[libc] [llvm] [libc][math] Refactor tanhf16 implementation to header-only in src/__support/math folder. (PR #178645)

[Muhammad Bassiouni via llvm-commits]

================
@@ -7,146 +7,10 @@
 //===----------------------------------------------------------------------===//
 
 #include "src/math/tanhf16.h"
-#include "hdr/fenv_macros.h"
-#include "src/__support/CPP/array.h"
-#include "src/__support/FPUtil/FEnvImpl.h"
-#include "src/__support/FPUtil/FPBits.h"
-#include "src/__support/FPUtil/PolyEval.h"
-#include "src/__support/FPUtil/cast.h"
-#include "src/__support/FPUtil/except_value_utils.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/common.h"
-#include "src/__support/macros/config.h"
-#include "src/__support/macros/optimization.h"
-#include "src/__support/math/expxf16_utils.h"
+#include "src/__support/math/tanhf16.h"
 
 namespace LIBC_NAMESPACE_DECL {
 
-#ifndef LIBC_MATH_HAS_SKIP_ACCURATE_PASS
-static constexpr fputil::ExceptValues<float16, 2> TANHF16_EXCEPTS = {{
-    // x = 0x1.f54p+0, tanhf16(x) = 0x1.ecp-1 (RZ)
-    {0x3fd5U, 0x3bb0U, 1U, 0U, 0U},
-    // x = -0x1.f54p+0, tanhf16(x) = -0x1.ecp-1 (RZ)
-    {0xbfd5U, 0xbbb0U, 0U, 1U, 0U},
-}};
-#endif // !LIBC_MATH_HAS_SKIP_ACCURATE_PASS
-
-LLVM_LIBC_FUNCTION(float16, tanhf16, (float16 x)) {
-  using namespace math::expxf16_internal;
-  using FPBits = fputil::FPBits<float16>;
-  FPBits x_bits(x);
-
-  uint16_t x_u = x_bits.uintval();
-  uint16_t x_abs = x_u & 0x7fffU;
-
-  // When -2^(-14) <= x <= -2^(-9), or |x| <= 0x1.d2p-4,
-  // or |x| >= atanh(1 - 2^(-11)), or x is NaN.
-  if (LIBC_UNLIKELY(x_abs <= 0x2f48U || x_abs >= 0x4429U)) {
-    // tanh(NaN) = NaN
-    if (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;
-    }
-
-    // When -2^(-14) <= x <= -2^(-9).
-    if (x_u >= 0x8400U && x_u <= 0x9800U) {
-      switch (fputil::quick_get_round()) {
-      case FE_TONEAREST:
-      case FE_DOWNWARD:
-        return x;
-      default:
-        return FPBits(static_cast<uint16_t>(x_u - 1U)).get_val();
-      }
-    }
-
-    // When |x| <= 0x1.d2p-4.
-    if (x_abs <= 0x2f48U) {
-      if (LIBC_UNLIKELY(x_abs == 0))
-        return x;
-
-      float xf = x;
-      float xf_sq = xf * xf;
-      // Degree-7 Taylor expansion generated by Sollya with the following
-      // commands:
-      //   > taylor(tanh(x), 7, 0);
-      //   > display = hexadecimal;
-      //   > // For each coefficient:
-      //   > round(/* put coefficient here */, SG, RN);
-      return fputil::cast<float16>(
-          xf * fputil::polyeval(xf_sq, 0x1p+0f, -0x1.555556p-2f, 0x1.111112p-3f,
-                                -0x1.ba1ba2p-5f));
-    }
-
-    // tanh(+/-inf) = +/-1
-    if (x_bits.is_inf())
-      return FPBits::one(x_bits.sign()).get_val();
-
-    // When |x| >= atanh(1 - 2^(-11)).
-    fputil::raise_except_if_required(FE_INEXACT);
-
-    int rounding_mode = fputil::quick_get_round();
-    if ((rounding_mode == FE_TONEAREST && x_abs >= 0x4482U) ||
-        (rounding_mode == FE_UPWARD && x_bits.is_pos()) ||
-        (rounding_mode == FE_DOWNWARD && x_bits.is_neg())) {
-      return FPBits::one(x_bits.sign()).get_val();
-    }
-    if (x_bits.is_pos())
-      return fputil::cast<float16>(0x1.ffcp-1);
-    return fputil::cast<float16>(-0x1.ffcp-1);
-  }
-
-#ifndef LIBC_MATH_HAS_SKIP_ACCURATE_PASS
-  if (auto r = TANHF16_EXCEPTS.lookup(x_u); LIBC_UNLIKELY(r.has_value()))
-    return r.value();
-#endif // !LIBC_MATH_HAS_SKIP_ACCURATE_PASS
-
-  // For atanh(-1 + 2^(-11)) < x < atanh(1 - 2^(-11)), to compute tanh(x), we
-  // perform the following range reduction: find hi, mid, lo, such that:
-  //   x = (hi + mid) * log(2) * 0.5 + lo, in which
-  //     hi is an integer,
-  //     mid * 2^5 is an integer,
-  //     -2^(-5) <= lo < 2^(-5).
-  // In particular,
-  //   hi + mid = round(x * log2(e) * 2 * 2^5) * 2^(-5).
-  // Then,
-  //   tanh(x) = sinh(x)/cosh(x)
-  //           = (e^x - e^(-x)) / (e^x + e^(-x))
-  //           = (e^(2x) - 1) / (e^(2x) + 1)
-  //           = (2^(hi + mid) * e^(2*lo) - 1) / (2^(hi + mid) * e^(2*lo) + 1)
-  //           = (e^(2*lo) - 2^(-hi - mid)) / (e^(2*lo) + 2^(-hi - mid))
-  // We store 2^(-mid) in the lookup table EXP2_MID_5_BITS, and compute
-  // 2^(-hi - mid) by adding -hi to the exponent field of 2^(-mid).
-  // e^lo is computed using a degree-3 minimax polynomial generated by Sollya.
-
-  float xf = x;
-  float kf = fputil::nearest_integer(xf * (LOG2F_E * 2.0f * 0x1.0p+5f));
-  int x_hi_mid = -static_cast<int>(kf);
-  unsigned x_hi = static_cast<unsigned>(x_hi_mid) >> 5;
-  unsigned x_mid = static_cast<unsigned>(x_hi_mid) & 0x1f;
-  // lo = x - (hi + mid)
-  //    = round(x * log2(e) * 2 * 2^5) * log(2) * 0.5 * (-2^(-5)) + x
-  float lo = fputil::multiply_add(kf, LOGF_2 * 0.5f * -0x1.0p-5f, xf);
-
-  uint32_t exp2_hi_mid_bits =
-      EXP2_MID_5_BITS[x_mid] +
-      static_cast<uint32_t>(x_hi << fputil::FPBits<float>::FRACTION_LEN);
-  // exp2_hi_mid = 2^(-hi - mid)
-  float exp2_hi_mid = fputil::FPBits<float>(exp2_hi_mid_bits).get_val();
-  // Degree-3 minimax polynomial generated by Sollya with the following
-  // commands:
-  //   > display = hexadecimal;
-  //   > P = fpminimax(expm1(2*x)/x, 2, [|SG...|], [-2^-5, 2^-5]);
-  //   > 1 + x * P;
-  float exp_2lo =
-      fputil::polyeval(lo, 0x1p+0f, 0x1p+1f, 0x1.001p+1f, 0x1.555ddep+0f);
-  return fputil::cast<float16>((exp_2lo - exp2_hi_mid) /
-                               (exp_2lo + exp2_hi_mid));
-}
+LLVM_LIBC_FUNCTION(float16, tanhf16, (float16 x)) { return math::tanhf16(x);}
----------------
bassiounix wrote:

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https://github.com/llvm/llvm-project/pull/178645