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[libc][math] Add float-only implementation for atanf. (#167004)

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Algorithm:
```
  1)  atan(x) = sign(x) * atan(|x|)

  2)  If |x| > 1 + 1/32, atan(|x|) = pi/2 - atan(1/|x|)

  3)  For 1/16 < |x| < 1 + 1/32, we find k such that: | |x| - k/16 | <= 1/32.
      Let y = |x| - k/16, then using the angle summation formula, we have:
    atan(|x|) = atan(k/16) + atan( (|x| - k/16) / (1 + |x| * k/16) )
              = atan(k/16) + atan( y / (1 + (y + k/16) * k/16 )
              = atan(k/16) + atan( y / ((1 + k^2/256) + y * k/16) )

  4)  Let u = y / (1 + k^2/256), then we can rewritten the above as:
    atan(|x|) = atan(k/16) + atan( u / (1 + u * k/16) )
              ~ atan(k/16) + (u - k/16 * u^2 + (k^2/256 - 1/3) * u^3 +
                              + (k/16 - (k/16)^3) * u^4) + O(u^5)
```

With all the computations are done in single precision (float), the
total of approximation errors and rounding errors is bounded by 4 ULPs.
This commit is contained in:
lntue 2025-11-20 09:42:13 -05:00 committed by GitHub
parent a74bfc06e8
commit aa3f930931
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GPG Key ID: B5690EEEBB952194
6 changed files with 245 additions and 3 deletions
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1
libc/src/__support/math/CMakeLists.txt
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@ -248,6 +248,7 @@ add_header_library(
add_header_library(
atanf
HDRS
atanf_float.h
atanf.h
DEPENDS
.inv_trigf_utils

10
libc/src/__support/math/atanf.h
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@ -18,6 +18,14 @@
#include "src/__support/macros/config.h"
#include "src/__support/macros/optimization.h" // LIBC_UNLIKELY
#if defined(LIBC_MATH_HAS_SKIP_ACCURATE_PASS) && \
defined(LIBC_MATH_HAS_INTERMEDIATE_COMP_IN_FLOAT)
// We use float-float implementation to reduce size.
#include "atanf_float.h"
#else
namespace LIBC_NAMESPACE_DECL {
namespace math {
@ -126,4 +134,6 @@ LIBC_INLINE static constexpr float atanf(float x) {
} // namespace LIBC_NAMESPACE_DECL
#endif // LIBC_MATH_HAS_SKIP_ACCURATE_PASS
#endif // LLVM_LIBC_SRC___SUPPORT_MATH_ATANF_H

168
libc/src/__support/math/atanf_float.h Normal file
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@ -0,0 +1,168 @@
//===-- Single-precision atanf float 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
//
//===----------------------------------------------------------------------===//
#ifndef LIBC_SRC___SUPPORT_MATH_ATANF_FLOAT_H
#define LIBC_SRC___SUPPORT_MATH_ATANF_FLOAT_H
#include "src/__support/FPUtil/FEnvImpl.h"
#include "src/__support/FPUtil/FPBits.h"
#include "src/__support/FPUtil/double_double.h"
#include "src/__support/FPUtil/multiply_add.h"
#include "src/__support/FPUtil/nearest_integer.h"
#include "src/__support/macros/config.h"
#include "src/__support/macros/optimization.h" // LIBC_UNLIKELY
namespace LIBC_NAMESPACE_DECL {
namespace math {
namespace atanf_internal {
using fputil::FloatFloat;
// atan(i/64) with i = 0..16, generated by Sollya with:
// > for i from 0 to 16 do {
// a = round(atan(i/16), SG, RN);
// b = round(atan(i/16) - a, SG, RN);
// print("{", b, ",", a, "},");
// };
static constexpr FloatFloat ATAN_I[17] = {
{0.0f, 0.0f},
{-0x1.1a6042p-30f, 0x1.ff55bcp-5f},
{-0x1.54f424p-30f, 0x1.fd5baap-4f},
{0x1.79cb6p-28f, 0x1.7b97b4p-3f},
{-0x1.b4dfc8p-29f, 0x1.f5b76p-3f},
{-0x1.1f0286p-27f, 0x1.362774p-2f},
{0x1.e4defp-30f, 0x1.6f6194p-2f},
{0x1.e611fep-29f, 0x1.a64eecp-2f},
{0x1.586ed4p-28f, 0x1.dac67p-2f},
{-0x1.6499e6p-26f, 0x1.0657eap-1f},
{0x1.7bdfd6p-26f, 0x1.1e00bap-1f},
{-0x1.98e422p-28f, 0x1.345f02p-1f},
{0x1.934f7p-28f, 0x1.4978fap-1f},
{0x1.c5a6c6p-27f, 0x1.5d5898p-1f},
{0x1.5e118cp-27f, 0x1.700a7cp-1f},
{-0x1.1d4eb6p-26f, 0x1.819d0cp-1f},
{-0x1.777a5cp-26f, 0x1.921fb6p-1f},
};
// 1 / (1 + (i/16)^2) with i = 0..16, generated by Sollya with:
// > for i from 0 to 16 do {
// a = round(1 / (1 + (i/16)^2), SG, RN);
// print(a, ",");
// };
static constexpr float ATANF_REDUCED_ARG[17] = {
0x1.0p0f, 0x1.fe01fep-1f, 0x1.f81f82p-1f, 0x1.ee9c8p-1f,
0x1.e1e1e2p-1f, 0x1.d272cap-1f, 0x1.c0e07p-1f, 0x1.adbe88p-1f,
0x1.99999ap-1f, 0x1.84f00cp-1f, 0x1.702e06p-1f, 0x1.5babccp-1f,
0x1.47ae14p-1f, 0x1.34679ap-1f, 0x1.21fb78p-1f, 0x1.107fbcp-1f,
0x1p-1f,
};
// Approximating atan( u / (1 + u * k/16) )
// atan( u / (1 + u * k/16) ) / u ~ 1 - k/16 * u + (k^2/256 - 1/3) * u^2 +
// + (k/16 - (k/16)^3) * u^3 + O(u^4)
LIBC_INLINE static float atanf_eval(float u, float k_over_16) {
// (k/16)^2
float c2 = k_over_16 * k_over_16;
// -(k/16)^3
float c3 = fputil::multiply_add(-k_over_16, c2, k_over_16);
float u2 = u * u;
// (k^2/256 - 1/3) + u * (k/16 - (k/16)^3)
float a0 = fputil::multiply_add(c3, u, c2 - 0x1.555556p-2f);
// -k/16 + u*(k^2/256 - 1/3) + u^2 * (k/16 - (k/16)^3)
float a1 = fputil::multiply_add(u, a0, -k_over_16);
// u - u^2 * k/16 + u^3 * ((k^2/256 - 1/3) + u^4 * (k/16 - (k/16)^3))
return fputil::multiply_add(u2, a1, u);
}
} // namespace atanf_internal
// There are several range reduction steps we can take for atan2(y, x) as
// follow:
LIBC_INLINE static float atanf(float x) {
using namespace atanf_internal;
using FPBits = typename fputil::FPBits<float>;
using FPBits = typename fputil::FPBits<float>;
constexpr float SIGN[2] = {1.0f, -1.0f};
constexpr FloatFloat PI_OVER_2 = {-0x1.777a5cp-25f, 0x1.921fb6p0f};
FPBits x_bits(x);
Sign s = x_bits.sign();
float sign = SIGN[s.is_neg()];
uint32_t x_abs = x_bits.uintval() & 0x7fff'ffffU;
// x is inf or nan, |x| <= 2^-11 or |x|= > 2^11.
if (LIBC_UNLIKELY(x_abs <= 0x3a00'0000U || x_abs >= 0x4500'0000U)) {
if (LIBC_UNLIKELY(x_bits.is_inf()))
return sign * PI_OVER_2.hi;
// atan(NaN) = NaN
if (LIBC_UNLIKELY(x_bits.is_nan())) {
if (x_bits.is_signaling_nan()) {
fputil::raise_except_if_required(FE_INVALID);
return FPBits::quiet_nan().get_val();
}
return x;
}
// |x| >= 2^11:
// atan(x) = sign(x) * pi/2 - atan(1/x)
// ~ sign(x) * pi/2 - 1/x
if (LIBC_UNLIKELY(x_abs >= 0x4200'0000))
return fputil::multiply_add(sign, PI_OVER_2.hi, -1.0f / x);
// x <= 2^-11:
// atan(x) ~ x
return x;
}
// Range reduction steps:
// 1) atan(x) = sign(x) * atan(|x|)
// 2) If |x| > 1, atan(|x|) = pi/2 - atan(1/|x|)
// 3) For 1/16 < |x| < 1 + 1/32, we find k such that: | |x| - k/16 | <= 1/32.
// Let y = |x| - k/16, then using the angle summation formula, we have:
// atan(|x|) = atan(k/16) + atan( (|x| - k/16) / (1 + |x| * k/16) )
// = atan(k/16) + atan( y / (1 + (y + k/16) * k/16 )
// = atan(k/16) + atan( y / ((1 + k^2/256) + y * k/16) )
// 4) Let u = y / (1 + k^2/256), then we can rewritten the above as:
// atan(|x|) = atan(k/16) + atan( u / (1 + u * k/16) )
// ~ atan(k/16) + (u - k/16 * u^2 + (k^2/256 - 1/3) * u^3 +
// + (k/16 - (k/16)^3) * u^4) + O(u^5)
float x_a = cpp::bit_cast<float>(x_abs);
// |x| > 1 + 1/32, we need to invert x, so we will perform the division in
// float-float.
if (x_abs > 0x3f84'0000U)
x_a = 1.0f / x_a;
// Perform range reduction.
// k = nearestint(x * 16)
float k_f = fputil::nearest_integer(x_a * 0x1.0p4f);
unsigned idx = static_cast<unsigned>(k_f);
float k_over_16 = k_f * 0x1.0p-4f;
float y = x_a - k_over_16;
// u = (x - k/16) / (1 + (k/16)^2)
float u = y * ATANF_REDUCED_ARG[idx];
// atan(x) = sign(x) * atan(|x|)
// = sign(x) * (atan(k/16) + atan(|))
// p ~ atan(u)
float p = atanf_eval(u, k_over_16);
// |x| > 1 + 1/32: q ~ (pi/2 - atan(1/|x|))
// |x| <= 1 + 1/32: q ~ atan(|x|)
float q = (p + ATAN_I[idx].lo) + ATAN_I[idx].hi;
if (x_abs > 0x3f84'0000U)
q = PI_OVER_2.hi + (PI_OVER_2.lo - q);
// |x| > 1 + 1/32: sign(x) * (pi/2 - atan(1/|x|))
// |x| <= 1 + 1/32: sign(x) * atan(|x|)
return sign * q;
}
} // namespace math
} // namespace LIBC_NAMESPACE_DECL
#endif // LIBC_SRC___SUPPORT_MATH_ATANF_FLOAT_H

13
libc/test/src/math/atanf_test.cpp
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@ -9,11 +9,18 @@
#include "hdr/math_macros.h"
#include "hdr/stdint_proxy.h"
#include "src/__support/FPUtil/FPBits.h"
#include "src/__support/macros/optimization.h"
#include "src/math/atanf.h"
#include "test/UnitTest/FPMatcher.h"
#include "test/UnitTest/Test.h"
#include "utils/MPFRWrapper/MPFRUtils.h"
#ifdef LIBC_MATH_HAS_SKIP_ACCURATE_PASS
#define TOLERANCE 4
#else
#define TOLERANCE 0
#endif // LIBC_MATH_HAS_SKIP_ACCURATE_PASS
using LlvmLibcAtanfTest = LIBC_NAMESPACE::testing::FPTest<float>;
namespace mpfr = LIBC_NAMESPACE::testing::mpfr;
@ -49,9 +56,9 @@ TEST_F(LlvmLibcAtanfTest, InFloatRange) {
for (uint32_t i = 0, v = 0; i <= COUNT; ++i, v += STEP) {
float x = FPBits(v).get_val();
EXPECT_MPFR_MATCH_ALL_ROUNDING(mpfr::Operation::Atan, x,
LIBC_NAMESPACE::atanf(x), 0.5);
LIBC_NAMESPACE::atanf(x), TOLERANCE + 0.5);
EXPECT_MPFR_MATCH_ALL_ROUNDING(mpfr::Operation::Atan, -x,
LIBC_NAMESPACE::atanf(-x), 0.5);
LIBC_NAMESPACE::atanf(-x), TOLERANCE + 0.5);
}
}
@ -72,6 +79,6 @@ TEST_F(LlvmLibcAtanfTest, SpecialValues) {
for (uint32_t v : val_arr) {
float x = FPBits(v).get_val();
EXPECT_MPFR_MATCH_ALL_ROUNDING(mpfr::Operation::Atan, x,
LIBC_NAMESPACE::atanf(x), 0.5);
LIBC_NAMESPACE::atanf(x), TOLERANCE + 0.5);
}
}

15
libc/test/src/math/exhaustive/CMakeLists.txt
View File

@ -550,6 +550,21 @@ add_fp_unittest(
-lpthread
)
add_fp_unittest(
atanf_float_test
NO_RUN_POSTBUILD
NEED_MPFR
SUITE
libc_math_exhaustive_tests
SRCS
atanf_float_test.cpp
LINK_LIBRARIES
-lpthread
DEPENDS
.exhaustive_test
libc.src.__support.math.atanf
)
add_fp_unittest(
asinf_test
NO_RUN_POSTBUILD

41
libc/test/src/math/exhaustive/atanf_float_test.cpp Normal file
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@ -0,0 +1,41 @@
//===-- Exhaustive test for atanf - float-only ----------------------------===//
//
// 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 "exhaustive_test.h"
#include "src/__support/math/atanf_float.h"
#include "utils/MPFRWrapper/MPFRUtils.h"
namespace mpfr = LIBC_NAMESPACE::testing::mpfr;
float atanf_fast(float x) { return LIBC_NAMESPACE::math::atanf(x); }
using LlvmLibcAtanfFloatExhaustiveTest =
LlvmLibcUnaryOpExhaustiveMathTest<float, mpfr::Operation::Atan, atanf_fast,
4>;
// Range: [0, Inf];
static constexpr uint32_t POS_START = 0x0000'0000U;
static constexpr uint32_t POS_STOP = 0x7f80'0000U;
TEST_F(LlvmLibcAtanfFloatExhaustiveTest, PostiveRange) {
std::cout << "-- Testing for FE_TONEAREST in range [0x" << std::hex
<< POS_START << ", 0x" << POS_STOP << ") --" << std::dec
<< std::endl;
test_full_range(mpfr::RoundingMode::Nearest, POS_START, POS_STOP);
}
// Range: [-Inf, 0];
static constexpr uint32_t NEG_START = 0x8000'0000U;
static constexpr uint32_t NEG_STOP = 0xff80'0000U;
TEST_F(LlvmLibcAtanfFloatExhaustiveTest, NegativeRange) {
std::cout << "-- Testing for FE_TONEAREST in range [0x" << std::hex
<< NEG_START << ", 0x" << NEG_STOP << ") --" << std::dec
<< std::endl;
test_full_range(mpfr::RoundingMode::Nearest, NEG_START, NEG_STOP);
}