[libc-commits] [libc] [llvm] [libc][math] Implement double-precision acosh (PR #199953)

[via libc-commits]

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+//===-- Implementation header for acosh -------------------------*- 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_ACOSH_H
+#define LLVM_LIBC_SRC___SUPPORT_MATH_ACOSH_H
+
+#include "log.h"
+#include "log1p.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/multiply_add.h"
+#include "src/__support/FPUtil/sqrt.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/optimization.h" // LIBC_UNLIKELY
+#include "src/__support/macros/properties/cpu_features.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+namespace math {
+
+// Compute log1p(u_hi + u_lo) using log1p's step-1 range reduction and
+// polynomial, with the Float128 accurate path for correct rounding.
+LIBC_INLINE double acosh_log1p_dd(double u_hi, double u_lo) {
+  using FPBits = fputil::FPBits<double>;
+  using DoubleDouble = fputil::DoubleDouble;
+  using namespace log1p_internal;
+  using namespace common_constants_internal;
+
+  constexpr int EXP_BIAS = FPBits::EXP_BIAS;
+  constexpr int FRACTION_LEN = FPBits::FRACTION_LEN;
+
+  // Knuth's 2Sum (exact_add<false>) is correct for all rounding modes.
+  DoubleDouble x_dd = fputil::exact_add<false>(u_hi, 1.0);
+  x_dd.lo += u_lo;
+
+  // Step-1 range reduction (identical to log1p's fast path).
----------------
lntue wrote:

Don't duplicate the core part of `log1p` implementation here.  Refactor it to a common function to be reused by both `log1p` and `acosh`.  The same with `math::log` usage below.

https://github.com/llvm/llvm-project/pull/199953