2) are stored in 2 x 64-element tables. |
| // - 2^(lo) ~ 1 + a0*lo + a1 * lo^2 + ... |
| // |
| // We compute (hi + mid1 + mid2) together by perform the rounding on x * 2^12. |
| // Since |x| < |-1075)| < 2^11, |
| // |x * 2^12| < 2^11 * 2^12 < 2^23, |
| // So we can fit the rounded result round(x * 2^12) in int32_t. |
| // Thus, the goal is to be able to use an additional addition and fixed width |
| // shift to get an int32_t representing round(x * 2^12). |
| // |
| // Assuming int32_t using 2-complement representation, since the mantissa part |
| // of a double precision is unsigned with the leading bit hidden, if we add an |
| // extra constant C = 2^e1 + 2^e2 with e1 > e2 >= 2^25 to the product, the |
| // part that are < 2^e2 in resulted mantissa of (x*2^12*L2E + C) can be |
| // considered as a proper 2-complement representations of x*2^12. |
| // |
| // One small problem with this approach is that the sum (x*2^12 + C) in |
| // double precision is rounded to the least significant bit of the dorminant |
| // factor C. In order to minimize the rounding errors from this addition, we |
| // want to minimize e1. Another constraint that we want is that after |
| // shifting the mantissa so that the least significant bit of int32_t |
| // corresponds to the unit bit of (x*2^12*L2E), the sign is correct without |
| // any adjustment. So combining these 2 requirements, we can choose |
| // C = 2^33 + 2^32, so that the sign bit corresponds to 2^31 bit, and hence |
| // after right shifting the mantissa, the resulting int32_t has correct sign. |
| // With this choice of C, the number of mantissa bits we need to shift to the |
| // right is: 52 - 33 = 19. |
| // |
| // Moreover, since the integer right shifts are equivalent to rounding down, |
| // we can add an extra 0.5 so that it will become round-to-nearest, tie-to- |
| // +infinity. So in particular, we can compute: |
| // hmm = x * 2^12 + C, |
| // where C = 2^33 + 2^32 + 2^-1, then if |
| // k = int32_t(lower 51 bits of double(x * 2^12 + C) >> 19), |
| // the reduced argument: |
| // lo = x - 2^-12 * k is bounded by: |
| // |lo| <= 2^-13 + 2^-12*2^-19 |
| // = 2^-13 + 2^-31. |
| // |
| // Finally, notice that k only uses the mantissa of x * 2^12, so the |
| // exponent 2^12 is not needed. So we can simply define |
| // C = 2^(33 - 12) + 2^(32 - 12) + 2^(-13 - 12), and |
| // k = int32_t(lower 51 bits of double(x + C) >> 19). |
| |
| // Rounding errors <= 2^-31. |
| int k = |
| static_cast<int>(cpp::bit_cast<uint64_t>(x + 0x1.8000'0000'4p21) >> 19); |
| double kd = static_cast<double>(k); |
| |
| uint32_t idx1 = (k >> 6) & 0x3f; |
| uint32_t idx2 = k & 0x3f; |
| |
| int hi = k >> 12; |
| |
| DoubleDouble exp_mid1{EXP2_MID1[idx1].mid, EXP2_MID1[idx1].hi}; |
| DoubleDouble exp_mid2{EXP2_MID2[idx2].mid, EXP2_MID2[idx2].hi}; |
| DoubleDouble exp_mid = fputil::quick_mult(exp_mid1, exp_mid2); |
| |
| // |dx| < 2^-13 + 2^-30. |
| double dx = fputil::multiply_add(kd, -0x1.0p-12, x); // exact |
| |
| // We use the degree-4 polynomial to approximate 2^(lo): |
| // 2^(lo) ~ 1 + a0 * lo + a1 * lo^2 + a2 * lo^3 + a3 * lo^4 = 1 + lo * P(lo) |
| // So that the errors are bounded by: |
| // |P(lo) - (2^lo - 1)/lo| < |lo|^4 / 64 < 2^(-13 * 4) / 64 = 2^-58 |
| // Let P_ be an evaluation of P where all intermediate computations are in |
| // double precision. Using either Horner's or Estrin's schemes, the evaluated |
| // errors can be bounded by: |
| // |P_(lo) - P(lo)| < 2^-51 |
| // => |lo * P_(lo) - (2^lo - 1) | < 2^-64 |
| // => 2^(mid1 + mid2) * |lo * P_(lo) - expm1(lo)| < 2^-63. |
| // Since we approximate |
| // 2^(mid1 + mid2) ~ exp_mid.hi + exp_mid.lo, |
| // We use the expression: |
| // (exp_mid.hi + exp_mid.lo) * (1 + dx * P_(dx)) ~ |
| // ~ exp_mid.hi + (exp_mid.hi * dx * P_(dx) + exp_mid.lo) |
| // with errors bounded by 2^-63. |
| |
| double mid_lo = dx * exp_mid.hi; |
| |
| // Approximate (2^dx - 1)/dx ~ 1 + a0*dx + a1*dx^2 + a2*dx^3 + a3*dx^4. |
| double p = poly_approx_d(dx); |
| |
| double lo = fputil::multiply_add(p, mid_lo, exp_mid.lo); |
| |
| double upper = exp_mid.hi + (lo + ERR_D); |
| double lower = exp_mid.hi + (lo - ERR_D); |
| |
| if (LIBC_LIKELY(upper == lower)) { |
| // To multiply by 2^hi, a fast way is to simply add hi to the exponent |
| // field. |
| int64_t exp_hi = static_cast<int64_t>(hi) << FPBits::FRACTION_LEN; |
| double r = cpp::bit_cast<double>(exp_hi + cpp::bit_cast<int64_t>(upper)); |
| return r; |
| } |
| |
| // Use double-double |
| DoubleDouble r_dd = exp2_double_double(dx, exp_mid); |
| |
| double upper_dd = r_dd.hi + (r_dd.lo + ERR_DD); |
| double lower_dd = r_dd.hi + (r_dd.lo - ERR_DD); |
| |
| if (LIBC_LIKELY(upper_dd == lower_dd)) { |
| // To multiply by 2^hi, a fast way is to simply add hi to the exponent |
| // field. |
| int64_t exp_hi = static_cast<int64_t>(hi) << FPBits::FRACTION_LEN; |
| double r = cpp::bit_cast<double>(exp_hi + cpp::bit_cast<int64_t>(upper_dd)); |
| return r; |
| } |
| |
| // Use 128-bit precision |
| Float128 r_f128 = exp2_f128(dx, hi, idx1, idx2); |
| |
| return static_cast<double>(r_f128); |
| } |
| |
| } // namespace LIBC_NAMESPACE |