[libc-commits] [libc] [libc][math][c23] Add tanf16 function (PR #121018)

[via libc-commits]

================
@@ -0,0 +1,112 @@
+//===-- Half-precision tan(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/tanf16.h"
+#include "hdr/errno_macros.h"
+#include "hdr/fenv_macros.h"
+#include "sincosf16_utils.h"
+#include "src/__support/FPUtil/FEnvImpl.h"
+#include "src/__support/FPUtil/FPBits.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/macros/optimization.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+constexpr size_t N_EXCEPTS = 9;
+
+constexpr fputil::ExceptValues<float16, N_EXCEPTS> TANF16_EXCEPTS{{
+    // (input, RZ output, RU offset, RD offset, RN offset)
+    {0x2894, 0x2894, 1, 0, 1},
+    {0x3091, 0x3099, 1, 0, 0},
+    {0x3098, 0x30a0, 1, 0, 0},
+    {0x55ed, 0x3911, 1, 0, 0},
+    {0x607b, 0xc638, 0, 1, 1},
+    {0x674e, 0x3b7d, 1, 0, 0},
+    {0x6807, 0x4014, 1, 0, 1},
+    {0x6f4d, 0xbe19, 0, 1, 1},
+    {0x7330, 0xcb62, 0, 1, 0},
+}};
+
+LLVM_LIBC_FUNCTION(float16, tanf16, (float16 x)) {
+  using FPBits = fputil::FPBits<float16>;
+  FPBits xbits(x);
+
+  uint16_t x_u = xbits.uintval();
+  uint16_t x_abs = x_u & 0x7fff;
+  bool x_sign = x_u >> 15;
+  float xf = x;
+
+  // Handle exceptional values
+  if (auto r = TANF16_EXCEPTS.lookup_odd(x_abs, x_sign);
+      LIBC_UNLIKELY(r.has_value()))
+    return r.value();
+
+  // |x| <= 0x1.d1p-5
+  if (LIBC_UNLIKELY(x_abs <= 0x2b44)) {
+    if (LIBC_UNLIKELY(x_abs <= 0x10e6)) {
+      // tan(+/-0) = +/-0
+      if (LIBC_UNLIKELY(x_abs == 0U))
+        return x;
+
+      int rounding = fputil::quick_get_round();
+
+      // Exhaustive tests show that, when:
+      // x > 0, and rounding upward or
+      // x < 0, and rounding downward then,
+      // tan(x) = x * 2^-11 + x
+      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));
+      else
+        return x;
+    }
+
+    float xsq = xf * xf;
+
+    float result = fputil::polyeval(xsq, 0x1p0f, 0x1.555556p-2f, 0x1.110ee4p-3f,
+                                    0x1.be80f6p-5f);
+
+    return fputil::cast<float16>(xf * result);
+  }
+
+  // tan(+/-inf)= NaN, and tan(NaN) = NaN
----------------
overmighty wrote:

Nit:

```suggestion
  // tan(+/-inf) = NaN, and tan(NaN) = NaN
```

https://github.com/llvm/llvm-project/pull/121018