[libc][math] Refactor exp2m1f implementation to header-only in src/__support/math folder.

libc/shared/math.h

@@ -50,6 +50,7 @@
 #include "math/exp2.h"
 #include "math/exp2f.h"
 #include "math/exp2f16.h"
+#include "math/exp2m1f.h"
 #include "math/expf.h"
 #include "math/expf16.h"
 #include "math/frexpf.h"

libc/shared/math/exp2m1f.h

@@ -0,0 +1,23 @@
+//===-- Shared exp2m1f 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_EXP2M1F_H
+#define LLVM_LIBC_SHARED_MATH_EXP2M1F_H
+
+#include "shared/libc_common.h"
+#include "src/__support/math/exp2m1f.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace shared {
+
+using math::exp2m1f;
+
+} // namespace shared
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SHARED_MATH_EXP2M1F_H

libc/src/__support/math/CMakeLists.txt

@@ -767,6 +767,24 @@ add_header_library(
     libc.src.__support.macros.optimization
 )
 
+add_header_library(
+  exp2m1f
+  HDRS
+    exp2m1f.h
+  DEPENDS
+    .exp10f_utils
+    libc.src.errno.errno
+    libc.src.__support.common
+    libc.src.__support.FPUtil.except_value_utils
+    libc.src.__support.FPUtil.fenv_impl
+    libc.src.__support.FPUtil.fp_bits
+    libc.src.__support.FPUtil.multiply_add
+    libc.src.__support.FPUtil.polyeval
+    libc.src.__support.FPUtil.rounding_mode
+    libc.src.__support.macros.optimization
+    libc.src.__support.macros.properties.cpu_features
+)
+
 add_header_library(
   exp10
   HDRS

libc/src/__support/math/exp2m1f.h

@@ -0,0 +1,195 @@
+//===-- Implementation header for exp2m1f ------------------------*- 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_EXP2M1F_H
+#define LLVM_LIBC_SRC___SUPPORT_MATH_EXP2M1F_H
+
+#include "exp10f_utils.h"
+#include "src/__support/FPUtil/FEnvImpl.h"
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/FPUtil/PolyEval.h"
+#include "src/__support/FPUtil/except_value_utils.h"
+#include "src/__support/FPUtil/multiply_add.h"
+#include "src/__support/FPUtil/rounding_mode.h"
+#include "src/__support/common.h"
+#include "src/__support/libc_errno.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/optimization.h"
+#include "src/__support/macros/properties/cpu_features.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+namespace math {
+
+LIBC_INLINE static constexpr float exp2m1f(float x) {
+#ifndef LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+  constexpr size_t N_EXCEPTS_LO = 8;
+
+  constexpr fputil::ExceptValues<float, N_EXCEPTS_LO> EXP2M1F_EXCEPTS_LO = {{
+      // (input, RZ output, RU offset, RD offset, RN offset)
+      // x = 0x1.36dc8ep-36, exp2m1f(x) = 0x1.aef212p-37 (RZ)
+      {0x2d9b'6e47U, 0x2d57'7909U, 1U, 0U, 0U},
+      // x = 0x1.224936p-19, exp2m1f(x) = 0x1.926c0ep-20 (RZ)
+      {0x3611'249bU, 0x35c9'3607U, 1U, 0U, 1U},
+      // x = 0x1.d16d2p-20, exp2m1f(x) = 0x1.429becp-20 (RZ)
+      {0x35e8'b690U, 0x35a1'4df6U, 1U, 0U, 1U},
+      // x = 0x1.17949ep-14, exp2m1f(x) = 0x1.8397p-15 (RZ)
+      {0x388b'ca4fU, 0x3841'cb80U, 1U, 0U, 1U},
+      // x = -0x1.9c3e1ep-38, exp2m1f(x) = -0x1.1dbeacp-38 (RZ)
+      {0xacce'1f0fU, 0xac8e'df56U, 0U, 1U, 0U},
+      // x = -0x1.4d89b4p-32, exp2m1f(x) = -0x1.ce61b6p-33 (RZ)
+      {0xafa6'c4daU, 0xaf67'30dbU, 0U, 1U, 1U},
+      // x = -0x1.a6eac4p-10, exp2m1f(x) = -0x1.24fadap-10 (RZ)
+      {0xbad3'7562U, 0xba92'7d6dU, 0U, 1U, 1U},
+      // x = -0x1.e7526ep-6, exp2m1f(x) = -0x1.4e53dep-6 (RZ)
+      {0xbcf3'a937U, 0xbca7'29efU, 0U, 1U, 1U},
+  }};
+
+  constexpr size_t N_EXCEPTS_HI = 3;
+
+  constexpr fputil::ExceptValues<float, N_EXCEPTS_HI> EXP2M1F_EXCEPTS_HI = {{
+      // (input, RZ output, RU offset, RD offset, RN offset)
+      // x = 0x1.16a972p-1, exp2m1f(x) = 0x1.d545b2p-2 (RZ)
+      {0x3f0b'54b9U, 0x3eea'a2d9U, 1U, 0U, 0U},
+      // x = -0x1.9f12acp-5, exp2m1f(x) = -0x1.1ab68cp-5 (RZ)
+      {0xbd4f'8956U, 0xbd0d'5b46U, 0U, 1U, 0U},
+      // x = -0x1.de7b9cp-5, exp2m1f(x) = -0x1.4508f4p-5 (RZ)
+      {0xbd6f'3dceU, 0xbd22'847aU, 0U, 1U, 1U},
+  }};
+#endif // !LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+
+  using FPBits = fputil::FPBits<float>;
+  FPBits xbits(x);
+
+  uint32_t x_u = xbits.uintval();
+  uint32_t x_abs = x_u & 0x7fff'ffffU;
+
+  // When |x| >= 128, or x is nan, or |x| <= 2^-5
+  if (LIBC_UNLIKELY(x_abs >= 0x4300'0000U || x_abs <= 0x3d00'0000U)) {
+    // |x| <= 2^-5
+    if (x_abs <= 0x3d00'0000U) {
+#ifndef LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+      if (auto r = EXP2M1F_EXCEPTS_LO.lookup(x_u); LIBC_UNLIKELY(r.has_value()))
+        return r.value();
+#endif // !LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+
+      // Minimax polynomial generated by Sollya with:
+      // > display = hexadecimal;
+      // > fpminimax((2^x - 1)/x, 5, [|D...|], [-2^-5, 2^-5]);
+      constexpr double COEFFS[] = {
+          0x1.62e42fefa39f3p-1, 0x1.ebfbdff82c57bp-3,  0x1.c6b08d6f2d7aap-5,
+          0x1.3b2ab6fc92f5dp-7, 0x1.5d897cfe27125p-10, 0x1.43090e61e6af1p-13};
+      double xd = x;
+      double xsq = xd * xd;
+      double c0 = fputil::multiply_add(xd, COEFFS[1], COEFFS[0]);
+      double c1 = fputil::multiply_add(xd, COEFFS[3], COEFFS[2]);
+      double c2 = fputil::multiply_add(xd, COEFFS[5], COEFFS[4]);
+      double p = fputil::polyeval(xsq, c0, c1, c2);
+      return static_cast<float>(p * xd);
+    }
+
+    // x >= 128, or x is nan
+    if (xbits.is_pos()) {
+      if (xbits.is_finite()) {
+        int rounding = fputil::quick_get_round();
+        if (rounding == FE_DOWNWARD || rounding == FE_TOWARDZERO)
+          return FPBits::max_normal().get_val();
+
+        fputil::set_errno_if_required(ERANGE);
+        fputil::raise_except_if_required(FE_OVERFLOW);
+      }
+
+      // x >= 128 and 2^x - 1 rounds to +inf, or x is +inf or nan
+      return x + FPBits::inf().get_val();
+    }
+  }
+
+  if (LIBC_UNLIKELY(x <= -25.0f)) {
+    // 2^(-inf) - 1 = -1
+    if (xbits.is_inf())
+      return -1.0f;
+    // 2^nan - 1 = nan
+    if (xbits.is_nan())
+      return x;
+
+    int rounding = fputil::quick_get_round();
+    if (rounding == FE_UPWARD || rounding == FE_TOWARDZERO)
+      return -0x1.ffff'fep-1f; // -1.0f + 0x1.0p-24f
+
+    fputil::set_errno_if_required(ERANGE);
+    fputil::raise_except_if_required(FE_UNDERFLOW);
+    return -1.0f;
+  }
+
+#ifndef LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+  if (auto r = EXP2M1F_EXCEPTS_HI.lookup(x_u); LIBC_UNLIKELY(r.has_value()))
+    return r.value();
+#endif // !LIBC_MATH_HAS_SKIP_ACCURATE_PASS
+
+  // For -25 < x < 128, to compute 2^x, we perform the following range
+  // reduction: find hi, mid, lo such that:
+  //   x = hi + mid + lo, in which:
+  //     hi is an integer,
+  //     0 <= mid * 2^5 < 32 is an integer,
+  //     -2^(-6) <= lo <= 2^(-6).
+  // In particular,
+  //   hi + mid = round(x * 2^5) * 2^(-5).
+  // Then,
+  //   2^x = 2^(hi + mid + lo) = 2^hi * 2^mid * 2^lo.
+  // 2^mid is stored in the lookup table of 32 elements.
+  // 2^lo is computed using a degree-4 minimax polynomial generated by Sollya.
+  // We perform 2^hi * 2^mid by simply add hi to the exponent field of 2^mid.
+
+  // kf = (hi + mid) * 2^5 = round(x * 2^5)
+  float kf = 0;
+  int k = 0;
+#ifdef LIBC_TARGET_CPU_HAS_NEAREST_INT
+  kf = fputil::nearest_integer(x * 32.0f);
+  k = static_cast<int>(kf);
+#else
+  constexpr float HALF[2] = {0.5f, -0.5f};
+  k = static_cast<int>(fputil::multiply_add(x, 32.0f, HALF[x < 0.0f]));
+  kf = static_cast<float>(k);
+#endif // LIBC_TARGET_CPU_HAS_NEAREST_INT
+
+  // lo = x - (hi + mid) = x - kf * 2^(-5)
+  double lo = fputil::multiply_add(-0x1.0p-5f, kf, x);
+
+  // hi = floor(kf * 2^(-4))
+  // exp2_hi = shift hi to the exponent field of double precision.
+  int64_t exp2_hi =
+      static_cast<int64_t>(static_cast<uint64_t>(k >> ExpBase::MID_BITS)
+                           << fputil::FPBits<double>::FRACTION_LEN);
+  // mh = 2^hi * 2^mid
+  // mh_bits = bit field of mh
+  int64_t mh_bits = ExpBase::EXP_2_MID[k & ExpBase::MID_MASK] + exp2_hi;
+  double mh = fputil::FPBits<double>(static_cast<uint64_t>(mh_bits)).get_val();
+
+  // Degree-4 polynomial approximating (2^x - 1)/x generated by Sollya with:
+  // > display = hexadecimal;
+  // > fpminimax((2^x - 1)/x, 4, [|D...|], [-2^-6, 2^-6]);
+  constexpr double COEFFS[5] = {0x1.62e42fefa39efp-1, 0x1.ebfbdff8131c4p-3,
+                                0x1.c6b08d7061695p-5, 0x1.3b2b1bee74b2ap-7,
+                                0x1.5d88091198529p-10};
+  double lo_sq = lo * lo;
+  double c1 = fputil::multiply_add(lo, COEFFS[0], 1.0);
+  double c2 = fputil::multiply_add(lo, COEFFS[2], COEFFS[1]);
+  double c3 = fputil::multiply_add(lo, COEFFS[4], COEFFS[3]);
+  double exp2_lo = fputil::polyeval(lo_sq, c1, c2, c3);
+  // 2^x - 1 = 2^(hi + mid + lo) - 1
+  //         = 2^(hi + mid) * 2^lo - 1
+  //         ~ mh * (1 + lo * P(lo)) - 1
+  //         = mh * exp2_lo - 1
+  return static_cast<float>(fputil::multiply_add(exp2_lo, mh, -1.0));
+}
+
+} // namespace math
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC___SUPPORT_MATH_EXP2M1F_H

libc/src/math/generic/CMakeLists.txt

@@ -1478,17 +1478,7 @@ add_entrypoint_object(
   HDRS
     ../exp2m1f.h
   DEPENDS
-    libc.src.errno.errno
-    libc.src.__support.common
-    libc.src.__support.FPUtil.except_value_utils
-    libc.src.__support.FPUtil.fenv_impl
-    libc.src.__support.FPUtil.fp_bits
-    libc.src.__support.FPUtil.multiply_add
-    libc.src.__support.FPUtil.polyeval
-    libc.src.__support.FPUtil.rounding_mode
-    libc.src.__support.macros.optimization
-    libc.src.__support.macros.properties.cpu_features
-    libc.src.__support.math.exp10f_utils
+    libc.src.__support.math.exp2m1f
 )
 
 add_entrypoint_object(