[libc][math] Refactor atan implementation to header-only in src/__support/math folder.

libc/shared/math.h

@@ -22,6 +22,7 @@
 #include "math/asinf16.h"
 #include "math/asinhf.h"
 #include "math/asinhf16.h"
+#include "math/atan.h"
 #include "math/erff.h"
 #include "math/exp.h"
 #include "math/exp10.h"

libc/shared/math/atan.h

@@ -0,0 +1,23 @@
+//===-- Shared atan function ------------------------------------*- 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_SHARED_MATH_ATAN_H
+#define LLVM_LIBC_SHARED_MATH_ATAN_H
+
+#include "shared/libc_common.h"
+#include "src/__support/math/atan.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace shared {
+
+using math::atan;
+
+} // namespace shared
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SHARED_MATH_ATAN_H

libc/src/__support/math/CMakeLists.txt

@@ -172,6 +172,33 @@ DEPENDS
     libc.src.__support.macros.optimization
 )
 
+add_header_library(
+  atan_utils
+  HDRS
+    atan_utils.h
+DEPENDS
+    libc.src.__support.integer_literals
+    libc.src.__support.FPUtil.double_double
+    libc.src.__support.FPUtil.dyadic_float
+    libc.src.__support.FPUtil.multiply_add
+    libc.src.__support.FPUtil.polyeval
+    libc.src.__support.macros.optimization
+)
+
+add_header_library(
+  atan
+  HDRS
+    atan.h
+DEPENDS
+    .atan_utils
+    libc.src.__support.FPUtil.double_double
+    libc.src.__support.FPUtil.fenv_impl
+    libc.src.__support.FPUtil.fp_bits
+    libc.src.__support.FPUtil.multiply_add
+    libc.src.__support.FPUtil.nearest_integer
+    libc.src.__support.macros.optimization
+)
+
 add_header_library(
   asinf
   HDRS

libc/src/__support/math/atan.h

@@ -0,0 +1,189 @@
+//===-- Implementation header for atan --------------------------*- 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_ATAN_H
+#define LLVM_LIBC_SRC___SUPPORT_MATH_ATAN_H
+
+#include "atan_utils.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 {
+
+// To compute atan(x), we divided it into the following cases:
+// * |x| < 2^-26:
+//      Since |x| > atan(|x|) > |x| - |x|^3/3, and |x|^3/3 < ulp(x)/2, we simply
+//      return atan(x) = x - sign(x) * epsilon.
+// * 2^-26 <= |x| < 1:
+//      We perform range reduction mod 2^-6 = 1/64 as follow:
+//      Let k = 2^(-6) * round(|x| * 2^6), then
+//        atan(x) = sign(x) * atan(|x|)
+//                = sign(x) * (atan(k) + atan((|x| - k) / (1 + |x|*k)).
+//      We store atan(k) in a look up table, and perform intermediate steps in
+//      double-double.
+// * 1 < |x| < 2^53:
+//      First we perform the transformation y = 1/|x|:
+//        atan(x) = sign(x) * (pi/2 - atan(1/|x|))
+//                = sign(x) * (pi/2 - atan(y)).
+//      Then we compute atan(y) using range reduction mod 2^-6 = 1/64 as the
+//      previous case:
+//      Let k = 2^(-6) * round(y * 2^6), then
+//        atan(y) = atan(k) + atan((y - k) / (1 + y*k))
+//                = atan(k) + atan((1/|x| - k) / (1 + k/|x|)
+//                = atan(k) + atan((1 - k*|x|) / (|x| + k)).
+// * |x| >= 2^53:
+//      Using the reciprocal transformation:
+//        atan(x) = sign(x) * (pi/2 - atan(1/|x|)).
+//      We have that:
+//        atan(1/|x|) <= 1/|x| <= 2^-53,
+//      which is smaller than ulp(pi/2) / 2.
+//      So we can return:
+//        atan(x) = sign(x) * (pi/2 - epsilon)
+
+LIBC_INLINE static constexpr double atan(double x) {
+
+  using namespace atan_internal;
+  using FPBits = fputil::FPBits<double>;
+
+  constexpr double IS_NEG[2] = {1.0, -1.0};
+  constexpr DoubleDouble PI_OVER_2 = {0x1.1a62633145c07p-54,
+                                      0x1.921fb54442d18p0};
+  constexpr DoubleDouble MPI_OVER_2 = {-0x1.1a62633145c07p-54,
+                                       -0x1.921fb54442d18p0};
+
+  FPBits xbits(x);
+  bool x_sign = xbits.is_neg();
+  xbits = xbits.abs();
+  uint64_t x_abs = xbits.uintval();
+  int x_exp =
+      static_cast<int>(x_abs >> FPBits::FRACTION_LEN) - FPBits::EXP_BIAS;
+
+  // |x| < 1.
+  if (x_exp < 0) {
+    if (LIBC_UNLIKELY(x_exp < -26)) {
+#ifdef LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+      return x;
+#else
+      if (x == 0.0)
+        return x;
+      // |x| < 2^-26
+      return fputil::multiply_add(-0x1.0p-54, x, x);
+#endif // LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+    }
+
+    double x_d = xbits.get_val();
+    // k = 2^-6 * round(2^6 * |x|)
+    double k = fputil::nearest_integer(0x1.0p6 * x_d);
+    unsigned idx = static_cast<unsigned>(k);
+    k *= 0x1.0p-6;
+
+    // numerator = |x| - k
+    DoubleDouble num, den;
+    num.lo = 0.0;
+    num.hi = x_d - k;
+
+    // denominator = 1 - k * |x|
+    den.hi = fputil::multiply_add(x_d, k, 1.0);
+    DoubleDouble prod = fputil::exact_mult(x_d, k);
+    // Using Dekker's 2SUM algorithm to compute the lower part.
+    den.lo = ((1.0 - den.hi) + prod.hi) + prod.lo;
+
+    // x_r = (|x| - k) / (1 + k * |x|)
+    DoubleDouble x_r = fputil::div(num, den);
+
+    // Approximating atan(x_r) using Taylor polynomial.
+    DoubleDouble p = atan_eval(x_r);
+
+    // atan(x) = sign(x) * (atan(k) + atan(x_r))
+    //         = sign(x) * (atan(k) + atan( (|x| - k) / (1 + k * |x|) ))
+#ifdef LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+    return IS_NEG[x_sign] * (ATAN_I[idx].hi + (p.hi + (p.lo + ATAN_I[idx].lo)));
+#else
+
+    DoubleDouble c0 = fputil::exact_add(ATAN_I[idx].hi, p.hi);
+    double c1 = c0.lo + (ATAN_I[idx].lo + p.lo);
+    double r = IS_NEG[x_sign] * (c0.hi + c1);
+
+    return r;
+#endif // LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+  }
+
+  // |x| >= 2^53 or x is NaN.
+  if (LIBC_UNLIKELY(x_exp >= 53)) {
+    // x is nan
+    if (xbits.is_nan()) {
+      if (xbits.is_signaling_nan()) {
+        fputil::raise_except_if_required(FE_INVALID);
+        return FPBits::quiet_nan().get_val();
+      }
+      return x;
+    }
+    // |x| >= 2^53
+    // atan(x) ~ sign(x) * pi/2.
+    if (x_exp >= 53)
+#ifdef LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+      return IS_NEG[x_sign] * PI_OVER_2.hi;
+#else
+      return fputil::multiply_add(IS_NEG[x_sign], PI_OVER_2.hi,
+                                  IS_NEG[x_sign] * PI_OVER_2.lo);
+#endif // LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+  }
+
+  double x_d = xbits.get_val();
+  double y = 1.0 / x_d;
+
+  // k = 2^-6 * round(2^6 / |x|)
+  double k = fputil::nearest_integer(0x1.0p6 * y);
+  unsigned idx = static_cast<unsigned>(k);
+  k *= 0x1.0p-6;
+
+  // denominator = |x| + k
+  DoubleDouble den = fputil::exact_add(x_d, k);
+  // numerator = 1 - k * |x|
+  DoubleDouble num;
+  num.hi = fputil::multiply_add(-x_d, k, 1.0);
+  DoubleDouble prod = fputil::exact_mult(x_d, k);
+  // Using Dekker's 2SUM algorithm to compute the lower part.
+  num.lo = ((1.0 - num.hi) - prod.hi) - prod.lo;
+
+  // x_r = (1/|x| - k) / (1 - k/|x|)
+  //     = (1 - k * |x|) / (|x| - k)
+  DoubleDouble x_r = fputil::div(num, den);
+
+  // Approximating atan(x_r) using Taylor polynomial.
+  DoubleDouble p = atan_eval(x_r);
+
+  // atan(x) = sign(x) * (pi/2 - atan(1/|x|))
+  //         = sign(x) * (pi/2 - atan(k) - atan(x_r))
+  //         = (-sign(x)) * (-pi/2 + atan(k) + atan((1 - k*|x|)/(|x| - k)))
+#ifdef LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+  double lo_part = p.lo + ATAN_I[idx].lo + MPI_OVER_2.lo;
+  return IS_NEG[!x_sign] * (MPI_OVER_2.hi + ATAN_I[idx].hi + (p.hi + lo_part));
+#else
+  DoubleDouble c0 = fputil::exact_add(MPI_OVER_2.hi, ATAN_I[idx].hi);
+  DoubleDouble c1 = fputil::exact_add(c0.hi, p.hi);
+  double c2 = c1.lo + (c0.lo + p.lo) + (ATAN_I[idx].lo + MPI_OVER_2.lo);
+
+  double r = IS_NEG[!x_sign] * (c1.hi + c2);
+
+  return r;
+#endif
+}
+
+} // namespace math
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC___SUPPORT_MATH_ATAN_H

libc/src/math/generic/atan_utils.h → libc/src/__support/math/atan_utils.h

@@ -18,7 +18,7 @@
 
 namespace LIBC_NAMESPACE_DECL {
 
-namespace {
+namespace atan_internal {
 
 using DoubleDouble = fputil::DoubleDouble;
 using Float128 = fputil::DyadicFloat<128>;
@@ -29,7 +29,7 @@ using Float128 = fputil::DyadicFloat<128>;
 //     b = round(atan(i/64) - a, D, RN);
 //     print("{", b, ",", a, "},");
 //   };
-constexpr DoubleDouble ATAN_I[65] = {
+static constexpr DoubleDouble ATAN_I[65] = {
     {0.0, 0.0},
     {-0x1.220c39d4dff5p-61, 0x1.fff555bbb729bp-7},
     {-0x1.5ec431444912cp-60, 0x1.ffd55bba97625p-6},
@@ -110,7 +110,8 @@ constexpr DoubleDouble ATAN_I[65] = {
 //        + x_lo * (1 - x_hi^2 + x_hi^4)
 // Since p.lo is ~ x^3/3, the relative error from rounding is bounded by:
 //   |(atan(x) - P(x))/atan(x)| < ulp(x^2) <= 2^(-14-52) = 2^-66.
-[[maybe_unused]] DoubleDouble atan_eval(const DoubleDouble &x) {
+[[maybe_unused]] LIBC_INLINE static DoubleDouble
+atan_eval(const DoubleDouble &x) {
   DoubleDouble p;
   p.hi = x.hi;
   double x_hi_sq = x.hi * x.hi;
@@ -142,7 +143,7 @@ constexpr DoubleDouble ATAN_I[65] = {
 //     b = 2^ll + a;
 //     print("{Sign::POS, ", 2^(ll - 128), ",", b, "},");
 // };
-constexpr Float128 ATAN_I_F128[65] = {
+static constexpr Float128 ATAN_I_F128[65] = {
     {Sign::POS, 0, 0_u128},
     {Sign::POS, -134, 0xfffaaadd'db94d5bb'e78c5640'15f76048_u128},
     {Sign::POS, -133, 0xffeaaddd'4bb12542'779d776d'da8c6214_u128},
@@ -215,7 +216,7 @@ constexpr Float128 ATAN_I_F128[65] = {
 //                 [0, 2^-7]);
 // > dirtyinfnorm(atan(x) - P, [0, 2^-7]);
 // 0x1.26016ad97f323875760f869684c0898d7b7bb8bep-122
-constexpr Float128 ATAN_POLY_F128[] = {
+static constexpr Float128 ATAN_POLY_F128[] = {
     {Sign::NEG, -129, 0xaaaaaaaa'aaaaaaaa'aaaaaaa6'003c5d1d_u128},
     {Sign::POS, -130, 0xcccccccc'cccccccc'cca00232'8776b063_u128},
     {Sign::NEG, -130, 0x92492492'49249201'27f5268a'cb24aec0_u128},
@@ -225,7 +226,8 @@ constexpr Float128 ATAN_POLY_F128[] = {
 };
 
 // Approximate atan for |x| <= 2^-7.
-[[maybe_unused]] Float128 atan_eval(const Float128 &x) {
+[[maybe_unused]] LIBC_INLINE static constexpr Float128
+atan_eval(const Float128 &x) {
   Float128 x_sq = fputil::quick_mul(x, x);
   Float128 x3 = fputil::quick_mul(x, x_sq);
   Float128 p = fputil::polyeval(x_sq, ATAN_POLY_F128[0], ATAN_POLY_F128[1],
@@ -234,7 +236,7 @@ constexpr Float128 ATAN_POLY_F128[] = {
   return fputil::multiply_add(x3, p, x);
 }
 
-} // anonymous namespace
+} // namespace atan_internal
 
 } // namespace LIBC_NAMESPACE_DECL
 

libc/src/math/generic/CMakeLists.txt

@@ -4020,19 +4020,6 @@ add_entrypoint_object(
     libc.src.errno.errno
 )
 
-add_header_library(
-  atan_utils
-  HDRS
-    atan_utils.h
-  DEPENDS
-    libc.src.__support.integer_literals
-    libc.src.__support.FPUtil.double_double
-    libc.src.__support.FPUtil.dyadic_float
-    libc.src.__support.FPUtil.multiply_add
-    libc.src.__support.FPUtil.polyeval
-    libc.src.__support.macros.optimization
-)
-
 add_entrypoint_object(
   atanf
   SRCS
@@ -4079,13 +4066,7 @@ add_entrypoint_object(
   COMPILE_OPTIONS
     -O3
   DEPENDS
-    .atan_utils
-    libc.src.__support.FPUtil.double_double
-    libc.src.__support.FPUtil.fenv_impl
-    libc.src.__support.FPUtil.fp_bits
-    libc.src.__support.FPUtil.multiply_add
-    libc.src.__support.FPUtil.nearest_integer
-    libc.src.__support.macros.optimization
+    libc.src.__support.math.atan
 )
 
 add_entrypoint_object(
@@ -4115,7 +4096,7 @@ add_entrypoint_object(
   HDRS
     ../atan2.h
   DEPENDS
-    .atan_utils
+    libc.src.__support.math.atan_utils
     libc.src.__support.FPUtil.double_double
     libc.src.__support.FPUtil.fenv_impl
     libc.src.__support.FPUtil.fp_bits
@@ -4141,7 +4122,7 @@ add_entrypoint_object(
   HDRS
     ../atan2f128.h
   DEPENDS
-    .atan_utils
+    libc.src.__support.math.atan_utils
     libc.src.__support.integer_literals
     libc.src.__support.uint128
     libc.src.__support.FPUtil.dyadic_float