[libc][math] Implement nexttoward functions (#72763)

Implements the `nexttoward`, `nexttowardf` and `nexttowardl` functions.
Also, raise excepts required by the standard in `nextafter` functions.

cc: @lntue

1 files changed, 22 insertions, 0 deletions

diff --git a/libc/src/math/generic/nexttowardl.cpp b/libc/src/math/generic/nexttowardl.cpp
new file mode 100644
index 000000000000..0c887ae0671b
--- /dev/null
+++ b/libc/src/math/generic/nexttowardl.cpp
@@ -0,0 +1,22 @@
+//===-- Implementation of nexttowardl 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/nexttowardl.h"
+#include "src/__support/FPUtil/ManipulationFunctions.h"
+#include "src/__support/common.h"
+
+namespace LIBC_NAMESPACE {
+
+LLVM_LIBC_FUNCTION(long double, nexttowardl, (long double x, long double y)) {
+  // We can reuse the nextafter implementation because nexttoward behaves
+  // exactly same as nextafter in case of long doubles. Also, we have explcitly
+  // handled the special 80-bit long doubles in nextafter implementation.
+  return fputil::nextafter(x, y);
+}
+
+} // namespace LIBC_NAMESPACE