[libc-commits] [libc] [libc][math][c23] Add asinf16() function (PR #124212)

[via libc-commits]

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@@ -0,0 +1,126 @@
+//===-- Half-precision asinf16(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/asinf16.h"
+#include "hdr/errno_macros.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/cast.h"
+#include "src/__support/FPUtil/multiply_add.h"
+#include "src/__support/FPUtil/sqrt.h"
+#include "src/__support/macros/optimization.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+// Generated by Sollya using the following command:
+// > round(pi/2, D, RN);
+static constexpr float PI_2 = 0x1.921fb54442d18p0f;
+
+LLVM_LIBC_FUNCTION(float16, asinf16, (float16 x)) {
+  using FPBits = fputil::FPBits<float16>;
+  FPBits xbits(x);
+
+  uint16_t x_u = xbits.uintval();
+  uint16_t x_abs = x_u & 0x7fff;
+  float xf = x;
+
+  float xsq = xf * xf;
+
+  // |x| <= 0x1p-1, |x| <= 0.5
+  if (x_abs <= 0x3800) {
+    if (LIBC_UNLIKELY(x_abs <= 0x1a1e)) {
+      // asinf16(+/-0) = +/-0
+      if (LIBC_UNLIKELY(x_abs == 0U)) {
+        return x;
+      }
+
+      // Exhaustive tests show that, when:
+      // x > 0, and rounding upward, or
+      // x < 0, and rounding downward, then,
+      // asin(x) = x * 2^-11 + x
+      // else, in other rounding modes,
+      // asin(x) = x
+      int rounding = fputil::quick_get_round();
+
+      if ((xbits.is_pos() && rounding == FE_UPWARD) ||
+          (xbits.is_neg() && rounding == FE_DOWNWARD)) {
+        return fputil::cast<float16>(fputil::multiply_add(xf, 0x1.0p-11f, xf));
+      }
+      return x;
+    }
+
+    // Degree-6 minimax odd polynomial of asin(x) generated by Sollya with:
+    // > P = fpminimax(asin(x)/x, [|0, 2, 4, 6, 8|], [|SG...|], [0, 0.5]);
+    float result =
+        fputil::polyeval(xsq, 0x1.000002p0f, 0x1.554c2ap-3f, 0x1.3541ccp-4f,
+                         0x1.43b2d6p-5f, 0x1.a0d73ep-5f);
+    return fputil::cast<float16>(xf * result);
+  }
+
+  // |x| > 1, asinf16(x) = NaN
+  if (LIBC_UNLIKELY(x_abs > 0x3c00)) {
+    // |x| <= +/-inf
+    if (LIBC_UNLIKELY(x_abs <= 0x7c00)) {
+      fputil::set_errno_if_required(EDOM);
+      fputil::raise_except_if_required(FE_INVALID);
+    }
+
+    return FPBits::quiet_nan().get_val();
----------------
wldfngrs wrote:

Should asinf16() set `EDOM` and raise `FE_INVALID` or not?

Also, here's libc `asinf()`:

https://github.com/llvm/llvm-project/blob/98d6dd39887361ff2161401614da0b7854234419/libc/src/math/generic/asinf.cpp#L105-L112

https://github.com/llvm/llvm-project/pull/124212