[libc-commits] [libc] [libc][math][c23] Add atanhf16 C23 math function. (PR #132612)

[Harrison Hao via libc-commits]

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@@ -0,0 +1,86 @@
+//===-- Implementation of atanh(x) 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/atanhf16.h"
+#include "explogxf.h"
+#include "hdr/errno_macros.h"
+#include "hdr/fenv_macros.h"
+#include "src/__support/FPUtil/FPBits.h"
+#include "src/__support/FPUtil/PolyEval.h"
+#include "src/__support/FPUtil/cast.h"
+#include "src/__support/FPUtil/multiply_add.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/optimization.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(float16, atanhf16, (float16 x)) {
+  using FPBits = typename fputil::FPBits<float16>;
+
+  FPBits xbits(x);
+  Sign sign = xbits.sign();
+  uint16_t x_abs = xbits.abs().uintval();
+
+  if (LIBC_UNLIKELY(x_abs >= 0x3c00U)) {
+    if (xbits.is_nan()) {
+      return x;
+    }
+    // |x| == 1.0
+    if (x_abs == 0x3c00U) {
+      fputil::set_errno_if_required(ERANGE);
+      fputil::raise_except_if_required(FE_DIVBYZERO);
+      return FPBits::inf(sign).get_val();
+    } else {
+      fputil::set_errno_if_required(EDOM);
+      fputil::raise_except_if_required(FE_INVALID);
+      return FPBits::quiet_nan().get_val();
+    }
+  }
+
+  // For |x| less than approximately 0.10
+  if (LIBC_UNLIKELY(x_abs <= 0x2e66U)) {
+    // The Taylor expansion of atanh(x) is:
+    //    atanh(x) = x + x^3/3 + x^5/5 + x^7/7 + x^9/9 + x^11/11
+    //             = x * [1 + x^2/3 + x^4/5 + x^6/7 + x^8/9 + x^10/11]
+    // When |x| < 0x0100U, this can be approximated by:
+    //    atanh(x) ≈ x + (1/3)*x^3
+    if (LIBC_UNLIKELY(x_abs < 0x0100U)) {
+      return static_cast<float16>(
+          LIBC_UNLIKELY(x_abs == 0) ? x : (x + 0x1.555556p-2 * x * x * x));
+    }
+
+    // For 0x0100U <= |x| <= 0x2e66U:
+    //   Let t = x^2.
+    //   Define P(t) ≈ (1/3)*t + (1/5)*t^2 + (1/7)*t^3 + (1/9)*t^4 + (1/11)*t^5.
+    // The coefficients below were derived using Sollya:
+    //   > display = hexadecimal;
+    //   > round(1/3, SG, RN);
+    //   > round(1/5, SG, RN);
+    //   > round(1/7, SG, RN);
+    //   > round(1/9, SG, RN);
+    //   > round(1/11, SG, RN);
+    // This yields:
+    //   0x1.555556p-2
+    //   0x1.99999ap-3
+    //   0x1.24924ap-3
+    //   0x1.c71c72p-4
+    //   0x1.745d18p-4f
----------------
harrisonGPU wrote:

I have updated it, what do you think about it? @overmighty 

https://github.com/llvm/llvm-project/pull/132612