[libc-commits] [libc] [libc][math] Implement double precision cbrt correctly rounded to all rounding modes. (PR #99262)

[via libc-commits]

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@@ -0,0 +1,340 @@
+//===-- Implementation of cbrt 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/math/cbrt.h"
+#include "hdr/fenv_macros.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/dyadic_float.h"
+#include "src/__support/FPUtil/multiply_add.h"
+#include "src/__support/common.h"
+#include "src/__support/integer_literals.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/optimization.h" // LIBC_UNLIKELY
+
+#if ((LIBC_MATH & LIBC_MATH_SKIP_ACCURATE_PASS) != 0)
+#define LIBC_MATH_CBRT_SKIP_ACCURATE_PASS
+#endif
+
+namespace LIBC_NAMESPACE_DECL {
+
+using DoubleDouble = fputil::DoubleDouble;
+using Float128 = typename fputil::DyadicFloat<128>;
+
+namespace {
+
+// Initial approximation of x^(-2/3) for 1 <= x < 2.
+// Polynomial generated by Sollya with:
+// > P = fpminimax(x^(-2/3), 7, [|D...|], [1, 2]);
+// > dirtyinfnorm(P/x^(-2/3) - 1, [1, 2]);
+// 0x1.28...p-21
+constexpr double intial_approximation(double x) {
+  constexpr double COEFFS[8] = {
+      0x1.bc52aedead5c6p1,  -0x1.b52bfebf110b3p2,  0x1.1d8d71d53d126p3,
+      -0x1.de2db9e81cf87p2, 0x1.0154ca06153bdp2,   -0x1.5973c66ee6da7p0,
+      0x1.07bf6ac832552p-2, -0x1.5e53d9ce41cb8p-6,
+  };
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overmighty wrote:

Do we have a policy on when to use C-style arrays, and when to use `cpp::array`?

https://github.com/llvm/llvm-project/pull/99262